743 research outputs found
The Mass Growth and Stellar Ages of Galaxies: Observations versus Simulations
Using observed stellar mass functions out to , we measure the main
progenitor stellar mass growth of descendant galaxies with masses of
at using an evolving
cumulative number density selection. From these mass growth histories, we are
able to measure the time at which half the total stellar mass of the descendant
galaxy was assembled, , which, in order of decreasing mass corresponds
to redshifts of and . We compare this to the
median light-weighted stellar age ( and
) of a sample of low redshift SDSS galaxies (from the literature) and
find the timescales are consistent with more massive galaxies forming a higher
fraction of their stars ex-situ compared to lower mass descendants. We find
that both and strongly correlate with mass which is in contrast
to what is found in the EAGLE hydrodynamical simulation which shows a flat
relationship between and . However, the semi-analytic model of
\citet{henriques2015} is consistent with the observations in both and
with , showing the most recent semi-analytic models are better
able to decouple the evolution of the baryons from the dark matter in
lower-mass galaxies.Comment: 6 pages, 3 figures, accepted for publication in ApJ
Multi-wavelength surveys: object detectability an nir luminosity function of galaxies
El estudio de la evolución de la FL se extendió al rango infrarrojo, en las bandas en reposo J y H, hasta redshift 3.5. Este fue el resutlado del análisis de tres catálogos públicos desde los proyectos MUSYC, FIRES y FIREWORKS. El análisis nos permitió medir por primera vez la FL en la banda H de reposo de galaxias de campo en el rango de redshift z ¿ [1.5,3.5]; al mismo tiempo, el mayor volumen nos permitió mejorar de manera significativa la medida del extremo brillante de la FL.
Desde el punto de vista espectroscópico, desarrollamos un método novedoso para el análisis de espectros con baja razón señal-ruido, parecido a los utilizados en el análisis de datos de rayos X. El método consiste en generar espectros bi-dimensionales a partir de espectros modelos, seleccionando el que mejor reproduce los datos observados a través de minimización de ¿2. El método se aplicó en concreto al caso del GRB090423, el GRB más lejano observado hasta el momento, demostrando que se puede extraer mas información de lo que generalmente se asume desde este tipo de datos
Using Cumulative Number Densities to Compare Galaxies across Cosmic Time
Comparing galaxies across redshifts at fixed cumulative number density is a
popular way to estimate the evolution of specific galaxy populations. This
method ignores scatter in mass accretion histories and galaxy-galaxy mergers,
which can lead to errors when comparing galaxies over large redshift ranges
(Delta z > 1). We use abundance matching in the LCDM paradigm to estimate the
median change in number density with redshift and provide a simple fit (+0.16
dex per unit Delta z) for progenitors of z = 0 galaxies. We find that galaxy
descendants do not evolve in the same way as galaxy progenitors, largely due to
scatter in mass accretion histories. We also provide estimates for the 1-sigma
range of number densities corresponding to galaxy progenitors and descendants.
Finally, we discuss some limits on number density comparisons, which arise due
to difficulties measuring physical quantities (e.g., stellar mass) consistently
across redshifts. A public tool to calculate number density evolution for
galaxies, as well as approximate halo masses, is available online.Comment: 5 pages, minor revisions to match ApJL accepted version. Code
available at: http://code.google.com/p/nd-redshif
VLT/X-Shooter Near-Infrared Spectroscopy and HST Imaging of Gravitationally-Lensed z~2 Compact Quiescent Galaxies
Quiescent massive galaxies at z~2 are thought to be the progenitors of
present-day massive ellipticals. Observations revealed them to be
extraordinarily compact. The determination of stellar ages, star formation
rates and dust properties via spectroscopic measurements has up to now only
been feasible for the most luminous and massive specimens (~3x M*). Here we
present a spectroscopic study of two near-infrared selected galaxies which are
close to the characteristic stellar mass M* (~0.9x M* and ~1.3x M*) and whose
observed brightness has been boosted by the gravitational lensing effect. We
measure the redshifts of the two galaxies to be z=1.71\pm0.02 and
z=2.15\pm0.01. By fitting stellar population synthesis models to their
spectro-photometric SEDs we determine their ages to be 2.4^{+0.8}_{-0.6} Gyr
and 1.7\pm0.3 Gyr, respectively, which implies that the two galaxies have
higher mass-to-light ratios than most quiescent z~2 galaxies in other studies.
