120 research outputs found
Staring at the Shadows of Archaic Galaxies: Damped Ly and Metal Absorbers toward a Young Weak-line Quasar
We characterize the Ly halo and absorption systems toward PSO
J083+11, a unique weak-line quasar, using Gemini/GNIRS,
Magellan/FIRE, and VLT/MUSE data. Strong absorptions by hydrogen and several
metal lines (e.g., CII, MgII, and OI) are discovered in the spectrum, which
indicates the presence of: (i) a proximate sub-damped Ly (sub-DLA)
system at and (ii) a MgII absorber at . To describe the
observed damping wing signal, we model the Ly absorption with a
combination of a sub-DLA with the neutral hydrogen column density of cm and absorption from the intergalactic
medium with a neutral fraction of around 10 percent. The sub-DLA toward PSO
J083+11 has an abundance ratio of [C/O] and metallicity of
[O/H] , similar to those of low-redshift metal-poor DLAs.
These measurements suggest that the sub-DLA might truncate PSO J083+11's
proximity zone size and complicate the quasar lifetime measurement. However,
this quasar shows no sign of a Ly halo in the MUSE datacube, where the
estimated limit of surface brightness is erg
s cm arcsec at aperture size of 1 arcsecond, or equivalent
to a Ly luminosity of erg s. This non-detection,
while being only weak independent evidence on its own, is at least consistent
with a young quasar scenario, as expected for a quasar with a short accretion
timescale.Comment: 22 pages, 12 figures, and 1 table. Accepted for publication in The
Astronomical Journal. We welcome the comments from the reader. Related paper:
arXiv:2009.0778
Evidence of major dry mergers at M* > 2 x 10^11 Msun from curvature in early-type galaxy scaling relations?
For early-type galaxies, the correlations between stellar mass and size,
velocity dispersion, surface brightness, color, axis ratio and color-gradient
all indicate that two mass scales, M* = 3 x 10^10 Msun and M* = 2 x 10^11 Msun,
are special. The smaller scale could mark the transition between wet and dry
mergers, or it could be related to the interplay between SN and AGN feedback,
although quantitative measures of this transition may be affected by
morphological contamination. At the more massive scale, mean axis ratios and
color gradients are maximal, and above it, the colors are redder, the sizes
larger and the velocity dispersions smaller than expected based on the scaling
at lower M*. In contrast, the color-sigma relation, and indeed, most scaling
relations with sigma, are not curved: they are well-described by a single power
law, or in some cases, are almost completely flat. When major dry mergers
change masses, sizes, axis ratios and color gradients, they are expected to
change the colors or velocity dispersions much less. Therefore, the fact that
scaling relations at sigma > 150 km/s show no features, whereas the size-M*,
b/a-M*, color-M* and color gradient-M* relations do, suggests that M* = 2 x
10^11 Msun is the scale above which major dry mergers dominate the assembly
histories of early-type galaxies.Comment: 5 pages, 3 figures. Accepted for publication in MNRA
Panchromatic Averaged Stellar Populations: PaasP
We study how the spectral fitting of galaxies, in terms of light fractions
derived in one spectral region translates into another region, by using results
from evolutionary synthesis models. In particular, we examine propagation
dependencies on Evolutionary Population Synthesis (EPS, {\sc grasil}, {\sc
galev}, Maraston and {\sc galaxev}) models, age, metallicity, and stellar
evolution tracks over the near-UV---near infrared (NUV---NIR, 3500\AA\ to
2.5\mc) spectral region. Our main results are: as expected, young (
400 Myr) stellar population fractions derived in the optical cannot be directly
compared to those derived in the NIR, and vice versa. In contrast, intermediate
to old age ( 500 Myr) fractions are similar over the whole spectral
region studied. The metallicity has a negligible effect on the propagation of
the stellar population fractions derived from NUV --- NIR. The same applies to
the different EPS models, but restricted to the range between 3800 \AA\ and
9000 \AA. However, a discrepancy between {\sc galev}/Maraston and {\sc
grasil}/{\sc galaxev} models occurs in the NIR. Also, the initial mass function
(IMF) is not important for the synthesis propagation. Compared to {\sc
starlight} synthesis results, our propagation predictions agree at 95%
confidence level in the optical, and 85% in the NIR. {\bf In summary,
spectral fitting} performed in a restricted spectral range should not be
directly propagated from the NIR to the UV/Optical, or vice versa. We provide
equations and an on-line form ({\bf Pa}nchromatic {\bf A}veraged {\bf S}tellar
{\bf P}opulation - \paasp) to be used for this purpose.Comment: 13 pages and 10 figures. Accepted by MNRA
Sizes and ages of SDSS ellipticals: Comparison with hierarchical galaxy formation models
In a sample of about 45,700 early-type galaxies extracted from SDSS, we find
that the shape, normalization, and dispersion around the mean size-stellar mass
relation is the same for young and old systems, provided the stellar mass is
greater than 3*10^10 Msun. This is difficult to reproduce in pure passive
evolution models, which generically predict older galaxies to be much more
compact than younger ones of the same stellar mass. However, this aspect of our
measurements is well reproduced by hierarchical models of galaxy formation.
