96 research outputs found
Chandra Detection of Intra-cluster X-ray sources in Virgo
We present a survey of X-ray point sources in the nearest and dynamically
young galaxy cluster, Virgo, using archival Chandra observations that sample
the vicinity of 80 early-type member galaxies. The X-ray source populations at
the outskirt of these galaxies are of particular interest. We detect a total of
1046 point sources (excluding galactic nuclei) out to a projected
galactocentric radius of 40 kpc and down to a limiting 0.5-8 keV
luminosity of . Based on the cumulative
spatial and flux distributions of these sources, we statistically identify
120 excess sources that are not associated with the main stellar content
of the individual galaxies, nor with the cosmic X-ray background. This excess
is significant at a 3.5 level, when Poisson error and cosmic variance
are taken into account. On the other hand, no significant excess sources are
found at the outskirt of a control sample of field galaxies, suggesting that at
least some fraction of the excess sources around the Virgo galaxies are truly
intra-cluster X-ray sources. Assisted with ground-based and HST optical imaging
of Virgo, we discuss the origins of these intra-cluster X-ray sources, in terms
of supernova-kicked low-mass X-ray binaries (LMXBs), globular clusters, LMXBs
associated with the diffuse intra-cluster light, stripped nucleated dwarf
galaxies and free-floating massive black holes.Comment: 29 pages, 8 figures. Accepted for publication in ApJ. Comments
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Improved Diffusion-based Image Colorization via Piggybacked Models
Image colorization has been attracting the research interests of the
community for decades. However, existing methods still struggle to provide
satisfactory colorized results given grayscale images due to a lack of
human-like global understanding of colors. Recently, large-scale Text-to-Image
(T2I) models have been exploited to transfer the semantic information from the
text prompts to the image domain, where text provides a global control for
semantic objects in the image. In this work, we introduce a colorization model
piggybacking on the existing powerful T2I diffusion model. Our key idea is to
exploit the color prior knowledge in the pre-trained T2I diffusion model for
realistic and diverse colorization. A diffusion guider is designed to
incorporate the pre-trained weights of the latent diffusion model to output a
latent color prior that conforms to the visual semantics of the grayscale
input. A lightness-aware VQVAE will then generate the colorized result with
pixel-perfect alignment to the given grayscale image. Our model can also
achieve conditional colorization with additional inputs (e.g. user hints and
texts). Extensive experiments show that our method achieves state-of-the-art
performance in terms of perceptual quality.Comment: project page: https://piggyback-color.github.io
Exploring the Cosmic Reionization Epoch in Frequency Space: An Improved Approach to Remove the Foreground in 21 cm Tomography
Aiming to correctly restore the redshifted 21 cm signals emitted by the
neutral hydrogen during the cosmic reionization processes, we re-examine the
separation approaches based on the quadratic polynomial fitting technique in
frequency space to investigate whether they works satisfactorily with complex
foreground, by quantitatively evaluate the quality of restored 21 cm signals in
terms of sample statistics. We construct the foreground model to characterize
both spatial and spectral substructures of the real sky, and use it to simulate
the observed radio spectra. By comparing between different separation
approaches through statistical analysis of restored 21 cm spectra and
corresponding power spectra, as well as their constraints on the mean halo bias
and average ionization fraction of the reionization processes, at
and the noise level of 60 mK we find that, although the complex
foreground can be well approximated with quadratic polynomial expansion, a
significant part of Mpc-scale components of the 21 cm signals (75% for Mpc scales and 34% for Mpc scales) is lost because
it tends to be mis-identified as part of the foreground when
single-narrow-segment separation approach is applied. The best restoration of
the 21 cm signals and the tightest determination of and can be
obtained with the three-narrow-segment fitting technique as proposed in this
paper. Similar results can be obtained at other redshifts.Comment: 33 pages, 14 figures. Accepted for publication in Ap
