Less Than 1% of Core-collapse Supernovae in the Local Universe Occur in Elliptical Galaxies

As of 2020 December 30, the BTS sample contains 4018 spectroscopically classified SNe. Of these, we select SNe satisfying the following criteria:

• 1.  
  The redshift is 0.015 < z < 0.1. The lower bound is meant to avoid shredding of nearby galaxies into multiple sources in galaxy catalogs and the upper bound is due to the reduced completeness of galaxy redshift catalogs (Fremling et al. 2020), required to ensure the SN and the galaxy are indeed associated.
• 2.  
  The likely host galaxies of the ZTF BTS SNe are continuously identified in the BTS sample explorer ³² (Perley et al. 2020). This is done by an automatic cross matching of the SN positions with the nearest galaxies in the Pan-STARRS (PS1; Chambers et al. 2016) or SDSS (Alam et al. 2015) photometric catalogs. Host-galaxy matching is complete out to an offset of <90'' and <30 kpc (projected distance) and employs a criterion to distinguish between multiple host-galaxy associations. We only select SNe that are associated with a host galaxy in this way.

Of the 4018 SNe in the sample, 3855 had a redshift in the required range. Among those, we could identify the most likely host galaxy of 3330 SNe. We cross match these host galaxies with the Galaxy Evolution Explorer (GALEX) Data Release (DR) 8/9 (Martin et al. 2005) and the ALLWISE (Cutri et al. 2021) catalogs and obtain the FUV (1542 Å), NUV (2274 Å) and W1–W4 [33,500 Å--220,000 Å] IR photometry. We correct the results for Galactic reddening using the maps of Schlafly & Finkbeiner (2011). Queries were performed from the VizieR Catalog (Ochsenbein et al. 2000) using astroquery(Ginsburg 2019). We use the host photometry to define a sample of CCSNe with early-type hosts. These are defined here as:

• 1.  
  Galaxies with W2 − W3 < 0.5 mag and NUV − r_PS1 >3 mag, if both the W2 − W3 color and the NUV − r_PS1 colors are available.
• 2.  
  Or galaxies with W2 − W3 < 0.3, if only the W2 − W3 color is available.

This definition is satisfied by 75% of elliptical galaxies in the Galaxy Zoo (Lintott et al. 2011) sample. It is chosen so that it includes all regions in the color–color parameter space that have a higher number of ellipticals compared to spirals, calculated in 0.5 mag bins.

Next, we search for any SN not classified as an SN Ia, and examine the host morphology visually. To be considered a CCSN in an elliptical host, we require:

• 1.  
  Independent and consistent spectroscopic redshift measurements of both the SN and the host galaxy from publicly available catalogs.
• 2.  
  Visual confirmation of the red color and absence of a bar, spiral arms, or a disk structure in the deepest available images.
• 3.  
  Once aperture-matched photometry and source deblending is performed, the host galaxy still occupies our double-color region for elliptical galaxies.
• 4.  
  Confirmation of a CCSN classification: SN classifications in the BTS (Fremling et al. 2020) are made using both human inspection and using SuperNova IDentification (SNID; Blondin & Tonry 2007), and are thus generally reliable. However, since CCSNe generally avoid elliptical galaxies, we take special care to avoid misclassified SNe Ia and require verification of the SN type through a visual inspection of the spectrum and by fitting the spectra using a python adaptation of Superfit (Howell et al. 2005) with an updated spectrum template bank (S. Goldwasser et al., in preparation).

Twenty-one CCSNe pass our color criteria, of which only 4 SN hosts emerged as having elliptical morphology—SN 2020uik (SN II), SN 2018fsh (SN II), SN 2019ape (SN Ic) and SN 2019cmv (SLSN-II). While SN 2019cmv passes our sample criteria, we exclude it due to the combination of having a high offset from the nearby elliptical galaxy, and the shallow limits on an underlying host at the SN site. At the distance of SN 2019cmv, we cannot rule out the presence of a faint, underlying host with a brightness of ≥14 mag. Such low-mass hosts have been observed for several SLSNe (Perley et al. 2016; Schulze et al. 2018), and are the most likely explanation for the unusual location of SN 2019cmv. SN 2020oce, classified as a SN Ic in the BTS, also passes our sample criteria, but we find that it is fit well by spectra of 91bg-like SNe Ia, and so exclude it from our sample. In Figure 1 we show PS1 image cutouts of the three CCSN host galaxies from the BTS matching our criteria. The images are constructed using the method described in Lupton et al. (2004).

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The data points in the left panel of Figure 2 show the UV–optical and mid-infrared (MIR) colors of CCSN hosts compared to the colors of elliptical and spiral galaxies from the Galaxy Zoo catalog (Lintott et al. 2011). In this plot, a good separation is achieved between the colors of elliptical and spiral galaxies. In the right panel of Figure 2 we show the same color–color plot for the host galaxies of SNe Ia. As expected, the vast majority of CCSNe occur in the region occupied by spirals. In Figure 3 we show the mean observed colors of spirals and ellipticals from the Galaxy Zoo catalog normalized to the W2 band and compare them with the colors of elliptical host galaxies of CCSNe. As this figure shows, the most significant difference between the spectral energy distribution (SED) of spiral and elliptical galaxies is seen in the UV and in the MIR. The elliptical host galaxies of CCSNe have SEDs similar to galaxy zoo ellipticals.

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In addition to the objects presented in this paper, we compile a sample of CCSNe in elliptical galaxies from the literature. We apply the criteria in Section 2.1 to the CCSN sample (Schulze et al. 2021) from the Palomar Transient Factory (PTF; Law et al. 2009) and the intermediate Palomar Transient Factory (iPTF; Kulkarni 2013), and to published CCSNe from Suh et al. (2011), Hakobyan et al. (2008), Graham et al. (2012), and Sanders et al. (2013). Our findings generally agree with the conclusions of these papers—that most CCSNe near elliptical galaxies occurred in either misclassified spirals or star-forming ellipticals. A small minority of the SNe analyzed in these papers do pass our criteria, and we combine these with our BTS objects to form a combined sample. We further require that objects in our sample have public spectra, so that the classification can be confirmed. In total, our combined sample includes 10 SNe, listed in Table 1, along with their classifications and estimated peak luminosities. We note that the host of the Type II SN Abell 399 11 19 0 (Graham et al. 2012) did not pass our sample inclusion criteria since it is not detected in both the NUV and W3 bands. However, the available limits (W2 − W3 < 0.06 mag, NUV − r_PS1 > 4.5 mag), along with the galaxy morphology, indicate this host is also an elliptical galaxy.

