Evolutionary paths among different red galaxy types at 0.3 < z < 1.5 and the late buildup of massive E-S0s through major mergers (original) (raw)
Related papers
Context. Several studies have tried to ascertain whether or not the increase in abundance of the early-type galaxies (E-S0a's) with time is mainly due to major mergers, reaching opposite conclusions. Aims. We have tested it directly through semi-analytical modelling, by studying how the massive early-type galaxies with log(M * /M ⊙ ) > 11 at z ∼ 0 (mETGs) would have evolved backwards-in-time, under the hypothesis that each major merger gives place to an early-type galaxy. Methods. The study was carried out just considering the major mergers strictly reported by observations at each redshift, and assuming that gas-rich major mergers experience transitory phases as dust-reddened, star-forming galaxies (DSFs). Results. The model is able to reproduce the observed evolution of the galaxy LFs at z 1, simultaneously for different rest-frame bands (B, I, and K) and for different selection criteria on color and morphology. It also provides a framework in which apparentlycontradictory results on the recent evolution of the luminosity function (LF) of massive, red galaxies can be reconciled, just considering that observational samples of red galaxies can be significantly contaminated by DSFs. The model proves that it is feasible to build up ∼ 50-60% of the present-day number density of mETGs at z 1 through the coordinated action of wet, mixed, and dry major mergers, fulfilling global trends that are in general agreement with mass-downsizing. The bulk of this assembly takes place during ∼ 1 Gyr elapsed at 0.8 < z < 1, providing a straightforward explanation to the observational fact that redshift z ∼ 0.8 is a transition epoch in the formation of mETGs. The gas-rich progenitors of these recently-assembled mETGs reproduce naturally the observational excess by a factor of ∼ 4-5 of late-type galaxies at 0.8 < z < 1, as compared to pure luminosity evolution (PLE) models. Conclusions. The model suggests that major mergers have been the main driver for the observational migration of mass from the massive-end of the blue galaxy cloud to that of the red sequence in the last ∼ 8 Gyr.
Astronomy and Astrophysics
Context. Several studies have tried to ascertain whether or not the increase in abundance of the early-type galaxies (E-S0a's) with time is mainly due to major mergers, reaching opposite conclusions. Aims. We have tested it directly through semi-analytical modelling, by studying how the massive early-type galaxies with log(M * /M ⊙ ) > 11 at z ∼ 0 (mETGs) would have evolved backwards-in-time, under the hypothesis that each major merger gives place to an early-type galaxy. Methods. The study was carried out just considering the major mergers strictly reported by observations at each redshift, and assuming that gas-rich major mergers experience transitory phases as dust-reddened, star-forming galaxies (DSFs). Results. The model is able to reproduce the observed evolution of the galaxy LFs at z 1, simultaneously for different rest-frame bands (B, I, and K) and for different selection criteria on color and morphology. It also provides a framework in which apparentlycontradictory results on the recent evolution of the luminosity function (LF) of massive, red galaxies can be reconciled, just considering that observational samples of red galaxies can be significantly contaminated by DSFs. The model proves that it is feasible to build up ∼ 50-60% of the present-day number density of mETGs at z 1 through the coordinated action of wet, mixed, and dry major mergers, fulfilling global trends that are in general agreement with mass-downsizing. The bulk of this assembly takes place during ∼ 1 Gyr elapsed at 0.8 < z < 1, providing a straightforward explanation to the observational fact that redshift z ∼ 0.8 is a transition epoch in the formation of mETGs. The gas-rich progenitors of these recently-assembled mETGs reproduce naturally the observational excess by a factor of ∼ 4-5 of late-type galaxies at 0.8 < z < 1, as compared to pure luminosity evolution (PLE) models. Conclusions. The model suggests that major mergers have been the main driver for the observational migration of mass from the massive-end of the blue galaxy cloud to that of the red sequence in the last ∼ 8 Gyr.
On the buildup of massive early-type galaxies at z la\lala 1
Astronomy and Astrophysics, 2010
Context. Several studies have tried to ascertain whether or not the increase in abundance of the early-type galaxies (E-S0a's) with time is mainly due to major mergers, reaching opposite conclusions. Aims. We have tested it directly through semi-analytical modelling, by studying how the massive early-type galaxies with log(M * /M ⊙ ) > 11 at z ∼ 0 (mETGs) would have evolved backwards-in-time, under the hypothesis that each major merger gives place to an early-type galaxy. Methods. The study was carried out just considering the major mergers strictly reported by observations at each redshift, and assuming that gas-rich major mergers experience transitory phases as dust-reddened, star-forming galaxies (DSFs). Results. The model is able to reproduce the observed evolution of the galaxy LFs at z 1, simultaneously for different rest-frame bands (B, I, and K) and for different selection criteria on color and morphology. It also provides a framework in which apparentlycontradictory results on the recent evolution of the luminosity function (LF) of massive, red galaxies can be reconciled, just considering that observational samples of red galaxies can be significantly contaminated by DSFs. The model proves that it is feasible to build up ∼ 50-60% of the present-day number density of mETGs at z 1 through the coordinated action of wet, mixed, and dry major mergers, fulfilling global trends that are in general agreement with mass-downsizing. The bulk of this assembly takes place during ∼ 1 Gyr elapsed at 0.8 < z < 1, providing a straightforward explanation to the observational fact that redshift z ∼ 0.8 is a transition epoch in the formation of mETGs. The gas-rich progenitors of these recently-assembled mETGs reproduce naturally the observational excess by a factor of ∼ 4-5 of late-type galaxies at 0.8 < z < 1, as compared to pure luminosity evolution (PLE) models. Conclusions. The model suggests that major mergers have been the main driver for the observational migration of mass from the massive-end of the blue galaxy cloud to that of the red sequence in the last ∼ 8 Gyr.