We find no direct evidence for active star-formation or AGN activity in either
of the two galaxies, based on the non-detection of emission lines. Based on the
derived redshifts and stellar ages we estimate the formation redshifts to be
z=4.3^{+3.4}_{-1.2} and z=4.3^{+1.0}_{-0.6}, respectively. We use the increased
spatial resolution due to the gravitational lensing to derive constraints on
the morphology. Fitting Sersic profiles to the de-lensed images of the two
galaxies confirms their compactness, with one of them being spheroid-like, and
the other providing the first confirmation of a passive lenticular galaxy at a
spectroscopically derived redshift z~2.Comment: accepted for publication in Ap
The Evolution of Early-type Galaxies Selected by Their Spatial Clustering
Aims: We present a new method that uses luminosity or stellar mass functions
combined with clustering measurements to select samples of galaxies at
different redshifts likely to follow a progenitor-to-descendant relationship.
As the method uses clustering information, we refer to galaxy samples selected
this way as clustering-selected samples. We apply this method to infer the
number of mergers during the evolution of MUSYC early-type galaxies (ETGs) from
z~1 to the present-day. Methods: The method consists in using clustering
information to infer the typical dark-matter halo mass of the hosts of the
selected progenitor galaxies. Using LambdaCDM predictions, it is then possible
to follow these haloes to a later time where the sample of descendants will be
that with the clustering of these descendant haloes. Results: This technique
shows that ETGs at a given redshift evolve into brighter galaxies at lower
redshifts (considering rest-frame, passively evolved optical luminosities).
This indicates that the stellar mass of these galaxies increases with time and
that, in principle, a stellar mass selection at different redshifts does not
provide samples of galaxies in a progenitor-descendant relationship.
Conclusions: The comparison between high redshift ETGs and their likely
descendants at z=0 points to a higher number density for the progenitors by a
factor 5.5+-4.0, implying the need for mergers to decrease their number density
by today. Because the luminosity densities of progenitors and descendants are
consistent, our results show no need for significant star-formation in ETGs
since z=1, which indicates that the needed mergers are dry, i.e. gas free.Comment: 11 pages, 8 figures, accepted for publication in A&
On the formation time scale of massive cluster ellipticals based on deep near-IR spectroscopy at z~2
We present improved constraints on the formation time scale of massive
cluster galaxies based on rest-frame optical spectra of galaxies in a forming
cluster located at z=2.16. The spectra are obtained with MOIRCS on the Subaru
telescope with an integration time of ~7 hours. We achieve accurate redshift
measurements by fitting SEDs using the spectra and broad-band photometry
simultaneously, allowing us to identify probable cluster members. Clusters at
low redshifts are dominated by quiescent galaxies, but we find that quiescent
galaxies and star forming galaxies co-exist in this z=2 system. Interestingly,
the quiescent galaxies form a weak red sequence in the process of forming. By
stacking the spectra of star forming galaxies, we observe strong emission lines
such as [OII] and [OIII] and we obtain a tentative hint of AGN activities in
these galaxies. On the other hand, the stacked spectrum of the quiescent
galaxies reveals a clear 4000A break with a possible CaII H+K absorption
feature and strong emission lines such as [OII] are absent in the spectrum,
confirming the quiescent nature of these galaxies. We then perform detailed
spectral analyses of the stacked spectrum, which suggest that these massive
quiescent galaxies formed at redshifts between 3 and 4 on a time scale of <~0.5
Gyr. This short formation time scale is not reproduced in recent numerical
simulations. We discuss possible mechanisms for how these galaxies form 10^11
Msun stellar mass on a short time scale and become red and quiescent by z=2.Comment: 17 pages, 14 figures, accepted for publication in Ap
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