ULTRALUMINOUS SUPERNOVAE AS A NEW PROBE OF THE INTERSTELLAR MEDIUM IN DISTANT GALAXIES

Studies of the interstellar medium (ISM) in distant galaxies have traditionally focused on two observational techniques: (1) direct spectroscopic measurements of emission lines from the aggregate H ii regions (e.g., Savaglio et al. 2005; Erb et al. 2006; Mannucci et al. 2009) and (2) absorption by intervening systems in spectra of background quasars (e.g., Steidel & Sargent 1992; Wolfe et al. 2005). These approaches are complementary: direct galaxy metallicity measurements at z ≳ 2 are challenging due to the relative faintness of galaxies and the redshifting of rest-frame optical emission lines into and beyond the near-IR band, while quasar absorption studies are most effective at z ≳ 2 where Lyα is redshifted into the observed optical window.

In recent years, these studies have been supplemented by absorption spectroscopy of bright gamma-ray burst (GRB) afterglows (e.g., Vreeswijk et al. 2004; Berger et al. 2006; Prochaska et al. 2007; Fynbo et al. 2009), which are detectable at least to z ∼ 9.5 (Tanvir et al. 2009; Cucchiara et al. 2011). GRBs provide a unique view of the ISM since their massive star progenitors are embedded within star-forming regions of their hosts (Bloom et al. 2002). As a result, GRBs probe galaxies (their hosts) at a small impact parameter of ≲ few kpc, and thus provide a complementary view to quasars, which tend to probe the outer halos of intervening galaxies due to the large cross section at large radii. As a consequence of this critical difference, GRBs have revealed higher neutral hydrogen and metal column density absorbers than quasars show (e.g., Berger et al. 2006; Prochaska et al. 2007).

Despite this success, GRB observations are not trivial. First, they require wide-field γ-ray satellites with real-time arcminute-scale localization capability (e.g., Swift). Second, while GRB optical/near-IR afterglows are initially more luminous than quasars (〈m(1 hr)〉 ∼ 17 mag, 〈M(1 hr)〉 ∼ −28 mag; e.g., Kann et al. 2010), the emission fades rapidly, by 3–4 mag in one day. Thus, the window of opportunity for GRB absorption spectroscopy is brief. In this context, an alternative astrophysical source with the advantages of GRBs (embedded massive star progenitor, large luminosity), but which overcomes the disadvantages of GRBs (rapid fading, γ-ray discovery), would provide a powerful probe of distant galaxies. Normal supernovae (SNe) are inadequate for this purpose since with M ≳ −19.5 mag they are too faint for absorption spectroscopy at cosmologically interesting redshifts⁸ (z ≳ 1). Moreover, the spectral energy distributions (SEDs) of Type I SNe are strongly suppressed at λ_r ≲ 4000 Å due to iron line blanketing, and they are therefore poorly suited to the detection of interstellar Lyα and metal UV lines in absorption. The SEDs of Type II SNe can extend into the UV, but they are generally less luminous than Type I events, although some Type IIn events are ultraluminous (e.g., Ofek et al. 2007; Smith et al. 2007).

Against this backdrop, a recently discovered class of ultraluminous SNe (ULSNe) with M ≈ −22 to −23 mag and SEDs that extend into the UV may provide a powerful probe of distant galaxies (Quimby et al. 2007, 2011; Barbary et al. 2009; Pastorello et al. 2010; Chomiuk et al. 2011). This potential was first noted by Quimby et al. (2011) and further discussed by Chomiuk et al. (2011). Here we present the first demonstration of this potential with spectroscopic observations of an ULSN at z = 1.566 discovered in the Pan-STARRS1 Medium-Deep Survey (PS1/MDS). This is one of the highest redshift spectroscopically confirmed SNe known to date. The spectra exhibit interstellar absorption features from Fe ii and Mg ii with equivalent widths that are intermediate between the populations of quasar and GRB absorbers. We also present PS1 detections of the host galaxy in several rest-frame UV bands that point to a substantial star formation rate, a young stellar population, and low stellar mass that are reminiscent of GRB host galaxies. These results pave the way for the use of ULSNe as probes of distant galaxies in the LSST and GSMT era.