Throughout this work, we use several SN and galaxy samples for comparison. These are:

• 1.  
  BTS CCSNe: we selected all BTS SNe classified as CCSNe that occurred in host galaxies with measured WISE W2 − W3 and UV–optical colors. This sample consists of 478 objects, of which 465 fall outside of the region that we associate with elliptical galaxies in Figure 2.
• 2.  
  SNe Ia elliptical galaxies: we selected all BTS SNe (Perley et al. 2020) classified as SNe Ia that occurred in host galaxies with both W2 − W3 < 0.5 mag and NUV − r_PS1 > 3 mag. This sample contains 240 objects.
• 3.  
  Galaxy Zoo (Lintott et al. 2011) galaxies: we randomly selected 50,000 galaxies from the Galaxy Zoo sample that have morphological classifications and with a redshift in the range 0.015 < z < 0.05. Of these galaxies, 1309 (10265) were classified as elliptical (spiral). Queries were performed using CasJobs (OMullane et al. 2005). We note that Lintott et al. (2011) use a broad categorization of galaxies into ellipticals and spirals. The elliptical group includes both elliptical (E) and lenticular (S0) galaxies.

2.4.1. SN 2018fsh

2.4.2. SN 2019ape

2.4.3. SN 2020uik

We queried the NASA/IPAC NED Galactic Extinction Calculator ³⁴ for the foreground Galactic extinction in the line of sight toward each of the 3 SNe presented in this work, derived from the maps of Schlafly & Finkbeiner (2011). We find a line-of-sight extinction of E_(B−V,MW) = 0.043 mag for SN 2018fsh E_(B−V,MW) = 0.031 mag for SN 2019ape and E_(B−V,MW) = 0.082 mag for SN 2020uik. We estimate the host extinction of SN 2019ape (along the line of sight to the SN) using the g − r color curve of SN 2019ape compared to intrinsic color curves of SNe Ic (Stritzinger et al. 2018; Drout et al. 2011). We find E_(B−V,Host) = 0.14 ± 0.03 mag. This value is broadly consistent with the value derived using the equivalent width of the Na D doublet absorption feature (Poznanski et al. 2012). We do not attempt to estimate the host extinction for SN 2018fsh or SN 2020uik, but estimate these are not significant due to the large offset from their host galaxies.

For all SNe discussed in this paper, we acquired gri photometry using the P48 ZTF camera (Dekany et al. 2020). These data were processed using the ZTF data-processing system (Masci et al. 2018). Light curves were obtained using the forced-photometry pipeline (Masci et al. 2018) on difference images produced using the optimal image subtraction algorithm of Zackay et al. (2016) at the position of the SN, as reported by the first ZTF alert. We report detections above a 3σ threshold. Same-night detections were binned in order to boost the signal. Additional photometry was acquired with:

• 1.  
  The Las Cumbres Observatory (LCO) network of 1 m telescopes through the Global Supernova Project (Howell 2019). Photometric data were reduced using the lcogtsnpipe pipeline which performs PSF-fitting photometry. Landolt standard field stars were used to calculate the zero-points for the filters UBV, whereas for gri bands we use Sloan magnitudes of stars in the same field as the object.
• 2.  
  The two ATLAS 0.5 m telescopes on Haleakala and Mauna Loa, Hawaii, USA (Tonry et al. 2018). Data were reduced using the forced-photometry service (Smith et al.2020).
• 3.  
  The 2.0 m robotic Liverpool Telescope (LT; Steele et al. 2004) at the Observatorio del Roque de los Muchachos Observatory on La Palma using the optical imager (IO:O) through the g, r, i, and z bands. Photometry was reduced using standard iraf routines within a custom python script and stacked using SWarp (Bertin et al. 2002). Digital image subtraction was performed versus PS1 reference imaging following the techniques of Fremling et al. (2016) and calibration was performed relative to PS1 photometric standards.
• 4.  
  The Rainbow Camera (Blagorodnova et al. 2018) on the Palomar 60 inch telescope (P60; Cenko et al. 2006). Reductions were performed using the automatic pipeline described by Fremling et al. (2016).

For SN 2019ape, instrument cross calibration was performed by applying constant shifts calculated using polynomial fits to band-specific light curves. In case a single instrument was available for a given band, the calibration was performed using synthetic photometry on the spectra scaled to r-band photometry to ensure accuracy relative to other bands. Following these procedures, a constant shift of −0.32 mag was applied to LCO g-band light curves to match the evolution of LT and ZTF light curves. No other offsets were required. The photometry in this work will be made available through the Weizmann Interactive Supernova data REPository (WISeREP;Yaron & Gal-Yam 2012) upon publication, and is provided in Table 2. The light curves of SN 2018fsh and SN 2020uik are shown in Figure 4, and the light curves of SN 2019ape are shown in Figure 5.

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Spectroscopic follow-up of SNe appearing in this work was performed using a variety of instruments:

• 1.  
  The 3.6 m ESO New Technology Telescope (NTT) at La Silla, Chile, using the ESO Faint Object Spectrograph and Camera (v.2) (EFOSC2; Buzzoni et al. 1984) as part of ePESSTO+. These data were reduced using the PESSTO pipeline (Smartt et al. 2015).
• 2.  
  The Nordic Optical telescope (NOT) using The Alhambra Faint Object Spectrograph and Camera (ALFOSC). Reductions were performed using foscgui. ³⁵
• 3.  
  The 200 inch Hale telescope at Palomar observatory using the Double Beam Spectrograph (Oke & Gunn 1982). These data were reduced following standard procedures using the P200/DBSP pipeline described in Bellm & Sesar (2016).
• 4.  
  The Spectral Energy Distribution machine (SEDm; Blagorodnova et al. 2018) mounted on the Palomar 60 inch telescope. Data were reduced using the automatic SEDm pipeline (Rigault et al. 2019).
• 5.  
  The Spectrograph for the Rapid Acquisition of Transients (SPRAT; Piascik et al. 2014) on the Liverpool Telescope. SPRAT spectra were reduced using the LT pipeline (Smith et al. 2016) and flux calibrated using a custom python routine.
• 6.  
  The Low Resolution Imaging Spectrometer (LRIS; Oke et al. 1995) on the 10-m Keck I telescope. The data were reduced using the LRIS automated reduction pipeline (LPipe; Perley 2019).

Figure 6 shows the spectrum of SN 2018fsh 96 days after explosion in comparison with the best-fitting SN II spectrum at a similar phase using Superfit. Figures 7 and 8 show the spectral evolution of SN 2019ape and SN 2020uik, respectively. Table 3 contains a log of spectroscopic observations presented in this work. All spectra will be made available to the public on WISeREP ³⁶ upon publication.

