309 research outputs found
Comparing the Evolution of the Galaxy Disk Sizes with CDM Models: The Hubble Deep Field
The intrinsic sizes of the field galaxies with I<26 in the Hubble and ESO-NTT
Deep Fields are shown as a function of their redshifts and absolute magnitudes
using photometric redshifts derived from the multicolor catalogs and are
compared with the CDM predictions. Extending to lower luminosities and to
higher z our previous analysis performed on the NTT field alone, we find that
the distribution of the galaxy disk sizes at different cosmic epochs is within
the range predicted by typical CDM models. However, the observed size
distribution of faint (M_B>-19) galaxies is skewed with respect to the CDM
predictions and an excess of small-size disks (R_d<2 kpc) is already present at
z~ 0.5. The excess persists up to z~3 and involves brighter galaxies . Such an
excess may be reduced if luminosity-dependent effects, like starburst activity
in interacting galaxies, are included in the physical mechanisms governing the
star formation history in CDM models.Comment: 9 pages, 3 figures, ApJ Letters in pres
Probing the evolution of the near-IR luminosity function of galaxies to z ~ 3 in the Hubble Deep Field South
[Abridged] We present the rest-frame Js-band and Ks-band luminosity function
of a sample of about 300 galaxies selected in the HDF-S at Ks<23 (Vega). We use
calibrated photometric redshift together with spectroscopic redshift for 25% of
the sample. The sample has allowed to probe the evolution of the LF in the
three redshift bins [0;0.8), [0.8;1.9) and [1.9;4) centered at the median
redshift z_m ~ [0.6,1.2,3]. The values of alpha we estimate are consistent with
the local value and do not show any trend with redshift. We do not see evidence
of evolution from z=0 to z_m ~ 0.6 suggesting that the population of local
bright galaxies was already formed at z<0.8. On the contrary, we clearly detect
an evolution of the LF to z_m ~ 1.2 characterized by a brightening of M* and by
a decline of phi*. To z_m ~ 1.2 M* brightens by about 0.4-0.6 mag and phi*
decreases by a factor 2-3. This trend persists, even if at a less extent, down
to z_m ~ 3 both in the Js-band and in the Ks-band LF. The decline of the number
density of bright galaxies seen at z>0.8 suggests that a significant fraction
of them increases their stellar mass at 1<z<2-3 and that they underwent a
strong evolution in this redshift range. On the other hand, this implies also
that a significant fraction of local bright/massive galaxies was already in
place at z>3. Thus, our results suggest that the assembly of high-mass galaxies
is spread over a large redshift range and that the increase of their stellar
mass has been very efficient also at very high redshift at least for a fraction
of them.Comment: 18 pages, 21 figures, Accepted for publication in MNRA
The B-Band Luminosity Function of Red and Blue Galaxies up to z=3.5
We have explored the redshift evolution of the luminosity function of red and
blue galaxies up to . This was possible joining a deep I band composite
galaxy sample, which includes the spectroscopic K20 sample and the HDFs
samples, with the deep and samples derived from the
deep NIR images of the Hubble Deep Fields North and South, respectively. About
30% of the sample has spectroscopic redshifts and the remaining fraction
well-calibrated photometric redshifts. This allowed to select and measure
galaxies in the rest-frame blue magnitude up to and to derive the
redshift evolution of the B-band luminosity function of galaxies separated by
their rest-frame color or specific (i.e. per unit mass) star-formation
rate. The class separation was derived from passive evolutionary tracks or from
their observed bimodal distributions. Both distributions appear bimodal at
least up to and the locus of red/early galaxies is clearly identified
up to these high redshifts. Both luminosity and density evolutions are needed
to describe the cosmological behaviour of the red/early and blue/late
populations. The density evolution is greater for the early population with a
decrease by one order of magnitude at with respect to the value at
. The luminosity densities of the early and late type galaxies with
. Indeed while star-forming
galaxies slightly increase or keep constant their luminosity density, "early"
galaxies decrease in their luminosity density by a factor from
to . A comparison with one of the latest versions of
the hierarchical CDM models shows a broad agreement with the observed number
and luminosity density evolutions of both populations.Comment: 41 pages, 14 figures, accepted for publication in Ap
Quasar Evolution Driven by Galaxy Encounters in Hierarchical Structures
We link the evolution of the galaxies in the hierarchical clustering scenario
with the changing accretion rates of cold gas onto the central massive black
holes that power the quasars. We base on galaxy interactions as main triggers
of accretion; the related scaling laws are taken up from Cavaliere & Vittorini
(2000), and grafted to a semi-analytic code for galaxy formation. As a result,
at high the protogalaxies grow rapidly by hierarchical merging; meanwhile,
much fresh gas is imported and also destabilized, so the holes are fueled at
their full Eddington rates. At lower the galactic dynamical events are
mostly encounters in hierarchically growing groups; now the refueling peters
out, as the residual gas is exhausted while the destabilizing encounters
dwindle. So, with no parameter tuning other than needed for stellar
observables, our model uniquely produces at a rise, and at a decline of the bright quasar population as steep as observed. In addition,
our results closely fit the observed luminosity functions of quasars, their
space density at different magnitudes from to , and
the local relation.Comment: 5 pages. Accepted for publication in ApJ Letter
A low escape fraction of ionizing photons of L>L* Lyman break galaxies at z=3.3
We present an upper limit for the relative escape fraction (f_{esc}^{rel}) of
ionizing radiation at z~3.3 using a sample of 11 Lyman Break Galaxies (LBGs)
with deep imaging in the U band obtained with the Large Binocular Camera,
mounted on the prime focus of the Large Binocular Telescope. We selected 11
LBGs with secure redshift in the range 3.27<z<3.35, from 3 independent fields.