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)
Monthly Notices of the Royal Astronomical Society, 2010
We combine SAURON integral field data of a representative sample of local early-type, red sequence galaxies with Spitzer/IRAC imaging in order to investigate the presence of trace star formation in these systems. With the Spitzer data, we identify galaxies hosting low-level star formation, as traced by PAH emission, with measured star formation rates that compare well to those estimated from other tracers. This star formation proceeds according to established scaling relations with molecular gas content, in surface density regimes characteristic of disk galaxies and circumnuclear starbursts. We find that star formation in early-type galaxies happens exclusively in fast-rotating systems and occurs in two distinct modes. In the first, star formation is a diffuse process, corresponding to widespread young stellar populations and high molecular gas content. The equal presence of co-and counter-rotating components in these systems strongly implies an external origin for the star-forming gas, and we argue that these star formation events may be the final stages of (mostly minor) mergers that build up the bulges of red sequence lenticulars. In the second mode of star formation, the process is concentrated into well-defined disk or ring morphologies, outside of which the host galaxies exhibit uniformly evolved stellar populations. This implies that these star formation events represent rejuvenations within previously quiescent stellar systems. Evidence for earlier star formation events similar to these in all fast rotating early-type galaxies suggests that this mode of star formation may be common to all such galaxies, with a duty cycle of roughly 1/10, and likely contributes to the embedded, co-rotating inner stellar disks ubiquitous in this population.
The Progenitors of the Compact Early-Type Galaxies at High Redshift
The Astrophysical Journal, 2014
We use GOODS and CANDELS images to identify progenitors of massive (M > 10 10 M ⊙) compact "early-type" galaxies (ETGs) at z ∼ 1.6. Since merging and accretion increase the size of the stellar component of galaxies, if the progenitors are among known star-forming galaxies, these must be compact themselves. We select candidate progenitors among compact Lyman-break galaxies at z ∼ 3 based on their mass, SFR and central stellar density and find that these account for a large fraction of, and possibly all, compact ETGs at z ∼ 1.6. We find that the average far-UV SED of the candidates is redder than that of the non-candidates, but the optical and mid-IR SED are the same, implying that the redder UV of the candidates is inconsistent with larger dust obscuration, and consistent with more evolved (aging) star-formation. This is in line with other evidence that compactness is a sensitive predictor of passivity among high-redshift massive galaxies. We also find that the light distribution of both the compact ETGs and their candidate progenitors does not show any extended "halos" surrounding the compact "core", both in individual images and in stacks. We argue that this is generally inconsistent with the morphology of merger remnants, even if gas-rich, as predicted by N-body simulations. This suggests that the compact ETGs formed via highly dissipative, mostly gaseous accretion of units whose stellar components are very small and undetected in the HST images, with their stellar mass assembling in-situ, and that they have not experienced any major merging until the epoch of observations at z ∼ 1.6.
The population of early-type galaxies at 1 < z < 2 - new clues on their formation and evolution
Monthly Notices of the Royal Astronomical Society, 2009
We present the morphological analysis based on Hubble Space Telescope HST-NICMOS (Near-Infrared Camera and Multi-Object Spectrometer) observations in the F160W filter (λ 1.6 μm) of a sample of 32 early-type galaxies (ETGs) at 1 < z < 2 with spectroscopic confirmation of their redshift and spectral type. The 32 ETGs at z ∼ 1.5 are placed on the ( μ e , R e ) plane according to the Kormendy relation (KR) with the same slope of the local one but with a different zero-point, which accounts for the evolution they undergo from z ∼ 1.5-2 to z = 0. The best fitting of their spectral energy distribution shows that ETGs at 1 < z < 2 are composed of two distinct populations, an older population (oETGs) and a younger population (yETGs) whose mean ages differ by about 1.5-2 Gyr. Young ETGs are not denser than local ones since they follow the size-mass relation of local ETGs, and luminosity evolution brings them on to the local KR and size-luminosity relations without the need of size evolution. Old ETGs do not follow the size-mass relation of local ETGs, and luminosity evolution does not account for the discrepancy they show with respect to the local size-luminosity relation and KR. An increase in their R e by a factor of 2.5-3 (a density decrease by a factor of 15-30) from z ∼ 1.5-2 to z ∼ 0 is required to bring these galaxies on to the local scaling relations. The different properties and the different behaviour shown by the two populations with respect to the scaling relations imply different formation and evolution scenarios. The older population of ETGs must have formed at a higher z in a sort of dissipative gas-rich collapse able to produce remnants which at z ∼ 2 are old and compact, a scenario which can be fitted qualitatively by some recent hydrodynamic simulations of gas-rich mergers. Given the typical time-scale of merging and the old age of their stellar population, oETGs should exist as they are up to z 3-3.5. The size evolution they must experience from z ∼ 2 to ∼0 must leave unchanged their mass to not exceed the local number of high-mass (M * > 5 × 10 11 M ) ETGs. Thus, major merging cannot fit this requirement. Satellite merging, close encounters and interactions can help at least qualitatively in solving this problem. The younger population of ETGs can be formed later through subsequent episodes of merging which increased progressively their size and assembled their mass down to z ∼ 2. At z < 2, they evolve purely in luminosity since episodes of major merging would bring them far from the local scaling relations.
