155 research outputs found
The non-universality of the low-mass end of the IMF is robust against the choice of SSP model
We perform a direct comparison of two state-of-the art single stellar
population (SSP) models that have been used to demonstrate the non-universality
of the low-mass end of the Initial Mass Function (IMF) slope. The two public
versions of the SSP models are restricted to either solar abundance patterns or
solar metallicity, too restrictive if one aims to disentangle elemental
enhancements, metallicity changes and IMF variations in massive early-type
galaxies (ETGs) with star formation histories different from the solar
neighborhood. We define response functions (to metallicity and
\alpha-abundance) to extend the parameter space of each set of models. We
compare these extended models with a sample of Sloan Digital Sky Survey (SDSS)
ETGs spectra with varying velocity dispersions. We measure equivalent widths of
optical IMF-sensitive stellar features to examine the effect of the underlying
model assumptions and ingredients, such as stellar libraries or isochrones, on
the inference of the IMF slope down to ~0.1 solar masses. We demonstrate that
the steepening of the low-mass end of the Initial Mass Function (IMF) based on
a non-degenerate set of spectroscopic optical indicators is robust against the
choice of the stellar population model. Although the models agree in a relative
sense (i.e. both imply more bottom-heavy IMFs for more massive systems), we
find non-negligible differences on the absolute values of the IMF slope
inferred at each velocity dispersion by using the two different models. In
particular, we find large inconsistency in the quantitative predictions of IMF
slope variations and abundance patterns when sodium lines are used. We
investigate the possible reasons for these inconsistencies.Comment: 16 pages, 9 figures, 2 tables, accepted for publication on Ap
VEXAS:VISTA EXtension to Auxiliary Surveys Data Release 1. The southern Galactic hemisphere
We present the first public data release of the VISTA EXtension to Auxiliary
Surveys (VEXAS), comprising of 9 cross-matched multi-wavelength photometric
catalogs where each object has a match in at least two surveys. We aim at a
spatial coverage as uniform as possible in the multi-wavelength sky, with the
purpose of providing the astronomical community with reference magnitudes and
colours for various scientific uses, including: object classification (e.g.
quasars, galaxies, and stars; high-z galaxies, white dwarfs, etc.); photometric
redshifts of large galaxy samples; searches of exotic objects such as, for
example, extremely red objects and lensed quasars. We have cross-matched the
wide-field VISTA catalogs (the VISTA Hemisphere Survey and the VISTA Kilo
Degree Infrared Galaxy Survey) with the AllWISE mid-infrared Survey, requiring
that a match exists within 10 arcsec. We have further matched this table with
X-Ray and radio data (ROSAT, XMM, SUMSS). We also performed a second
cross-match between VISTA and AllWISE, with a smaller matching radius (3"),
including WISE magnitudes. We have then cross-matched this resulting table
( objects) with three photometric wide-sky optical deep
surveys (DES, SkyMapper, PanSTARRS). We finally include matches to objects with
spectroscopic follow-up by the SDSS and 6dFGS. To demonstrate the power of
all-sky multi-wavelength cross-match tables, we show two examples of scientific
applications of VEXAS, in particular using the publicly released tables to
discover strong gravitational lenses (beyond the reach of previous searches),
and to build a statistically large sample of extremely red objects. The VEXAS
catalog is currently the widest and deepest, public, optical-to-IR photometric
and spectroscopic database in the Southern Hemisphere.Comment: 13 pages, 10 figures, accepted for publication on Astronomy and
Astrophysic
VEXAS: VISTA EXtension to Auxiliary Surveys -- Data Release 2: Machine-learning based classification of sources in the Southern Hemisphere
We present the second public data release (DR) of the VISTA EXtension to
Auxiliary Surveys (VEXAS), where we classify objects into stars, galaxies and
quasars based on an ensemble of machine learning algorithms. The aim of VEXAS
is to build the widest multi-wavelength catalogue, providing reference
magnitudes, colours and morphological information for a large number of
scientific uses. We apply an ensemble of 32 different machine learning models,
based on three different algorithms and on different magnitude sets, training
samples and classification problems on the three VEXAS DR1 optical+infrared
(IR) tables. The tables were created in DR1 cross-matching VISTA near-IR data
with WISE far-IR data and with optical magnitudes from the Dark Energy Survey
(VEXAS-DESW), the Sky Mapper Survey (VEXAS-SMW), and the PanSTARRS (VEXAS-PSW).
