269 research outputs found
Deep Galaxy Counts, Extragalactic Background Light, and the Stellar Baryon Budget
We assess the constraints imposed by the observed extragalactic background
light (EBL) on the cosmic history of star formation and the stellar mass
density today. The logarithmic slope of the galaxy number-magnitude relation
from the Southern Hubble Deep Field} imaging survey is flatter than 0.4 in all
seven UBVIJHK optical bandpasses, i.e. the light from resolved galaxies has
converged from the UV to the near-IR. We find a lower limit to the surface
brightness of the optical extragalactic sky of about 15 nW/m^2/sr, comparable
to the intensity of the far-IR background from COBE data. Assuming a Salpeter
initial mass function with a lower cutoff consistent with observations of M
subdwarf disk stars, we set a lower limit of Omega_g+s h^2>0.0013 I_50 to the
visible (processed gas + stars) mass density required to generate an EBL at a
level of 50 I_50 nW/m^2/sr; our `best-guess' value is Omega_g+s h^2=0.003 I_50.
Motivated by the recent microlensing results of the MACHO collaboration, we
consider the possibility that massive dark halos around spiral galaxies are
composed of faint white dwarfs, and show that only a small fraction (<5%) of
the nucleosynthetic baryons can be locked in the remnants of intermediate-mass
stars forming at z_F<5, as the bright early phases of such halos would
otherwise overproduce the observed EBL.Comment: LaTeX, 17 pages, 5 figures, revised version accepted for publication
in the MNRA
On the Luminosity Function of Early--Type Galaxies
In a recent paper Loveday et al. (1992) have presented new results on the
luminosity function for a sample of galaxies with . After having
morphologically classified each galaxy (early--type, late--type, merged or
uncertain), they have estimated the parameters of a Schechter luminosity
function for early-- and late--type galaxies. However, in their sample there is
a bias against identifying early--type galaxies at large distances and/or faint
magnitudes: in fact, many of the early--type galaxies at faint magnitudes have
probably been classified as ``uncertain". As discussed in Loveday et al., the
existence of such a bias is indicated by the fact that for these galaxies
. In this paper we show, both theoretically and through the
use of simulated samples, that this incompleteness strongly biases the derived
parameters of the luminosity function for early--type galaxies. If no
correction for such incompleteness is applied to the data (as done by Loveday
et al.), one obtains a flatter slope and a brighter with respect
to the real parameters.Comment: accepted for publication on MNRAS, Standard TeX, for tables and
figures contact [email protected] BAP 03-1994-04-IR
Search Instructions for Globular Clusters in Formation at High Redshifts
The formation of globular clusters (GC), with their multiple stellar
generations, is still an unsolved puzzle. Thus, interest is rising on the
possibility to detect their precursors at high redshift, hence directly
witnessing their formation. A simple set of assumptions are empirically
justified and then used to predict how many such precursors formed between
redshift 3 and 10 could actually be detected by the NIRCam instrument on board
of JWST. It is shown that the near power-law shape of the rest-frame UV
continuum of young globular cluster precursors (GCP) implies that both colours
and luminosities in NIRCam long-wavelength passbands depend remarkably weakly
on formation redshift. Thus, the predicted number counts depend only little on
the actual formation redshifts in the mentioned range, with the exception of
the bluest passbands for which counts can be strongly suppressed by
intergalactic absorption along the line of sight. Instead, counts depend
strongly on the actual mass of GCPs, in such a way that one NIRCam pointing
should detect of the order of 10 GCPs to mag if their mass
distribution was the same of today GCs, or over 1,000 if their mass was 10
times higher. Therefore, GCP number counts will set fairly tight constraints on
the initial mass of GCs. An encouraging agreement with the number density of
candidate GCPs at , revealed by the Hubble Frontier Fields (HFF)
program, suggests that their initial mass could be at least 4 times higher than
that of their local descendants if all were to end up as GCs.Comment: MNRAS accepte
Catching galaxies in the act of quenching star formation
Detecting galaxies when their star-formation is being quenched is crucial to
understand the mechanisms driving their evolution. We identify for the first
time a sample of quenching galaxies selected just after the interruption of
their star formation by exploiting the [O III]5007/Halpha ratio and searching
for galaxies with undetected [O III]. Using a sample of ~174000 star-forming
galaxies extracted from the SDSS-DR8 at 0.04 < z < 0.21,we identify the ~300
quenching galaxy best candidates with low [O III]/Halpha, out of ~26000
galaxies without [O III] emission. They have masses between 10^9.7 and 10^10.8
Mo, consistently with the corresponding growth of the quiescent population at
these redshifts. Their main properties (i.e. star-formation rate, colours and
metallicities) are comparable to those of the star-forming population,
coherently with the hypothesis of recent quenching, but preferably reside in
higher-density environments.Most candidates have morphologies similar to
star-forming galaxies, suggesting that no morphological transformation has
occurred yet. From a survival analysis we find a low fraction of candidates
(~0.58% of the star-forming population), leading to a short quenching timescale
of tQ~50Myr and an e-folding time for the quenching history of tauQ~90Myr, and
their upper limits of tQ<0.76 Gyr and tauQ<1.5Gyr, assuming as quenching
galaxies 50% of objects without [O III] (~7.5%).Our results are compatible with
a 'rapid' quenching scenario of satellites galaxies due to the final phase of
strangulation or ram-pressure stripping. This approach represents a robust
alternative to methods used so far to select quenched galaxies (e.g. colours,
specific star-formation rate, or post-starburst spectra).Comment: 22 pages, 23 figures, accepted for publication in MNRA
A methodology to select galaxies just after the quenching of star formation
We propose a new methodology aimed at finding star-forming galaxies in the
phase which immediately follows the star-formation (SF) quenching, based on the
use of high- to low-ionization emission line ratios. These ratios rapidly
disappear after the SF halt, due to the softening of the UV ionizing radiation.