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We retrieved archival images of all SN host galaxies discussed in this work from Galaxy Evolution Explorer (GALEX) Data Release (DR) 8/9 (Martin et al. 2005), SDSS DR9 (Ahn et al. 2012), PS1 DR1 (Chambers et al. 2016), the Two-Micron All Sky Survey (2MASS; Skrutskie et al. 2006), and the unWISE (Lang 2014) images from the NEOWISE (Meisner et al. 2017) Reactivation Year 3. For SNe included in our sample we use the matched-aperture photometry software package Lambda Adaptive Multi-Band Deblending Algorithm in R (LAMBDAR; Wright et al. 2016) that is based on a photometry software package developed by Bourne et al. (2012) and the tools presented by Schulze et al. (2021). The photometry was either calibrated against zero-points (GALEX, PS1, SDSS, and NEOWISE) or against a set of stars (2MASS). We correct the measurements for Milky Way extinction based on Schlafly & Finkbeiner (2011). The resulting photometry is summarized in Tables 4 and 5. We note that in the case of SN 2016hil, the W3 measurements are estimated from the W2 − W3 color reported in Irani et al. (2019), where MIR photometry is measured from ALLWISE data using the same methodology described here.

4.1.1. Light-curve Properties

4.1.2. Spectra

The spectral observations of SN 2018fsh and SN 2020uik show that both appear to be spectroscopically normal SNe II. SN 2018fsh was observed late in its evolution, and shows a typical spectrum for a SN IIL at this phase. We show a comparison with the closest Superfit match SN 2014G (de Jaeger et al. 2019) in Figure 6. The spectrum shows strong Hα and moderate Ca ii emission. We interpret the absorption feature at λ5900 as Na λ5890 and not He λ5876 in the absence of additional features at λ6678 and λ7065 (Gal-Yam 2017). Figure 9 shows a later nebular spectrum with Mg and Ca ii emission lines as well as a complex Hα emission profile. The Hα feature has a broad and boxy profile, with velocities ranging from −15,000 to 10,000 km s⁻¹. Similar profiles have been previously attributed to interaction of the fast outer ejecta with CSM (Filippenko et al. 1994; Patat et al. 1995). This interpretation is supported by the flattening of the light curve at late times, e.g., as observed in all bands for SN 1993J (Jerkstrand et al. 2015).

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SN 2020uik has a more detailed spectroscopic sequence with a typical Hα P-Cygni profile, Na i absorption at λ5890 as well as Fe ii absorption lines (λ4924, λ5018 and λ5169) and with typical Ca ii emission developing over time. Neither of the two events shows significantly strong Ca emission compared to other SNe II to be considered peculiar.

4.1.3. Limits on an Underlying Host Galaxy

We derive limits on the presence of a compact source at the sites of SN 2018fsh and SN 2020uik using deep archival imaging as described in Irani et al. (2019). For SN 2018fsh we use Legacy Survey images (Dey et al. 2019) from the Beijing-Arizona Sky Survey fields (Zou et al. 2017), and for SN 2020uik we use deep PS1 imaging (Flewelling et al. 2020). Our 5σ limits are m_g > 24.69 mag at the site of SN 2018fsh and m_g > 23.45 mag at the site of SN 2020uik corresponding to luminosity limits of M_g = −10.73 mag and M_g = − 11.91 mag respectively. In Figure 10 we show cutouts of the explosion sites. In both cases, there are no sources within a few kpc of the SN location.

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4.2.1. Light-curve Properties

4.2.2. Spectroscopic Comparison of SN 2019ape and Other SNe Ic

Qualitatively, the spectra of SN 2019ape are similar to those of SNe Ic. In Figure 11 the spectra are compared with the well-observed SNe Ic SN 1994I (Filippenko et al. 1995), SN 2004aw, the best-fitting SN to SN 2019ape with Superfit (Taubenberger et al. 2006), and SN 2007gr (Valenti et al. 2008; Modjaz et al. 2014) around maximum light and at two weeks past maximum. Around maximum light, the spectroscopic features are similar in position and in strength among the objects, in particular those associated with Fe ii and Mg ii in the region bluewards of 5500 Å. Of note is that the spectrum of SN 2019ape at −7.5 days peaked at around 5500 Å. This is unlike the early spectra of SNe 2004aw and 2007gr which are bluer despite being observed later in their evolution (e.g., the spectrum of SN 2007gr at −4.6 days peaks at ∼4000 Å). Over the course of a week, the spectra of the comparison objects gradually redden as they reach maximum light and the ejecta cool. The SN 2019ape spectrum at −3.5 days is similar in this respect to the comparison objects around peak or slightly after. The spectrum is a very good match to the spectrum of SN 2004aw at around +5 days. As time progresses the objects gradually evolve to look similar, although SN 2019ape does not display the narrow lines of SN 2007gr and retains the slight blending of the Fe ii lines in the blue part of the spectrum as seen in SNe 1994I and 2004aw (for a discussion of this, see Prentice & Mazzali 2017). It can be concluded that SN 2019ape is a normal SN Ic supernova, albeit with some differences in its pre-peak spectra, and the peculiar secondary peak in g-band.

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4.2.3. Photospheric-phase Spectral Modelling

We model the spectra of SN 2019ape with a 1D radiative transfer code which has been used extensively to model the spectra of stripped-envelope SNe (SE-SNe) (e.g., Mazzali et al.2000, 2002, 2006; Sauer et al. 2006; Prentice et al. 2018b; Ashall et al. 2019; Teffs et al. 2021). The code is described in detail by Mazzali & Lucy (1993), Lucy (1999), Mazzali (2000), and is the source of a parameter study by Ashall & Mazzali (2020). The model requires an input density profile, ejecta composition, photospheric velocity, epoch, and luminosity which it then uses to approximate the expanding SN ejecta, which are assumed to be in homologous expansion. Radiation is assumed to be emitted at a sharp inner boundary (the "the inner photosphere"), following a blackbody distribution with temperature T_bb, which is found through an iterative MC process. The code follows the propagation of "photon packets" through the SN atmosphere, as represented by the density and abundance profiles. These packets can be subject to Thomson scattering and line absorption in the model ejecta, both fluorescence and reverse fluorescence are possible. The interaction of photons and the gas redistributes the temperature and determines the ionization and excitation states of the gas, which are computed self-consistently using the nebular approximation (Abbott & Lucy 1985; Pauldrach et al. 1996). Finally, the emergent spectrum is found by calculating the formal integral of the radiation field.

