The Rest‐Frame Optical Spectra of Lyman Break Galaxies: Star Formation, Extinction, Abundances, and Kinematics

Abstract

We present the Ðrst results of a spectroscopic survey of Lyman break galaxies (LBGs) in the nearinfrared aimed at detecting the emission lines of [O II], [O III], and Hb from the H II regions of normal star-forming galaxies at z^3. From observations of 15 objects with the Keck telescope and the Very Large Telescope augmented with data from the literature for an additional four objects, we reach the following main conclusions. The rest-frame optical properties of LBGs at the bright end of the luminosity function are remarkably uniform, their spectra are dominated by emission lines, [O III] is always stronger than Hb and [O II], and projected velocity dispersions are between 50 and 115 km s~1. Contrary to expectations, the star formation rates deduced from the Hb luminosity are on average no larger than those implied by the stellar continuum at 1500 presumably any di †erential extinction between A ; rest-frame optical and UV wavelengths is small compared to the relative uncertainties in the calibrations of these two star formation tracers. For the galaxies in our sample, the abundance of oxygen can only be determined to within 1 order of magnitude without recourse to other emission lines ([N II] and Ha), which are generally not available. Even so, it seems well established that LBGs are the most metalenriched structures at z^3, apart from quasi-stellar objects, with abundances greater than about 1/10 solar and generally higher than those of damped Lya systems at the same epoch. They are also signiÐ-cantly overluminous for their metallicities ; this is probably an indication that their mass-to-light ratios are low compared to present-day galaxies. At face value, the measured velocity dispersions imply virial masses of about 1010 within half-light radii of 2.5 kpc. The corresponding mass-to-light ratios, M _ M/L B 0.15 in solar units, are indicative of stellar populations with ages between 108 and 109 yr, consistent with the UV-optical spectral energy distributions. However, we are unable to establish conclusively whether or not the widths of the emission lines reÑect the motions of the H II regions within the gravitational potential of the galaxies, even though in two cases we see hints of rotation curves. All 19 LBGs observed show evidence for galactic-scale superwinds ; such outÑows have important consequences for regulating star formation, distributing metals over large volumes, and allowing Lyman continuum photons to escape and ionize the intergalactic medium.

The impact of starbursts and post-starbursts on the photometric evolution of high redshift galaxies

“…A consistent inclusion of appropriate amounts of dust in the course of the redshift evolution of the various model galaxy types is difficult, in particular during bursty phases of SF. Optical as well as FUV detected starbursts in the local universe along with LBGs at high redshift, all do show dust extinction, albeit typically at rather moderate values of A FUV ∼ 0.5−2 mag (Buat et al 2005;Shapley et al 2001;Pettini et al 2001), corresponding to visual extinctions on the order of A V ∼ 0.2−0.9 mag. A comparison between FIR (60 µm) and NUV (GALEX) selected starburst galaxy samples shows that all but one out of 118 FIR-selected starbursts are also detected in the NUV and that the median NUV dust attenuation for the FIR-selected starbursts is only ∼2 mag (Buat et al 2005).…”

“…Many Lyman Break Galaxies (LBGs) are also called starbursts by some authors, albeit with more moderate SF rates than ULIRGs or SCUBA galaxies, typically in the range 10−100 M /yr Pettini et al 2001;Giavalisco 2002). Although some of them feature irregular or knotty structures, they are not clearly related to (major) mergers.…”

The Launching of Cold Clouds by Galaxy Outflows. Ii. The Role of Thermal Conduction

“…Such powerful outbursts are well observed in high-surface density galaxies over a wide range of masses and redshifts (e.g., Martin 1999;Pettini et al 2001;Heckman 2002;Veilleux et al 2005), including the ultraluminous infrared galaxies that host as much as half of the z 1 star formation in the universe (Rupke et al 2005). Their impact is extensive: they are believed to set the correlation between galaxy stellar mass and interstellar medium metallicity, (e.g., Tremonti et al 2004;Kewley & Ellison 2008;Mannucci et al 2010), they enrich the intergalactic medium with metals (e.g., Pichon et al 2003;Ferrara et al 2005;Martin et al 2010;Peeples et al 2014;Turner et al 2015), and they help determine the number density of faint galaxies (e.g., Scannapieco et al 2002;Benson et al 2003).…”

Stellar Masses of High-Redshift Galaxies

“…Some of this arises from assumptions in the population-synthesis models (e.g., metallicity, IMF; see [2]). The metallicities of high-redshift galaxies are only weakly constrained; optical and near-IR spectra suggest ∼ 1/4 − 1/3 Z ⊙ (e.g., [6]). Varying the metallicity assumed in the synthesis models causes systematic shifts in the distribution of best-fitmodel parameters, including ∼ 0.3 dex in the inferred stellar masses.…”