Whereas the models predict more compact galaxies at high redshifts, subsequent
minor, dry mergers increase the sizes of the more massive objects, resulting in
a flat size-age relation at the present time. At lower masses, the models
predict that mergers are less frequent, so that the expected anti-correlation
between age and size is not completely erased. This is in good agreement with
our data: below 3*10^10 Msun, the effective radius R_e is a factor of ~2 lower
for older galaxies. These successes of the models are offset by the fact that
the predicted sizes have other serious problems, which we discuss.Comment: 13 pages, 9 Figures, 1 Table. Accepted by MNRA
The stellar populations of early-type galaxies -- II. The effects of environment and mass
The degree of influence that environment and mass have on the stellar
populations of early-type galaxies is uncertain. In this paper we present the
results of a spectroscopic analysis of the stellar populations of early-type
galaxies aimed at addressing this question. The sample of galaxies is drawn
from four clusters, with =0.04, and their surrounding structure extending to
~10R_{vir}. We find that the distributions of the absorption-line strengths and
the stellar population parameters age, metallicity and alpha-element abundance
ratio do not differ significantly between the clusters and their outskirts, but
the tight correlations found between these quantities and velocity dispersion
within the clusters are weaker in their outskirts. All three stellar population
parameters of cluster galaxies are positively correlated with velocity
dispersion. Galaxies in clusters form a homogeneous class of objects that have
similar distributions of line-strengths and stellar population parameters, and
follow similar scaling relations regardless of cluster richness or morphology.
We estimate the intrinsic scatter of the Gaussian distribution of metallicities
to be 0.3 dex, while that of the alpha-element abundance ratio is 0.07 dex. The
e-folding time of the exponential distribution of galaxy ages is estimated to
be 900 Myr. The intrinsic scatters of the metallicity and alpha-element
abundance ratio distributions can almost entirely be accounted for by the
correlations with velocity dispersion and the intrinsic scatter about these
relations. This implies that a galaxies mass plays the major role in
determining its stellar population.Comment: 20 pages, 12 figures, 5 tables, accepted by MNRA
Are disk galaxies the progenitors of giant ellipticals?
A popular formation scenario for giant elliptical galaxies proposes that they
might have formed from binary mergers of disk galaxies. Difficulties with the
scenario that emerged from earlier studies included providing the necessary
stellar mass and metallicity, maintaining the tight color-magnitude relation
and avoiding phase space limits. In this paper we revisit the issue and put
constraints on the binary disc merger scenario based on the stellar populations
of disc galaxies. We draw the following conclusions: Low redshift collisionless
or gaseous mergers of present day Milky Way like disc galaxies do not form
present day elliptical galaxies. Binary mergers of the progenitors of present
day Milky Way like disc galaxies can have evolved into intermediate mass
elliptical galaxies () if they have merged earlier than 3-4
Gyrs ago. Assuming that most present day disk galaxies formed in a similar way
to the Milky Way model presented here, more massive giant ellipticals in
general can not have formed from binary mergers of the progenitors of present
day disc galaxies. A major reason for these conclusions is that the mass in
metals of typical disk galaxy is approximately a factor of 4-8 smaller than the
mass in metals of a typical early-type galaxy and this ratio grows to larger
values with increasing redshift.Comment: accepted by ApJ, to appear in Jan 200
Building galaxies by accretion and in-situ star formation
We examine galaxy formation in a cosmological AMR simulation, which includes
two high resolution boxes, one centered on a 3 \times 10^14 M\odot cluster, and
one centered on a void. We examine the evolution of 611 massive (M\ast >
10^10M\odot) galaxies. We find that the fraction of the final stellar mass
which is accreted from other galaxies is between 15 and 40% and increases with
stellar mass. The accreted fraction does not depend strongly on environment at
a given stellar mass, but the galaxies in groups and cluster environments are
older and underwent mergers earlier than galaxies in lower density
environments. On average, the accreted stars are ~2.5 Gyrs older, and ~0.15 dex
more metal poor than the stars formed in-situ. Accreted stellar material
typically lies on the outskirts of galaxies; the average half-light radius of
the accreted stars is 2.6 times larger than that of the in-situ stars. This
leads to radial gradients in age and metallicity for massive galaxies, in
qualitative agreement with observations. Massive galaxies grow by mergers at a
rate of approximately 2.6% per Gyr. These mergers have a median (mass-weighted)
mass ratio less than 0.26 \pm 0.21, with an absolute lower limit of 0.20, for
