838 research outputs found
zCOSMOS â 10k-bright spectroscopic sample : The bimodality in the galaxy stellar mass function: exploring its evolution with redshift
We present the galaxy stellar mass function (GSMF) to redshift z â 1, based on the analysis of about 8500 galaxies with I < 22.5 (AB mag)
over 1.4 deg^2, which are part of the zCOSMOS-bright 10k spectroscopic sample. We investigate the total GSMF, as well as the contributions of
early- and late-type galaxies (ETGs and LTGs, respectively), defined by different criteria (broad-band spectral energy distribution, morphology,
spectral properties, or star formation activities). We unveil a galaxy bimodality in the global GSMF, whose shape is more accurately represented
by 2 Schechter functions, one linked to the ETG and the other to the LTG populations. For the global population, we confirm a mass-dependent
evolution (âmass-assembly downsizingâ), i.e., galaxy number density increases with cosmic time by a factor of two between z = 1 and z = 0 for
intermediate-to-low mass (log(M/M_â) ~ 10.5) galaxies but less than 15% for log(M/M_â) > 11.We find that the GSMF evolution at intermediate-to-
low values of M(log(M/M_â) < 10.6) is mostly explained by the growth in stellar mass driven by smoothly decreasing star formation activities,
despite the redder colours predicted in particular at low redshift. The low residual evolution is consistent, on average, with ~0.16 merger per
galaxy per Gyr (of which fewer than 0.1 are major), with a hint of a decrease with cosmic time but not a clear dependence on the mass. From
the analysis of different galaxy types, we find that ETGs, regardless of the classification method, increase in number density with cosmic time
more rapidly with decreasing M, i.e., follow a top-down building history, with a median âbuilding redshiftâ increasing with mass (z > 1 for
log(M/M_â) > 11), in contrast to hierarchical model predictions. For LTGs, we find that the number density of blue or spiral galaxies with
log(M/M_â) > 10 remains almost constant with cosmic time from z ~ 1. Instead, the most extreme population of star-forming galaxies (with
high specific star formation), at intermediate/high-mass, rapidly decreases in number density with cosmic time. Our data can be interpreted as
a combination of different effects. Firstly, we suggest a transformation, driven mainly by SFH, from blue, active, spiral galaxies of intermediate
mass to blue quiescent and subsequently (1â2 Gyr after) red, passive types of low specific star formation. We find an indication that the complete
morphological transformation, probably driven by dynamical processes, into red spheroidal galaxies, occurred on longer timescales or followed
after 1â2 Gyr. A continuous replacement of blue galaxies is expected to be accomplished by low-mass active spirals increasing their stellar
mass. We estimate the growth rate in number and mass density of the red galaxies at different redshifts and masses. The corresponding fraction
of blue galaxies that, at any given time, is transforming into red galaxies per Gyr, due to the quenching of their SFR, is on average ~25% for
log(M/M_â) < 11. We conclude that the build-up of galaxies and in particular of ETGs follows the same downsizing trend with mass (i.e. occurs
earlier for high-mass galaxies) as the formation of their stars and follows the converse of the trend predicted by current SAMs. In this scenario, we
expect there to be a negligible evolution of the galaxy baryonic mass function (GBMF) for the global population at all masses and a decrease with
cosmic time in the GBMF for the blue galaxy population at intermediate-high masses
On the robustness of the H Lick index as a cosmic clock in passive early-type galaxies
We examine the H Lick index in a sample of massive () and passive early-type galaxies extracted from SDSS at
z<0.3, in order to assess the reliability of this index to constrain the epoch
of formation and age evolution of these systems. We further investigate the
possibility of exploiting this index as "cosmic chronometer", i.e. to derive
the Hubble parameter from its differential evolution with redshift, hence
constraining cosmological models independently of other probes. We find that
the H strength increases with redshift as expected in passive evolution
models, and shows at each redshift weaker values in more massive galaxies.
