Max Pettini

Lyman Break Galaxies at z>4 and the Evolution of the UV Luminosity Density at High Redshift

We present initial results of a survey for star-forming galaxies in the redshift range 3.8 z 4.5. This sample consists of a photometric catalog of 244 galaxies culled from a total solid angle of 0.23 deg2 to an apparent magnitude of IAB = 25.0. Spectroscopic redshifts in the range 3.61 ? z ? 4.81 have been obtained for 48 of these galaxies; their median redshift is z = 4.13. Selecting these galaxies in a manner entirely analogous to our large survey for Lyman-break galaxies at smaller redshift (2.7 z 3.4) allows a relatively clean differential comparison between the populations and integrated luminosity density at these two cosmic epochs. Over the same range of UV luminosity, the spectroscopic properties of the galaxy samples at z ~ 4 and z ~ 3 are indistinguishable, as are the luminosity function shapes and the total integrated UV luminosity densities [?UV(z = 3)/?UV(z = 4) = 1.1 ? 0.3]. We see no evidence at these bright magnitudes for the steep decline in the star formation density inferred from fainter photometric Lyman-break galaxies in the Hubble deep field (HDF). The HDF provides the only existing data on Lyman-break galaxy number densities at fainter magnitudes. We have reanalyzed the z ~ 3 and z ~ 4 Lyman-break galaxies in the HDF using our improved knowledge of the spectral energy distributions of these galaxies, and we find, like previous authors, that faint Lyman-break galaxies appear to be rarer at z ~ 4 than z ~ 3. This might signal a large change in the faint-end slope of the Lyman-break galaxy luminosity function between redshifts z ~ 3 and z ~ 4, or, more likely, be due to significant variance in the number counts within the small volumes probed by the HDF at high redshifts (~160 times smaller than the ground-based surveys discussed here). If the true luminosity density at z ~ 4 is somewhat higher than implied by the HDF, as our ground-based sample suggests, then the emissivity of star formation as a function of redshift would appear essentially constant for all z > 1 once internally consistent corrections for dust are made. This suggests that there may be no obvious peak in star formation activity and that the onset of substantial star formation in galaxies might occur at z 4.5.

Rest - frame ultraviolet spectra of Z~3 Lyman Break Galaxies

We present the results of a systematic study of the rest-frame UV spectroscopic properties of Lyman break galaxies (LBGs). The database of almost 1000 LBG spectra proves useful for constructing high signal-to-noise composite spectra. The composite spectrum of the entire sample reveals a wealth of features attributable to hot stars, H II regions, dust, and outflowing neutral and ionized gas. By grouping the database according to galaxy parameters such as Lyα equivalent width, UV spectral slope, and interstellar kinematics, we isolate some of the major trends in LBG spectra that are least compromised by selection effects. We find that LBGs with stronger Lyα emission have bluer UV continua, weaker low-ionization interstellar absorption lines, smaller kinematic offsets between Lyα and the interstellar absorption lines, and lower star formation rates. There is a decoupling between the dependence of low- and high-ionization outflow features on other spectral properties. Additionally, galaxies with rest-frame WLyα ≥ 20 A in emission have weaker than average high-ionization lines and nebular emission lines that are significantly stronger than in the sample as a whole. Most of the above trends can be explained in terms of the properties of the large-scale outflows seen in LBGs. According to this scenario, the appearance of LBG spectra is determined by a combination of the covering fraction of outflowing neutral gas, which contains dust and the range of velocities over which this gas is absorbing. In contrast, the strengths of collisionally excited nebular emission lines should not be affected by the nature of the outflow, and variations in these lines may indicate differences in the temperatures and metallicities in H II regions of galaxies with very strong Lyα emission. Higher sensitivity and spectral resolution observations are still required for a full understanding of the covering fraction and velocity dispersion of the outflowing neutral gas in LBGs and its relationship to the escape fraction of Lyman continuum radiation in galaxies at z ~ 3.