ELUCID - Exploring the Local Universe with reConstructed Initial Density field III: Constrained Simulation in the SDSS Volume
A method we developed recently for the reconstruction of the initial density
field in the nearby Universe is applied to the Sloan Digital Sky Survey Data
Release 7. A high-resolution N-body constrained simulation (CS) of the
reconstructed initial condition, with particles evolved in a 500 Mpc/h
box, is carried out and analyzed in terms of the statistical properties of the
final density field and its relation with the distribution of SDSS galaxies. We
find that the statistical properties of the cosmic web and the halo populations
are accurately reproduced in the CS. The galaxy density field is strongly
correlated with the CS density field, with a bias that depend on both galaxy
luminosity and color. Our further investigations show that the CS provides
robust quantities describing the environments within which the observed
galaxies and galaxy systems reside. Cosmic variance is greatly reduced in the
CS so that the statistical uncertainties can be controlled effectively even for
samples of small volumes.Comment: submitted to ApJ, 19 pages, 22 figures. Please download the
high-resolution version at http://staff.ustc.edu.cn/~whywang/paper
A Study of the Merger History of the Galaxy Group HCG 62 Based on X-Ray Observations and SPH Simulations
We choose the bright compact group HCG 62, which was found to exhibit both
excess X-ray emission and high Fe abundance to the southwest of its core, as an
example to study the impact of mergers on chemical enrichment in the intragroup
medium. We first reanalyze the high-quality Chandra and XMM-Newton archive data
to search for the evidence for additional SN II yields, which is expected as a
direct result of the possible merger-induced starburst. We reveal that, similar
to the Fe abundance, the Mg abundance also shows a high value in both the
innermost region and the southwest substructure, forming a high-abundance
plateau, meanwhile all the SN Ia and SN II yields show rather flat
distributions in in favor of an early enrichment. Then we carry
out a series of idealized numerical simulations to model the collision of two
initially isolated galaxy groups by using the TreePM-SPH GADGET-3 code. We find
that the observed X-ray emission and metal distributions, as well as the
relative positions of the two bright central galaxies with reference to the
X-ray peak, can be well reproduced in a major merger with a mass ratio of 3
when the merger-induced starburst is assumed. The `best-match' snapshot is
pinpointed after the third pericentric passage when the southwest substructure
is formed due to gas sloshing. By following the evolution of the simulated
merging system, we conclude that the effects of such a major merger on chemical
enrichment are mostly restricted within the core region when the final relaxed
state is reached.Comment: Accepted for publication in the Astrophysical Journa
Mapping the real space distributions of galaxies in SDSS DR7: I. Two Point Correlation Functions
Using a method to correct redshift space distortion (RSD) for individual
galaxies, we mapped the real space distributions of galaxies in the Sloan
Digital Sky Survey (SDSS) Data Release 7 (DR7). We use an ensemble of mock
catalogs to demonstrate the reliability of our method. Here as the first paper
in a series, we mainly focus on the two point correlation function (2PCF) of
galaxies. Overall the 2PCF measured in the reconstructed real space for
galaxies brighter than agrees with the direct
measurement to an accuracy better than the measurement error due to cosmic
variance, if the reconstruction uses the correct cosmology. Applying the method
to the SDSS DR7, we construct a real space version of the main galaxy catalog,
which contains 396,068 galaxies in the North Galactic Cap with redshifts in the
range . The Sloan Great Wall, the largest known
structure in the nearby Universe, is not as dominant an over-dense structure as
appears to be in redshift space. We measure the 2PCFs in reconstructed real
space for galaxies of different luminosities and colors. All of them show clear
deviations from single power-law forms, and reveal clear transitions from
1-halo to 2-halo terms. A comparison with the corresponding 2PCFs in redshift
space nicely demonstrates how RSDs boost the clustering power on large scales
(by about at scales ) and suppress it on
small scales (by about at a scale of ).Comment: 19 pages, 13 figure
