The Old Host-galaxy Environment of SSS17a, the First Electromagnetic Counterpart to a Gravitational-wave Source^*

On 2017 August 17 (UT), the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo interferometer detected a gravitational-wave source from a binary neutron star (BNS) merger, GW170817 (LIGO/Virgo Collaboration 2017b; LIGO Scientific Collaboration and Virgo Collaboration 2017, in preparation). Two seconds after the LIGO/Virgo detection, the Fermi Gamma-ray Space Telescope and INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) detected a short-duration gamma-ray burst (sGRB; INTEGRAL 2017; LIGO/Virgo Collaboration 2017a). About 11 hr after the LIGO/Virgo trigger, our team discovered an optical transient in NGC 4993 coincident with GW170817, called Swope Supernova Survey 2017a (SSS17a; One-Meter Two-Hemisphere (1M2H) Collaboration 2017; Coulter et al. 2017).

SSS17a is the first detection of an electromagnetic counterpart to a gravitational-wave source. This discovery marks a milestone and opens a new era in modern astronomy. The gravitational-wave data suggests that SSS17a is a BNS merger, the most popular progenitor model of sGRBs (e.g., Eichler et al. 1989; Lee & Ramirez-Ruiz 2007; Berger 2014).

The host environments of astrophysical transients have long been a profitable route to understanding the nature of their progenitor systems and placing broad constraints on their properties. For example, the long-duration GRBs and sGRBs have very different host environments. While long GRBs predominantly occur in star-forming galaxies (e.g., Bloom et al. 2002), sGRBs can be found in both star-forming and early-type galaxies (Prochaska et al. 2006; Fong et al. 2013), indicating an older population. In addition, sGRBs tend to be found in more massive galaxies and generally show larger offsets from their hosts than long GRBs do (Zheng & Ramirez-Ruiz 2007; Behroozi et al. 2014). The distinct host properties suggest they are likely to arise from different progenitor populations.

In this work, we investigate the host environment of SSS17a, both globally and locally. By comparing our results to those from different kinds of astrophysical transients, we constrain the nature of the progenitor system.

A plan of the paper follows. In Section 2, we describe the observations and data reduction, and Section 3 discusses the methods used to analyze the data and show the determined host properties. The discussion and conclusions are presented in Sections 4 and 5, respectively. Throughout this paper, we assume ${H}_{0}=70$ km s⁻¹ Mpc⁻¹ and a flat universe with ${{\rm{\Omega }}}_{{\rm{M}}}=0.3$ when necessary.

SSS17a was discovered $5\buildrel{\prime\prime}\over{.} 3$ E and $8\buildrel{\prime\prime}\over{.} 7\,{\rm{N}}$ of NGC 4993 (1M2H Collaboration 2017; Coulter et al. 2017), an early-type S0 galaxy with redshift $z=0.009727\pm 0.000050$ (de Vaucouleurs et al. 1991) in a galaxy group (Makarov & Karachentsev 2011). The transient is only 1.9 kpc offset (projected) from NGC 4993, assuming the distance to NGC 4993 of 39.5 Mpc based on the Tully–Fisher method (Freedman et al. 2001).

NGC 4993 was observed by the Hubble Space Telescope (HST) with the Advanced Camera for Surveys (ACS) on 2017 April 28 (UT) in the F606W filter as part of the "Schedule Gap Pilot" program (Program 14840; PI: Bellini). We obtained the HST images from the Mikulski Archive for Space Telescopes. We reduced the HST image using the drizzlepac pipeline (Avila et al. 2015). The calibrated frames were further corrected for geometric distortion, sky background, and cosmic-rays, and combined with astrodrizzle. We registered the final combined images using TweakReg.

We performed photometry on the combined HST/ACS image following standard procedures with dolphot.¹² The dolphot photometry was calibrated using the ACS/WFC $F606W$ zeropoint for 2017 April 28 from the ACS zeropoint calculator.¹³

We obtained Pan-STARRS1 (PS1) griz imaging of NGC 4993 from the PS1 image cutout server¹⁴ (Chambers et al. 2016). These data had been calibrated to the PS1 system following procedures described in Magnier et al. (2016).

To measure the photometry of NGC 4993, we fit an elliptical isophote to the galaxy profile using the IRAF package isophote. We measured an HST/ACS $F606W$ AB magnitude of 12.23 ± 0.01 mag. Using the same method, we measured PS1 griz AB magnitudes of 12.45 ± 0.02, 12.14 ± 0.02, 11.78 ± 0.02, and 12.62 ± 0.02 mag, respectively. In addition, we obtained far-UV and near-UV (NUV) photometry from the Galaxy Evolution Explorer (GALEX; Bianchi et al. 2017), $JH{K}_{s}$ near-infrared photometry from the Two Micron All-Sky Survey (2MASS; Skrutskie et al. 2006), and 3.6–22 μm IR photometry from the Wide-field Infrared Survey Explorer (Wright et al. 2010).