2.1. PS1 Survey Summary

2.2. Discovery and Photometric Observations of PS1-11bam

PS1-11bam was discovered in PS1/MDS data at R.A. = 08^h41^m14192, decl. = +44°01'5695 (J2000), about 10 days before maximum light, with the time of peak corresponding to UT 2011 November 22 (Figure 1). The object was detected in the first images of the season, so the actual rise time is likely ≳ 10 days. PS1-11bam was detected for nearly 100 days in the i_P1 and z_P1 bands, and for a shorter period of time near peak in the g_P1 and r_P1 bands. The peak absolute magnitude in i_P1 (λ_r ≈ 2930 Å) is M_AB = −22.3 ± 0.1 mag, matching the most luminous SNe to date (cf. Chomiuk et al. 2011; Quimby et al. 2011). The light curve declines by 1 mag from maximum in about 25 days (rest frame), somewhat faster than 30–40 days for previous ULSNe (e.g., Chomiuk et al. 2011; Quimby et al. 2011).

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The SED within ±1 day of peak brightness can be fit with a blackbody function, with a temperature of about 1.7 × 10⁴ K (Figure 1). The flux density in the g_P1 band (λ_r ≈ 1880 Å) is suppressed compared to the blackbody model due to broad absorption blueward of λ_r ≈ 2000 Å (Section 2.3 and Figure 1).

2.3. Spectroscopy of PS1-11bam

We obtained spectra of PS1-11bam with the Blue Channel spectrograph (Schmidt et al. 1989) on the MMT 6.5 m telescope on UT 2011 November 29. The spectrum consisted of 3 × 1200 s exposures obtained at an airmass of 1.1 with a 1'' slit aligned at the parallactic angle. The data were processed using standard procedures in IRAF, and the resulting spectrum covers ≈3320–8530 Å. As shown in Figure 1, the spectrum exhibits broad absorption features at about 4500, 5650, 6150, and 6800 Å, corresponding to features seen in previous ULSNe (Quimby et al. 2007, 2011; Pastorello et al. 2010; Chomiuk et al. 2011). Accounting for an expansion velocity of about −15,000 km s⁻¹ (Chomiuk et al. 2011), the resulting redshift is z ≈ 1.55. At this redshift, emission is detected to λ_r ≈ 1300 Å.

Following this identification, we obtained additional spectra with the Gemini Multi-Object Spectrograph (GMOS; Hook et al. 2004) on the Gemini North 8 m telescope on UT 2011 December 5 and 2012 January 1. The observations consisted of 2 × 1500 s exposures with the R400 grating (December 5), 2 × 1200 s with the B600 grating (January 1), and 2 × 1050 s with the R400 grating (January 1) all taken at airmass of 1.1–1.2 and with a 1'' slit aligned at the parallactic angle. The spectra were processed using the gemini package in IRAF, and cover 4800–9050 Å and 3850–9600 Å, respectively, with a resolution of about 7 Å. In addition to the broad SN features, the first Gemini spectrum reveals narrow absorption lines of Fe ii and Mg ii at a common redshift of z = 1.5657 ± 0.0003 (Table 1 and Figure 2). The second Gemini spectrum further reveals a narrow [O ii]λ3727 emission line at z = 1.567 ± 0.001 (Figure 2), confirming that the absorption features arise in the host galaxy of PS1-11bam, with a potential velocity offset of −150 ± 80 km s⁻¹. The emission/absorption redshift also validates our identification of the broad SN features.

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Table 1. ISM Absorption Lines in PS1-11bam

λobs	Line	fij	z	Wr	log N
(Å)	(Å)			(Å)	(cm−2)
6014.29	Fe ii 2344.214	0.114	1.5656	1.0 ± 0.2	14.3
6092.81	Fe ii 2374.461	0.031	1.5660	0.7 ± 0.2	14.7
6114.45	Fe ii 2382.765	0.320	1.5661	0.7 ± 0.2	13.6
6638.80	Fe ii 2586.650	0.069	1.5666	0.8 ± 0.2	14.3
6672.35	Fe ii 2600.173	0.239	1.5661	1.2 ± 0.1	13.9
7174.50	Mg ii 2796.352	0.612	1.5657	1.0 ± 0.2	13.4
7192.93	Mg ii 2803.531	0.305	1.5657	1.3 ± 0.2	13.8

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The detailed properties of PS1-11bam will be discussed in a separate paper (R. Lunnan et al. 2012, in preparation); here we focus on the interstellar absorption and the properties of the associated host galaxy.

