PHYSICAL CONDITIONS IN A YOUNG, UNREDDENED, LOW-METALLICITY GALAXY AT HIGH REDSHIFT

By D. Erb, M. Pettini, A. Shapley, C. Steidel, D. Law, Naveen A. Reddy. 2010

206 citations recorded by the source corpus.

Increasingly large samples of galaxies are now being discovered at redshifts z ∼ 5–6 and higher. Many of these objects are inferred to be young, low in mass, and relatively unreddened, but detailed analysis of their high quality spectra will not be possible until the advent of future facilities. In this paper, we shed light on the physical conditions in a plausibly similar low-mass galaxy by presenting the analysis of the rest-frame optical and UV spectra of Q2343-BX418, an L* galaxy at z = 2.3 with a very low mass-to-light ratio and unusual properties: BX418 is young (<100 Myr), low mass (M⋆ ∼ 109 M☉), low in metallicity (Z ∼ 1/6 Z☉), and unreddened (E(B − V) ≃ 0.02, UV continuum slope β = −2.1). We infer a metallicity 12 + log(O/H) = 7.9 ± 0.2 from the rest-frame optical emission lines. We also determine the metallicity via the direct, electron temperature method, using the ratio O iii] λλ1661, 1666/[O iii] λ5007 to determine the electron temperature and finding 12 + log(O/H) = 7.8 ± 0.1. These measurements place BX418 among the most metal-poor galaxies observed in emission at high redshift. The rest-frame UV spectrum, which represents ∼12 hr of integration with the Keck telescope, contains strong emission from Lyα (with rest-frame equivalent width 54 Å), He ii λ1640 (both stellar and nebular), C iii] λλ1907, 1909 and O iii] λλ1661, 1666. The C iv/C iii] ratio indicates that the source of ionization is unlikely to be an active galactic nucleus. Analysis of the He ii, O iii], and C iii] line strengths indicates a very high ionization parameter log U ∼ −1, while Lyα and the interstellar absorption lines indicate that outflowing gas is highly ionized over a wide range of velocities. It remains to be determined how many of BX418's unique spectral features are due to its global properties, such as low metallicity and dust extinction, and how many are indicative of a short-lived phase in the early evolution of an otherwise normal star-forming galaxy.

Subfields: Astronomy and Astrophysics, Electrical and Electronic Engineering, Instrumentation

