98 research outputs found
Inferring the photometric and size evolution of galaxies from image simulations
Current constraints on models of galaxy evolution rely on morphometric
catalogs extracted from multi-band photometric surveys. However, these catalogs
are altered by selection effects that are difficult to model, that correlate in
non trivial ways, and that can lead to contradictory predictions if not taken
into account carefully. To address this issue, we have developed a new approach
combining parametric Bayesian indirect likelihood (pBIL) techniques and
empirical modeling with realistic image simulations that reproduce a large
fraction of these selection effects. This allows us to perform a direct
comparison between observed and simulated images and to infer robust
constraints on model parameters. We use a semi-empirical forward model to
generate a distribution of mock galaxies from a set of physical parameters.
These galaxies are passed through an image simulator reproducing the
instrumental characteristics of any survey and are then extracted in the same
way as the observed data. The discrepancy between the simulated and observed
data is quantified, and minimized with a custom sampling process based on
adaptive Monte Carlo Markov Chain methods. Using synthetic data matching most
of the properties of a CFHTLS Deep field, we demonstrate the robustness and
internal consistency of our approach by inferring the parameters governing the
size and luminosity functions and their evolutions for different realistic
populations of galaxies. We also compare the results of our approach with those
obtained from the classical spectral energy distribution fitting and
photometric redshift approach.Our pipeline infers efficiently the luminosity
and size distribution and evolution parameters with a very limited number of
observables (3 photometric bands). When compared to SED fitting based on the
same set of observables, our method yields results that are more accurate and
free from systematic biases.Comment: 24 pages, 12 figures, accepted for publication in A&
Photometric redshifts from evolutionary synthesis with PEGASE: the code ZPEG and the z=0 age constraint
Photometric redshifts are estimated on the basis of template scenarios with
the help of the code ZPEG, an extension of the galaxy evolution model PEGASE.2
and available on the PEGASE web site. The spectral energy distribution (SED)
templates are computed for nine spectral types including starburst, irregular,
spiral and elliptical. Dust, extinction and metal effects are coherently taken
into account, depending on evolution scenarios. The sensitivity of results to
adding near-infrared colors and IGM absorption is analyzed. A comparison with
results of other models without evolution measures the evolution factor which
systematically increases the estimated photometric redshift values by > 0.2 for z > 1.5. Moreover we systematically check that the evolution
scenarios match observational standard templates of nearby galaxies, implying
an age constraint of the stellar population at z=0 for each type. The respect
of this constraint makes it possible to significantly improve the accuracy of
photometric redshifts by decreasing the well-known degeneracy problem. The
method is applied to the HDF-N sample. From fits on SED templates by a
-minimization procedure, not only is the photometric redshift derived
but also the corresponding spectral type and the formation redshift
when stars first formed. Early epochs of galaxy formation z > 5 are found from
this new method and results are compared to faint galaxy count interpretations.
