54 research outputs found
Soft band X/K luminosity ratios for gas-poor early-type galaxies
We aim to place upper limits on the combined X-ray emission from the
population of steady nuclear-burning white dwarfs in galaxies. In the framework
of the single-degenerate scenario these systems are believed to be likely
progenitors of Type Ia supernovae. From the Chandra archive, we selected normal
early-type galaxies with the point source detection sensitivity better than
10^37 erg/s to minimize the contribution of unresolved low-mass X-ray binaries.
The galaxies, contaminated by emission from ionized ISM, were identified based
on the analysis of radial surface brightness profiles and energy spectra. The
sample was complemented by the bulge of M31 and the data for the solar
neighborhood. To cover a broad range of ages, we also included NGC3377 and
NGC3585. Our final sample includes eight gas-poor galaxies for which we
determine L_X/L_K ratios in the 0.3-0.7 keV energy band. In computing the L_X
we included both unresolved emission and soft resolved sources with the color
temperature of kT_bb <= 200 eV. We find that the X/K luminosity ratios are in
the range of (1.7-3.2) x 10^27 erg/s/L_K,sun. The data show no obvious trends
with mass, age, or metallicity of the host galaxy, although a weak
anti-correlation with the Galactic NH appears to exist. It is much flatter than
predicted for a blackbody emission spectrum with temperature of ~50-75 eV,
suggesting that sources with such soft spectra contribute significantly less
than a half to the observed X/K ratios. However, the correlation of the X/K
ratios with NH has a significant scatter and in the strict statistical sense
cannot be adequately described by a superposition of a power law and a
blackbody components with reasonable parameters, thus precluding quantitative
constraints on the contribution from soft sources. (abbr.)Comment: 9 pages, 5 figures, 4 tables, accepted for publication in Astronomy
and Astrophysic
Soft band X/K luminosity ratios in late-type galaxies and constraints on the population of supersoft X-ray sources
We study X-ray to K-band luminosity ratios (L_X/L_K) of late-type galaxies in
the 0.3-0.7 keV energy range. From the Chandra archive, we selected nine spiral
and three irregular galaxies with point source detection sensitivity better
than 5 x 10^36 erg/s in order to minimize the contribution of unresolved X-ray
binaries. In late-type galaxies cold gas and dust may cause significant
interstellar absorption, therefore we also demanded the existence of publicly
available HI maps. The obtained L_X/L_K ratios vary between (5.4-68) x 10^27
erg/s/L_K,sun exceeding by factor of 2-20 the values obtained for gas-poor
early-type galaxies. Based on these results we constrain the role of supersoft
X-ray sources as progenitors of type Ia supernovae (SNe Ia). For majority of
galaxies the upper limits range from ~3% to ~15% of the SN Ia frequency
inferred from K-band luminosity, but for a few of them no meaningful
constraints can be placed. On a more detailed level, we study individual
structural components of spiral galaxies: bulge and disk, and, for grand design
spiral galaxies, arm and interarm regions.Comment: 10 pages, 2 tables, 6 figures, accepted for publication in MNRAS,
minor change
2MASS Galaxies in the Fornax Cluster Spectroscopic Survey
The Fornax Cluster Spectroscopic Survey (FCSS) is an all-object survey of a
region around the Fornax Cluster of galaxies undertaken using the 2dF
multi-object spectrograph on the Anglo-Australian Telescope. Its aim was to
obtain spectra for a complete sample of all objects with 16.5 < b_j < 19.7
irrespective of their morphology (i.e. including `stars', `galaxies' and
`merged' images). We explore the extent to which (nearby) cluster galaxies are
present in 2MASS. We consider the reasons for the omission of 2MASS galaxies
from the FCSS and vice versa. We consider the intersection (2.9 square degrees
on the sky) of our data set with the infra-red 2 Micron All-Sky Survey (2MASS),
using both the 2MASS Extended Source Catalogue (XSC) and the Point Source
Catalogue (PSC). We match all the XSC objects to FCSS counterparts by position
and also extract a sample of galaxies, selected by their FCSS redshifts, from
the PSC. We confirm that all 114 XSC objects in the overlap sample are
galaxies, on the basis of their FCSS velocities. A total of 23 Fornax Cluster
galaxies appear in the matched data, while, as expected, the remainder of the
sample lie at redshifts out to z = 0.2 (the spectra show that 61% are early
type galaxies, 18% are intermediate types and 21% are strongly star
forming).The PSC sample turns out to contain twice as many galaxies as does the
XSC. However, only one of these 225 galaxies is a (dwarf) cluster member. On
the other hand, galaxies which are unresolved in the 2MASS data (though almost
all are resolved in the optical) amount to 71% of the non-cluster galaxies with
2MASS detections and have redshifts out to z=0.32.Comment: 5 pages, accepted by A&A, resubmitted due to missing reference
Universal X-ray emissivity of the stellar population in early-type galaxies: unresolved X-ray sources in NGC 3379
We use deep Chandra observations to measure the emissivity of the unresolved
X-ray emission in the elliptical galaxy NGC 3379. After elimination of bright,
low-mass X-ray binaries with luminosities >10^{36 erg/sec, we find that the
remaining unresolved X-ray emission is characterized by an emissivity per unit
stellar mass L_x/M_stars ~8.2x10^{27} erg/s/M_sun in the 0.5-2 keV energy band.