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Citations in the corpus

Nuclear activity versus star formation: emission-line diagnostics at ultraviolet and optical wavelengths (2015, background): “…active and inactive galaxies in Sections 4.2–4.6, and which involve emission lines currently detected in observations of high-redshift galaxies (Erb et al. 2010; Stark et al. 2014), i.e., Nvλ1240, C ivλ1550, He iiλ1640, O iii]λ1663 (and the corresponding individual doublet components), N…”
Lyα and C iii] emission in z = 7–9 Galaxies: accelerated reionization around luminous star-forming systems? (2016, background): “The total flux in the resolved CIII] doublet is close to 50% that of Lyα, nearly 10× greater than is seen in the most extreme CIII] emitting galaxies at lower redshift (e.g., Erb et al. 2010; Christensen et al. 2012; Stark et al. 2014).”
A critical analysis of the ultraviolet continuum slopes (β) of high-redshift galaxies: no evidence (yet) for extreme stellar populations at z > 6 (2011, background): “…aroundλrest ≃ 1500Å than have been detected to date in galaxies discovered at any lower redshiftz < 6.5 (e.g. Steidel et al. 1999; Meurer et al. 1999; Adelberger & Steidel2000; Ouchi et al. 2004; Stanway et al. 2005; Bouwens et al. 2006; Hathi et al. 2008; Bouwens et al. 2009; Erb et al. 2010).”
Rest-frame ultraviolet-to-optical spectral characteristics of extremely metal-poor and metal-free galaxies (2011, methodology): “In fact, the line has been detected from z = 2–3 LBGs but its origin is attributed to the stellar winds of these stars (Shapley et al. 2006; Erb et al. 2010).”
Ionization state of inter-stellar medium in galaxies: evolution, SFR-M * -Z dependence, and ionizing photon escape (2013, methodology): “For high-z galaxies with a Lyα flux measurement, we classify galaxies with EWrest (Lyα) > 20 Å as LAE; Lynx arc (Fosbury et al. 2003), BX418 (Erb et al. 2010), Abell 1689 31.1 arc, and SMACS J2031 arc (Christensen et al. 2012a,b).”
RECONCILING THE STELLAR AND NEBULAR SPECTRA OF HIGH-REDSHIFT GALAXIES (2016, background): “He II emission can be present as either a stellar feature, broadened by processes associated with winds from massive stars, or as a nebular feature, from recombination in H II regions; of course, it is possible that both mechanisms may contribute in any single case (see, e.g., Erb et al. 2010).”
EVIDENCE FOR PopIII-LIKE STELLAR POPULATIONS IN THE MOST LUMINOUS Lyα EMITTERS AT THE EPOCH OF REIONIZATION: SPECTROSCOPIC CONFIRMATION (2015, background): “…et al. 2000), with typical FWHM of lines being ∼ 1000 km s−1; 2) Wolf-Rayet (WR) stars (many cases known in e.g. SDSS: Shirazi & Brinchmann 2012; or see Erb et al. (2010) for a higher redshift example), with typical FWHM of lines being ∼ 3000 km s−1; 3) Direct collapse black hole (DCBH), which…”
A CRITICAL ASSESSMENT OF PHOTOMETRIC REDSHIFT METHODS: A CANDELS INVESTIGATION (2013, methodology): “However, there are some differences in the details of the template sets used in each case, e.g., code 4C includes a template of BX418 (Erb et al. 2010), a metal poor galaxy with strong Lyα emission, code 7C uses the EAZY templates, plus a template with deep Lyα absorption (constructed from an…”
Spectroscopic detection of C iv λ1548 in a galaxy at z = 7.045: implications for the ionizing spectra of reionization-era galaxies (2015, background): “Recent studies have proposed that the [CIII]λ1907, CIII]λ1909 doublet provides a feasible route toward spectroscopic confirmation of reionization-era galaxies in which Lyα is strongly attenuated by the partially neutral IGM (Erb et al. 2010; Stark et al. 2014b).”
Lyα EMISSION FROM GREEN PEAS: THE ROLE OF CIRCUMGALACTIC GAS DENSITY, COVERING, AND KINEMATICS (2015, background): “Efforts to understand Lyα escape from high-redshift galaxies have generally suffered from the limitations associated with studying faint, distant sources (e.g., Shapley et al. 2003; Erb et al. 2010, 2014; Jones et al. 2012, 2013).”
DIFFUSE Lyα EMITTING HALOS: A GENERIC PROPERTY OF HIGH-REDSHIFT STAR-FORMING GALAXIES (2011, background): “…the strongest Lyα emission (among the continuum-selected samples) invariably have much weaker than average low-ionization IS absorption lines (see Erb et al. 2010 for a well-observed example), while those with the Lyα appearing strongly in absorption have correspondingly strong IS absorption…”
THE RELATION BETWEEN STAR FORMATION RATE AND STELLAR MASS FOR GALAXIES AT 3.5 ⩽ z ⩽ 6.5 IN CANDELS (2014, background): “Nebular emission is important because many galaxies at high redshift are observed to have intense star formation and high equivalent widths (EW) from emission lines (Erb et al. 2010; van der Wel et al. 2011; Atek et al. 2011; Brammer et al. 2013).”
THEORETICAL EVOLUTION OF OPTICAL STRONG LINES ACROSS COSMIC TIME (2013, background): “Many high redshift galaxies have high ionization parameters (−2.9 < logU < −2.0 Pettini et al. 2001; Lemoine & Pelletier 2003; Maiolino et al. 2008; Hainline et al. 2009; Erb et al. 2010; Richard et al. 2011; Wuyts et al. 2012a).”
PHYSICAL PROPERTIES OF EMISSION-LINE GALAXIES AT z ∼ 2 FROM NEAR-INFRARED SPECTROSCOPY WITH MAGELLAN FIRE (2014, background): “For this reason, relatively few near-infrared spectra of galaxies at z ∼ 2 that cover all of the important rest-frame optical emission lines have been published to date (e.g., Erb et al. 2006; Hainline et al. 2009; Erb et al. 2010; Rigby et al. 2011; Belli et al. 2013).”
Near infrared spectroscopy and star-formation histories of 3 ≤ z ≤ 4 quiescent galaxies (2018, background): “…with the upcoming James Webb Space Telescope (JWST), although given the expected change in gas-phase metallicity and ionization parameters in high redshift galaxies (e.g., Erb et al. 2010; Steidel et al. 2016; Shapley et al. 2017), an evolution of the [O ii] SFR calibration would not be surprising.”
STRONG NEBULAR LINE RATIOS IN THE SPECTRA of z ∼ 2–3 STAR FORMING GALAXIES: FIRST RESULTS FROM KBSS-MOSFIRE (2014, methodology): “Q2343BX418 (z = 2.3053) was studied in detail by Erb et al. (2010) as a prototypical example of a UV-bright galaxy with little or no reddening, strong Lyman α emission, and unusually strong rest-UV nebular lines of OIII]λλ1661,1666 and CIII] λλ1906,1909.”
KECK SPECTROSCOPY OF 3 < z < 7 FAINT LYMAN BREAK GALAXIES: THE IMPORTANCE OF NEBULAR EMISSION IN UNDERSTANDING THE SPECIFIC STAR FORMATION RATE AND STELLAR MASS DENSITY (2012, result): “…2.0× (the exact value chosen at random from a uniform distribution), consistent with the flux ratios observed in sub-solar galaxies at z ≃ 2−3 (e.g., Hainline et al. 2009, Bian et al. 2010, Erb et al. 2010, Richard et al. 2011) and those predicted by our nebular+stellar population synthesis models.”