We modeled three spectra of SN 2019ape, one prior to r-band maximum, one at maximum, and one a few weeks later. As SN 2019ape shows spectral similarities to SN 2004aw, we started with the density profile and abundance distribution from the model found in Mazzali et al. (2017), which in turn is based on the CO21 1D hydrodynamical model developed for SN 1994I (Nomoto et al. 1994). This initial model was modified using the scaling relations from Hachinger et al. (2009) for the ejected mass M_ej and the explosion kinetic energy E_k until a model that reasonably reproduces the spectral flux and line profiles is found, and then the abundances are iteratively modified until the bulk of the features are well reproduced. As we did not model the light curve in parallel with the spectra, the values of M_ej and E_k that we determined are approximate. Additionally, the lack of early-time spectra and photometry limit the accuracy of the determination of E_k using this method (see Mazzali et al. 2017). Our modeling suggests that a good fit is obtained using a density profile with M_ej = 2 M_⊙ and E_k = 2 × 10⁵¹ erg, and a specific kinetic energy that is similar to SN 2004aw (E_k,51/M_ej ≈ 1). As the first modeled epoch is approximately 10 days after t ${}_{\exp }$ as estimated in Section 2.4.2, we used two dummy shells to model the high-velocity material properly. As we have no early spectra, these two shells were primarily used to reduce high-velocity line formation of certain elements. In addition, we use an additional dummy shell between the second and third modeled epochs (t ∼ 14 days and t ∼ 27 days respectively) as this is an extensive gap in spectral evolution. The resulting models are shown in Figure 12 with the abundances of each shell given in Table 6.

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Table 6. Parameters and Selected Abundance Fractions for the Model Shells

t (days)	vph (km s−1)	Tbb (K)	C	O	Ne	Na	Mg	Si	S	Ca	56Fe	56Ni
Dummy shell	23000	...	0.7	0.004	0.29	0.0	0.005	0.0005	0.00025	0.0	0.0001	0.0001
Dummy shell	17000	...	0.7	0.0065	0.29	0.0	0.0025	0.0005	0.00025	3 × 10−6	0.0001	0.0001
10	12500	7249.6	0.66	0.035	0.3	0.0005	0.0025	0.0015	0.0006	7 × 10−6	0.0001	0.0005
14	10500	7028.2	0.5	0.2	0.3	0.00055	0.001	0.0022	0.002	0.0	0.0005	0.002
Dummy shell	8500	...	0.5	0.3	0.15	0.001	0.001	0.015	0.035	2 × 10−6	0.0025	0.0025
27	6500	6327	0.5	0.3	0.02	0.002	0.001	0.015	0.05	1 × 10−6	0.0025	0.05

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The first modeled spectrum was obtained on 2019 February 8, ∼10 days from estimated explosion time and ∼7.6 days prior to r-band peak. For this spectrum we find v_ph = 12,500 km s⁻¹ and L = 1.40 × 10⁴² erg s⁻¹. The model manages to capture many of the spectral features well, with a few issues (see first panel of Figure 12). The abundance at this phase is primarily carbon (∼66%), with ∼30% neon and only 3% oxygen. A larger oxygen abundance leads to an excessively strong O i λ7774 feature in the synthetic spectrum. In the dummy shells above this photosphere, the oxygen mass is reduced even further, leaving a C and Ne rich outer shell. Comparatively, at this epoch the models shown in Mazzali et al. (2017) had approximately equal abundances of carbon and oxygen (35%). The observed O i λ7774 line in this epoch is also noisy and likely contaminated with a telluric line, which makes it difficult to fit. As mentioned previously, a key difference between SN 2004aw and SN 2019ape is that the spectrum of SN 2019ape is redder and peaks at ∼5500 Å rather than ∼4500 Å. Two noticeable issues with the model are the inability to replicate the peak near 5500 Å and the weak NIR Ca ii line. The peak could be driven by re-emission from the iron features near 5000 Å or possibly other Fe-group elements, however, the high abundances of Fe or Fe-group elements needed to drive the flux upward in this region produce absorption features that are far too strong as well as other lines that are not observed. Increasing the total luminosity can also reproduce the peak at 5500 Å but produces far too high a flux level at wavelengths redder than 6000 Å. The second issue is that the Ca ii NIR feature is too weak, which is likely due to the model having too high a temperature at this phase, leading to the over-ionization of Ca. Lowering the temperature produces a worse fit to other parts of the spectrum, and increasing the Ca abundance is inconsistent with the later spectra.

We next model a spectrum that was obtained four days later, on 2019 Feb. 12 (∼14 days after the explosion and 2.8 days before the peak). The input parameters were v_ph = 11000 km s⁻¹ and L = 1.64 × 10⁴² erg s⁻¹. This spectrum is quite similar to the one at $t-{t}_{\exp }\sim 10$ d but is marginally redder, suggesting a lower temperature. The abundances are similar to those of the previous spectrum, as the velocities only differ by 1500 km s⁻¹ and therefore represent shells located close within the ejecta profile. Although the main features of the spectrum are replicated, owing to the lower temperature the Ca ii NIR and H&K features are stronger in this model than in the previous one.

The final spectrum modeled was obtained on 2019 Feb. 25, ∼27 days after the explosion and 9.4 days after the peak. Here, we used v_ph = 6500 km s⁻¹ and L = 1.23 × 10⁴² erg s⁻¹. As before, many of the features in the spectrum are replicated by the model, apart from the peak at 5500 Å, which shows an Fe ii line with a rest wavelength of 5534 Å that is clearly not visible in the observed spectrum. Comparing to the other SNe of similar epochs in Figure 11, the line is seen in SN 2004aw, SN 2007gr, and possibly in SN 1994I. However, the Fe abundance is required to reproduce other features, suggesting that the Fe abundance at higher velocities may be too low. But, as discussed previously, increasing the Fe abundance in the outermost regions produces unwanted lines in those epochs. Several lines are observed in the spectrum in the region between 6600 and 7100 Å that are not reproduced by the model. These are often blends of C i and Fe-group elements. Carbon is still mostly singly ionized, and the issues with the Fe distribution still holds. The O i abundance is still low during this phase, but the O i 7774 Å line is saturated, and is less responsive to abundances changes at this epoch. The absorption near 9300 Å is possibly due to strong C i features that are only partially replicated.