However, a detailed comparison of the observed index with the predictions of
stellar population synthesis models highlights a significant tension, with the
observed index being systematically lower than expected. By analyzing the
stacked spectra, we find a weak [NII] emission line (not
detectable in the single spectra) which anti-correlates with the mass, that can
be interpreted as a hint of the presence of ionized gas. We estimated the
correction of the H index by the residual emission component exploiting
different approaches, but find it very uncertain and model-dependent. We
conclude that, while the qualitative trends of the observed H-z
relations are consistent with the expected passive and downsizing scenario, the
possible presence of ionized gas even in the most massive and passive galaxies
prevents to use this index for a quantitative estimate of the age evolution and
for cosmological applications.Comment: 20 pages, 11 figures, 1 table. Accepted for publication in MNRAS Main
Journa
Pure Luminosity Evolution models for faint field galaxy samples
We have examined a set of pure luminosity evolution (PLE) models in order to
explore up to what extent the rapidly increasing observational constraints from
faint galaxy samples can be understood in this simple framework. We find that a
PLE model, in which galaxies evolve mildly in time even in the rest frame UV,
can reproduce most of the observed properties of faint galaxies assuming an
open () universe. In particular, such a model is able to fit
reasonably well the number counts in the , and bands, as
well as the colour and redshift distributions derived from most of the existing
samples. The most significant discrepancy between the predictions of this model
and the data is the distribution of faint -selected galaxies.
Significantly worse fits are obtained with PLE models for the theoretically
attractive value of , although a simple number luminosity evolution
model with a significant amount of merger events fits the data also in this
cosmology.Comment: 15 pages, plain tex (insert encapsulated postscript figures), plus an
extra figure Fig3c.ps and the tex-macro mn.tex uuencoded, gzipp'ed tar file
-- accepted by MNRA
Listening to galaxies tuning at z ~ 2.5 - 3.0: The first strikes of the Hubble fork
We investigate the morphological properties of 494 galaxies selected from the
GMASS survey at z>1, primarily in their optical rest frame, using HST images,
from the CANDELS survey. We propose that the Hubble sequence of galaxy
morphologies takes shape at redshift 2.5<z<3. The fractions of both ellipticals
and disks decrease with increasing lookback time at z>1, such that at redshifts
z=2.5-2.7 and above, the Hubble types cannot be identified, and most galaxies
are classified as irregular. The quantitative morphological analysis shows
that, at 1<z<3, morphological parameters are not as effective in distinguishing
the different morphological Hubble types as they are at low redshift. No
significant morphological k-correction was found to be required for the Hubble
type classification, with some exceptions. In general, different morphological
types occupy the two peaks of the rest-frame (U-B) colour bimodality of
galaxies: most irregulars occupy the blue peak, while ellipticals are mainly
found in the red peak, though with some level of contamination. Disks are more
evenly distributed than either irregulars and ellipticals. We find that the
position of a galaxy in a UVJ diagram is related to its morphological type: the
"quiescent" region of the plot is mainly occupied by ellipticals and, to a
lesser extent, by disks. We find that only ~33% of all morphological
ellipticals in our sample are red and passively evolving galaxies. Blue
galaxies morphologically classified as ellipticals show a remarkable structural
similarity to red ones. Almost all irregulars have a star-forming galaxy
spectrum. In addition, the majority of disks show some sign of star-formation
activity in their spectra, though in some cases their red continuum is
indicative of old stellar populations. Finally, an elliptical morphology may be
associated with either passively evolving or strongly star-forming galaxies.Comment: 27 pages, 16 figures, 5 tables. "Morphological atlas" in the
appendix. Revised version accepted for publication in A&
On the Determination of Star Formation Rates in Evolving Galaxy Populations
The redshift dependence of the luminosity density in certain wavebands (e.g.