The Mass-Metallicity Relation at z≳2*

We use a sample of 87 rest-frame UV-selected star-forming galaxies with mean spectroscopic redshift z = 2.26 ± 0.17 to study the correlation between metallicity and stellar mass at high redshift. Using stellar masses determined from SED fitting to observed 0.3-8 μm photometry, we divide the sample into six bins in stellar mass and construct six composite Hα + [N ] spectra from all of the objects in each bin. We estimate the mean oxygen abundance in each bin from the [N II]/Hα ratio and find a monotonic increase in metallicity with increasing stellar mass, from 12 + log(O/H) < 8.2 for galaxies with M = 2.7 × 109 M☉ to 12 + log(O/H) = 8.6 for galaxies with M = 1.0 × 1011 M☉. We use the empirical relation between SFR density and gas density to estimate the gas fractions of the galaxies, finding an increase in gas fraction with decreasing stellar mass. These gas fractions, combined with the observed metallicities, allow the estimation of the effective yield yeff as a function of stellar mass; in constrast to observations in the local universe, which show a decrease in yeff with decreasing baryonic mass, we find a slight increase. Such a variation of metallicity with gas fraction is best fitted by a model with supersolar yield and an outflow rate ~4 times higher than the SFR. We conclude that the mass-metallicity relation at high redshift is driven by the increase in metallicity as the gas fraction decreases through star formation and is likely modulated by metal loss from strong outflows in galaxies of all masses.

Lyman break galaxies at redshift Z ~ 3: Survey description and full data set

We present the basic data for a large ground-based spectroscopic survey for z ~ 3 Lyman break galaxies (LBGs), photometrically selected using rest-UV colors from very deep images in 17 high Galactic latitude fields. The total survey covers an area of 0.38 deg2 and includes 2347 photometrically selected candidate LBGs to an apparent AB magnitude limit of 25.5. Approximately half of these objects have been observed spectroscopically using the Keck telescopes, yielding 940 redshifts with z = 2.96 ± 0.29. We discuss the images, photometry, target selection, and spectroscopic program in some detail and present catalogs of the photometric and spectroscopic data, made available in electronic form. We discuss the general utility of conducting nearly volume-limited redshift surveys in prescribed redshift intervals using judicious application of photometric preselection.

The rest-frame optical properties of Z 3 galaxies

We present the results of a near-infrared imaging survey of z ~ 3 Lyman break galaxies (LBGs). The survey covers a total of 30 arcmin2 and includes 118 photometrically selected LBGs with Ks-band measurements, 63 of which also have J-band measurements, and 81 of which have spectroscopic redshifts. Using the distribution of optical magnitudes from previous work and -Ks colors for this subsample, we compute the rest-frame optical luminosity function of LBGs. This luminosity function is described by an analytic Schechter fit with a very steep faint-end slope of ? = -1.85 ? 0.15, and it strikingly exceeds locally determined optical luminosity functions at brighter magnitudes, where it is fairly well constrained. The V-band luminosity density of only the observed bright end of the z ~ 3 LBG luminosity function already approaches that of all stars in the local universe. For the 81 galaxies with measured redshifts, we investigate the range of LBG stellar populations implied by the photometry that generally spans the range 900-5500 ? in the rest frame. The parameters under consideration are the star formation rate as a function of time, the time since the onset of star formation, and the degree of reddening and extinction by dust. While there are only weak constraints on the parameters for most of the individual galaxies, there are strong trends in the sample as a whole. With a wider wavelength baseline than most previous studies at similar redshifts, we confirm the trend that intrinsically more luminous galaxies are dustier. We also find that there is a strong correlation between extinction and the age of the star formation episode, in the sense that younger galaxies are dustier and have much higher star formation rates. The strong correlation between extinction and age, which we show is unlikely to be an artifact of the modeling procedure, has important implications for an evolutionary sequence among LBGs. A unified scenario that accounts for the observed trends in bright LBGs is one in which a relatively short period of very rapid star formation (hundreds of M? yr-1) lasts for roughly 50-100 Myr, after which both the extinction and star formation rate are considerably reduced and stars are formed at a more quiescent, but still rapid, rate for at least a few hundred megayears. In our sample, a considerable fraction (~20%) of the LBGs have best-fit star formation ages 1 Gyr, implied stellar masses of 1010 M?, and are still forming stars at ~30 M? yr-1.