The Next Generation Virgo Cluster Survey XVI. The Angular Momentum of Dwarf Early-Type Galaxies from Globular Cluster Satellites
We analyze the kinematics of six Virgo cluster dwarf early-type galaxies
(dEs) from their globular cluster (GC) systems. We present new Keck/DEIMOS
spectroscopy for three of them and reanalyze the data found in the literature
for the remaining three. We use two independent methods to estimate the
rotation amplitude (Vmax) and velocity dispersion (sigma_GC) of the GC systems
and evaluate their statistical significance by simulating non-rotating GC
systems with the same number of GC satellites and velocity uncertainties. Our
measured kinematics agree with the published values for the three galaxies from
the literature and, in all cases, some rotation is measured. However, our
simulations show that the null hypothesis of being non-rotating GC systems
cannot be ruled out. In the case of VCC1861, the measured Vmax and the
simulations indicate that it is not rotating. In the case of VCC1528, the null
hypothesis can be marginally ruled out, thus, it might be rotating although
further confirmation is needed. In our analysis, we find that, in general, the
measured Vmax tends to be overestimated and the measured sigma_GC tends to be
underestimated by amounts that depend on the intrinsic Vmax/sigma_GC, the
number of observed GCs (N_GC), and the velocity uncertainties. The bias is
negligible when N_GC>~20. In those cases where a large N_GC is not available,
it is imperative to obtain data with small velocity uncertainties. For
instance, errors of <2km/s lead to Vmax<10km/s for a system that is
intrinsically not rotating.Comment: ApJ in press. 20 pages, 17 figures, 5 table
Mapping the Real Space Distributions of Galaxies in SDSS DR7: II. Measuring the growth rate, clustering amplitude of matter and biases of galaxies at redshift
We extend the real-space mapping method developed in Shi et at. (2016) so
that it can be applied to flux-limited galaxy samples. We use an ensemble of
mock catalogs to demonstrate the reliability of this extension, showing that it
allows for an accurate recovery of the real-space correlation functions and
galaxy biases. We also demonstrate that, using an iterative method applied to
intermediate-scale clustering data, we can obtain an unbiased estimate of the
growth rate of structure , which is related to the clustering
amplitude of matter, to an accuracy of . Applying this method to the
Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7), we construct a real-space
galaxy catalog spanning the redshift range , which
contains 584,473 galaxies in the north Galactic cap (NGC). Using this data, we
infer \fss at a median redshift , which is consistent with the WMAP9
cosmology at the level. By combining this measurement with the
real-space clustering of galaxies and with galaxy-galaxy weak lensing
measurements for the same sets of galaxies, we are able to break the degeneracy
between , , and . From the SDSS DR7 data alone, we obtain the
following cosmological constraints at redshift :
, , and
, ,
, and for galaxies within
different absolute magnitude bins and , respectively
A Gemini/GMOS Study of Intermediate Luminosity Early-Type Virgo Cluster Galaxies. I. Globular Cluster and Stellar Kinematics
We present a kinematic analysis of the globular cluster systems and diffuse
stellar light of four intermediate luminosity (sub-) early-type
galaxies in the Virgo cluster based on Gemini/GMOS data. Our galaxy sample is
fainter () than most previous studies, nearly doubling the
number of galaxies in this magnitude range that now have GC kinematics. The
data for the diffuse light extends to , and the data for the globular
clusters reaches 8--. We find that the kinematics in these outer regions
are all different despite the fact that these four galaxies have similar
photometric properties, and are uniformly classified as "fast rotators" from
their stellar kinematics within . The globular cluster systems exhibit a
wide range of kinematic morphology. The rotation axis and amplitude can change
between the inner and outer regions, including a case of counter-rotation. This
difference shows the importance of wide-field kinematic studies, and shows that
stellar and GC kinematics can change significantly as one moves beyond the
inner regions of galaxies. Moreover, the kinematics of the globular cluster
systems can differ from that of the stars, suggesting that the formation of the
two populations are also distinct.Comment: 24 pages, 21 figures, 9 table, ApJ in pres
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