231 research outputs found
Quiescent galaxies at : observations vs. models
The presence of massive quiescent galaxies at high redshifts is still a
challenge for most models of galaxy formation. The aim of this work is to
compare the observed number density and properties of these galaxies with the
predictions of state-of-the-art models. The sample of massive quiescent
galaxies has been selected from the COSMOS2015 photometric catalogue with
, and
. The photometric SEDs of the selected
galaxies have been thoroughly analyzed based on different stellar population
synthesis models. The final sample includes only those galaxies qualified as
quiescent in all SED fitting runs. The observed properties have been compared
to theoretical models: the number density of quiescent galaxies with is reproduced by some models, although there is a
large scatter in their predictions. Instead, very massive
are underpredicted by most of the current
models of galaxy formation: some of them, built on the CARNage simulation, are
consistent with data up to , while at higher redshifts the volume of
the considered simulation is too small to find such rare objects. Simulated
galaxies which match the observed properties in the plane
at have been analyzed by reconstructing their evolutionary paths:
their merger trees suggest that AGN feedback could be the key process allowing
for a rapid quenching of the star formation at and that its
treatment should be improved in models.Comment: Accepted for publication in ApJL, 9 pages, 6 figure
Massive and old quiescent galaxies at high redshift
Massive quiescent galaxies at high redshift can shed light on the processes
of galaxy mass assembly and quenching of the star formation at early epochs. We
present observer-frame color-color diagrams designed to identify candidate
quiescent galaxies from z=2.5 up to the highest redshifts, that can be then be
selected for spectroscopic follow-up observations. The application to the
COSMOS2015 catalog shows that, after refining the selection with SED fitting,
the number of massive old quiescent galaxies exceeds the forecast of
state-of-the-art semi-analytic models, pointing out the need of an improvement
of the implemented quenching mechanisms at high redshifts.Comment: Astronomy & Astrophysics, accepted; Matching version in press; 21
pages, 13 figure
The stellar-to-halo mass relation over the past 12 Gyr
Understanding how galaxy properties are linked to the dark matter halos they
reside in, and how they co-evolve is a powerful tool to constrain the processes
related to galaxy formation. The stellar-to-halo mass relation (SHMR) and its
evolution over the history of the Universe provides insights on galaxy
formation models and allows to assign galaxy masses to halos in N-body dark
matter simulations. We use a statistical approach to link the observed galaxy
stellar mass functions on the COSMOS field to dark matter halo mass functions
from the DUSTGRAIN simulation and from a theoretical parametrization from z=0
to z=4. We also propose an empirical model to describe the evolution of the
stellar-to-halo mass relation as a function of redshift. We calculate the
star-formation efficiency (SFE) of galaxies and compare results with previous
works and semi-analytical models.Comment: accepted for publication in A&A, matching version in pres
X-ray redshifts for obscured AGN: a case study in the J1030 deep field
We present a procedure to constrain the redshifts of obscured ( cm) Active Galactic Nuclei (AGN) based on low-count statistics
X-ray spectra, which can be adopted when photometric and/or spectroscopic
redshifts are unavailable or difficult to obtain. We selected a sample of 54
obscured AGN candidates on the basis of their X-ray hardness ratio, ,
in the Chandra deep field (479 ks, 335 arcmin) around the QSO
SDSS J1030+0524. The sample has a median value of net counts in the
0.5-7 keV energy band. We estimate reliable X-ray redshift solutions taking
advantage of the main features in obscured AGN spectra, like the Fe 6.4 keV
K emission line, the 7.1 keV Fe absorption edge and the
photoelectric absorption cut-off. The significance of such features is
investigated through spectral simulations, and the derived X-ray redshift
solutions are then compared with photometric redshifts. Both photometric and
X-ray redshifts are derived for 33 sources. When multiple solutions are derived
by any method, we find that combining the redshift solutions of the two
techniques improves the rms by a factor of two. Using our redshift estimates
(), we derived absorbing column densities in the
range cm and absorption-corrected, 2-10 keV
rest-frame luminosities between and erg s, with
median values of cm and erg s, respectively. Our results suggest that
the adopted procedure can be applied to current and future X-ray surveys, for
sources detected only in the X-rays or that have uncertain photometric or
single-line spectroscopic redshifts.Comment: 22 pages, 18 figure
The star formation histories of z ∼ 1 post-starburst galaxies
Funding: LTA acknowledges support from the Ministry of Higher Education and Scientific Research (MOHESR), Iraq. AW acknowledges financial support from the Royal Society Newton Fund (grant NAF/R1/180403, PI Natalia Vale Asari) and Fundação de à Amparo Pesquisa do Estado de São Paulo (FAPESP) process number 2019/01768-6.We present the star formation histories of 39 galaxies with high-quality rest-frame optical spectra at 0.5 <z <1.3 selected to have strong Balmer absorption lines and/or Balmer break, and compare to a sample of spectroscopically selected quiescent galaxies at the same redshift. Photometric selection identifies a majority of objects that have clear evidence for a recent short-lived burst of star formation within the last 1.5 Gyr, i.e. 'post-starburst' galaxies, however we show that good quality continuum spectra are required to obtain physical parameters such as burst mass fraction and burst age. Dust attenuation appears to be the primary cause for misidentification of post-starburst galaxies, leading to contamination in spectroscopic samples where only the [O II] emission line is available, as well as a small fraction of objects lost from photometric samples. The 31 confirmed post-starburst galaxies have formed 40-90 per cent of their stellar mass in the last1-1.5 Gyr. We use the derived star formation histories to find that the post-starburst galaxies are visible photometrically for 0.5-1 Gyr. This allows us to update a previous analysis to suggest that 25-50 per cent of the growth of the red sequence at z ∼ 1 could be caused by a starburst followed by rapid quenching. We use the inferred maximum historical star formation rates of several 100-1000 M⊙yr-1 and updated visibility times to confirm that sub-mm galaxies are likely progenitors of post-starburst galaxies. The short quenching time-scales of 100-200 Myr are consistent with cosmological hydrodynamic models in which rapid quenching is caused by the mechanical expulsion of gas due to an acive galactic neucleus.Publisher PDFPeer reviewe
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