3D-HST GRISM SPECTROSCOPY OF A GRAVITATIONALLY LENSED, LOW-METALLICITY STARBURST GALAXY AT z = 1.847^*

Tracing galaxy populations over cosmic time is key to understanding their evolutionary processes. Observations of a particular galaxy type over a range of redshifts yield complementary information, from spatially resolved, high signal-to-noise studies of individual local examples to more distant statistical samples (e.g., Overzier et al. 2009). While local low-mass dwarfs (≲ 10⁹ M_☉) can be studied relatively easily, more distant examples are usually missing in typical flux-, mass-, or star-formation-rate-limited surveys. The best-studied examples at cosmological distances are usually strongly magnified by gravitational lenses (e.g., Fosbury et al. 2003; Yuan & Kewley 2009; Wuyts et al. 2012).

The lowest mass galaxies tend to have the lowest metallicities (Tremonti et al. 2004), and the normalization of this mass–metallicity relation evolves such that galaxies at a given stellar mass have lower metallicities at higher redshifts to at least z ∼ 3 (Erb et al. 2006; Mannucci et al. 2009). The physical conditions of low-metallicity star-forming galaxies can be very different than their higher-mass counterparts, characterized by a hard radiation field and strong stellar winds (e.g., Erb et al. 2010). Consequently, they may have strong emission lines, which can be used to select them (Atek et al. 2011; van der Wel et al. 2011) and also must be accounted for when modeling their broadband photometry (Atek et al. 2011; Finkelstein et al. 2011). Active galactic nuclei (AGNs) are also sources of hard ionizing radiation that can result in similar spectral features, so accurately separating the influence of AGNs and metallicity/star formation is crucial for understanding the dominant processes that shape galaxies at z > 1 (e.g., Trump et al. 2011).

In this Letter, we present Hubble Space Telescope (HST) observations of a gravitational lens system discovered by the Strong Lensing Legacy Survey consisting of a bright cusp arc at z = 1.8470 that is lensed by a massive galaxy at z = 0.6459 (SL2SJ02176-0513; Tu et al. 2009). The system lies within the CANDELS imaging (Grogin et al. 2011; Koekemoer et al. 2011) and 3D-HST spectroscopic (Brammer et al. 2012) surveys of the UKIDSS/UDS field. We use the unique combination of the HST data sets and the natural lens to demonstrate that the lensed galaxy is a low-mass, low-metallicity galaxy undergoing an extreme starburst, and that it shares many characteristics of local low-metallicity blue compact dwarf galaxies. We adopt cosmological parameters h = 0.7, Ω_m = 0.3, and Ω_Λ = 0.7 throughout.

We use the "ver. 1.0" reduction of the 1–2 orbit HST ACS F606W, F814W and WFC3 F125W, and F160W images provided by the CANDELS team (Koekemoer et al. 2011). Three-color combinations of the CANDELS images are shown in Figure 1. The arc is very blue through the observed optical and F125W bands, with the UV slope β = −1.7  ±  0.2 (f_λ∝λ^β) determined from the two Advanced Camera for Surveys (ACS) bands (λ_rest =2100–2800 Å). The arc becomes surprisingly red when the F160W band is included; Cooray et al. (2011) hypothesized that the red F125W−F160W color is caused by strong [O iii] emission that dominates the flux in the redder band, similar to the extreme equivalent-width galaxies discovered by van der Wel et al. (2011). In addition to the HST observations, we extract photometry of the arc from CFHT-LS ugriz and UKIDSS/UDS JHK imaging following Cooray et al. (2011) and removing the roughly symmetric lens galaxy by subtracting a flipped version of the ground-based images.

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SL2SJ02176-0513 was observed with the WFC3 G141 grism on 2011 December 21 as part of the 3D-HST survey. The spectrum was reduced as described in detail by Brammer et al. (2012). Figure 2 shows the remarkable G141 spectrum of the lens system, with strong emission lines of [O iii] λ4959+5007, Hβ, and Hγ (potentially blended with [O iii] λ4363) that explain the colors in Figure 1 and confirm the prediction of Cooray et al. (2011). All of the emission lines are extended, showing the same morphology as the UV (ACS) continuum. The emission lines prove that the arc and counter image lie at the same redshift.

