530 research outputs found
Constraining the Warm Dark Matter Particle Mass through Ultra-Deep UV Luminosity Functions at z=2
We compute the mass function of galactic dark matter halos for different
values of the Warm Dark Matter (WDM) particle mass m_X and compare it with the
abundance of ultra-faint galaxies derived from the deepest UV luminosity
function available so far at redshift z~2. The magnitude limit M_UV=-13 reached
by such observations allows us to probe the WDM mass functions down to scales
close to or smaller than the half-mass mode mass scale ~10^9 M_sun. This
allowed for an efficient discrimination among predictions for different m_X
which turn out to be independent of the star formation efficiency adopted to
associate the observed UV luminosities of galaxies to the corresponding dark
matter masses. Adopting a conservative approach to take into account the
existing theoretical uncertainties in the galaxy halo mass function, we derive
a robust limit m_X>1.8 keV for the mass of thermal relic WDM particles when
comparing with the measured abundance of the faintest galaxies, while m_X>1.5
keV is obtained when we compare with the Schechter fit to the observed
luminosity function. The corresponding lower limit for sterile neutrinos
depends on the modeling of the production mechanism; for instance m_sterile > 4
keV holds for the Shi-Fuller mechanism. We discuss the impact of observational
uncertainties on the above bound on m_X. As a baseline for comparison with
forthcoming observations from the HST Frontier Field, we provide predictions
for the abundance of faint galaxies with M_UV=-13 for different values of m_X
and of the star formation efficiency, valid up to z~4.Comment: 14 pages, 3 figures. Accepted for publication in The Astrophysical
Journa
Observing the very low-surface brightness dwarfs in a deep field in the VIRGO cluster: constraints on Dark Matter scenarios
We report the discovery of 11 very faint (r< 23), low surface brightness
({\mu}_r< 27 mag/arcsec^2) dwarf galaxies in one deep field in the Virgo
cluster, obtained by the prime focus cameras (LBC) at the Large Binocular
Telescope (LBT). These extend our previous sample to reach a total number of 27
galaxies in a field of just of 0.17 deg^2 located at a median distance of 390
kpc from the cluster center. Their association with the Virgo cluster is
supported by their separate position in the central surface brightness - total
magnitude plane with respect to the background galaxies of similar total
magnitude. For a significant fraction (26\%) of the sample the association to
the cluster is confirmed by spectroscopic follow-up. We show that the mere
abundance of satellite galaxies corresponding to our observed number in the
target field provides extremely tight constraints on Dark Matter models with
suppressed power spectrum compared to the Cold Dark Matter case, independently
of the galaxy luminosity distribution. In particular, requiring the observed
number of satellite galaxies not to exceed the predicted abundance of Dark
Matter sub-halos yields a limit m_X >3 keV at 1-{\sigma} and m_X > 2.3 keV at
2-{\sigma} confidence level for the mass of thermal Warm Dark Matter particles.
Such a limit is competitive with other limits set by the abundance of
ultra-faint satellite galaxies in the Milky Way, is completely independent of
baryon physics involved in galaxy formation, and has the potentiality for
appreciable improvements with next observations. We extend our analysis to Dark
Matter models based on sterile neutrinos, showing that our observations set
tight constraints on the combination of sterile neutrino mass m_{\nu} and
mixing parameter sin^2(2{\theta}). We discuss the robustness of our results
with respect to systematics.Comment: Accepted for publication in Astronomy & Astrophysic
The Lyman continuum escape fraction of galaxies at z = 3.3 in the VUDS-LBC/COSMOS field
Context. The ionizing Lyman continuum flux escaping from high-redshift galaxies into the intergalactic medium is a fundamental quantity to understand the physical processes involved in the reionization epoch. However, from an observational point of view, direct detections of HI ionizing photons at high redshifts are feasible for galaxies mainly in the interval z ~ 3−4.
Aims. We have investigated a sample of star-forming galaxies at z ~ 3.3 to search for possible detections of Lyman continuum ionizing photons escaping from galaxy halos.
Methods. We used deep ultraviolet (UV) imaging in the COSMOS field, obtained with the prime focus camera LBC at the LBT telescope, along with a catalogue of spectroscopic redshifts obtained by the VIMOS Ultra Deep Survey (VUDS) to build a sample of 45 galaxies at z ~ 3.3 with L > 0.5 L^∗. We obtained deep LBC images of galaxies with spectroscopic redshifts in the interval 3.27 < z < 3.40 both in the R- and deep U-bands (magnitude limit U ~ 29.7 at S/N = 1). At these redshifts, the R-band samples the non-ionizing 1500 Å rest-frame luminosity and the U-band samples the rest-frame spectral region just short-ward of the Lyman edge at 912 Å. Their flux ratio is related to the ionizing escape fraction after statistical removal of the absorption by the intergalactic medium along the line of sight.
Results. A subsample of ten galaxies apparently shows escape fractions >28%, but a detailed analysis of their properties reveals that, with the exception of two marginal detections (S/N ~ 2) in the U-band, all the other eight galaxies are most likely contaminated by the UV flux of low-redshift interlopers located close (in angular position) to the high-z targets. The average escape fraction derived from the stacking of the cleaned sample was constrained to f_(esc)^(rel) < 2%. The implied hydrogen photoionization rate is a factor two lower than that needed to keep the intergalactic medium ionized at z ~ 3, as observed in the Lyman-α forest of high-z quasar spectra or by the proximity effect.