The detected interstellar Fe ii and Mg ii features are shown in Figure 2 and summarized in Table 1. The rest-frame equivalent widths of the strongest lines are W_r(Fe ii λ2600) = 1.2 ± 0.1 Å and W_r(Mg ii λ2803) = 1.3 ± 0.2 Å. We use the Mg ii λ2803 line since the Mg ii λ2796 line is mildly contaminated by an atmospheric sky line. The equivalent widths allow us to place a lower limit on the ion column densities, assuming the optically thin regime of the curve of growth, N ≳ 1.13 × 10²⁰ cm⁻² (W_r/fλ²_r); here W_r and λ_r are rest-frame values in units of Å and f is the oscillator strength. This is a lower limit due to likely line saturation and the low spectral resolution. From the strong lines we find logN(Fe ii) ≳ 13.9 and logN(Mg ii) ≳ 13.8, although the weakest oscillator strength line (Fe ii λ2374) gives logN(ii) ≳ 14.7. We note that the redshift of PS1-11bam is not large enough for a detection of Lyα, and we therefore cannot determine the gas-phase metallicity.

In Figure 3 we compare the Fe ii λ2600 and Mg ii λ2803 equivalent widths to those measured for quasar intervening systems at z ≈ 0.4–2.3 from the SDSS (Quider et al. 2011), and for intrinsic absorbers (i.e., host galaxies) from GRB spectra (Fynbo et al. 2009). We find that the equivalent widths for PS1-11bam are in the top quartile of the quasar absorption system distribution, but are in the bottom quartile of the GRB intrinsic absorbers. In comparison to the composite GRB spectrum of Christensen et al. (2011), the Mg ii λ2803 equivalent width is comparable, with 1.3 Å for PS1-11bam and 1.5 Å for the GRB composite. The Fe ii λ2600 equivalent width is lower, with 1.2 Å for PS1-11bam and 1.85 Å for the GRB composite. However, we note that the Fe ii λ2600 line in the GRB composite spectrum is blended with the fine-structure line Fe ii*λ2599, so the actual equivalent width is ≲ 1.85 Å. The fine-structure line is due to UV pumping by the early afterglow intense radiation field (Dessauges-Zavadsky et al. 2006; Vreeswijk et al. 2007), which we do not expect⁹ in the case of PS1-11bam. Finally, we find that the equivalent widths for PS1-11bam are about three times smaller than the values measured from a composite spectrum of 13 star-forming galaxies at a mean redshift of z ≈ 1.6 from the Gemini Deep Deep Survey (GDDS; Savaglio et al. 2004). For subsequent comparison with the host of PS1-bam (Section 4), we note that the GDDS galaxies have a mean rest-frame UV absolute magnitude of M₂₀₀₀ ≈ −20.3 AB mag.

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The larger equivalent widths for the PS1-11bam absorber compared to the bulk of quasar intervening systems, and the similarity to the GRB composite spectrum, which is mainly composed of damped Lyα absorbers (DLAs; logN(H i) ≳ 20.3), suggest that PS1-11bam would also uncover a DLA if we could measure its Lyα column density. Indeed, quasar studies suggest that about 80% of all absorbers with Mg ii λ2796 ≳ 1.3 Å and about 50% of absorbers with Fe ii λ2600 ≳ 1.2 Å are DLAs (Rao et al. 2006).

We detect the host galaxy of PS1-11bam in pre-explosion stacks of the PS1/MDS data with apparent magnitudes¹⁰ of g_P1 = 23.62 ± 0.13, r_P1 = 23.62 ± 0.12, i_P1 = 23.78 ± 0.13, z_P1 = 23.73 ± 0.14, and y_P1 ≳ 23.4 (3σ); see Figure 4. The blue colors of the galaxy are indicative of a young stellar population. The host SED covers rest-frame wavelengths of about 1900–3800 Å, with the y_P1-band limit constraining the 4000 Å/Balmer break (Figure 4). We fit the SED with the Maraston (2005) evolutionary stellar population synthesis models, using a Salpeter initial mass function and a red horizontal branch morphology, with the stellar population age (τ_*) and stellar mass (M_*) as free parameters. For a metallicity range of 0.05–1 Z_☉ we find τ_* ≈ 15–45 Myr and M_* ≈ (1.1–2.6) × 10⁹M_☉ (χ² = 1.2 for 3 degrees of freedom; Figure 4); lower metallicity leads to older ages and larger stellar masses. We also find that A^host_V ≲ 0.5 mag for an assumed LMC extinction curve.