We stacked the images of our sources in the R and U band, which correspond to
an effective rest-frame wavelength of 1500\AA and 900\AA respectively,
obtaining a limit in the U band image of >=30.7(AB)mag at 1 sigma. We derive a
1 sigma upper limit of f_{esc}^{rel}~5%, which is one of the lowest values
found in the literature so far at z~3.3. Assuming that the upper limit for the
escape fraction that we derived from our sample holds for all galaxies at this
redshift, the hydrogen ionization rate that we obtain (Gamma_{-12}<0.3 s^{-1})
is not enough to keep the IGM ionized and a substantial contribution to the UV
background by faint AGNs is required. Since our sample is clearly still limited
in size, larger z~3 LBG samples, at similar or even greater depths are
necessary to confirm these results on a more firm statistical basis.Comment: 15 pages, 2 figures, 1 table, accepted for publication in Ap
On the Escape of Ionizing Radiation from Starbursts
Far-ultraviolet spectra obtained with show that the strong
1036 interstellar absorption-line is essentially black in five of
the UV-brightest local starburst galaxies. Since the opacity of the neutral ISM
below the Lyman-edge will be significantly larger than in the line, these
data provide strong constraints on the escape of ionizing radiation from these
starbursts. Interpreted as a a uniform absorbing slab, the implied optical
depth at the Lyman edge is huge (). Alternatively, the areal
covering factor of opaque material is typically 94%. Thus, the fraction
of ionizing stellar photons that escape the ISM of each galaxy is small: our
conservative estimates typically yield . Inclusion of
extinction due to dust will further decrease . An analogous analysis
of the rest-UV spectrum of the star-forming galaxy at =2.7
leads to similar constraints on . These new results agree with the
constraints provided by direct observations below the Lyman edge in a few other
local starbursts. However, they differ from the recently reported properties of
star-forming galaxies at 3. We assess the idea that the strong
galactic winds seen in many powerful starbursts clear channels through their
neutral ISM. We show empirically that such outflows may be a necessary - but
not sufficient - part of the process for creating a relatively porous ISM. We
note that observations will soon document the cosmic evolution in the
contribution of star-forming galaxies to the metagalactic ionizing background,
with important implications for the evolution of the IGM.Comment: 17 pages; ApJ, in pres
The Evolution of the Luminosity Function in Deep Fields: A Comparison with CDM Models
The galaxy Luminosity Function (LF) has been estimated in the rest frame B
luminosity at 0<z<1.25 and at 1700 {\AA} for 2.5<z<4.5 from deep multicolor
surveys in the HDF-N, HDF-S, NTT-DF. The results have been compared with a
recent version of galaxy formation models in the framework of hierarchical
clustering in a flat Cold Dark Matter Universe with cosmological constant. The
results show a general agreement for z<= 1, although the model LF has a steeper
average slope at the faint end; at z~3 such feature results in an
overprediction of the number of faint (I_{AB}~ 27) galaxies, while the
agreement at the bright end becomes critically sensitive to the details of dust
absorption at such redshifts. The discrepancies at the faint end show that a
refined treatement of the physical processes involving smaller galaxies is to
be pursued in the models, in terms of aggregation processes and/or stellar
feedback heavily affecting the luminosity of the low luminosity objects. The
implications of our results on the evolution of the cosmological star formation
rate are discussed.Comment: Revised version; corrected magnitudes at 1700 Angstrom in figure 2;
ApJ
The discovery of the optical/IR counterpart of the 12s transient X-ray pulsar GS 0834-43
We report the discovery of the optical/infra-red counterpart of the 12.3s
transient X-ray pulsar GS0834-43. We re-analysed archival ROSAT PSPC
observations of GS0834-43, obtaining two new refined positions, about 14" and
18" away from the previously published one, and a new spin period measurement.
Within the new error circles we found a relatively faint (V=20.1) early type
reddened star (V-R=2.24). The optical spectrum shows a strong Halpha emission
line. The IR observations of the field confirm the presence of an IR excess for
the Halpha-emitting star (K'=11.4, J-K'=1.94) which is likely surrounded by a
conspicuous circumstellar envelope. Spectroscopic and photometric data indicate
a B0-2 V-IIIe spectral-type star located at a distance of 3-5kpc and confirm
the Be-star/X-ray binary nature of GS0834-43.Comment: 6 pages. Accepted for publication in MNRA
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