The Stellar, Gas, and Dynamical Masses of Star-forming Galaxies at z ~ 2

We present analysis of the near-infrared spectra of 114 rest-frame UV-selected star-forming galaxies at z ~ 2. By combining the Hα spectra with photometric measurements from observed 0.3-8 μm, we assess the relationships among kinematics, dynamical masses, inferred gas fractions, and stellar masses and ages. The Hα line widths give a mean dynamical mass M_(dyn) = (6.9 ± 0.6) × 10^(10) M_⊙ within a typical radius of ~6 kpc, after excluding AGNs. The average dynamical mass is ~2 times larger than the average stellar mass, and the two agree to within a factor of several for most objects. However, ~15% of the sample has M_(dyn)≫ M. These objects are best fit by young stellar populations and tend to have high Hα equivalent widths, W_(Hα) ≳ 200 A, suggesting that they are young starbursts with large gas masses. Rest-frame optical luminosity and velocity dispersion are correlated with 4 σ significance. Using the local empirical correlation between star formation rate per unit area and gas surface density, we estimate the mass of the gas associated with star formation and find a mean gas fraction of ~50% and a strong decrease in gas fraction with increasing stellar mass. The masses of gas and stars combined are considerably better correlated with the dynamical masses than are the stellar masses alone, and agree to within a factor of 3 for 85% of the sample. The combination of kinematic measurements, estimates of gas masses, and stellar population properties suggest that the factor of ~500 range in stellar mass across the sample cannot be fully explained by intrinsic differences in the total masses of the galaxies, which vary by a factor of ~40; the remaining variation is due to the evolution of the stellar population and the conversion of gas into stars.

The Ultraviolet Spectrum of MS 1512–cB58: An Insight into Lyman-Break Galaxies*

We present an intermediate-resolution, high signal-to-noise ratio spectrum of the z = 2.7268 galaxy MS 1512-cB58 obtained with the Low Resolution Imaging Spectrograph (LRIS) on the Keck I telescope and covering the rest frame far-UV from 1150 to 1930 A. Gravitational lensing by a foreground cluster boosts the flux from MS 1512-cB58 by a factor of ~30 and provides the opportunity for a first quantitative study of the physical properties of star-forming galaxies at high redshift. The spectrum we have recorded is very rich in stellar and interstellar features; from our analysis of them, we deduce the following main results. The ultraviolet spectral properties of MS 1512-cB58 are remarkably similar to those of nearby star-forming galaxies and spectral synthesis models based on libraries of O and B stars can reproduce accurately the fine detail of the integrated stellar spectrum. The P Cygni profiles of C IV and N V are best matched by continuous star formation with a Salpeter initial mass function (IMF) extending beyond M = 50 M☉—we find no evidence for either a flatter IMF (at the high-mass end) or an IMF deficient in the most massive stars. There are clues in our data that the metallicity of both the stars and the gas is a few times below solar. Our best estimate, ZcB58 ≈ Z☉, is ≈3 times higher than the typical metallicity of damped Lyα systems at the same redshift, which is consistent with the proposal that the galaxies which dominate the H I absorption cross section are generally forming stars at a slower rate than L* Lyman-break galaxies like MS 1512-cB58. The relative velocities of the stellar lines, interstellar absorption, and H II emission indicate the existence of large-scale outflows in the interstellar medium of MS 1512-cB58, with a bulk outward motion of 200 km s-1 and a mass-loss rate of ≈60 M☉ yr-1, which is roughly comparable to the star formation rate. Such galactic winds seem to be a common feature of starburst galaxies at all redshifts and may well be the mechanism that self-regulates star formation, distributes metals over large volumes, and allows the escape of ionizing photons into the intergalactic medium. We suggest further observations of MS 1512-cB58 that would provide more precise measurements of element abundances and of detailed physical parameters and highlight the need to identify other examples of gravitationally lensed galaxies for a comprehensive study of star formation at early times.

New Observations of the Interstellar Medium in the Lyman Break Galaxy MS 1512?cB58