We assemble a large table of spectroscopically confirmed objects (415 628
unique objects), based on the combination of 6 different spectroscopic surveys
that we use for training. We develop feature imputation to classify also
objects for which magnitudes in one or more bands are missing. We classify in
total ~90 million objects in the Southern Hemisphere. Among these,~62.9M
(~52.6M) are classified as 'high confidence' ('secure') stars, ~920k (~750k) as
'high confidence' ('secure') quasars and ~34.8M (~34.1M) as 'high confidence'
('secure') galaxies, with probabilities (). The density of high-confidence extragalactic objects varies
strongly with the survey depth: at , there are
111/deg quasars in the VEXAS-DESW footprint and 103/deg in the
VEXAS-PSW footprint, while only 10.7/deg in the VEXAS-SM footprint.
Improved depth in the midIR and coverage in the optical and nearIR are needed
for the SM footprint that is not already covered by DESW and PSW.Comment: 25 pages, 18 figures, 8 tables. Accepted for publication on A&A. The
VEXAS tables are publicly available through the ESO Phase 3 here:
https://archive.eso.org/scienceportal/home?data_collection=VEXAS. The DR2
tables update the DR1 with the addition of imputed magnitudes and membership
probabilities to each of the three classe
KiDS0239-3211: A new gravitational quadruple lens candidate
We report the discovery of a candidate to quadrupole gravitationally lensed
system KiDS0239-3211 based on the public data release 3 of the KiDS survey and
machine learning techniques
Catalog of quasars from the Kilo-Degree Survey Data Release 3
We present a catalog of quasars selected from broad-band photometric ugri
data of the Kilo-Degree Survey Data Release 3 (KiDS DR3). The QSOs are
identified by the random forest (RF) supervised machine learning model, trained
on SDSS DR14 spectroscopic data. We first cleaned the input KiDS data from
entries with excessively noisy, missing or otherwise problematic measurements.
Applying a feature importance analysis, we then tune the algorithm and identify
in the KiDS multiband catalog the 17 most useful features for the
classification, namely magnitudes, colors, magnitude ratios, and the stellarity
index. We used the t-SNE algorithm to map the multi-dimensional photometric
data onto 2D planes and compare the coverage of the training and inference
sets. We limited the inference set to r<22 to avoid extrapolation beyond the
feature space covered by training, as the SDSS spectroscopic sample is
considerably shallower than KiDS. This gives 3.4 million objects in the final
inference sample, from which the random forest identified 190,000 quasar
candidates. Accuracy of 97%, purity of 91%, and completeness of 87%, as derived
from a test set extracted from SDSS and not used in the training, are confirmed
by comparison with external spectroscopic and photometric QSO catalogs
overlapping with the KiDS footprint. The robustness of our results is
strengthened by number counts of the quasar candidates in the r band, as well
as by their mid-infrared colors available from WISE. An analysis of parallaxes
and proper motions of our QSO candidates found also in Gaia DR2 suggests that a
probability cut of p(QSO)>0.8 is optimal for purity, whereas p(QSO)>0.7 is
preferable for better completeness. Our study presents the first comprehensive
quasar selection from deep high-quality KiDS data and will serve as the basis
for versatile studies of the QSO population detected by this survey.Comment: Data available from the KiDS website at
http://kids.strw.leidenuniv.nl/DR3/quasarcatalog.php and the source code from
https://github.com/snakoneczny/kids-quasar
The X-shooter Lens Survey - II. Sample presentation and spatially resolved kinematics
We present the X-shooter Lens Survey (XLENS) data. The main goal of XLENS is
to disentangle the stellar and dark matter content of massive early-type
galaxies (ETGs), through combined strong gravitational lensing, dynamics and
spectroscopic stellar population studies. The sample consists of 11 lens
galaxies covering the redshift range from to and having stellar
velocity dispersions between and . All
galaxies have multi-band, high-quality HST imaging. We have obtained long-slit
spectra of the lens galaxies with X-shooter on the VLT. We are able to
disentangle the dark and luminous mass components by combining lensing and
extended kinematics data-sets, and we are also able to precisely constrain
stellar mass-to-light ratios and infer the value of the low-mass cut-off of the
IMF, by adding spectroscopic stellar population information. Our goal is to
correlate these IMF parameters with ETG masses and investigate the relation
between baryonic and non-baryonic matter during the mass assembly and structure
formation processes. In this paper we provide an overview of the survey,
highlighting its scientific motivations, main goals and techniques. We present
the current sample, briefly describing the data reduction and analysis process,
and we present the first results on spatially resolved kinematics.Comment: Accepted for publication in MNRA
TDCOSMO. XI. New lensing galaxy redshift and velocity dispersion measurements from Keck spectroscopy of eight lensed quasar systems
We have measured the redshifts and single-aperture velocity dispersions of
eight lens galaxies using the data collected by the Echellette Spectrograph and
Imager (ESI) and Low Resolution Imaging Spectrometer (LRIS) at W.M. Keck
observatory on different observing nights spread over three years (2018-2020).