The new tool is available at: http://www.iap.fr/pegaseComment: 10 pages, 10 postscript figures, 2 tables; accepted for publication
in Astronomy & Astrophysics; to compute redshifts see
http://www.iap.fr/pegase
Tracing Recent Star Formation of Red Early-type Galaxies out to 1
We study the mid-infrared (IR) excess emission of early-type galaxies (ETGs)
on the red-sequence at 1 using a spectroscopic sample of galaxies in the
fields of Great Observatories Origins Deep Survey (GOODS). In the mass-limited
sample of 1025 galaxies with 10 and
, we identify 696 24 m detected (above the
5) galaxies and find them to have a wide range of NUV- and -[12
m] colors despite their red optical colors. Even in the sample of
very massive ETGs on the red sequence with 10
, more than 18% show excess emission over the photospheric emission
in the mid-IR. The combination with the results of red ETGs in the local
universe suggests that the recent star formation is not rare among quiescent,
red ETGs at least out to if the mid-IR excess emission results from
intermediate-age stars or/and from low-level ongoing star formation. Our
colorcolor diagram including near-UV and mid-IR emissions are efficient not
only for identifying ETGs with recent star formation, but also for
distinguishing quiescent galaxies from dusty star-forming galaxies.Comment: 25 pages, 9 figures, accepted for publication in Ap
Analysis of stellar populations with large empirical libraries at high spectral resolution
The stellar population models dramatically progressed with the arrival of
large and complete libraries, ELODIE, CFLIB (=Indo-US) and MILES at a
relatively high resolution. We show that the quality of the fits is not anymore
limited by the size of the stellar libraries in a large range of ages (0.1 to
10 Gyrs) and metallicities (-2 to +0.4 dex). The main limitations of the
empirical stellar libraries are (i) the coverage of the parameters space (lack
of hot stars of low metallicity), (ii) the precision and homogeneity of the
atmospheric parameters and (iii) the non-resolution of individual element
abundances (in particular [/Fe]). Detailed abundance measurements in
the large libraries, and usage of theoretical libraries are probably the next
steps, and we show that a combination between an empirical (ELODIE) and a
theoretical library (Coelho et al. 2005) immediately improves the modeling of
(-enhanced) globular clusters.Comment: 4 pages; proceedings of IAU Symposium No. 241, "Stellar Populations
as Building Blocks of Galaxies", editors A. Vazdekis and R. Peletie
The spin of late-type galaxies at redshifts z < 1.2
We study the evolution of the galactic spin using data of high redshift
galaxies in the fields of the Great Observatories Origins Deep Survey (GOODS).
Through simple dynamical considerations we estimate the spin for the disc
galaxies in our sample and find that its distribution is consistent with that
found for nearby galaxies. Defining a dimensionless angular momentum parameter
for the disc component of the galaxies (), we do not find signs of
evolution in the redshift range . We find that the mass
and environmental dependence of the spin of our high redshift galaxies are
similar to that of low- galaxies; showing a strong dependence on mass, in
the sense that low-mass systems present higher values than
high-mass galaxies, with no significant dependence on the environmental
density. These results lead us to conclude that, although individual disc
galaxies might occasionally suffer from strong evolution, they evolve in such a
way that the overall spin distribution of the galactic population remains
constant from to the present epoch.Comment: 8 pages, 4 figures, 1 Table. Accepted for publication in MNRA
Aprendizaje cooperativo: una nueva metodología motivadora para el alumno
International audienc
Swirling around filaments: are large-scale structure vortices spinning up dark halos?
The kinematic analysis of dark matter and hydrodynamical simulations suggests
that the vorticity in large-scale structure is mostly confined to, and
predominantly aligned with their filaments, with an excess of probability of 20
per cent to have the angle between vorticity and filaments direction lower than
60 degrees relative to random orientations. The cross sections of these
filaments are typically partitioned into four quadrants with opposite vorticity
sign, arising from multiple flows, originating from neighbouring walls. The
spins of halos embedded within these filaments are consistently aligned with
this vorticity for any halo mass, with a stronger alignment for the most
massive structures up to an excess of probability of 165 per cent. On large
scales, adiabatic/cooling hydrodynamical simulations display the same vorticity
in the gas as in the dark matter. The global geometry of the flow within the
cosmic web is therefore qualitatively consistent with a spin acquisition for
smaller halos induced by this large-scale coherence, as argued in Codis et al.
(2012). In effect, secondary anisotropic infall (originating from the
vortex-rich filament within which these lower-mass halos form) dominates the
angular momentum budget of these halos. The transition mass from alignment to
orthogonality is related to the size of a given multi-flow region with a given
polarity. This transition may be reconciled with the standard tidal torque
theory if the latter is augmented so as to account for the larger scale
anisotropic environment of walls and filaments.Comment: 17 pages, 19 figures, 3 tables. accepted for publication in MNRA
Modelling and interpreting optical spectra of galaxies at R=10000
One way to extract more information from the integrated light of galaxies is
to improve the spectral resolution at which observations and analysis are
carried out. The population synthesis code currently providing the highest
spectral resolution is Pegase-HR, which was made available by D. Le Borgne et
al. in 2004. Based on an empirical stellar library, it provides synthetic
spectra between 4000 and 6800 A at lambda/d(lambda)=10000 for any star
formation history, with or without chemical evolution. Such a resolution is
particularly useful for the study of low mass galaxies, massive star clusters,
or other galaxy regions with low internal velocity dispersions.