We examined the position of SSS17a in the HST/ACS $F606W$ image and did not detect any sources at the transient location. Placing artificial stars on similar surface-brightness areas, we determined an AB magnitude limit at the position of SSS17a of ${m}_{V}\gt 27.2$ mag, corresponding to ${M}_{V}\gt -5.8$ mag at the distance of NGC 4993, consistent with limits initially reported by HST (2017).

Zoom In Zoom Out Reset image size

We obtained an optical spectrum of NGC 4993 on 2017 September 5 (UT) using the f/4 camera of the Inamori-Magellan Areal Camera and Spectrograph (ImacS; Dressler et al. 2006) on the 6.5 m Magellan/Baade telescope at Las Campanas Observatory. We used the 600 ℓ/mm grating with a blaze angle of $8\buildrel{\circ}\over{.} 6$ to cover the wavelength range 3500–6500 Å at a spectral resolution of $R\approx 2500$. We obtained three 600 s exposures on NGC 4993 with a $0\buildrel{\prime\prime}\over{.} 7$-wide long slit in mediocre conditions with some clouds. We carried out basic reductions of the spectra (bias subtraction, wavelength calibration, flatfielding, and coaddition) using the COSMOS software package (Dressler et al. 2011).¹⁵ We then extracted the spectrum over a $3\buildrel{\prime\prime}\over{.} 7$-diameter aperture in IRAF and applied a flux calibration derived from observations of the standard star LTT 6248. The flux-calibrated spectrum of NGC 4993 is displayed in the upper panel of Figure 2.

Zoom In Zoom Out Reset image size

3.1. Stellar Mass and Star Formation Rate

We use the photometric redshift code z-peg (Le Borgne & Rocca-Volmerange 2002), which is based on the spectral synthesis code PÉGASE.2 (Fioc & Rocca-Volmerange 1997), to estimate the host-galaxy stellar mass (${M}_{\mathrm{stellar}}$) and star formation rate (SFR). z-peg fits the observed galaxy colors with galaxy SED templates corresponding to nine spectral types (SB, Im, Sd, Sc, Sbc, Sb, Sa, S0, and E). We assume a Salpeter (1955) initial-mass function. The photometry is corrected for foreground Milky Way reddening of $E(B-V)=0.109$ mag (Schlafly & Finkbeiner 2011; Shappee et al. 2017) with ${R}_{V}=3.1$ and a Cardelli et al. (1989, CCM) reddening law.

Using our 14-band photometry (see Section 2), we measure a host ${M}_{\mathrm{stellar}}$ of $\mathrm{log}(M/{M}_{\odot })={10.49}_{-0.20}^{+0.08}$, corresponding to a halo mass of $\mathrm{log}({M}_{\mathrm{halo}}/{M}_{\odot })=11.96$ using the ${M}_{\mathrm{stellar}}$–${M}_{\mathrm{halo}}$ relation derived in Yang et al. (2008), assuming $\mathrm{log}({M}_{0}/{M}_{\odot })=9.8$, $\mathrm{log}({M}_{{\rm{h}}}/{M}_{\odot })=10.7$, $\alpha =0.6$ and $\beta =2.9$ in their Equation (7). The observed photometry and best-fit template can be found in Figure 3.

Zoom In Zoom Out Reset image size

In Figure 4 we compare the measured ${M}_{\mathrm{stellar}}$ to that for the host galaxies of supernovae (SNe) and both short and long GRBs. Similar to SNe Ia and core-collapse SNe, sGRBs can be found in galaxies with a wide range of ${M}_{\mathrm{stellar}}$. By contrast, long GRBs are predominantly found in low-mass galaxies. We find that NGC 4993 is more massive than 50% of host galaxies for all classes. In fact, NGC 4993 is more massive than every long GRB host galaxy in the Leibler & Berger (2010) sample.

Zoom In Zoom Out Reset image size

z-peg also indicates negligible recent star formation (at least over the past 0.5 Gyr) in the host galaxy. The same result is obtained by intentionally forcing z-peg to better fit the UV photometry (but sacrificing the goodness of the full SED fitting; see the gray curve in Figure 3). This is further supported by the non-detection of nebular emission lines in the host spectrum. Using the GALEX NUV photometry, we estimate an SFR of only 0.003 ${M}_{\odot }$ yr⁻¹ (see also VAST 2017) based on the conversion from Kennicutt (1998).