Conclusions. These results support a scenario where high redshift, relatively bright (L ≥ 0.5L^∗) star-forming galaxies alone are unable to sustain the level of ionization observed in the cosmic intergalactic medium at z ~ 3. Star-forming galaxies at higher redshift and at fainter luminosities (L ≪ L^∗) can only be major contributors to the reionization of the Universe if their physical properties are subject to rapid changes from z ~ 3 to z ~ 6–10. Alternatively, ionizing sources could be discovered looking for fainter sources among the active galactic nuclei population at high redshift
The Luminosity Function of high-redshift QSOs - A combined analysis of GOODS and SDSS
Aims: In this work the luminosity function of QSOs is measured in the
redshift range 3.5<z<5.2 for the absolute magnitude interval -21<M_{145}<-28.
The determination of the faint end of the luminosity function at these
redshifts provides important constraints on models of joint evolution of
galaxies and AGNs. Methods: We have defined suitable criteria to select faint
QSOs in the GOODS fields, checking in detail their effectiveness and
completeness. Spectroscopic follow-up of the resulting QSO candidates has been
carried out. The confirmed sample of faint QSOs is compared with a brighter one
derived from the SDSS. We have used a Monte-Carlo technique to estimate the
properties of the luminosity function, checking various parameterizations for
its shape and evolution. Results: Models based on Pure Density Evolution show
better agreement with observation than models based on Pure Luminosity
Evolution. However a different break magnitude with respect to z~2.1 is
required at 3.5<z<5.2. Models with a steeper faint end score a higher
probability. We do not find any evidence for a flattening of the bright end at
redshift z>3.5. Conclusions: The estimated space density evolution of QSOs
indicates a suppression of the formation and/or feeding of Supermassive Black
Holes at these redshifts. The QSO contribution to the UV background is
insufficient to ionize the IGM at 3.5<z<5.2.Comment: 17 pages, 13 ps figures, A&A accepted. Updated to journal versio
The evolution of the AGN content in groups up to z~1
Determining the AGN content in structures of different mass/velocity
dispersion and comparing them to higher mass/lower redshift analogs is
important to understand how the AGN formation process is related to
environmental properties. We use our well-tested cluster finding algorithm to
identify structures in the GOODS North and South fields, exploiting the
available spectroscopic redshifts and accurate photometric redshifts. We
identify 9 structures in GOODS-south (presented in a previous paper) and 8 new
structures in GOODS-north. We only consider structures where at least 2/3 of
the members brighter than M_R=-20 have a spectroscopic redshift. For those
group members that coincide with X-ray sources in the 4 and 2 Msec Chandra
source catalogs respectively, we determine if the X-ray emission originates
from AGN activity or it is related to the galaxies' star-formation activity. We
find that the fraction of AGN with Log L_H > 42 erg/s in galaxies with M_R <
-20 is on average 6.3+-1.3%, much higher than in lower redshift groups of
similar mass and more than double the fraction found in massive clusters at a
similarly high redshift. We then explore the spatial distribution of AGN in the
structures and find that they preferentially populate the outer regions. The
colors of AGN host galaxies in structures tend to be confined to the green
valley, thus avoiding the blue cloud and, partially, also the red-sequence,
contrary to what happens in the field. We finally compare our results to the
predictions of two sets of semi analytic models to investigate the evolution of
AGN and evaluate potential triggering and fueling mechanisms. The outcome of
this comparison attests the importance of galaxy encounters, not necessarily
leading to mergers, as an efficient AGN triggering mechanism. (abridged)Comment: 11 pages, 8 figures, Accepted accepted for publication in A&
Galaxy Formation in Sterile Neutrino Dark Matter Models
We investigate galaxy formation in models with dark matter (DM) constituted
by sterile neutrinos. Given their large parameter space, defined by the
combinations of sterile neutrino mass and mixing parameter
with active neutrinos, we focus on models with
keV, consistent with the tentative 3.5 keV line detected in several X-ray
spectra of clusters and galaxies. We consider i) two resonant production models
with and , to cover
the range of mixing parameter consistent with the 3.5 keV line; ii) two
scalar-decay models, representative of the two possible cases characterizing
such a scenario: a freeze-in and a freeze-out case. We also consider thermal
Warm Dark Matter with particle mass keV. Using a semi-analytic model,
we compare the predictions for the different DM scenarios with a wide set of
observables. We find that comparing the predicted evolution of the stellar mass
function, the abundance of satellites of Milky Way-like galaxies, and the
global star formation history of galaxies with observations does not allow to
disentangle the effects of the baryonic physics from those related to the
different DM models. On the other hand, the distribution of the stellar-to-halo
mass ratios, the abundance of faint galaxies in the UV luminosity function at
, and the specific star formation and age distribution of local,
low-mass galaxies constitute potential probes for the considered DM scenarios.
We discuss how next observations with upcoming facilities will enable to rule
out or to strongly support DM models based on sterile neutrinos.Comment: 21 pages, accepted for publication in The Astrophysical Journa
Multi-wavelength analysis of the field of the dark burst GRB 031220
We have collected and analyzed data taken in different spectral bands (from
X-ray to optical and infrared) of the field of GRB031220 and we present results
of such multiband observations. Comparison between images taken at different
epochs in the same filters did not reveal any strong variable source in the
field of this burst. X-ray analysis shows that only two of the seven Chandra
sources have a significant flux decrease and seem to be the most likely
afterglow candidates. Both sources do not show the typical values of the R-K
colour but they appear to be redder. However, only one source has an X-ray
decay index (1.3 +/- 0.1) that is typical for observed afterglows. We assume
that this source is the best afterglow candidate and we estimate a redshift of
1.90 +/- 0.30. Photometric analysis and redshift estimation for this object
suggest that this GRB can be classified as a Dark Burst and that the
obscuration is the result of dust extinction in the circum burst medium or
inside the host galaxy.Comment: 7 pages, 5 figures, accepted for publication on A&
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