This value is in good agreement with those previousely determined for the dwarf
elliptical galaxy M32, the bulge of the spiral galaxy M31 and the Milky Way, as
well as with the integrated X-ray emissivity of cataclysmic variables and
coronally active binaries in the Solar neighborhood. This strongly suggests
that i) the bulk of the unresolved X-ray emission in NGC 3379 is produced by
its old stellar population and ii) the old stellar populations in all galaxies
can be characterized by a universal value of X-ray emissivity per unit stellar
mass or per unit K band luminosity.Comment: 9 pages, 7 figures, Accepted for publication in A&
Unresolved X-ray emission in M31 and constraints on progenitors of Classical Novae
We investigate unresolved X-ray emission from M31 based on an extensive set
of archival XMM-Newton and Chandra data. We show that extended emission, found
previously in the bulge and thought to be associated with a large number of
faint compact sources, extends to the disk of the galaxy with similar X-ray to
K-band luminosity ratio. We also detect excess X-ray emission associated with
the 10-kpc star-forming ring. The L_X/SFR ratio in the 0.5-2 keV band ranges
from zero to ~1.8 x 10^38 (erg/s)/(M_sun/yr), excluding the regions near the
minor axis of the galaxy where it is ~1.5-2 times higher. The latter is likely
associated with warm ionized gas of the galactic wind rather than with the
star-forming ring itself.
Based on this data, we constrain the nature of Classical Nova (CN)
progenitors. We use the fact that hydrogen-rich material, required to trigger
the explosion, accumulates on the white dwarf surface via accretion. Depending
on the type of the system, the energy of accretion may be radiated at X-ray
energies, thus contributing to the unresolved X-ray emission. Based on the CN
rate in the bulge of M31 and its X-ray surface brightness, we show that no more
than ~10 per cent of CNe can be produced in magnetic cataclysmic variables, the
upper limit being ~3 per cent for parameters typical for CN progenitors. In
dwarf novae, >~90-95 per cent of the material must be accreted during
outbursts, when the emission spectrum is soft, and only a small fraction in
quiescent periods, characterized by rather hard spectra.Comment: 11 pages, 10 figures, 4 tables, accepted for publication in MNRA
Dependence of the LMXB population on stellar age
We investigate the dependence of the low-mass X-ray binary (LMXB) population
in early-type galaxies on stellar age, by selecting 20 massive nearby
early-type galaxies from the Chandra archive occupying a relatively narrow
range of masses and spanning a broad range of ages, from 1.6 Gyr to more than
10 Gyrs, with the median value of 6 Gyrs. With the ~ 2000 X-ray point sources
detected in total, we correlated the specific number of LMXBs in each galaxy
with its stellar age and globular cluster (GC) content. We found a correlation
between the LMXB population and stellar age: older galaxies tend to possess
about ~50% more LMXBs (per unit stellar mass) than the younger ones. The
interpretation of this dependence is complicated by large scatter and a rather
strong correlation between stellar age and GC content of galaxies in our
sample. We present evidence suggesting that the more important factor may be
the evolution of the LMXB population with time. Its effect is further amplified
by the larger GC content of older galaxies and correspondingly, the larger
numbers of dynamically formed binaries in them. We also found clear evolution
of the X-ray luminosity function (XLF) with age, that younger galaxies have
more bright sources and fewer faint sources per unit stellar mass. The XLF of
LMXBs in younger galaxies appears to extend significantly beyond E39 erg/s.
Such bright sources seem to be less frequent in older galaxies. We found that 6
out of ~ 12 (ultra-) luminous sources are located in GCs.Comment: 13 pages, 8 figures, accepted by A&A on 03/08/201
The M33 Globular Cluster System with PAndAS Data: The Last Outer Halo Cluster?
