Rapid Growth of Massive Galaxies: A Paradox for Hierarchical Formation Models (original) (raw)
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Astrophysical Journal, 2005
We present deep 3.6 - 8 micron imaging of the Hubble Deep Field South with IRAC on the Spitzer Space Telescope. We study Distant Red Galaxies (DRGs) at z>2 selected by Js - Ks > 2.3 and compare them to a sample of Lyman Break Galaxies (LBGs) at z=2-3. The observed UV-to-8 micron spectral energy distributions are fit with stellar population models to constrain star formation histories and derive stellar masses. We find that 70% of the DRGs are best described by dust-reddened star forming models and 30% are very well fit with old and ``dead'' models. Using only the I - Ks and Ks - 4.5 micron colors we can effectively separate the two groups. The dead systems are among the most massive at z~2.5 (mean stellar mass <M*> = 0.8 x 10^11 Msun) and likely formed most of their stellar mass at z>5. To a limit of 0.5 x 10^11 Msun their number density is ~10 x lower than that of local early-type galaxies. Furthermore, we use the IRAC photometry to derive rest-frame near-infrared J, H, and K fluxes. The DRGs and LBGs together show a large variation (a factor of 6) in the rest-frame K-band mass-to-light ratios (M/L_K), implying that even a Spitzer 8 micron-selected sample would be very different from a mass-selected sample. The average M/L_K of the DRGs is about three times higher than that of the LBGs, and DRGs dominate the high-mass end. The M/L_K ratios and ages of the two samples appear to correlate with derived stellar mass, with the most massive galaxies being the oldest and having the highest mass-to-light ratios, similar as found in the low-redshift universe.
Astrophysical Journal, 2004
We recently identified a substantial population of galaxies at z>2 with red rest-frame optical colors. These distant red galaxies (DRGs) are efficiently selected by the simple observed color criterion J-K>2.3. In this paper we present NIR spectroscopy with Keck/NIRSPEC of six DRGs at 2.4<z<3.2. We detect continuum emission and emission lines of all observed galaxies. Equivalent widths of H alpha are 20-30 Ang, smaller than measured for LBGs and nearby LIRGs, and comparable to normal nearby galaxies. The modest equivalent widths imply that the galaxies either have a decreasing star formation rate, or that they are very dusty. Fitting both the photometry and the H alpha lines, we find continuum extinction A_V=1-2 mag, ages 1-2.5 Gyr, star formation rates 200-400 solar masses/yr, and stellar masses 1-5x10^11 solar masses for models with constant star formation rates. From [NII]/H alpha ratios we infer that the metallicities are high, 1-1.5 x Solar. For four galaxies we can determine line widths from the optical emission lines. The widths are high, ranging from 130-240 km/s, and by combining data for LBGs and DRGs we find significant correlations between linewidth and restframe U-V color, and between linewidth and stellar mass. The latter correlation has a similar slope and offset as the ``baryonic Tully-Fisher relation'' for nearby galaxies. The median dynamical mass is ~2x10^11 solar masses, supporting the high stellar masses inferred from the photometry. We find that the median M/L_V ~ 0.8, a factor of ~5 higher than measured for LBGs. We infer from our small sample that DRGs are dustier, more metal rich, more massive, and have higher ages than z=3 LBGs of the same rest-frame V-band luminosity. Their high M/L ratios imply that they contribute significantly to the stellar mass density at z~2.5. [ABRIDGED]
The Astrophysical Journal, 2003
We present the results of a Very Large Telescope (VLT) and Hubble Space Telescope imaging survey aimed at the identification of 4:5 < z < 6 galaxies. In the VLT data, a set of broadband and intermediate-band filters has been used to select 13 high-z candidates in a Z AB 25 mag catalog, over an area of about 30 arcmin 2. Discrimination against lower redshift interlopers (mainly early-type galaxies at high redshift and cool Galactic stars) has been done combining morphological and spectral classification. This sample has been combined with a deeper I AB 27:2 mag sample obtained from the Hubble Deep Field (HDF) campaigns. The VLT final sample consists of 13 high-z candidates, four of which are identified with high confidence as z > 4:5 galaxies. The resulting integral surface density of the Z AB < 25 candidates at z > 4:5 is in the range 0.13-0.44 arcmin À2 , and that in the highest redshift bin 5 < z 6 is between 0.07 and 0.13 arcmin À2. In the two HDFs, we identify 25 galaxies at I AB 27:2 in the range 4:5 z < 5 and 16 at 5 z 6, corresponding to surface densities of 3.1 and 2 arcmin À2 , respectively. We show that the observed Z AB < 25 UV luminosity density (LD) appears to drop by about 1 order of magnitude from z ' 3 to 6. However, if we apply a threshold to obtain an absolute magnitude-limited sample, the UV LD is roughly constant up to z ' 6. We finally show that recent semianalytic hierarchical models for galaxy formation, while predicting a nearly constant total UV LD up to z ' 6, underpredict the observed UV LD at Z AB 25 and overpredict the I AB 27:2 one. This behavior can be understood in terms of a poor match to the slope of the UV luminosity function.