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From the observed g_P1 band we infer a rest-frame UV luminosity of L_{ν, UV} ≈ 8 × 10²⁸ erg s⁻¹ Hz⁻¹, corresponding to a star formation rate of SFR ≈ 12M_☉ yr⁻¹ (Kennicutt 1998). The upper bound on the extinction (A^host₁₉₀₀ ≈ 1.3 mag) indicates that SFR ≲ 40 M_☉ yr⁻¹. From the [O ii]λ3727 line flux, F([O ii]) ≈ 7 × 10⁻¹⁷ erg cm⁻² s⁻¹, we find SFR ≈ 6 M_☉ yr⁻¹ (Kennicutt 1998). The star formation rate is substantially larger than for previous ULSNe hosts, with SFR ≈ 0.1–1 M_☉ yr⁻¹ (Chomiuk et al. 2011; Neill et al. 2011). The young stellar population age, low stellar mass, and appreciable star formation rate are reminiscent of long GRB host galaxies for which 〈τ_*〉 ∼ 60 Myr, 〈M_*〉 ∼ 1.5 × 10⁹ M_☉, and 〈SFR〉 ∼ 10 M_☉ yr⁻¹ (Christensen et al. 2004; Savaglio et al. 2009; Leibler & Berger 2010).

The rest-frame UV absolute magnitude of the host galaxy of PS1-11bam, M₂₀₀₀ ≈ −20.7 AB mag, is brighter than the mean of the GDDS composite spectrum, M₂₀₀₀ ≈ −20.3 AB mag (Savaglio et al. 2004). This may appear puzzling given the much larger Fe ii λ2600 and Mg ii λ2803 equivalent widths in the GDDS composite compared to PS1-11bam, but we note that the GDDS galaxies are K-band (i.e., stellar mass) selected, while the host of PS1-11bam was selected through a massive star explosion (i.e., star formation). As a result, despite its larger UV luminosity, the host of PS1-11bam is most likely a lower stellar mass system than the mean GDDS galaxy.

We presented the discovery of the ULSN PS1-11bam and follow-up spectroscopy that reveals broad SN features and interstellar absorption from Fe ii and Mg ii. The ISM line equivalent widths are larger than the median of the quasar intervening absorber sample, but smaller than the GRB intrinsic absorber sample. They are substantially lower compared to a stack of GDDS galaxies at z ≈ 1.6. Although the potential of ULSNe as probes of distant galaxies has been proposed previously (Quimby et al. 2011), this is the first direct demonstration of this approach at a cosmologically interesting redshift that overlaps with the GRB and quasar samples. This is also the redshift range at which direct galaxy metallicity measurements become progressively more challenging.

We also detect the host galaxy of PS1-11bam in pre-explosion images and find that it exhibits a substantial star formation rate, a young stellar population age, and a low stellar mass, similar to the long GRB host sample. The star formation rate is substantially larger than in previous ULSN hosts (Chomiuk et al. 2011; Neill et al. 2011), indicating that they likely span a wide range of properties.

The spectrum of PS1-11bam demonstrates that ISM measurements using ULSNe are both promising and challenging. First, the luminosities are about a factor of 10–100 times lower than for GRB afterglows, but this can be partially overcome with repeated observations during the broad peak and in the future with 30 m class telescopes. Second, with a typical blackbody temperature of ∼2 × 10⁴ K, the continuum flux at λ_r ∼ 1200 Å, required for absorption measurements of Lyα, is about 5 times lower than at the peak of the SED. However, our observations of PS1-11bam demonstrate that continuum emission is indeed detectable to at least λ_r ∼ 1300 Å. Third, any broad SN features near λ_r ∼ 1200 Å may complicate measurements of Lyα absorption; existing data do not extend sufficiently blueward to assess this point. Fourth, since circumstellar (CSM) interaction has been invoked to explain the large luminosities of ULSNe (Chevalier & Irwin 2011), absorption in the CSM may complicate measurements of ISM features. However, CSM interaction is generally accompanied by strong emission or P Cygni line profiles, which are also time variable (e.g., Bowen et al. 2000). This is not seen in PS1-11bam, and it may serve as a discriminant between ISM and CSM absorption for future events. Finally, our observations demonstrate that at present PS1 is uniquely capable of discovering UV-bright ULSNe to z ∼ 2, thanks to its superior red sensitivity and use of i_P1z_P1-band filters, and that follow-up spectroscopy with 8 m class telescopes can probe ISM absorption features; the upcoming Dark Energy Survey and PS2 will have the same potential. With the even greater depth of LSST, in conjunction with follow-up spectroscopy from 30 m class telescopes, ULSNe will ultimately probe galaxies to z ∼ 4.

PS1 has been made possible through contributions of the Institute for Astronomy, the University of Hawaii, the Pan-STARRS1 Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, Queen's University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network, Incorporated, the National Central University of Taiwan, and the National Aeronautics and Space Administration under grant NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate. This work is based in part on observations obtained at the Gemini Observatory (Program GN-2011B-Q-3; PI: Berger), which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil), and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina). Observations were also obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona. Some of the computations in this paper were run on the Odyssey cluster supported by the FAS Science Division Research Computing Group at Harvard University.