2,878 research outputs found
On the formation of dwarf galaxies and stellar halos
Using analytic arguments and a suite of very high resolution (10^3 Msun per
particle) cosmological hydro-dynamical simulations, we argue that high
redshift, z ~ 10, M ~ 10^8 Msun halos, form the smallest `baryonic building
block' (BBB) for galaxy formation. These halos are just massive enough to
efficiently form stars through atomic line cooling and to hold onto their gas
in the presence of supernovae winds and reionisation. These combined effects,
in particular that of the supernovae feedback, create a sharp transition: over
the mass range 3-10x10^7 Msun, the BBBs drop two orders ofmagnitude in stellar
mass. Below ~2x10^7 Msun, galaxies will be dark with almost no stars and no
gas. Above this scale is the smallest unit of galaxy formation: the BBB.
A small fraction (~100) of these gas rich BBBs fall in to a galaxy the size
of the Milky Way. Ten percent of these survive to become the observed LG dwarf
galaxies at the present epoch. Those in-falling halos on benign orbits which
keep them far away from the Milky Way or Andromeda manage to retain their gas
and slowly form stars - these become the smallest dwarf irregular galax ies;
those on more severe orbits lose their gas faster than they can form stars and
become the dwarf spheroidals. The remaining 90% of the BBBs will be accreted.
We show that this gives a metallicity and total stellar mass consistent with
the Milky Way old stellar halo (abridged).Comment: 15 pages, 7 figures, one figure added to match accepted version. Some
typos fixed. MNRAS in pres
The effect of neutrinos on the matter distribution as probed by the Intergalactic Medium
We present a suite of full hydrodynamical cosmological simulations that
quantitatively address the impact of neutrinos on the (mildly non-linear)
spatial distribution of matter and in particular on the neutral hydrogen
distribution in the Intergalactic Medium (IGM), which is responsible for the
intervening Lyman-alpha absorption in quasar spectra. The free-streaming of
neutrinos results in a (non-linear) scale-dependent suppression of power
spectrum of the total matter distribution at scales probed by Lyman-alpha
forest data which is larger than the linear theory prediction by about 25% and
strongly redshift dependent. By extracting a set of realistic mock quasar
spectra, we quantify the effect of neutrinos on the flux probability
distribution function and flux power spectrum. The differences in the matter
power spectra translate into a ~2.5% (5%) difference in the flux power spectrum
for neutrino masses with Sigma m_{\nu} = 0.3 eV (0.6 eV). This rather small
effect is difficult to detect from present Lyman-alpha forest data and nearly
perfectly degenerate with the overall amplitude of the matter power spectrum as
characterised by sigma_8. If the results of the numerical simulations are
normalized to have the same sigma_8 in the initial conditions, then neutrinos
produce a smaller suppression in the flux power of about 3% (5%) for Sigma
m_{\nu} = 0.6 eV (2
sigma C.L.), comparable to constraints obtained from the cosmic microwave
background data or other large scale structure probes.Comment: 38 pages, 21 figures. One section and references added. JCAP in pres
Detecting X-ray filaments in the low redshift Universe with XEUS and Constellation-X
We propose a possible way to detect baryons at low redshifts from the
analysis of X-ray absorption spectra of bright AGN pairs. A simple
semi-analytical model to simulate the spectra is presented. We model the
diffuse warm-hot intergalactic medium (WHIM) component, responsible for the
X-ray absorption, using inputs from high-resolution hydro-dynamical simulations
and analytical prescriptions. We show that the number of OVII absorbers per
unit redshift with column density larger than cm -
corresponding to an equivalent width of 1 km/s - which will be possibly
detectable by {\it XEUS}, is \magcir 30 per unit redshift. {\it
Constellation-X} will detect OVII absorptions per unit redshift with
an equivalent width of 10 km/s. Our results show that, in a CDM
Universe, the characteristic size of these absorbers at is
Mpc. The filamentary structure of WHIM can be probed by finding
coincident absorption lines in the spectra of background AGN pairs. We estimate
that at least 20 AGN pairs at separation \mincir 20 arcmin are needed to
detect this filamentary structure at a 3 level. Assuming observations
of distant sources using {\it XEUS} for exposure times of 500 ksec, we find
that the minimum source flux to probe the filamentary structure is erg cm s, in the 0.1-2.4 keV energy band. Thus,
most pairs of these extragalactic X-ray bright sources have already been
identified in the {\it ROSAT} All-Sky Survey. Re-observation of these objects
by future missions could be a powerful way to search for baryons in the low
redshift Universe.Comment: 18 pages, 10 Figures. Two figures added, Sections 2 and 3 expanded.