Astrophysical Journal, 2005
We combine HST/ACS imaging from the GEMS survey with redshifts and rest-frame quantities from COMBO-17 to study the evolution of morphologically early-type galaxies with red colors since z=1. We use a new large sample of 728 galaxies with centrally-concentrated radial profiles (Sersic n>2.5) and rest-frame U-V colors on the red sequence. By appropriate comparison with the local relations from SDSS, we find that the luminosity-size (L-R) and stellar mass-size (M-R) relations evolve in a manner that is consistent with the passive aging of ancient stars. By itself, this result is consistent with a completely passive evolution of the red early-type galaxy population. If instead, as demonstrated by a number of recent surveys, the early-type galaxy population builds up in mass by a factor of 2 since z=1, our results imply that new additions to the early-type galaxy population follow similar L-R and M-R correlations, compared to the older subset of early-type galaxies. Adding early-type galaxies to the red sequence through disk fading appears to be consistent with the data. Through comparison with models, the role of dissipationless merging is limited to <1 major merger on average since z=1 for the most massive galaxies. Predictions from models of gas-rich mergers are not yet mature enough to allow a detailed comparison to our observations. We find tentative evidence that the amount of luminosity evolution depends on galaxy stellar mass, such that the least massive galaxies show stronger luminosity evolution compared to more massive early types. This could reflect a different origin of low-mass early-type galaxies and/or younger stellar populations; the present data is insufficient to discriminate between these possibilities. (abridged)
Star formation and mass assembly in high redshift galaxies
Astronomy and Astrophysics, 2009
Aims. The goal of this work is to infer the star formation properties and the mass assembly process of high redshift (0.3 ≤ z < 2.5) galaxies from their IR emission using the 24 µm band of MIPS-Spitzer. Methods. We used an updated version of the GOODS-MUSIC catalog, which has multiwavelength coverage from 0.3 to 24 µm and either spectroscopic or accurate photometric redshifts. We describe how the catalog has been extended by the addition of mid-IR fluxes derived from the MIPS 24 µm image. We compared two different estimators of the star formation rate (SFR hereafter). One is the total infrared emission derived from 24 µm, estimated using both synthetic and empirical IR templates. The other one is a multiwavelength fit to the full galaxy SED, which automatically accounts for dust reddening and age-star formation activity degeneracies. For both estimates, we computed the SFR density and the specific SFR. Results. We show that the two SFR indicators are roughly consistent, once the uncertainties involved are taken into account. However, they show a systematic trend, IR-based estimates exceeding the fit-based ones as the star formation rate increases. With this new catalog, we show that: a) at z > 0.3, the star formation rate is correlated well with stellar mass, and this relationship seems to steepen with redshift if one relies on IR-based estimates of the SFR; b) the contribution to the global SFRD by massive galaxies increases with redshift up to ≃ 2.5, more rapidly than for galaxies of lower mass, but appears to flatten at higher z; c) despite this increase, the most important contributors to the SFRD at any z are galaxies of about, or immediately lower than, the characteristic stellar mass; d) at z ≃ 2, massive galaxies are actively star-forming, with a median SFR ≃ 300M ⊙ yr −1 . During this epoch, our targeted galaxies assemble a substantial part of their final stellar mass; e) the specific SFR (SSFR) shows a clear bimodal distribution.
