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Register a new userLSD: Lyman-break galaxies Stellar populations and Dynamics. I: Mass, metallicity and gas at z~3.1
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We present the first results of a project, LSD, aimed at obtaining spatially-resolved, near-infrared spectroscopy of a complete sample of Lyman-Break Galaxies at z~3. Deep observations with adaptive optics resulted in the detection of the main optical lines, such as [OII], Hbeta and [OIII], which are used to study sizes, SFRs, morphologies, gas-phase metallicities, gas fractions and effective yields. Optical, near-IR and Spitzer/IRAC photometry is used to measure stellar mass. We obtain that morphologies are usually complex, with the presence of several peaks of emissions and companions that are not detected in broad-band images. Typical metallicities are 10-50% solar, with a strong evolution of the mass-metallicity relation from lower redshifts. Stellar masses, gas fraction, and evolutionary stages vary significantly among the galaxies, with less massive galaxies showing larger fractions of gas. In contrast with observations in the local universe, effective yields decrease with stellar mass and reach solar values at the low-mass end of the sample. This effect can be reproduced by gas infall with rates of the order of the SFRs. Outflows are present but are not needed to explain the mass-metallicity relation. We conclude that a large fraction of these galaxies are actively creating stars after major episodes of gas infall or merging.
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We present a sample of 33 spectroscopically confirmed z ~ 3.1 Ly$alpha$-emitting galaxies (LAEs) in the Cosmological Evolution Survey (COSMOS) field. This paper details the narrow-band survey we conducted to detect the LAE sample, the optical spectroscopy we performed to confirm the nature of these LAEs, and a new near-infrared spectroscopic detection of the [O III] 5007 AA line in one of these LAEs. This detection is in addition to two [O III] detections in two z ~ 3.1 LAEs we have reported on previously (McLinden et al 2011). The bulk of the paper then presents detailed constraints on the physical characteristics of the entire LAE sample from spectral energy distribution (SED) fitting. These characteristics include mass, age, star-formation history, dust content, and metallicity. We also detail an approach to account for nebular emission lines in the SED fitting process - wherein our models predict the strength of the [O III] line in an LAE spectrum. We are able to study the success of this prediction because we can compare the model predictions to our actual near-infrared observations both in galaxies that have [O III] detections and those that yielded non-detections. We find a median stellar mass of 6.9 $times$ 10$^8$ M$_{odot}$ and a median star formation rate weighted stellar population age of 4.5 $times$ 10$^6$ yr. In addition to SED fitting, we quantify the velocity offset between the [O III] and Ly$alpha$ lines in the galaxy with the new [O III] detection, finding that the Ly$alpha$ line is shifted 52 km s$^{-1}$ redwards of the [O III] line, which defines the systemic velocity of the galaxy.
We take advantage of the exceptional photometric coverage provided by the combination of GALEX data in the UV and the ALHAMBRA survey in the optical and near-IR to analyze the physical properties of a sample of 1225 GALEX-selected Lyman break galaxies (LBGs) at $0.8 lesssim z lesssim 1.2$ located in the COSMOS field. This is the largest sample of LBGs studied at that redshift range so far. According to a spectral energy distribution (SED) fitting with synthetic stellar population templates, we find that LBGs at $z sim 1$ are mostly young galaxies with a median age of 341 Myr and have intermediate dust attenuation, $ < E_s (B-V) > sim 0.20$. Due to their selection criterion, LBGs at $z sim 1$ are UV-bright galaxies and have high dust-corrected total SFR, with a median value of 16.9 $M_odot {rm yr}^{-1}$. Their median stellar mass is $log{left(M_*/M_odot right)} = 9.74$. We obtain that the dust-corrected total SFR of LBGs increases with stellar mass and the specific SFR is lower for more massive galaxies. Only 2% of the galaxies selected through the Lyman break criterion have an AGN nature. LBGs at $z sim 1$ are mostly located over the blue cloud of the color-magnitude diagram of galaxies at their redshift, with only the oldest and/or the dustiest deviating towards the green valley and red sequence. Morphologically, 69% of LBGs are disk-like galaxies, with the fraction of interacting, compact, or irregular systems being much lower, below 12%. LBGs have a median effective radius of 2.5 kpc and bigger galaxies have higher total SFR and stellar mass. Comparing to their high-redshift analogues, we find evidence that LBGs at lower redshifts are bigger, redder in the UV continuum, and have a major presence of older stellar populations in their SEDs. However, we do not find significant difference in the distributions of stellar mass or dust attenuation.