3.2. Age and Metallicity

3.3. Offset and Fractional Flux

3.4. Morphology

NGC 4993 is clearly an S0 galaxy (Capaccioli et al. 2015). To further quantify its morphology, we use galfit (Peng et al. 2002) to fit the surface-brightness profile of NGC 4993. We fit the galaxy profile with a single Sérsic model given by

$$\begin{eqnarray}&&{\rm{\Sigma }}{(r)={{\rm{\Sigma }}}_{e}\exp \{-\kappa [(r/{r}_{e})}^{1/n}-1]\},\end{eqnarray} \tag{ 1 }$$

where r_e is the effective radius such that half of the total flux is enclosed within r_e, ${{\rm{\Sigma }}}_{e}$ is the surface brightness at the effective radius, r_e, n is the Sérsic index (a concentration parameter), and κ is a variable coupled to n.

Fitting the HST image of NGC 4993, galfit gives a concentration parameter $n\approx 4$ (the de Vaucouleurs profile), which is similar to typical elliptical galaxies. The effective radius r_e is $17^{\prime\prime}$, corresponding to a physical size of 3.3 kpc. A residual image is created by subtracting the best-fit model from the original image (see Figure 5).

Zoom In Zoom Out Reset image size

Dust lanes are clearly seen in the residual image, extending several kpc from the galactic center (see both Figures 1 and 5) roughly in the direction of SSS17a (HST 2017). However, the dust lanes do not appear to reach the position of SSS17a, providing further evidence that SSS17a does not suffer strong extinction and is consistent with the results of Shappee et al. (2017). The dust lanes found in early-type galaxies are usually indications of recent minor mergers and likely to host active galactic nuclei (Shabala et al. 2012).

3.5. Globular Clusters

Globular clusters contain very high densities of stars. This high stellar density increases the probability of close interactions and leads to mergers more frequently than for field stars (Grindlay et al. 2006; Lee et al. 2010; Samsing et al. 2014). Here we investigate the possibility that SSS17a originated from a globular cluster in NGC 4993.

To better detect sources hidden in the diffuse stellar light, we use the galfit residual image (Section 3.4) and identify sources using sextractor (Bertin & Arnouts 1996, see Figure 5). To identify possible globular clusters, we require that each source have the following properties: (1) not obviously a foreground star (we cross-check this by using a catalog such as USNO-B1.0), (2) point-like PSF, and (3) a brightness consistent with a globular cluster at 40 Mpc given the globular cluster luminosity function (e.g., Faifer et al. 2011), specifically those with $21\leqslant {m}_{\mathrm{AB}}\leqslant 24$ mag (corresponding to $-10\leqslant {M}_{\mathrm{AB}}\leqslant -7$ mag). A total of 119 sources pass these cuts and are selected as potential globular clusters, with the closest one being ∼290 pc away in projection from the position of SSS17a. In principle, we should be able to detect all of the globular clusters in the image (the detection limit is ∼27 mag). However, the number estimated here could be underestimated due to the dust extinction or the relatively bright background near the host nucleus.

Previous studies (e.g., Peng et al. 2008) showed that the total mass of globular clusters (${M}_{\mathrm{GCS}}$) within the host galaxy can be estimated by a simple scaling relation to the host-galaxy halo mass (${M}_{\mathrm{halo}}$) via

$$\begin{eqnarray}&&{M}_{\mathrm{GCS}}/{M}_{\mathrm{halo}}=\eta ,\end{eqnarray} \tag{ 2 }$$

where η represents the absolute efficiency of globular cluster formation. Assuming an efficiency $\eta \simeq 4\times {10}^{-5}$ (Harris et al. 2015) and an average globular cluster mass of $4\times {10}^{5}\,{M}_{\odot }$ (Spitler & Forbes 2009), the number of globular clusters (${N}_{\mathrm{GCS}}$) within a galaxy of ${M}_{\mathrm{halo}}$ can be estimated by

$$\begin{eqnarray}&&{N}_{\mathrm{GCS}}=(1.0\times {10}^{-10})\times {M}_{\mathrm{halo}}.\end{eqnarray} \tag{ 3 }$$

Using ${N}_{\mathrm{GCS}}=119$ (the number of likely globular clusters detected in the HST image), we determine $\mathrm{log}({M}_{\mathrm{halo}}/{M}_{\odot })=12.07$, which is close to the value that we found using the ${M}_{\mathrm{stellar}}$–${M}_{\mathrm{halo}}$ relation (see Section 3.1).