After a summary of the main features of Pegase-HR and comparisons with other
population synthesis codes, this paper focuses on the inversion of optical
galaxy spectra. We explore the limits of what information can or can not be
recovered, based on theoretical principles and extensive simulations. First
applications to extragalactic objects are shown.Comment: Inv. talk in "The Spectral Energy Distribution of Gas-Rich Galaxies:
Confronting Models with Data", Heidelberg, 4-8 Oct. 2004, eds. C.C. Popescu
and R.J. Tuffs, AIP Conf. Ser., in pres
Evolved Galaxies at z > 1.5 from the Gemini Deep Deep Survey: The Formation Epoch of Massive Stellar Systems
We present spectroscopic evidence from the Gemini Deep Deep Survey (GDDS) for
a significant population of color-selected red galaxies at 1.3 < z < 2.2 whose
integrated light is dominated by evolved stars. Unlike radio-selected objects,
the z > 1.5 old galaxies have a sky density > 0.1 per sq. arcmin. Conservative
age estimates for 20 galaxies with z > 1.3; = 1.49, give a median age of
1.2 Gyr and = 2.4. One quarter of the galaxies have inferred z_f > 4.
Models restricted to abundances less than or equal to solar give median ages
and z_f of 2.3 Gyr and 3.3, respectively. These galaxies are among the most
massive and contribute approximately 50% of the stellar mass density at 1 < z <
2. The derived ages and most probable star formation histories suggest a high
star-formation-rate (300-500 solar masses per year) phase in the progenitor
population. We argue that most of the red galaxies are not descendants of the
typical z=3 Lyman break galaxies. Galaxies associated with luminous sub-mm
sources have the requisite star formation rates to be the progenitor
population. Our results point toward early and rapid formation for a
significant fraction of present day massive galaxies.Comment: 12 pages, 2 figures, 1 table, Accepted for publication, ApJ Letter
Cosmic Star Formation History and its Dependence on Galaxy Stellar Mass
We examine the cosmic star formation rate (SFR) and its dependence on galaxy
stellar mass over the redshift range 0.8 < z < 2 using data from the Gemini
Deep Deep Survey (GDDS). The SFR in the most massive galaxies (M > 10^{10.8}
M_sun) was six times higher at z = 2 than it is today. It drops steeply from z
= 2, reaching the present day value at z ~ 1. In contrast, the SFR density of
intermediate mass galaxies (10^{10.2} < M < 10^{10.8} M_sun) declines more
slowly and may peak or plateau at z ~ 1.5. We use the characteristic growth
time t_SFR = rho_M / rho_SFR to provide evidence of an associated transition in
massive galaxies from a burst to a quiescent star formation mode at z ~ 2.
Intermediate mass systems transit from burst to quiescent mode at z ~ 1, while
the lowest mass objects undergo bursts throughout our redshift range. Our
results show unambiguously that the formation era for galaxies was extended and
proceeded from high to low mass systems. The most massive galaxies formed most
of their stars in the first ~3 Gyr of cosmic history. Intermediate mass objects
continued to form their dominant stellar mass for an additional ~2 Gyr, while
the lowest mass systems have been forming over the whole cosmic epoch spanned
by the GDDS. This view of galaxy formation clearly supports `downsizing' in the
SFR where the most massive galaxies form first and galaxy formation proceeds
from larger to smaller mass scales.Comment: Accepted for publication in ApJ
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