We present the results of Spectral Energy Distribution(SED) fitting analysis for Lyman Break Galaxies(LBGs) at z~5 in the GOODS-N and its flanking fields (the GOODS-FF). With the publicly available IRAC images in the GOODS-N and IRAC data in the GOODS-FF, we constructed the rest-frame UV to optical SEDs for a large sample (~100) of UV-selected galaxies at z~5. Comparing the observed SEDs with model SEDs generated with a population synthesis code, we derived a best-fit set of parameters (stellar mass, age, color excess, and star formation rate) for each of sample LBGs. The derived stellar masses range from 10^8 to 10^11M_sun with a median value of 4.1x10^9M_sun. The comparison with z=2-3 LBGs shows that the stellar masses of z~5 LBGs are systematically smaller by a factor of 3-4 than those of z=2-3 LBGs in a similar rest-frame UV luminosity range. The star formation ages are relatively younger than those of the z=2-3 LBGs. We also compared the results for our sample with other studies for the z=5-6 galaxies. Although there seem to be similarities and differences in the properties, we could not conclude its significance. We also derived a stellar mass function of our sample by correcting for incompletenesses. Although the number densities in the massive end are comparable to the theoretical predictions from semi-analytic models, the number densities in the low-mass part are smaller than the model predictions. By integrating the stellar mass function down to 10^8 M_sun, the stellar mass density at z~5 is calculated to be (0.7-2.4)x10^7M_sun Mpc^-3. The stellar mass density at z~5 is dominated by massive part of the stellar mass function. Compared with other observational studies and the model predictions, the mass density of our sample is consistent with general trend of the increase of the stellar mass density with time.
We present the results of a photometric and spectroscopic survey of 321 Lyman break galaxies (LBGs) at z ~ 3 to investigate systematically the relationship between Lya emission and stellar populations. Lya equivalent widths (EW) were calculated from rest-frame UV spectroscopy and optical/near-infrared/Spitzer photometry was used in population synthesis modeling to derive the key properties of age, dust extinction, star formation rate (SFR), and stellar mass. We directly compare the stellar populations of LBGs with and without strong Lya emission, where we designate the former group (EW > 20 AA) as Lya emitters (LAEs) and the latter group (EW < 20 AA) as non-LAEs. This controlled method of comparing objects from the same UV luminosity distribution represents an improvement over previous studies in which the stellar populations of LBGs and narrowband-selected LAEs were contrasted, where the latter were often intrinsically fainter in broadband filters by an order of magnitude simply due to different selection criteria. Using a variety of statistical tests, we find that Lya equivalent width and age, SFR, and dust extinction, respectively, are significantly correlated in the sense that objects with strong Lya emission also tend to be older, lower in star formation rate, and less dusty than objects with weak Lya emission, or the line in absorption. We accordingly conclude that, within the LBG sample, objects with strong Lya emission represent a later stage of galaxy evolution in which supernovae-induced outflows have reduced the dust covering fraction. We also examined the hypothesis that the attenuation of Lya photons is lower than that of the continuum, as proposed by some, but found no evidence to support this picture.
We present results of a search for bright Lyman break galaxies at 1.5<=z<=2.5 in the GOODS-S field using a NUV-dropout technique in combination with color-selection. We derived a sample of 73 LBG candidates. We compare our selection efficiencies to BM/BX- and BzK methods (techniques solely based on ground-based data sets), and find the NUV data to provide greater efficiency for selecting star-forming galaxies. We estimate LBG candidate ages, masses, star formation rates, and extinction from fitting PEGASE synthesis evolution models. We find about 20% of our LBG candidates are comparable to infrared luminous LBGs or sub-millimeter galaxies which are thought to be precursors of massive elliptical galaxies today. Overall, we can show that although BM/BX and BzK methods do identify star-forming galaxies at z~2, the sample they provide biases against those star-forming galaxies which are more massive and contain sizeable red stellar populations. A true Lyman break criterion at z~2 is therefore more directly comparable to the populations found at z~3, which does contain a red fraction.
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