We present the results of a detailed study of the interstellar medium (ISM) of MS 1512-cB58 (cB58 for short), an ~L* Lyman break galaxy at z = 2.7276, based on new spectral observations obtained with the Echelle Spectrograph and Imager on the Keck II telescope at 58 km s-1 resolution. We focus in particular on the chemical abundances and kinematics of the interstellar gas deduced from the analysis of 48 ultraviolet absorption lines, at rest wavelengths between 1134 and 2576 ?, due to elements from H to Zn. Our main findings are as follows. Even at this relatively early epoch, the ISM of this galaxy is already highly enriched in elements released by Type II supernovae; the abundances of O, Mg, Si, P, and S are all ~2/5 of their solar values. In contrast, N and the Fe-peak elements Mn, Fe, and Ni are underabundant by a factor of ~3. Based on current ideas of stellar nucleosynthesis, these results can be understood if most of the metal enrichment in cB58 has taken place within the last ~300 Myr, the timescale for the release of N from intermediate-mass stars. Such a young age is consistent with the UV-optical spectral energy distribution. Thus, cB58 seems to be an example of a galaxy in the process of converting its gas into stars on a few dynamical timescales?quite possibly we are witnessing the formation of a galactic bulge or an elliptical galaxy. The energetic star formation activity has stirred the interstellar medium to high velocities; the strongest absorption lines span a velocity interval of ~1000 km s-1. The net effect is a bulk outflow of the ISM at a speed of ~255 km s-1 and at a rate that exceeds the star formation rate. It is unclear whether this gas will be lost or retained by the galaxy. On the one hand, the outflow probably has sufficient energy to escape the potential well of cB58, for which we derive a baryonic mass of ~1010 M?. On the other hand, at least some of the elements manufactured by previous generations of stars must have mixed efficiently with the ambient, neutral, ISM to give the high abundances we measure. We point out that the chemical and kinematic properties of cB58 are markedly different from those of most damped Ly? systems at the same redshift.

Hα Observations of a Large Sample of Galaxies at z ~ 2: Implications for Star Formation in High-Redshift Galaxies*

Using Hα spectra of 114 rest-frame UV-selected galaxies at z ~ 2, we compare inferred star formation rates (SFRs) with those determined from the UV continuum luminosity. After correcting for extinction using standard techniques based on the UV continuum slope, we find excellent agreement between the indicators, with = 31 M_☉ yr^(-1) and = 29 M_☉ yr^(-1). The agreement between the indicators suggests that the UV luminosity is attenuated by a typical factor of ~4.5 (ranging from no attenuation to a factor of ~100 for the most obscured object in the sample), in good agreement with estimates of obscuration from X-ray, radio, and mid-IR data. The Hα luminosity is attenuated by a factor of ~1.7 on average, and the maximum Hα attenuation is a factor of ~5. In agreement with X-ray and mid-IR studies, we find that the SFR increases with increasing stellar mass and at brighter K magnitudes to ~ 60 M_☉ yr^(-1) for galaxies with K_s 10^(11) M_☉) have had higher SFRs in the past.

Multiwavelength Constraints on the Cosmic Star Formation History from Spectroscopy: The Rest-Frame Ultraviolet, Hα, and Infrared Luminosity Functions at Redshifts 1.9 ≲ z ≲ 3.4*

We use a sample of rest-frame UV-selected and spectroscopically observed galaxies at redshifts 1.9 ≤ z < 3.4, combined with ground-based spectroscopic Hα and Spitzer MIPS 24 μm data, to derive the most robust measurements of the rest-frame UV, Hα, and infrared (IR) luminosity functions (LFs) at these redshifts. Our sample is by far the largest of its kind, with over 2000 spectroscopic redshifts in the range 1.9 ≤ z < 3.4 and ~15,000 photometric candidates in 29 independent fields covering a total area of almost a square degree. Our method for computing the LFs takes into account a number of systematic effects, including photometric scatter, Lyα line perturbations to the observed optical colors of galaxies, and contaminants. Taking into account the latter, we find no evidence for an excess of UV-bright galaxies over what was inferred in early z ~ 3 LBG studies. The UV LF appears to undergo little evolution between z ~ 4 and z ~ 2. Corrected for extinction, the UV luminosity density (LD) at z ~ 2 is at least as large as the value at z ~ 3 and a factor of ~9 larger than the value at z ~ 6, primarily reflecting an increase in the number density of bright galaxies between z ~ 6 and z ~ 2. Our analysis yields the first constraints anchored by extensive spectroscopy on the infrared and bolometric LFs for faint and moderately luminous (L_(bol) ≾ 10^(12) L⊙) galaxies. Adding the IR to the emergent UV luminosity, incorporating independent measurements of the LD from ULIRGs, and assuming realistic dust attenuation values for UV-faint galaxies, indicates that galaxies with L_(bol) < 10^(12) L⊙ account for ≈80% of the bolometric LD and SFRD at z ~ 2–3. This suggests that previous estimates of the faint end of the Lbol LF may have underestimated the steepness of the faint-end slope at L_(bol) < 10^(12) L⊙. Our multiwavelength constraints on the global SFRD indicate that approximately one-third of the present-day stellar mass density was formed in subultraluminous galaxies between redshifts z = 1.9–3.4.