These results, combined with other ancillary data, such as high-resolution
images of the lens systems, and time delays, are necessary to increase the
sample size of the quasar-galaxy lens systems for which the Hubble constant can
be measured, using the time-delay strong lensing method, hence increasing the
precision of its inference. Typically, the 2D spectra of the quasar-galaxy lens
systems get spatially blended due to seeing by ground-based observations. As a
result, the extracted lensing galaxy (deflector) spectra become significantly
contaminated by quasar light, which affects the ability to extract meaningful
information about the deflector. To account for spatial blending and extract
less contaminated and higher signal-to-noise ratio (S/N) 1D spectra of the
deflectors, a forward modeling method has been implemented. From the extracted
spectra, we have measured redshifts using prominent absorption lines and single
aperture velocity dispersions using the penalized pixel fitting code pPXF. In
this paper, we report the redshifts and single aperture velocity dispersions of
eight lens galaxies - J0147+4630, B0445+123, B0631+519, J0659+1629, J0818-2613,
J0924+0219, J1433+6007, and J1817+2729. Among these systems, six do not have
previously measured velocity dispersions; for the other two, our measurements
are consistent with previously reported values. Additionally, we have measured
the previously unknown redshifts of the deflectors in J0818-2613 and J1817+2729
to be and , respectively.Comment: 13 pages, 6 figures, 3 tables; accepted in A&
What does (not) drive the variation of the low-mass end of the stellar initial mass function of early-type galaxies
The stellar initial mass function (IMF) seems to be variable and not
universal, as argued in the literature in the last three decades. Several
relations among the low-mass end of the IMF slope and other stellar population,
photometric or kinematic parameters of massive early-type galaxies (ETGs) have
been proposed, but a consolidated agreement on a factual cause of the observed
variations has not been reached yet. We investigate the relations between the
IMF and other stellar population parameters in NGC 3311, the central galaxy of
the Hydra I cluster. NGC 3311 is characterized by old and metal-rich stars,
like other massive ETGs, but has unusual increasing stellar velocity dispersion
and [Fe] profiles. We use spatially resolved MUSE observations to
obtain stellar population properties using Bayesian full-spectrum fitting in
the central part of NGC 3311 to compare the IMF slope against other stellar
parameters with the goal of assessing their relations/dependencies. For NGC
3311, we unambiguously invalidate the previously observed direct correlation
between the IMF slope and the local stellar velocity dispersion, confirming
some doubts already raised in the literature. This relation may arise as a
spatial coincidence only, between the region with the largest stellar velocity
dispersion, with that where the oldest, population is found
and dominates. We also show robust evidence that the proposed IMF-metallicity
relation is contaminated by the degeneracy between these two parameters. The
tightest correlations we found are those between stellar age and IMF and
between galactocentric radius and IMF. The variation of the IMF is not due to
kinematical, dynamical, or global properties in NGC 3311. We speculate that IMF
might be dwarf-dominated in the "red-nuggets" formed at high redshifts that
ended up being the central cores of today's giant ellipticals. [Abridged]Comment: 5 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
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