We focus on [O III] 5007/H and [Ne III] 3869/[O II]
3727, studying them with simulations obtained with the CLOUDY
photoionization code. If a sharp quenching is assumed, we find that the two
ratios are very sensitive tracers as they drop by a factor 10 within
10 Myr from the interruption of the SF; instead, if a smoother and
slower SF decline is assumed (i.e. an exponentially declining star-formation
history with -folding time 200 Myr), they decrease by a factor
2 within 80 Myr. We mitigate the ionization -- metallicity
degeneracy affecting our methodology using pairs of emission line ratios
separately related to metallicity and ionization, adopting the [N II]
6584/[O II] 3727 ratio as metallicity diagnostic. Using a
Sloan Digital Sky Survey galaxy sample, we identify 10 examples among the most
extreme quenching candidates within the [O III] 5007/H vs. [N
II] 6584/[O II] 3727 plane, characterized by low [O III]
5007/H, faint [Ne III] 3869, and by blue
dust-corrected spectra and colours, as expected if the SF quenching has
occurred in the very recent past. Our results also suggest that the observed
fractions of quenching candidates can be used to constrain the quenching
mechanism at work and its time-scales.Comment: Accepted for publication in MNRAS; 19 pages, 21 figures, 1 tabl
Reconstructing the galaxy density field with photometric redshifts: II. Environment-dependent galaxy evolution since
Although extensively investigated, the role of the environment in galaxy
formation is still not well understood. In this context, the Galaxy Stellar
Mass Function (GSMF) is a powerful tool to understand how environment relates
to galaxy mass assembly and the quenching of star-formation. In this work, we
make use of the high-precision photometric redshifts of the UltraVISTA Survey
to study the GSMF in different environments up to , on physical
scales from 0.3 to 2 Mpc, down to masses of . We
witness the appearance of environmental signatures for both quiescent and
star-forming galaxies. We find that the shape of the GSMF of quiescent galaxies
is different in high- and low-density environments up to with the
high-mass end () being enhanced in high-density
environments. On the contrary, for star-forming galaxies a difference between
the GSMF in high- and low density environments is present for masses . Star-forming galaxies in this mass range appear to
be more frequent in low-density environments up to . Differences in
the shape of the GSMF are not visible anymore at . Our results, in terms
of general trends in the shape of the GSMF, are in agreement with a scenario in
which galaxies are quenched when they enter hot gas-dominated massive haloes
which are preferentially in high-density environments.Comment: 18 pages, 10 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Constraining the time evolution of dark energy, curvature and neutrino properties with cosmic chronometers
We use the latest compilation of observational H(z) measurements obtained
with cosmic chronometers in the redshift range to place constraints on
cosmological parameters. We consider the sample alone and in combination with
other state-of-the art cosmological probes: CMB data from the latest Planck
2015 release, the most recent estimate of the Hubble constant , a
compilation of recent BAO data, and the latest SNe sample. Since cosmic
chronometers are independent of the assumed cosmological model, we are able to
provide constraints on the parameters that govern the expansion history of the
Universe in a way that can be used to test cosmological models. We show that
the H(z) measurements obtained with cosmic chronometer from the BOSS survey
provide enough constraining power in combination with CMB data to constrain the
time evolution of dark energy, yielding constraints competitive with those
obtained using SNe and/or BAO. From late-Universe probes alone we find that
and , and when combining also CMB data we
obtain and . These new constraints imply
that nearly all quintessence models are disfavoured, only phantom models or a
pure cosmological constant being allowed. For the curvature we find
, including CMB data. Cosmic chronometers data are
important also to constrain neutrino properties by breaking or reducing
degeneracies with other parameters. We find that , thus
excluding the possibility of an extra (sterile) neutrino at more than
, and put competitive limits on the sum of neutrino masses, eV at 95% confidence level. Finally, we constrain the redshift
evolution of dark energy, and find w(z) consistent with the CDM model
at the 40% level over the entire redshift range . [abridged]Comment: 19 pages, 9 figures, 6 tables, submitted to JCAP. The cosmic
chronometers data used in this analysis can be downloaded at
http://www.physics-astronomy.unibo.it/en/research/areas/astrophysics/cosmology-with-cosmic-chronometer
The Star Formation History of Field Galaxies
We develop a method for interpreting faint galaxy data which focuses on the integrated light radiated from the galaxy population as a whole. The emission history of the universe at ultraviolet, optical, and near-infrared wavelengths is modeled from the present epoch to z~4 by tracing the evolution with cosmic time of the galaxy luminosity density, as determined from several deep spectroscopic samples and the Hubble Deep Field (HDF) imaging survey. The global spectrophotometric properties of field galaxies can be well fit by a simple stellar evolution model, defined by a time-dependent star formation rate (SFR) per unit comoving volume and a universal IMF extending from 0.1 to 125 M_sun. In the best-fit models, the global SFR rises sharply, by about an order of magnitude, from a redshift of zero to a peak value at z~1.5, to fall again at higher redshifts. The models are able to account for the entire background light recorded in the galaxy counts down to the very faint magnitude levels probed by the HDF. Since only 20% of the current stellar content of galaxies is produced at z>2, a rather low cosmic metallicity is expected at these early times, in good agreement with the observed enrichment history of the damped Lyman-\alpha systems. A ``monolithic collapse'' model, where half of the present-day stars formed at z>2.5 and were shrouded by dust, can be made consistent with the global history of light, but overpredicts the metal mass density at high redshifts as sampled by QSO absorbers
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