UV and H-alpha) can be used to infer the history of star formation in the
populations of galaxies producing this luminosity. This history is a useful
datum in studies of galaxy evolution. It is therefore important to understand
the errors that attend the inference of star formation rate densities from
luminosity densities. This paper explores the self-consistency of star
formation rate diagnostics by reproducing commonly used observational
procedures in a model with known galaxy populations, evolutionary histories and
spectral emission properties. The study reveals a number of potential sources
of error in the diagnostic processes arising from the differential evolution of
different galaxy types. We argue that multi-wavelength observations can help to
reduce these errors.Comment: 13 pages (including 5 encapsulated postscript figures), aastex,
accepted for publication in Ap
Are z>2 Herschel galaxies proto-spheroids?
We present a backward approach for the interpretation of the evolution of the
near-infrared and the far-infrared luminosity functions across the redshift
range 0<z<3. In our method, late-type galaxies are treated by means of a
parametric phenomenological method based on PEP/HerMES data up to z~4, whereas
spheroids are described by means of a physically motivated backward model. The
spectral evolution of spheroids is modelled by means of a single-mass model,
associated to a present-day elliptical with K-band luminosity comparable to the
break of the local early-type luminosity function. The formation of
proto-spheroids is assumed to occurr across the redshift range 1< z < 5. The
key parameter is represented by the redshift z_0.5 at which half
proto-spheroids are already formed. A statistical study indicates for this
parameter values between z_0.5=1.5 and z_0.5=3. We assume as fiducial value
z_0.5~2, and show that this assumption allows us to describe accourately the
redshift distributions and the source counts. By assuming z_0.5 ~ 2 at the
far-IR flux limit of the PEP-COSMOS survey, the PEP-selected sources observed
at z>2 can be explained as progenitors of local spheroids caught during their
formation. We also test the effects of mass downsizing by dividing the
spheroids into three populations of different present-day stellar masses. The
results obtained in this case confirm the validity of our approach, i.e. that
the bulk of proto-spheroids can be modelled by means of a single model which
describes the evolution of galaxies at the break of the present-day early type
K-band LF.Comment: Accepted for publication in ApJ; 26 pages; 13 figure
A new photometric technique for the joint selection of star-forming and passive galaxies at 1.4<z<2.5
A simple two color selection based on B-, z-, and K- band photometry is
proposed for culling galaxies at 1.4<z<2.5 in K-selected samples and
classifying them as star-forming or passive systems. The method is calibrated
on the highly complete spectroscopic redshift database of the K20 survey,
verified with simulations and tested on other datasets. Requiring
BzK=(z-K)-(B-z)>-0.2 (AB) allows to select actively star-forming galaxies at
z>1.4, independently on their dust reddening. Instead, objects with BzK<-0.2
and (z-K)>2.5 (AB) colors include passively evolving galaxies at z>1.4, often
with spheroidal morphologies. Simple recipes to estimate the reddening, SFRs
and masses of BzK-selected galaxies are derived, and calibrated on K<20
galaxies. Based on their UV (reddening-corrected), X-ray and radio
luminosities, the BzK-selected star-forming galaxies with K<20 turn out to have
average SFR ~ 200 Msun yr^-1, and median reddening E(B-V)~0.4. Besides missing
the passively evolving galaxies, the UV selection appears to miss some relevant
fraction of the z~2 star-forming galaxies with K<20, and hence of the
(obscured) star-formation rate density at this redshift. The high SFRs and
masses add to other existing evidence that these z=2 star-forming galaxies may
be among the precursors of z=0 early-type galaxies. Theoretical models cannot
reproduce simultaneously the space density of both passively evolving and
highly star-forming galaxies at z=2. In view of Spitzer Space Telescope
observations, an analogous technique based on the RJL photometry is proposed to
complement the BzK selection and to identify massive galaxies at 2.5<z<4.0.
These color criteria should help in completing the census of the stellar mass
and of the star-formation rate density at high redshift (abridged).Comment: 19 pages, 17 figures, to appear on ApJ (20 December 2004 issue
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