More optimistic results for Constellation-X. Accepted by MNRA
Compact strain-sensitive flexible photonic crystals for sensors
A promising fabrication route to produce absorbing flexible photonic crystals is presented, which exploits self-assembly during the shear processing of multi-shelled polymer spheres. When absorbing material is incorporated in the interstitial space surrounding high-refractive-index spheres, a dramatic enhancement in the transmission edge on the short-wavelength side of the band gap is observed. This effect originates from the shifting optical field spatial distribution as the incident wavelength is tuned around the band gap, and results in a contrast up to 100 times better than similar but nonabsorbing photonic crystals. An order-of-magnitude improvement in strain sensitivity is shown, suggesting the use of these thin films in photonic sensors
Possible evidence for an inverted temperature-density relation in the intergalactic medium from the flux distribution of the Lyman-alpha forest
We compare the improved measurement of the Lya forest flux probability
distribution at 1.7<z<3.2 presented by Kim et al. (2007) to a large set of
hydrodynamical simulations of the Lya forest with different cosmological
parameters and thermal histories. The simulations are in good agreement with
the observational data if the temperature-density relation for the low density
intergalactic medium (IGM), T=T_0 Delta^{gamma-1}, is either close to
isothermal or inverted (gamma<1). Our results suggest that the voids in the IGM
may be significantly hotter and the thermal state of the low density IGM may be
substantially more complex than is usually assumed at these redshifts. We
discuss radiative transfer effects which alter the spectral shape of ionising
radiation during the epoch of HeII reionisation as a possible physical
mechanism for achieving an inverted temperature-density relation at z~3.Comment: 16 pages, 6 figures, accepted for publication in MNRAS following
minor revision. The accepted version includes an expanded discussion of the
flux power spectru
How cold is cold dark matter? Small scales constraints from the flux power spectrum of the high-redshift Lyman-alpha forest
We present constraints on the mass of warm dark matter (WDM) particles
derived from the Lyman-alpha flux power spectrum of 55 high- resolution HIRES
spectra at 2.0 < z < 6.4. From the HIRES spectra, we obtain a lower limit of
mwdm > 1.2 keV 2 sigma if the WDM consists of early decoupled thermal relics
and mwdm > 5.6 keV (2 sigma) for sterile neutrinos. Adding the Sloan Digital
Sky Survey Lyman-alpha flux power spectrum, we get mwdm > 4 keV and mwdm > 28
keV (2 sigma) for thermal relics and sterile neutrinos. These results improve
previous constraints by a factor two.Comment: Some issues clarified (especially resolution related). Conclusions
unchanged. Accepted version by PR
Probing 3-D matter distribution at z~2 with QSO multiple lines of sight
We investigate the 3-D matter distribution at z~2 with high resolution (R ~
40000) spectra of QSO pairs and groups obtained with the UVES spectrograph at
ESO VLT. Our sample is unique for the number density of objects and the variety
of separations, between 0.5 and 7 proper Mpc. We compute the real space
cross-correlation function of the Lyman-alpha forest transmitted fluxes. There
is a significant clustering signal up to ~2 proper Mpc, which is still present
when absorption lines with high column density (log N > 13.8) are excluded.Comment: Poster paper presented at the IAU Colloquium #199 on "Probing
Galaxies through Quasar Absorption Lines" held in Shanghai, China from March
14th to 18th, 200
Strong MgII systems in quasar and gamma-ray burst spectra
The incidence of strong MgII systems in gamma-ray burst (GRB) spectra is a
few times higher than in quasar (QSO) spectra. We investigate several possible
explanations for this effect, including: dust obscuration bias, clustering of
the absorbers, different beam sizes of the sources, multiband magnification
bias of GRBs, association of the absorbers with the GRB event or the
circumburst environment. We find that: i) the incidence rate of MgII systems in
QSO spectra could be underestimated by a factor 1.3-2 due to dust obscuration;
ii) the equivalent-width distribution of the MgII absorbers along GRBs is
consistent with that observed along QSOs thus suggesting that the absorbers are
more extended than the beam sizes of the sources; iii) on average, GRB
afterglows showing more than one MgII system are a factor of 1.7 brighter than
the others, suggesting a lensing origin of the observed discrepancy; iv)
gravitational lensing (in different forms, from galaxy lensing to microlensing)
can bias high the counts of MgII systems along GRBs if the luminosity functions
of the prompt gamma-ray emission and of the optical afterglows have a mean
faint-end slope approaching -5/3 -- -2; v) some of the absorbers can be
associated with the circumburst environment or produced by supernova remnants
unrelated to the GRB event itself but lying in the same star-forming region.
With the possible exception of magnification bias, it is unlikely that one of
these effects on its own can fully account for the observed counts. However,
the combined action of some of them can substantially reduce the statistical
significance of the discrepancy.Comment: 8 pages, 3 figures, ApJ in press, comments welcom
Tomography of the intergalactic medium with Ly-alpha forests in close QSO pairs
We study the three-dimensional distribution of non virialised matter at z~2
using high resolution spectra of QSO pairs and simulated spectra drawn from
cosmological hydrodynamical simulations. We have collected the largest sample
of QSO pairs ever observed with UVES at the ESO-VLT, with angular separations
between ~1 and 14 arcmin. The observed correlation functions of the transmitted
flux in the HI Lyman alpha forest along and transverse to the lines of sight
are in good agreement implying that the distortions in redshift space due to
peculiar velocities are small. The clustering signal is significant up to
velocity separations of ~200 km/s, or about 3 h^{-1} comoving Mpc. The regions
at lower overdensity (rho/ < 6.5) are still clustered but on smaller
scales (Delta v < 100 km/s). The observed and simulated correlation functions
are compatible at the 3 sigma level. A better concordance is obtained when only
the low overdensity regions are selected for the analysis or when the effective
optical depth of the simulated spectra is increased artificially, suggesting a
deficiency of strong lines in the simulated spectra. We found that also a lower
value of the power-law index of the temperature-density relation for the Lyman
alpha forest gas improves the agreement between observed and simulated results.
If confirmed, this would be consistent with other observations favouring a late
HeII reionization epoch (at z~3). We remark the detection of a significant
clustering signal in the cross correlation coefficient at a transverse velocity
separation Delta v_{\perp} ~500 km/s whose origin needs further investigation.Comment: Accepted for publication in MNRAS, revised version matching the
accepted on
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