galaxies with M\ast ~ 10^12 M\odot. This suggests that major mergers do not
dominate in the accretion history of massive galaxies. All of these results
agree qualitatively with results from SPH simulations by Oser et al. (2010,
2012).Comment: 18 pages, 12 figures, submitted to MNRA
Data and 2D scaling relations for galaxies in Abell 1689: a hint of size evolution at z~0.2
{abridged} We present imaging and spectroscopy of Abell 1689 (z=0.183) from
GEMINI/GMOS-N and HST/ACS. We measure integrated photometry from the GMOS g'
and r' images (for 531 galaxies) and surface photometry from the HST F625W
image (for 43 galaxies) as well as velocities and velocity dispersions from the
GMOS spectra (for 71 galaxies). We construct the Kormendy relation (KR),
Faber-Jackson relation (FJR) and colour-magnitude relation (CMR) for early-type
galaxies in Abell 1689 using this data and compare them to those of the Coma
cluster. We measure the intrinsic scatter of the CMR in Abell 1689 to be 0.054
\pm 0.004 mag which places degenerate constraints on the ratio of the assembly
timescale to the time available (beta) and the age of the population. Making
the assumption that galaxies in Abell 1689 will evolve into those of Coma over
an interval of 2.26 Gyr breaks this degeneracy and limits beta to be > 0.6 and
the age of the red sequence to be > 5.5 Gyr (formed at z > 0.55). Without
corrections for size evolution but accounting for magnitude cuts and selection
effects, the KR & FJR are inconsistent and disagree at the 2 sigma level
regarding the amount of luminosity evolution in the last 2.26 Gyr. However,
after correcting for size evolution the KR & FJR show similar changes in
luminosity (0.22 \pm 0.11 mag) that are consistent with the passive evolution
of the stellar populations from a single burst of star formation 10.2 \pm 3.3
Gyr ago (z = 1.8+inf-0.9). Thus the changes in the KR, FJR & CMR of Abell 1689
relative to Coma all agree and suggest old galaxy populations with little or no
synchronisation in the star formation histories. Furthermore, the weak evidence
for size evolution in the cluster environment in the last 2.26 Gyr places
interesting constraints on the possible mechanisms at work, favouring
harassment or secular processes over merger scenarios.Comment: Accepted for publication in MNRA
Young ages and other intriguing properties of massive compact galaxies in the Local Universe
We characterize the kinematics, morphology, stellar populations and star
formation histories of a sample of massive compact galaxies in the nearby
Universe, which might provide a closer look to the nature of their high
redshift (z > 1.0) massive counterparts. We find that nearby compact massive
objects show elongated morphologies and are fast rotators. New high-quality
long-slit spectra show that they have young mean luminosity-weighted ages (<
2Gyr) and solar metallicities or above ([Z/H]> 0.0). No significant stellar
population gradients are found. The analysis of their star formation histories
suggests that these objects have experienced recently enormous bursts which, in
some cases, represent unprecedented large fractions of their total stellar
mass. These galaxies seem to be truly unique, as they do not follow the
characteristic kinematical and stellar population patterns of present-day
massive ellipticals, spirals or even dwarfs.Comment: 16 pages, 11 figures; Accepted for publication in MNRA
Galaxy kinematics and mass estimates at z ∼ 1 from ionised gas and stars
We compare ionised gas and stellar kinematics of 16 star-forming galaxies (log (M⋆/M⊙) = 9.7 − 11.2, SFR =6 − 86M⊙/yr) at z ∼ 1 using near-infrared integral field spectroscopy (IFS) of Hα emission from the KMOS3D survey and optical slit spectroscopy of stellar absorption and gas emission from the LEGA-C survey. Hα is dynamically colder than stars, with higher disc rotation velocities (by ∼45 per cent) and lower disc velocity dispersions (by a factor ∼2). This is similar to trends observed in the local Universe. We find higher rotational support for Hα relative to [OII], potentially explaining systematic offsets in kinematic scaling relations found in the literature. Regarding dynamical mass measurements, for six galaxies with cumulative mass profiles from Jeans Anisotropic Multi-Gaussian Expansion (JAM) models the Hα dynamical mass models agree remarkably well out to ∼10 kpc for all but one galaxy (average ΔΜdyn(Re, F814W) < 0.1 dex). Simpler dynamical mass estimates based on integrated stellar velocity dispersion are less accurate (standard deviation 0.24 dex). Differences in dynamical mass estimates are larger, for example, for galaxies with stronger misalignments of the Hα kinematic major axis and the photometric position angle, highlighting the added value of IFS observations for dynamics studies. The good agreement between the JAM models and the dynamical models based on Hα kinematics at z ∼ 1 corroborates the validity of dynamical mass measurements from Hα IFS observations, which can be more easily obtained for higher redshift galaxies
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