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The SL2SJ02176-0513 system is a bright MIPS 24 μm source⁸ with a total flux of 0.565 mJy, offset from the lens galaxy and centered on the arc. From a simultaneous fit (see Labbé et al. 2006) of the 24 μm contributions from the arc, the lens, and the additional z = 2.3 lensed galaxy to the north, we find that the arc contributes 85% of the 24 μm flux, or 0.476 ± 0.025 mJy.

3.1. Lens Magnification

3.2. Modeling the Grism Spectrum

In order to extract quantities from the grism spectrum, we generate and fit a model of the two-dimensional (2D) spectrum that essentially convolves an arbitrary one-dimensional spectrum with an assumed object morphology, given the grism dispersion configuration files provided by STScI. To model the contribution of the lens to the flux at the location of the arc, we adopt as the lens morphology an analytical Sérsic profile with parameters determined by running galfit (Peng et al. 2002) on the 3D-HST F140W image. For the arc, we adopt the observed (lens-subtracted) F140W morphology and a one-dimensional spectrum that consists of a Z = 0.008 Bruzual & Charlot (2003) single stellar population model for the continuum and individual emission lines.

The model is fit to the observed spectrum with parameters optimized by the emcee Markov chain Monte Carlo (MCMC) sampler (Foreman-Mackey et al. 2012), where the free parameters are (1) a spatial shift and a spectral scaling to improve the subtraction of the lens; (2) the redshift of the arc; (3) the age, stellar mass, and reddening of the arc continuum following a Calzetti et al. (2000) reddening law; and (4) individual strengths of the arc emission lines [O iii] λ4959+5007, Hβ, Hγ, [O iii] λ4363, Hδ, H, and Ne iii λ3869. While the spectral resolution of the G141 grism (R ∼ 130) is insufficient to resolve the Hγ and [O iii] λ4363 lines, the sum of these lines is well constrained by the G141 spectrum.

The best-fit model of the 2D arc spectrum is shown in the middle panel of Figure 2, and the residuals of the full lens+arc model fit are shown in the bottom panel. The parameters of the model, including the line strengths and observed-frame equivalent widths, are summarized in Table 1. The uncertainties on all parameters come from the full marginalized posterior distribution function of the MCMC chain. The most robust fits are for the λ4959+5007 and Hβ emission line strengths, with rest-frame equivalent widths of 2000 ± 100 and 520 ± 40 Å, respectively. We obtain a marginal detection of [O iii] λ4363 at 5 ± 3 × 10⁻¹⁷ erg s⁻¹ cm⁻² assuming an intrinsic Balmer line ratio Hγ/Hβ = 0.468 and no reddening of the Balmer lines. The [O iii] λ4363 flux will be higher for any non-zero Balmer decrement.

Table 1. Observables and Derived Quantities

Parameter	Value
R.A., decl.	02:17:37.237,  −05:13:29.78
Photometry
F606W (V) [AB]	22.07 ± 0.03
F814W (i)	21.96 ± 0.03
F125W (J)	21.71 ± 0.02
F140W (Hwide)	21.10 ± 0.04
F160W (H)	20.90 ± 0.02
MIPS 24 μm (mJy)	0.476 ± 0.025
μ⋆	25 ± 1a
μ'	∼1.4a
Spectrum + SED fit
z	1.84691 ± 0.0004rand ± 0.002sys
log (age/yr)	7.2 ± 0.2
AV (continuum)	0.09 ± 0.15
log (μ · M/M☉)	9.5 ± 0.1
f([O iii] λλ4959 + 5007)	309 ± 2b
f(Hβ)	74 ± 2b
f(Hγ +[O iii] λ4363)	40 ± 3b
f(Hδ)	13 ± 2b
f(Ne iii λ3869)	30 ± 4b
${\rm EW}_{{\rm O\,\mathsc{iii}}}$	5690 ± 290 Åc
EWHβ	1470 ± 110 Åc
Derived parameters
β (2000–2800 Å)	−1.7 ± 0.2
μ · SFRHβ (M☉ yr−1)	390 ± 9
12 + log (O/H)	7.5 ± 0.2
$\sqrt{\mu ^\prime }\cdot r_e$	350 pc
ΣSFR(r < re, M☉ yr−1 kpc−2)	20

Notes. ^aμ_⋆ is the relative magnification between the integrated arc and the counter image. μ' is the brightness magnification of the counter image. The total lens magnification is μ = μ_⋆ · μ'. ^bLine fluxes are in 10⁻¹⁷ erg s⁻¹ cm⁻², uncorrected for lens magnification. ^cObserved frame.