4.2.4. Star Formation of the Host Galaxy

Massive stars are associated with regions of elevated star formation which can be traced by UV continuum and Hα emission (Kennicutt 1998). Here we consider both star formation in the local environment of SN 2019ape and the total star formation rate of its host galaxy. De et al. (2020) acquired a deep nebular spectrum of SN 2019ape ∼ 300 days after the explosion. The 2D spectrum reveals a compact Hα-emitting region in close proximity to the SN site on top of the narrow absorption feature observed throughout the slit. In Figure 13, we show a cutout of the 2D image with the trace of SN 2019ape and the Hα emitting region highlighted. Using SAOImageDS9 (Joye & Mandel 2003) we measured the strength of the Hα emission relative to the local absorption background. After correcting for MW extinction we find a flux of f_Hα = 1.4 ± 0.5 × 10⁻¹⁶ erg s⁻¹ cm⁻² and an integrated Hα luminosity of 3.1 ± 1 × 10³⁸ erg s⁻¹ - a typical value for an H ii region (Kennicutt & Edgar 1989). We convert this to a star formation rate (SFR) using the calibration by Kennicutt (1998) and find that this regions forms 2.4 × 10⁻³ M_⊙ yr⁻¹. Globally the host galaxy is relatively UV bright, with a color of NUV − r_PS1 = 4.03 ± 0.03 mag indicating it has undergone an episode of recent (<100 Myr) star formation. (Suh et al. 2011; Yi et al. 2005).

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We use the host photometry of the objects in our sample and the archival photometry for the BTS SN hosts to derive their SFR and stellar masses. Traditionally, SFR can be derived using UV luminosities that are dominated by massive stars (e.g., Kennicutt 1998; Salim et al. 2007). However, this tracer is sensitive to dust attenuation (e.g., see Calzetti et al. 1995; Buat et al. 1999). To compensate for the effects of dust attenuation, we use the UV SFR indicator calibrated by Salim et al. (2007). Stellar mass values are estimated from the W2 brightness using the calibration of Wen et al. (2013). We report the derived SFR and stellar mass estimates in Table 7, along with the mean SFR and stellar mass estimates for Galaxy Zoo spirals and ellipticals. In Figure 14 we plot the SFR of BTS CCSNe hosts, our sample of CCSNe in elliptical galaxies and BTS SNe Ia in elliptical galaxies. The elliptical host galaxies of CCSNe show more star formation on average compared to the general elliptical galaxy population - ${0.41}_{-0.23}^{+0.53}\ {M}_{\odot }\ {\mathrm{yr}}^{-1}$ compared to ${0.09}_{-0.05}^{+0.09}\ {M}_{\odot }\ {\mathrm{yr}}^{-1}$ for Galaxy Zoo ellipticals and ${0.76}_{-0.72}^{+1.47}\ {M}_{\odot }\ {\mathrm{yr}}^{-1}$ for the general CCSN host-galaxy population. CCSNe in elliptical galaxies are also more massive than most elliptical galaxies, with ${2.76}_{-1.49}^{+3.24}\times {10}^{10}\,{M}_{\odot }$ compared to an average of ${0.83}_{-0.58}^{+1.95}\times {10}^{10}\,{M}_{\odot }$ for the Galaxy Zoo ellipticals. The high SFR of CCSNe elliptical hosts can be explained by their higher mass. The specific SFR (sSFR; i.e., the SFR per unit mass) of CCSNe ellitpicals hosts is ${1.49}_{-0.87}^{+2.08}\times {10}^{-11}\ {\mathrm{yr}}^{-1}$ compared to ${1.06}_{-0.52}^{+1.03}\times {10}^{-11}\ {\mathrm{yr}}^{-1}$ forgalaxy zoo ellitpicals, and ${2.60}_{-2.54}^{+4.99}\times {10}^{-10}\ {\mathrm{yr}}^{-1}$ for the hosts of BTS CCSNe.

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Table 7. SN Host-galaxy Properties Derived in This Work

SN	Host	SFR (M⊙ yr−1)	Mass (109 M⊙)
SN2003ky	NGC 4001	1.18 ± 0.09	37.5 ± 1.0
SN2006ee	NGC 774	0.17 ± 0.01	19.6 ± 0.5
SN2006gy	NGC 1260	0.18 ± 0.04	36.4 ± 1.0
PTF10gqf	WISEA J150350.31+553738.	0.26 ± 0.01	13.9 ± 0.4
PS1-12sk	RXC J0844.9+4258	1.62 ± 0.09	98.7 ± 3.0
SN2016hil	WISEA J011024.51+141238.	0.26 ± 0.05	50.9 ± 1.9
PTF16pq	CGCG 091-056	0.16 ± 0.01	32.7 ± 0.9
SN2018fsh	MCG +07-18-013	0.43 ± 0.19	15.2 ± 0.4
SN2019ape	NGC 3426	1.22 ± 0.02	49.0 ± 1.3
SN2020uik	WISEA J080154.84-064527.	0.4 ± 0.02	5.6 ± 0.2
...	Galaxy Zoo ellipticals a	${0.09}_{-0.05}^{+0.09}$	${8.27}_{-5.79}^{+19.33}$
...	Galaxy Zoo spirals a	${0.43}_{-0.28}^{+0.78}$	${2.43}_{-1.71}^{+5.69}$

Note.

^a Values quoted here represent the sample mean and standard deviation.

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Finding a CCSN in an elliptical galaxy can be either a sign of residual star formation, or that the progenitor of the event in question was not a massive star. The most recent example of the later are the class of Ca-rich Type Ib supernovae(Perets et al. 2010). While these transients have spectra consistent with SNe Ib near maximum light, they occur predominantly in passive environments and galaxy outskirts, which argues against a massive star origin (e.g., Perets et al. 2010; Kasliwal et al. 2012; Lunnan et al. 2017, and most recently De et al. 2020). In addition to their remarkable locations, Ca-rich SNe display peculiar features—strong Ca emission lines in their nebular phase, and a lower luminosity compared to typical SNe Ib.

A non-massive-star origin has been suggested for individual CCSNe offset from elliptical galaxies, such as (1) for PS1-12sk (Sanders et al. 2013; Hosseinzadeh et al. 2019), where deep HST UV imaging excludes local star formation, and (2) for SN 2016hil (Irani et al. 2019), where a double peaked light curve and a low metallicity spectrum were observed. However, conclusions regarding the (potentially) peculiar properties of CCSNe in elliptical host galaxies were difficult to reach based on isolated events, and due to noisy and sparsely sampled photometry and spectroscopy in the case of SN 2016hil.

Here, we consider the combined properties of SNe II in ellipticals, which we can now study as a population. Figure 15 shows the peak luminosities of Type II SNe analyzed in this work, compared to the peak-luminosity distribution of BTS SNe II in our comparison sample. With only 7 SNe II, our sample is too small for a meaningful two-sample Kolmogorov–Smirnov (KS) test of the two peak luminosity distributions. We compare the mean and the standard deviations of the two distributions instead. Our sample of SNe II in ellitpicals has a mean peak absolute magnitude of −17.3 ± 1.0 mag, consistent with −17.8 ± 0.8 mag for all spectroscopically regular SNe II in the BTS sample.