The star formation history of luminous infrared galaxies
2006
Aims. We constrain the past star formation histories of a sample of 25 distant (z ∼ 0.7) luminous infrared galaxies (LIRGs) detected with the mid infrared cameras ISOCAM and MIPS onboard the ISO and Spitzer satellites. Methods. We used high-resolution VLT-FORS2 spectroscopy in addition to a comprehensive library of 200 000 model optical spectra to derive Bayesian likelihood estimates of the star formation histories of these galaxies, based on analysis of Balmer absorption lines and the 4000 Å break. Results. The locus of distant LIRGs in the diagram defined by Hδ A and D4000 is roughly comparable to that of local LIRGs observed with IRAS, suggesting that no trend toward an evolution is detected between the local and distant LIRGs. We obtain similar results when using either the H8 or the Hδ A Balmer absorption-line indices in combination with D4000. By computing a birthrate parameter (b = SFR/ SFR) of 4 ± 1, we confirme that the distant LIRGs are currently experiencing a major phase of star formation. The most likely duration of the bursts is 0.10 +0.16 −0.06 Gyr, during which the LIRGs produce ∼5-10% of their current stellar mass. No evidence was found for successive starbursts on the scale of a few times 10 7 yr, such as those predicted by some numerical simulations of major mergers. However, the high number density of those galaxies suggests that they could have experienced between two and four LIRG phases until the present epoch. This scenario is not consistent with the formation of the z ∼ 0.7 LIRGs through the continuous star formation characterizing isolated spiral galaxies as has been independently argued based on their morphology. Instead, minor mergers, tidal interactions, or gas accretion remain plausible triggering mechanisms for more than half of the distant LIRGs that do not harbor the morphology of major mergers.
The Las Campanas Infrared Survey: Early-Type Galaxy Progenitors beyond [CLC][ITAL]z[/ITAL][/CLC] = 1
The Astrophysical Journal, 2001
We have identified a population of faint red galaxies from a 0.62 square degree region of the Las Campanas Infrared Survey whose properties are consistent with their being the progenitors of early-type galaxies. The optical and IR colors, number-magnitude relation and angular clustering together indicate modest evolution and increased star formation rates among the early-type field population at redshifts between one and two. The counts of red galaxies with H magnitudes between 17 and 20 rise with a slope that is much steeper than that of the total H sample. The surface density of red galaxies drops from roughly 3000 per square degree at H = 20.5, I − H > 3 to ∼ 20 per square degree at H = 20, I − H > 5. The V − I colors are approximately 1.5 magnitudes bluer on average than a pure old population and span a range of more than three magnitudes. The strength of the angular clustering of the red galaxies is an order of magnitude larger than that of the full galaxy sample. The colors, and photometric redshifts derived from them, indicate that the red galaxies have redshift distributions adequately described by Gaussians with σ z ≃ 0.2 centered near redshift one, with the exception that galaxies having V − I < 1.6 and I − H > 3 are primarily in the 1.5 z 2 range. We invert the angular correlation functions using these n(z) and find co-moving correlation lengths of r 0 ≃ 9 − 10h −1 Mpc at z ≃ 1, comparable to, or larger than, those found for early-type galaxies at lower redshifts. A simple photometric evolution model reproduces the counts of the red galaxies, with only a ∼ 30% decline in the underlying space density of early-type galaxies at z ∼ 1.2. The colors indicate characteristic star formation rates of ∼ 1M ⊙ /yr per 10 10 M ⊙ . We suggest on the basis of the colors, counts, and clustering that these red galaxies are the bulk of the progenitors of present day early-type galaxies.
The 2dF-SDSS LRG and QSO Survey: the star formation histories of luminous red galaxies
Monthly Notices of the Royal Astronomical Society, 2006
We present a detailed investigation into the recent star formation histories of 5,697 Luminous Red Galaxies (LRGs) based on the Hδ(4101Å), and [O ii] (3727Å) lines and the D4000 index. LRGs are luminous (L> 3L*), galaxies which have been selected to have photometric properties consistent with an old, passively evolving stellar population. For this study we utilise LRGs from the recently completed 2dF-SDSS LRG and QSO survey (2SLAQ). Equivalent widths of the Hδ and [O ii] lines are measured and used to define three spectral types, those with only strong Hδ absorption (k+a), those with strong [O ii] in emission (em) and those with both (em+a). All other LRGs are considered to have passive star formation histories. The vast majority of LRGs are found to be passive (∼80 per cent), however significant numbers of k+a (2.7 per cent), em+a (1.2 per cent) and em LRGs (8.6 per cent) are identified. An investigation into the redshift dependence of the fractions is also performed. A sample of SDSS MAIN galaxies with colours and luminosities consistent with the 2SLAQ LRGs is selected to provide a low redshift comparison. While the em and em+a fractions are consistent with the low redshift SDSS sample, the fraction of k+a LRGs is found to increase significantly with redshift. This result is interpreted as an indication of an increasing amount of recent star formation activity in LRGs with redshift. By considering the expected life time of the k+a phase, the number of LRGs which will undergo a k+a phase can be estimated. A crude comparison of this estimate with the predictions from semi-analytic models of galaxy formation shows that the predicted level of k+a and em+a activity is not sufficient to reconcile the predicted mass growth for massive early-types in a hierarchical merging scenario.