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3.3. Fundamental Quantities: Star Formation Rate, Stellar Mass, and Metallicity

While the unique observational data of SL2SJ02176-0513 paint a self-consistent picture of an extremely low-metallicity, low-mass starburst galaxy, many of the simplifying assumptions underlying its derived properties can result in significant systematic uncertainties. To place the properties of SL2SJ02176-0513 in context with nearby galaxies, we select a comparison sample from the Sloan Digital Sky Survey (SDSS DR7; Abazajian et al. 2009) with ${\rm EW}_{{\rm O\,\mathsc{iii}}} > 1500$ Å and $\log \left(f_{{\rm O\,\mathsc{iii}}}/f_{\mathrm{H}\beta }\right) < 0.75$⁹ as measured by Brinchmann et al. (2004).

This simple selection criterion reduces the full SDSS spectroscopic sample of 930,000 galaxies to just 14. Of these, eight objects have 12 + log (O/H) > 8 (within the fiber aperture; Tremonti et al. 2004) and are in fact compact, especially blue, individual H ii regions in spiral/irregular galaxies (M101 and M106 among them). The remaining six objects are blue compact dwarf galaxies with 12 + log (O/H) < 8. Four are found in the extremely metal-poor galaxy compilation by Morales-Luis et al. (2011) with 12 + log (O/H) ∼ 7.6 and the final two have 12 + log (O/H) ∼ 7.8 (Brinchmann et al. 2008; Engelbracht et al. 2008).

4.1. Spectral Energy Distributions

Three of these local analog galaxies are compared to SL2SJ02176-0513 in Figure 3. The left panel demonstrates the similarity of the SEDs of SL2SJ02176-0513 and its local analogs at all observed wavelengths: all have similar blue UV continua, and the optical/NIR photometry of the analogs agrees well with the extrapolation of the lens stellar continuum fit. SL2SJ02176-0513 is 2–20 times more luminous than the local analogs shown, qualitatively consistent with the order of magnitude increase of stellar mass at constant metallicity to z ∼ 2 (e.g., Erb et al. 2006). Both samples show a marked upturn at mid-IR wavelengths sampled by Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010) for the local galaxies and by Spitzer-MIPS for SL2SJ02176-0513.

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Hot dust in low-metallicity blue compact dwarf galaxies has been known and studied for some time (e.g., Thuan et al. 1999; Hirashita & Hunt 2004; Engelbracht et al. 2008) and has even recently been used to select new examples with red WISE colors (Griffith et al. 2011; Izotov et al. 2011). The dust can be produced rapidly by Type II supernovae even in primordial bursts (e.g., Todini & Ferrara 2001) and it can be heated by the substantial UV radiation of the intense starbursts (Izotov et al. 2011).

There is significant variation in the IR properties of the local galaxies whose colors and spectral properties are similar in the UV/optical. While they are rare, examples exist of local galaxies with such rapidly rising mid-IR dust SEDs as SL2SJ02176-0513. The SED of one such "mid-IR peaker" (Engelbracht et al. 2008) is shown in Figure 3 and agrees well with the SL2SJ02176-0513 SED. This object, SBS 0335-052E, is one of the most metal-poor galaxies known in the local universe (12 + log (O/H) = 7.23 ± 0.01; Izotov et al. 1997, 2009).

4.2. Emission Lines

4.3. Morphology

The optical morphologies of three of the SDSS analog galaxies are shown in the thumbnails of Figure 3. All of the analogs, like SL2SJ02176-0513, are dominated by multiple bright blue clumps with sizes of a few hundred parsecs. SBS 0335-052E is also composed of multiple "super star clusters" with ages 3–15 Myr contained within a scale of 500 pc (Thuan et al. 1997; Reines et al. 2008). Since all of the individual clumps share the properties of young bursts, the star formation must be somewhat synchronized across the entire galaxy.