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Unfortunately, we do not have enough data to compare the spectral properties of SNe II in ellipticals to the general SN II population. However, we point out that absence of strong Ca emission in the nebular spectrum of SN 2018fsh and SN 2019ape, (compared to Ca-rich SNe Ib/c or Ia; De et al. 2020), and the typical spectral evolution of SN 2020uik, suggest these two SNe are typical SNe II. The complex Hα profile seen in the nebular spectrum of SN 2018fsh has a broad and boxy profile, extending to high-velocities. These were previously seen for the well observed SN 1993J and SN 1998S and interpreted as signatures of late-time interaction with circumstellar material (Filippenko et al. 1994; Patat et al. 1995; Pozzo et al. 2004). Sollerman et al. (2021) discuss SNe with similar nebular Hα extensively. The presence of extended circumstellar material (CSM) around the progenitor of SN 2018fsh could indicate some SNe II form through exotic formation channels, such as mergers of intermediate-mass stars as outlined in Zapartas et al. (2017), or a common-envelope phase as suggested by Soker (2019). Zapartas et al. (2017) estimates that such binary interaction could account for up to 15% of SNe II. It remains to be seen if such signatures are common for SNe II with elliptical host galaxies.

While our sample only contains a single SN Ic, we demonstrated in Section 4.2.2 that SN 2019ape is not a unique event in most respects—both by comparison to other events and by our modeling of SN 2019ape in 4.2.3. The later indicates that the abundances, ejected mass and kinetic energy are typical of a normal SN Ic when compared to a sample of events (Prentice et al. 2016, 2018a). Its peak luminosity of M_r = −16.75 mag is close to the mean peak luminosity in the BTS sample (−17.3 ± 0.5 mag) and the light-curve evolution is typical (Prentice et al. 2018a). The only unusual aspect is the secondary g-band peak. It might indicate interaction with extended CSM at later times, as for example observed by Ben-Ami et al. (2014) or Gutiérrez et al. (2021). Similarly to SN 2018fsh, the presence of extended CSM might indicate binary evolution is responsible for the formation of the progenitor star of SN 2019ape. However, given the rest of its properties and the location of the event, we consider it unlikely that SN 2019ape emerged from an unusual SN Ic progenitor. In particular, the estimated ejected mass, chemical abundance (dominated by unburned carbon) and energy point toward a massive progenitor. We conclude that our sample of events in elliptical galaxies is overall consistent with having properties similar to the general population of CCSNe.

Based on our observations of the SN sites, we can separate the sample of CCSNe in ellipticals into two groups—the SNe which are located close to compact star-forming knots, and those that are not. A good example of the former is SN 2006gy, which exploded near the center of an early-type galaxy (Ofek et al. 2007; Smith et al. 2007). SN 2006gy exploded near a region showing signs of localized star formation. This is indicated by a nearby dust lane and compact Hα emission interpreted as an H ii region (Ofek et al. 2007; Smith et al. 2007). However Jerkstrand et al. (2020) identify neutral iron lines in one of the spectra of SN 2006gy, and argue it is consistent with a SN Ia embedded in a shell of circumstellar material, possibly contradicting a massive star origin. Similarly to the site of SN 2006gy, the localized Hα emission extending to the site of SN 2019ape is most likely an indication of a nearby H ii region where the progenitor star could have been formed. SNe Ic are often associated with Hα emission in their host galaxies, supporting a young progenitor population even when compared to other CCSNe types (Anderson et al. 2015). The same might be true for SNe Ic observed in elliptical galaxies. This would imply that the initial mass function of stars formed in H ii regions in elliptical galaxies extends to massive (≥8 M_⊙) stars.

In addition to SNe occurring near sites of active formation, we identify a second group of SNe that occurs in the outskirts of their elliptical hosts, with no signs of localized star formation. In all cases in which the background from the apparent host galaxy was faint enough, we obtain strong limits on the presence of an underlying host galaxy. Limits on the luminosity of hypothetical faint hosts range from −10 mag to −12 mag, putting all these hosts at the low end of the CCSN host-galaxy luminosity distribution as presented in Schulze et al. (2021).

Next, we consider whether CCSNe are typically located at larger offsets from their elliptical host galaxies compared to the host-offset distribution of the general CCSN population, and compared to offsets of SNe Ia in elliptical galaxies. We begin by inspecting the comparison between the offset distributions of SNe Ia in ellipticals and our sample of CCSNe in ellipticals. The right panel of Figure 16 shows the offset distribution of the BTS SNe Ia in ellipticals, CCSNe, and the combined sample of CCSNe in ellipticals. We show only SNe that are offset by less than 30 kpc (projected) or 90'', the limits of the automatic cross checks presented by Perley et al. (2020). We expect this to cover the vast majority of CCSNe, as the largest offsets in the (i)PTF sample were 37 kpc (Schulze et al. 2021). On the left panel of the same figure we show the physical offsets and the corresponding host-galaxy mass, along with the average 80% light radius, r₈₀[M_*], derived using the low-redshift mass-size relation of Mowla et al. (2019). Schulze et al. (2021) demonstrated (their Figure 11) that the majority (>85%) of CCSNe occur at distances smaller than r₈₀.

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The number of SNe in our sample is too low for significant results from a two-sample KS test, but we can formulate an alternative statistical test. We observe that 5% of SNe Ia with elliptical host galaxies are detected at projected offsets larger than 22.5 kpc. We define the null hypothesis as the case in which CCSNe in ellipticals come from the same offset distribution as SNe Ia: For a total of 9 SNe, we expect on average ∼0.5 SNe at offsets larger than 22.5 kpc (upper 5% of the Ia sample distribution). Assuming a Poisson distribution, the probability of finding three or more SNe in this range is then 1%, so that the null hypothesis is rejected with a confidence of 99%. We note that this test is sensitive to the position of the threshold. For example, repeating this test for all SNe at distances larger than 17 kpc (upper 10% of the Ia sample distribution) will result in less significant results (95% rejection). Repeating this analysis with r/r₈₀[M_*] gives similar results—the probability that 3 of 9 SNe have offsets larger than >2 r₈₀ is less than 3%, assuming the offset distribution of SNe Ia with elliptical hosts. To conclude, we find tentative evidence that CCSNe are typically located at larger offsets from their host galaxies compared to SNe Ia in the same host-galaxy population. However, a larger sample is needed in order to reach a high statistical significance.