Monthly Notices of the Royal Astronomical Society, 2010
We use the Spitzer Wide-area InfraRed Extragalactic Legacy Survey (SWIRE) to explore the specific star-formation activity of galaxies and their evolution near the peak of the cosmic far-infrared background at 70 and 160µm. We use a stacking analysis to determine the mean far-infrared properties of well defined subsets of galaxies at flux levels well below the far-infrared catalogue detection limits of SWIRE and other Spitzer surveys. We tabulate the contribution of different subsets of galaxies to the far-infrared background at 70µm and 160µm. These long wavelengths provide a good constraint on the bolometric obscured emission. The large area provides good constraints at low z and in finer redshift bins than previous work. At all redshifts we find that the specific far-infrared luminosity decreases with increasing mass, following a trend L FIR /M * ∝ M β * with β = −0.38 ± 0.14. This is a more continuous change than expected from the De Lucia & Blaizot (2007) semi-analytic model suggesting modifications to the feedback prescriptions. We see an increase in the specific far-infrared luminosity by about a factor of ∼ 100 from 0 < z < 2 and find that the specific far infrared luminosity evolves as (1 + z) α with α = 4.4 ± 0.3 for galaxies with 10.5 < log 10 M * /M ⊙ 12. This is considerably steeper than the De Lucia & Blaizot (2007) semi-analytic model (α ∼ 2.5). When separating galaxies into early and late types on the basis of the optical/IR spectral energy distributions we find that the decrease in specific far-infrared luminosity with stellar mass is stronger in early type galaxies (β ∼ −0.46), while late type galaxies exhibit a flatter trend (β ∼ −0.15). The evolution is strong for both classes but stronger for the early type galaxies. The early types show a trend of decreasing strength of evolution as we move from lower to higher masses while the evolution of the late type galaxies has little dependence on stellar mass. We suggest that in late-type galaxies we are seeing a consistently declining specific star-formation rate α = 3.36 ± 0.16 through a common phenomenon e.g. exhaustion of gas supply i.e. not systematically dependent on the local properties of the galaxy.
A long-wavelength view on galaxy evolution from deep surveys by the Infrared Space Observatory
2001
We discuss the constraints set on galaxy evolution by a variety of data from deep extragalactic surveys performed in the mid-IR and far-IR with the Infrared Space Observatory and with millimetric telescopes at longer wavelengths. These observations indicate extremely high rates of evolution for IR galaxies, exceeding those measured for galaxies at other wavelengths and comparable or larger than the rates observed for quasars. We also match the modelled integrated emission by IR galaxies at any redshifts with the observed spectral intensity of the extragalactic IR background (CIRB), as a further constraint. The multi-wavelength statistics on IR galaxies can be reconciled with each other by assuming for the bulk of the population spectral energy distributions (SED) as typical for starbursts, which we take as an indication that stellar (rather than AGN, see also Fadda et al. 2001) activity powers IR emission by faint galaxies. According to our model and following the analysis of Elbaz et al. (2001), the deep ISO surveys at 15 µm may have already resolved more than 50% of the bolometric CIRB intensity: the faint ISO 15 µm source samples, relatively easy to identify in deep optical images (Aussel et al. 1999), can then allow to investigate the origin of the CIRB background. From our fits to the observed optical-IR SEDs, these objects appear to mostly involve massive galaxies hosting luminous starbursts (SF R ∼ 100 M⊙/yr). The evolutionary scheme we infer from these data considers a bimodal star formation (SF), including a phase of long-lived quiescent SF, and enhanced SF taking place during transient events recurrently triggered by interactions and merging. We interpret the strong observed evolution as an increase with z of the rate of interactions between galaxies (density evolution) and an increase of their IR luminosity due to the more abundant fuel available in the past (luminosity evolution): both factors enhance the probability to detect a galaxy during the "active" phase at higher z. Very schematically, we associate the origin of the bulk of the optical/NIR background to the quiescent evolution, while the CIRB is interpreted as mostly due the dusty starburst phase. The latter possibly leads to the formation of galaxy spheroids, when the dynamical events triggering the starburst redistribute already present stellar populations. The large energy contents in the CIRB and optical backgrounds are not easily explained, considering the moderate efficiency of energy generation by stars: a top-heavy stellar IMF associated with the starburst phase (and compared with a more standard IMF during the quiescent SF) would alleviate the problem. The evolution of the IR emissivity of galaxies from the present time to z ∼ 1 is so strong that the combined set of constraints by the observed z-distributions and the CIRB spectrum impose it to turnover at z > 1: scenarios in which a dominant fraction of stellar formation occurs at very high-z are not supported by our analysis.