The Evolution of Galaxy Mergers and Morphology at z< 1.2 in the Extended Groth Strip
2008
We present the quantitative rest-frame B morphological evolution and galaxy merger fractions at 0.2 < z < 1.2 as observed by the All-wavelength Extended Groth Strip International Survey (AEGIS). We use the Gini coefficent and M 20 to identify major mergers and classify galaxy morphology for a volume-limited sample of 3009 galaxies brighter than 0.4L * B , assuming pure luminosity evolution of 1.3 M B per unit redshift. We find that the merger fraction remains roughly constant at 10 ± 2% for 0.2 < z < 1.2. The fraction of E/S0/Sa increases from 21 ± 3% at z ∼ 1.1 to 44 ± 9% at z ∼ 0.3, while the fraction of Sb-Ir decreases from 64 ± 6% at z ∼ 1.1 to 47 ± 9% at z ∼ 0.3. The majority of z < 1.2 Spitzer MIPS 24 µm sources with L(IR) > 10 11 L ⊙ are disk galaxies, and only ∼ 15% are classified as major merger candidates. Edge-on and dusty disk galaxies (Sb-Ir) are almost a third of the red sequence at z ∼ 1.1, while E/S0/Sa make up over 90% of the red sequence at z ∼ 0.3. Approximately 2% of our full sample are red mergers. We conclude (1) the merger rate does not evolve strongly between 0.2 < z < 1.2; (2) the decrease in the volume-averaged star-formation rate density since z ∼ 1 is a result of declining star-formation in disk galaxies rather than a disappearing population of major mergers; (3) the build-up of the red sequence at z < 1 can be explained by a doubling in the number of spheroidal galaxies since z ∼ 1.2.
Hierarchical models predict that massive early-type galaxies (mETGs) derive from the most massive and violent merging sequences occurred in the Universe. However, the role of wet, mixed, and dry major mergers in the assembly of mETGs is questioned by some recent observations. We have developed a semi-analytical model to test the feasibility of the major-merger origin hypothesis for mETGs, just accounting for the effects on galaxy evolution of the major mergers strictly reported by observations. The model proves that it is feasible to reproduce the observed number density evolution of mETGs since z ∼ 1, just accounting for the coordinated effects of wet/mixed/dry major mergers. It can also reconcile the different assembly redshifts derived by hierarchical models and by mass downsizing data for mETGs, just considering that a mETG observed at a certain redshift is not necessarily in place since then. The model predicts that wet major mergers have controlled the mETGs buildup since z∼1, although dry and mixed mergers have also played an essential role in it. The bulk of this assembly took place at 0.7< z <1, being nearly frozen at z 0.7 due to the negligible number of major mergers occurred per existing mETG since then. The model suggests that major mergers have been the main driver for the observational migration of mass from the massive end of the blue galaxy cloud to that of the red sequence in the last ∼ 8 Gyr.
Galaxy Luminosity Functions to z ∼1 from DEEP2 and COMBO‐17: Implications for Red Galaxy Formation
The Astrophysical Journal, 2007
The DEEP2 and COMBO-17 surveys are used to study the evolution of the luminosity function of red and blue galaxies to z ∼ 1. Schechter function fits show that, since z = 1, M * B dims by ∼ 1.3 mag per unit redshift for both color classes, φ * of blue galaxies shows little change, while φ * for red galaxies has formally nearly quadrupled. At face value, the number density of blue galaxies has remained roughly constant since z = 1, whereas that of red galaxies has been rising. Luminosity densities support both conclusions, but we note that most red-galaxy evolution occurs between our data and local surveys and in our highest redshift bin, where the data are weakest. We discuss the implications of having most red galaxies emerge after z = 1 from precursors among the blue population, taking into account the properties of local and distant E/S0s. We suggest a "mixed" scenario in which some blue galaxies have their star-formation quenched in gas-rich mergers, migrate to the red sequence with a variety of masses, and merge further on the red sequence in one or more purely stellar mergers. E/S0s of a given mass today will have formed via different routes, in a manner that may help to explain the fundamental plane and other local scaling laws.