A possible explanation for the high offset of CCSNe in elliptical galaxies is a reduced detection efficiency at low offsets on top of the high surface brightness center of their elliptical hosts. Foley (2015) suggested this option to explain the lack of Ca-rich SNe Ib at low offsets from their host galaxies. Frohmaier et al. (2017) calculated the recovery efficiency of the PTF pipeline and found it was lower in regions of high surface brightness, but Frohmaier et al. (2017) later found that this is not enough to explain the large offsets observed for Ca-rich transients (Kasliwal et al. 2012). ZTF data should be less susceptible to this bias than PTF was, since the new ZTF camera provides substantially higher image quality, and the optimal image subtraction of Zackay et al. (2016) provides much cleaner subtraction; we thus estimate this effect should be even weaker in our BTS sample. Still, a larger homogeneous sample is needed to quantify this effect for CCSNe in elliptical host galaxies.

An extended offset distribution for CCSNe in elliptical host galaxies compared to the progenitors of SNe Ia has significant implications for the formation of their progenitor stars.

An offset between the locations of SNe Ia and CCSNe in ellitpicals could reflect an inside-out growth (Sánchez-Blázquez et al. 2007; Pérez et al. 2013) in some massive ellipticals. While the SNe Ia originate from an older stellar population closer to the center of the massive hosts, CCSNe could explode in regions where the host galaxy is accreting gas from the IGM. This would be in agreement with the findings of Salim et al. (2012), who found galaxy-scale star formation in UV-excess early-type galaxies can extend to large offsets. Gomes et al. (2016) found the extended star formation in the periphery of early-type galaxies can take the form of faint (24 ≲μ_r (mag/□'') ≲ 26) spiral features which possibly indicate inside-out growth. However, such low-surface-brightness features are excluded for both SN 2016hil and PS1-12sk, but are still an option for SN 2018fsh SN 2020uik and PTF10gqf.

It is possible that the SNe we observe do not originate from the ellipticals themselves, but from a different stellar population nearby. This option is supported by some evidence that the space between elliptical galaxies might not be completely empty. in addition to having a large offset distribution extending to regions with apparently no stellar populations (Kasliwal et al. 2012), Ca-rich SNe prefer group and cluster environments (Lunnan et al. 2017). Interestingly enough, PS1-12sk exploded in a bright cluster environment as noted by Sanders et al. (2013), who raised the possibility this might be evidence for star formation in galaxy cluster cooling flow filaments. Similarly, SN 2018fsh occurred in the compact group V1CG 538 (Lee et al. 2017).

However, out of five objects that do not coincide with their elliptical host galaxy, only two are possibly associated with a group or cluster environment. We consider this preliminary evidence that star-forming cooling flows are not the main channel of star formation near elliptical galaxies. Gal-Yam et al. (2003) measured the fraction of cluster SNe Ia originating from an intergalactic stellar population (i.e., those which originate outside of galaxies) to be 20%. In our sample, all SNe detected in group or cluster environment were offset from their putative host, indicating that this fraction is significantly different in CCSNe near ellipticals. However, a larger sample is needed to measure this fraction with high significance. While not part of our sample, we also note that the SN II A399 11 19 0 (Graham et al. 2012) occurred in a nearby galaxy cluster, and is not offset from its host galaxy. Ruiz et al. (2014) explore a sample of 1000 ellipticals and find that the majority have small satellites (down to a mass ratio of 1:400) with an average projected distance of ∼59 kpc from their parent galaxy, that contain 8% of their stellar mass. However, such galaxies are excluded by our deep limits at the SN sites. Another possibility for the origins of offset CCSNe can be found in the work of Sedgwick et al. (2019), who recently demonstrated that a population of star-forming low-surface-brightness galaxies (LSBGs) host many of the seemingly hostless SNe. These LSBGs can have significant star formation activity, like the nearby UGC 2162. This ultra-diffuse galaxy has a low (but not negligible) SFR of 0.01 M_⊙ yr⁻¹, but with a very low surface brightness of 24.4 mag/□'' (Trujillo et al. 2017). This would translate to an integrated absolute magnitude of −7 mag in the optical. Such a faint and extended object would be difficult to detect when located on top of the diffuse emission at the outskirts of a massive elliptical at a redshift of 0.02. While such an ultra-diffuse host is ruled out in the case of PS1-12sk (Hosseinzadeh et al. 2019), it has not been excluded for SNe II in the outskirts of elliptical galaxies.

If the host galaxies of offset CCSNe are satellites of massive galaxies, they would constitute a small fraction of CCSNe in their typical host galaxies, and could erroneously be associated with nearby more luminous hosts. However, in elliptical galaxies which host much fewer CCSNe, such satellite galaxies could host a larger fraction of the observed SNe. To test this hypothesis, we select CCSNe from the BTS and PTF samples that occur at offset of ≥15 kpc from massive ($\mathrm{log}(M[{M}_{\mathrm{odot}}])\gt 10$) galaxies. We manually examine the deepest publicly available survey data (if possible using the Legacy Survey or otherwise PS1 images) and exclude all cases where the SN occurred on or very close to pronounced structures that are part of the host galaxy, such as spiral arms extending to the SN location, or elevated (>3σ) emission compared to the local background. We find the fraction of highly offset CCSNe occurring in elliptical compared to nonelliptical hosts is 0.33 (0.17–1.57; 95% confidence interval). This value is consistent with the fraction of massive ellipticals of all massive ($\mathrm{log}(M[{M}_{\odot }])\gt 10$) galaxies in the Galaxy Zoo sample—0.33 (0.30–0.40; 95% confidence interval). To reduce the selection bias, we selected a low-z (0.025 < z < 0.032) sample such that the redshift distributions of elliptical galaxies, spiral galaxies, and all galaxies (including those with irregular or uncertain morphological classifications) match.

We conclude that the most likely explanation for the highly offset distribution of CCSNe near ellipticals is one of the following

• 1.  
  a population of ultra-diffuse satellite star-forming galaxies near massive elliptical galaxies. The rate of highly offset CCSNe around massive ellipticals is hence similar to their rate around all massive galaxies. Due to the large number of CCSNe occurring in massive star-forming galaxies, the few offset events are unremarkable, but with very few events occurring in massive ellipticals, the offset population, perhaps arising from very faint satellites, stands out. Deeper observations of the location of our sample events are needed to establish this.
• 2.  
  Some cases can be explained by extended and faint spiral features, or extended galaxy-scale star formation due to accretion of gas from the IGM. The latter would indicate inside-out growth of their elliptical host galaxies. Future studies of the host galaxies of the CCSNe in our sample can also confirm or exclude this possibility.