In Figure 4 we take different combinations of the HST ACS and WFC3 images of SL2SJ02176-0513 to compare the properties of the two main clumps resolved in the arc. The ratio F606W/F814W provides a rough map of the UV slope across the arc, and we find that the source "b" is significantly redder than source "a" (Figure 1, the color image itself shows this difference). The redder source has an [O iii] equivalent width ∼50% higher than the bluer source in the ratio of the F160W (line) and F814W (continuum) images, however, the F160W ([O iii]) to F125W (Balmer line) ratio is roughly constant across the arc. The differences between the sources are most apparent for the a₁ and b₁ components that are best separated by the lens, though the other pairs of conjugate images show roughly the same trends. The individual super star clusters in SBS 0335-052E show differences in broadband colors and line ratios comparable to those in Figure 4 (Thuan et al. 1997; Izotov et al. 2009).

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We take advantage of the unique combination of a natural gravitational lens and high spatial resolution HST imaging and near-IR spectroscopy to extract the detailed properties of SL2SJ02176-0513. The near-IR spectrum is dominated by extremely strong emission lines of Hγ, Hβ, and [O iii] at z = 1.847. From the UV/optical spectrum and photometry, we determine that the source of SL2SJ02176-0513 is a young star-bursting (sSFR ∼ 100 Gyr⁻¹) dwarf galaxy (M ∼ 1.3 × 10⁸ M_☉) with an extremely low gas-phase metallicity (12 + log (O/H) ∼ 7.5). Even with so few metals, SL2SJ02176-0513 shows detectable hot dust emission observed at λ_obs = 24 μm.

We find the unique properties of SL2SJ02176-0513 to be remarkably similar to those of nearby low-metallicity blue compact dwarf galaxies selected to have similar [O iii] equivalent widths and ([O iii]/Hβ) line ratios. Such galaxies are frequently considered to be local analogs to galaxies expected to be more common at earlier cosmic times and in SL2SJ02176-0513 we have discovered a compelling connection at high redshift (which itself is likely a bright, magnified example of the galaxies discovered by van der Wel et al. 2011).

SL2SJ02176-0513 has characteristics that are frequently attributed to AGNs at similar redshifts: extreme equivalent widths and line ratios of the optical emission lines, high-ionization UV lines, and the presence of a strong IR excess from heated dust. We demonstrate that all of these properties are largely consistent with a hard ionization field produced by a compact, low-metallicity starburst (see also examples from Fosbury et al. 2003; Erb et al. 2010 and discussion by Hunt et al. 2010). Further evidence comes from the fact that the lens resolves two line-emitting components of SL2SJ02176-0513, though extended narrow-line regions excited by nuclear activity have been observed (e.g., Unger et al. 1987). Both star formation and AGNs reach a peak in their activity at z ∼ 2 so robust identification and separation of the two contributions is critical for understanding their effect on galaxy evolution.

We conclude by noting that we will obtain a substantial sample of (unlensed) galaxies with similar, if not quite as extreme, properties to SL2SJ02176-0513 at 1.3 < z < 2.2 (covering [O iii]+Hβ) in the full 3D-HST survey. Without the factor of ∼25 lens magnification, such an object would have m_F140W = 25.2, where 3D-HST is sensitive to line equivalent widths ≳1000 Å (Brammer et al. 2012). Indeed, many such galaxies have recently been found with WFC3 (Atek et al. 2011; van der Wel et al. 2011). While individual unlensed objects will not allow such detailed study as performed here, statistical samples of these galaxies will offer insights into the metallicity and star formation properties of the low-mass building blocks of galaxies observed today.

We thank the referee, M. Malkan, for constructive comments that greatly improved the manuscript and R. Gavazzi and T. Treu for helpful discussions and for providing the LRIS spectrum of SL2SJ02176-0513. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. Funding for this research was provided in part by the Marie Curie Actions of the European Commission (FP7-COFUND) and ERC grant HIGHZ No. 227749.

Facility: HST (WFC3) - Hubble Space Telescope satellite