We use CFHT/MegaCam data to search for outer halo star clusters in M33 as
part of the Pan-Andromeda Archaeological Survey (PAndAS). This work extends
previous studies out to a projected radius of 50 kpc and covers over 40 square
degrees. We find only one new unambiguous star cluster in addition to the five
previously known in the M33 outer halo (10 kpc <= r <= 50 kpc). Although we
identify 2440 cluster candidates of various degrees of confidence from our
objective image search procedure, almost all of these are likely background
contaminants, mostly faint unresolved galaxies. We measure the luminosity,
color and structural parameters of the new cluster in addition to the five
previously-known outer halo clusters. At a projected radius of 22 kpc, the new
cluster is slightly smaller, fainter and redder than all but one of the other
outer halo clusters, and has g' ~ 19.9, (g'-i') ~ 0.6, concentration parameter
c ~ 1.0, a core radius r_c ~ 3.5 pc, and a half-light radius r_h ~ 5.5 pc. For
M33 to have so few outer halo clusters compared to M31 suggests either tidal
stripping of M33's outer halo clusters by M31, or a very different, much calmer
accretion history of M33.Comment: 37 pages, 9 figures. Accepted by the Astrophysical Journa
Photometric Selection of Emission Line Galaxies, Clustering Analysis and a Search for the ISW effect
We investigate the use of simple colour cuts applied to the SDSS optical
imaging to perform photometric selections of emission line galaxies out to z<1.
From colour-cuts using the SDSS g, r and i bands, we obtain mean photometric
redshifts of z=0.32+-0.08, z=0.44+-0.12 and z=0.65+-0.21. We further calibrate
our high redshift selection using spectroscopic observations with the AAOmega
spectrograph on the 4m Anglo-Australian Telescope (AAT), observing ~50-200
galaxy candidates in 4 separate fields. With just 1-hour of integration time
and with seeing of ~1.6", we successfully determined redshifts for ~65% of the
targeted candidates. We calculate the angular correlation functions of the
samples and find correlation lengths of r0=2.64 h-1 Mpc, r0=3.62 h-1 Mpc and
r0=5.88 h-1 Mpc for the low, mid and high redshift samples respectively.
Comparing these results with predicted dark matter clustering, we estimate the
bias parameter for each sample to be b=0.70, b=0.92 and b=1.46. We calculate
the 2-point redshift-space correlation function at z~0.6 and find a clustering
amplitude of s0=6.4 h-1 Mpc. Finally, we use our photometric sample to search
for the Integrated Sachs-Wolfe signal in the WMAP 5yr data. We cross-correlate
our three redshift samples with the WMAP W, V, Q and K bands and find an
overall trend for a positive signal similar to that expected from models.
However, the signal in each is relatively weak. Combining all three galaxy
samples we find a signal of wTg(<100')=0.20+-0.12 microK in the WMAP W-band, a
significance of 1.7sigma.Comment: 14 pages, 17 figures, submitted to MNRA
Unresolved and diffuse components of X-ray emission and X/K luminosity ratios in nearby early-type and late-type galaxies
We explore the nature of unresolved X-ray emission in a broad sample of
galaxies of all morphological types based on archival Chandra data. After
removing bright compact sources, we study L_X/L_K luminosity ratios of
unresolved emission, and compare them with the Solar neighborhood values. We
conclude that unresolved emission is determined by four main components, three
of which were known before: (i) The population of faint unresolved sources
associated with old stellar population. In early-type galaxies, their 2-10 keV
band luminosity scales with the stellar mass with L_X/L_K = (3.1\pm 0.9) x
10^27 erg/s/L_K,sun; (ii) The ISM with kT ~ 0.2-0.8 keV present in galaxies of
all types. Because of the large dispersion in the gas content of galaxies, the
size of our sample is insufficient to obtain reliable scaling law for this
component; (iii) The population of unresolved young stars and young stellar
objects in late-type galaxies. Their 2-10 keV band luminosity scales with the
star-formation rate with L_X/SFR = (1.7\pm0.9) x 10^38 (erg/s)/(M_sun/yr); (iv)
In four old and massive Virgo ellipticals (M49, M60, M84, NGC4636) we find
anomalously high X-ray emission in the 2-10 keV band. Its presence has not been
recognized before and its nature is unclear. Although it appears to be stronger
in galaxies having stronger ISM component, its existence cannot be explained in
terms of an extrapolation of the warm ISM spectrum. Association with Virgo
cluster of galaxies suggests that the excess emission may be due to
intracluster gas accreted in the gravitational well of a massive galaxy. We
investigate this and other possibilities.Comment: 12 pages, 6 figures, 3 tables, accepted for publication in MNRA
Apolipoprotein E and Alzheimer’s disease: The influence of apolipoprotein E on amyloid- and other amyloidogenic proteins
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