We attempt to measure the formation rate of massive stars in elliptical galaxies through the fraction of CCSNe occurring in such environments in a fixed volume. The connection between the SFR and the CCSN rate has been previously established. Kennicutt (1984) found a linear correlation between the SFR and the CCSN rate, and used it to estimate a lower mass limit for the progenitor mass of CCSNe. Botticella et al. (2012) assume a lower mass limit of 8 M_⊙ and derive a SFR for a complete sample of galaxies in the local universe. Their estimate agrees with the SFR implied from their FUV luminosities. Maoz et al. (2011) used a sample of 119 CCSNe from the Lick Observatory SN search (LOSS; Leaman et al. 2011; Li et al. 2011) and estimated a rate of 0.01 ± 0.002 SNe per M_⊙. If the CCSN rate per M_⊙ is fixed across galaxy type, the fraction of CCSNe occurring in elliptical galaxies should reflect the fraction of the total star formation in their host galaxy population. Recently, Schulze et al. (2021) provided additional support for this claim by showing that the SN host-galaxy mass distributions are consistent with those of star-forming galaxies weighted by their star formation activity.

When measuring the fraction of CCSNe in elliptical galaxies, the different colors of elliptical and spiral galaxies could create a selection effect especially when considering galaxies with both UV–optical and MIR color information. Most notably, the redder UV–optical color of elliptical galaxies makes them less likely to have archival GALEX photometry compared to spirals at a similar optical brightness. However, our MIR photometry is not biased against ellipticals in the same way due to the brighter W2 magnitudes of massive ellipticals. W3 magnitudes distribute similarly for spirals and ellipticals. Figure 17 shows the magnitude distribution for the WISE/W2 band and GALEX/NUV band for all galaxies in the BTS Ia sample for elliptical and nonelliptical hosts. We illustrate that while ellipticals are less likely to be detected by GALEX at larger distances due to their faint NUV brightness, they are more likely to be detected by WISE. Thus, we derive estimates based on hosts with both UV–optical and MIR colors and based on hosts with MIR colors alone (using both definitions discussed in Section 2.1). A consistent result between both calculations will ensure that the selection effects in our samples are not strong. We also include a third estimate of the rate compared to all CCSNe, regardless of their host-galaxy association. The resulting fractions are:

• 1.  
  ${R}_{{CC}}=\tfrac{2}{478}={0.4}_{-0.3}^{+0.5} \%$ using both W2 − W3 and NUV-r color
• 2.  
  ${R}_{{CC}}=\tfrac{3}{888}={0.3}_{-0.1}^{+0.4} \%$ based on our definition for ellipticals using only the W2 − W3 color and compared to all CCSNe associated with host galaxies.
• 3.  
  ${R}_{{CC}}=\tfrac{3}{959}={0.3}_{-0.1}^{+0.3} \%$ based on our definition for ellipticals using only the W2 − W3 color, and compared to all CCSNe regardless of their host-galaxy association.

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Statistical errors reflect the exact binomial 68% confidence interval on the reported values. We now compare the measured fraction of CCSNe in ellipticals (∼0.4%) with the fraction of star formation ellipticals accounted for out of the total star formation produced by all galaxy types, calculated using the method of Cortese (2012) applied to UV and MIR observations.

We sum the total SFR of all Galaxy Zoo ellipticals which satisfy the color criteria in Section 2.1, and are within the range 0.025 < z < 0.032, and compare it to the fraction of the total SFR in this redshift range in all galaxy types. We find that in this sample, the fraction of the total SFR produced by ellipticals is 1 ± 0.01%, slightly higher (∼2σ) than the star formation inferred from the fraction of CCSNe in elliptical galaxies compared to all CCSNe. This could be explained due to additional UV emission by Active Galactic Nuclei (AGN) and old stellar populations (Jura 1982; Crocker et al. 2011), or due to an incomplete sample in the low-mass range of spiral galaxies (which host a significant fraction of CCSNe; Taggart & Perley 2021). Alternatively, this difference could reflect an underestimate of the SFR in spiral compared to elliptical galaxies due to the different total extinction of the host galaxies populations, or due to a different rate of failed SNe in both environments.

Our results are in significant tension with those of Kaviraj (2014), who claim that early-type galaxies (elliptical and lenticular) account for 14% of the star formation budget at z < 0.07. The sample in the study by Kaviraj (2014) is limited to bright <16.8 mag galaxies, which account for 52% of the total star formation in our general Galaxy Zoo sample, but only for 5% of star formation in ellitpicals. Thus ellipticals are over-represented in their sample.

Our findings highlight the importance of CCSN rates as independent tracers of SFR across a large range of sSFR and galaxy brightness, while preventing the severe selection effects associated with the low end of the galaxy brightness distribution. The planned Rubin Observatory Legacy Survey of Space and Time (LSST; Ivezić et al. 2019) is expected to increase the rate of CCSN detections by an order of magnitude in upcoming years. This will increase the number of CCSNe detected in elliptical hosts by an order-of-magnitude, allowing an in-depth study of the nature of star formation in and around elliptical hosts. However, we caution against using CCSNe as estimators of star formation without taking proper care to remove the significant contamination due to misclassified galaxies or SNe, or due to a false association of the SN with the host galaxy.

Della Valle et al. (2005) previously estimated elliptical galaxies might host up to 3% of CCSNe, based on a sample of SNe Ia in ellitpicals from the Asiago SN survey (Cappellaro et al. 1999). Our findings place more stringent constraints on the rate of CCSNe in ellipticals compared to those obtained in previous studies by an order of magnitude. Recently, Sedgwick et al. (2021) published a study of 36 CCSNe occurring in elliptical galaxies out of red photometrically classified CCSNe isolated from the SDSS-II survey at z < 0.2, suggesting that the fraction of CCSNe occurring in elliptical galaxies is significantly higher than the fraction we measure in this study (∼8% compared to our ∼0.4%). We point out that a population comprising of 8% of CCSNe would stand out in the spectroscopically complete BTS. Applying this rate to the 478 CCSN hosts with UV and MIR color information, we would expect to find 38 ± 6 CCSNe in elliptical galaxies, compared to the 2 found in our sample—a >5σ difference. We conclude that the fraction of CCSNe in ellipticals given by Sedgwick et al. (2021) is over estimated. This could be due to a contamination of their sample by misclassified SNe Ia. Sedgwick et al. (2021) define a confidently classified CCSN as having probability of P_Ia < 0.05 for being a SN Ia, based on their light curves alone. Since SNe Ia vastly outnumber CCSNe in elliptical hosts (in our sample, 240:2) the contamination of the CCSN sample due to falsely classified SNe Ia is likely significant, and could dominates their sample.