In Section 3.3 we show that sGRBs tend to have larger offsets from their host galaxies than other kinds of transients. The observed offset distribution is generally consistent with the predictions for compact object mergers (e.g., Behroozi et al. 2014). Simulations show that these progenitor systems experience a natal kick when the stars transition to white dwarfs, neutron stars, or black holes. The kick velocity can be up to several hundreds of kilometers per second (Fryer & Kalogera 1997; Fryer et al. 1998)—potentially larger than the escape velocity of its host galaxy, which could expel the progenitor system and result in a large offset from the host galaxy.

However, SSS17a has a relatively small offset compared to the typical offsets of sGRBs. Combined with its likely old age, the location close to the center of the host galaxy suggests that the progenitor system of SSS17a was bound to NGC 4993. Assuming a stellar mass of $\mathrm{log}(M/{M}_{\odot })=10.49$ (Section 3.1), the escape velocity of NGC 4993 is 350 km s⁻¹ at the transient location. We therefore have a constraint on the SSS17a progenitor system kick of $\leqslant 350$ km s⁻¹, which is consistent with the kicks seen for Milky Way neutron star binaries (Fryer & Kalogera 1997; Wang et al. 2006; Wong et al. 2010).

Assuming the distance to the nearest likely globular cluster (290 pc; see Section 3.5) and the age of the youngest stellar population (2.8 Gyr; see Section 3.2), a velocity of ∼0.1 km s⁻¹ is sufficient for the progenitor to travel from a globular cluster to its the current location. Thus, the progenitor kick should be dominated by the escape velocity of the globular cluster (typically several tens of kilometers per second), which makes it hard to exclude the possibility that the progenitor originated in a globular cluster.

In this work, we investigate the host environment of SSS17a, the first electromagnetic counterpart to a gravitational-wave source. We use optical spectroscopy and broadband UV-through-IR photometry of the host galaxy to constrain the host properties, such as stellar mass, SFR, age, and metallicity. Below we summarize our main findings.

• 1.  
  NGC 4993, the host galaxy of SSS17a, is an S0 galaxy at 40 Mpc. It is massive and shows negligible recent star formation. Its mean stellar age is high, suggesting that the progenitor system likely originated from an old stellar population (an age of >2.8 Gyr). NGC 4993 is similar to galaxies that have hosted sGRBs and the expected host galaxies of BNS mergers. It is unlike typical host galaxies for other transient classes, being the most distinct from long GRB host galaxies.
• 2.  
  Its small projected offset combined with its likely old age suggests that the progenitor system of SSS17a was gravitationally bound to NGC 4993. This then implies a limit on the kick velocity of the progenitor system that is ≤350 km s⁻¹.
• 3.  
  Many likely globular clusters are detected in the host galaxy, including close to the position of SSS17a. We cannot exclude the possibility that the progenitor of SSS17a originated from a globular cluster.

The galactic environment of SSS17a provides additional constraints on its progenitor system beyond that extracted from the GW data and the EM observations of SSS17a itself. With larger samples of BNS merger host galaxies, we will be able to determine if they differ in any way from sGRB host galaxies.

We thank the University of Copenhagen, DARK Cosmology Centre, and the Niels Bohr International Academy for hosting D.A.C., R.J.F., A.M.B., E.R., and M.R.S. during the discovery of GW170817/SSS17a. R.J.F., A.M.B., and E.R. were participating in the Kavli Summer Program in Astrophysics, "Astrophysics with gravitational wave detections." This program was supported by the the Kavli Foundation, Danish National Research Foundation, the Niels Bohr International Academy, and the DARK Cosmology Centre. We would also like to thank J. Mulchaey (Carnegie Observatories Director), L. Infante (Las Campanas Observatory Director), and the entire Las Campanas staff for their extreme dedication, professionalism, and excitement, all of which were critical in the discovery of the first gravitational-wave optical counterpart and its host galaxy as well as the observations used in this study.

The UCSC group is supported in part by NSF grant AST-1518052, the Gordon & Betty Moore Foundation, the Heising-Simons Foundation, generous donations from many individuals through a UCSC Giving Day grant, and from fellowships from the Alfred P. Sloan Foundation (R.J.F.), the David and Lucile Packard Foundation (R.J.F. and E.R.) and the Niels Bohr Professorship from the DNRF (E.R.). A.M.B. acknowledges support from a UCMEXUS-CONACYT Doctoral Fellowship. D.K. is supported in part by a Department of Energy (DOE) Early Career award DE-SC0008067, a DOE Office of Nuclear Physics award DE-SC0017616, and a DOE SciDAC award DE-SC0018297, and by the Director, Office of Energy Research, Office of High Energy and Nuclear Physics, Divisions of Nuclear Physics, of the U.S. Department of Energy under contract No. DE-AC02-05CH11231. M.R.D. and B.J.S. were partially supported by NASA through Hubble Fellowship grants HST-HF-51373.001 and HST-HF-51348.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with program GO-14840.