1,574 research outputs found
On the Integrated Spectrum of the X-ray Binaries and the Origin of Soft X-ray Emission from the Bulge of M31
Using ROSAT PSPC data, we have performed several tests aimed at understanding
the origin of the soft X-ray spectral component detected from the bulge of M31.
We find that a significant soft component in the spectrum of the bulge is
spatially correlated with the unresolved X-ray emission near the core of M31,
which is probably a hot interstellar medium or perhaps a population of multiple
faint sources. For the first time, we extracted the spectrum of this unresolved
emission, by removing point sources dominating the integral spectrum of the
bulge, and found it to be responsible for the most of soft excess. A soft
spectral component is not at all needed to fit the point source spectrum that
remains after subtracting the unresolved emission. The integral spectra of
bright point sources, both inside and outside of the M31 bulge, can be fitted
with a single power-law in the ROSAT band. Our analysis rules out the previous
suggestion that all bulge emission in M31 may be generated by low mass X-ray
binaries (Irwin & Bregman, 1999).Comment: 11 pages incl. 2 figures, 2 tables, accepted to ApJ
Small-scale systems of galaxies. IV. Searching for the faint galaxy population associated with X-ray detected isolated E+S pairs
In hierarchical evolutionary scenarios, isolated, physical pairs may
represent an intermediate phase, or "way station", between collapsing groups
and isolated elliptical (E) galaxies (or fossil groups). We started a
comprehensive study of a sample of galaxy pairs composed of a giant E and a
spiral (S) with the aim of investigating their formation/evolutionary history
from observed optical and X-ray properties. Here we present VLT-VIMOS
observations designed to identify faint galaxies associated with the E+S
systems from candidate lists generated using photometric criteria on WFI images
covering an area of ~ 0.2 h^{-1} Mpc radius around the pairs.
The results are discussed in the context of the evolution of poor galaxy
group associations. A comparison between the Optical Luminosity Functions
(OLFs) of our E+S systems and a sample of X-ray bright poor groups suggest that
the OLF of X-ray detected poor galaxy systems is not universal. The OLF of our
X-ray bright systems suggests that they are more dynamically evolved than our
X-ray faint sample and some X-ray bright groups in the literature. However, we
suggest that the X-ray faint E+S pairs represent a phase in the dynamical
evolution of some X-ray bright poor galaxy groups. The recent or ongoing
interaction in which the E member of the X-ray faint pairs is involved could
have decreased the luminosity of any surrounding X-ray emitting gas.Comment: accepted for publication in Astronomy and Astrophysic
X-ray Tail in NGC 7619
We present new observational results of NGC 7619, an elliptical galaxy with a
prominent X-ray tail and a dominant member of the Pegasus group. With Chandra
and XMM-Newton observations, we confirm the presence of a long X-ray tail in
the SW direction; moreover, we identify for the first time a sharp
discontinuity of the X-ray surface brightness in the opposite (NE) side of the
galaxy. The density, temperature and pressure jump at the NE discontinuity
suggest a Mach number ~1, corresponding to a galaxy velocity of ~500 km s-1,
relative to the surrounding hot gas. Spectral analysis of these data shows that
the Iron abundance of the hot gaseous medium is much higher (1-2 solar) near
the center of NGC 7619 and in the tail extending from the core than in the
surrounding regions (< 1/2 solar), indicating that the gas in the tail is
originated from the galaxy. The possible origin of the head-tail structure is
either on-going ram-pressure stripping or sloshing. The morphology of the
structure is more in line with a ram pressure stripping phenomenon, while the
position of NGC 7619 at the center of the Pegasus I group, and its dominance,
would prefer sloshing.Comment: ApJ accepted to appear in the 2008 December 1 issue; Added discussion
on sloshin
A multi-wavelength study of the evolution of Early-Type Galaxies in Groups: the ultraviolet view
ABRIDGED- The UV-optical color magnitude diagram (CMD) of rich galaxy groups
is characterised by a well developed Red Sequence (RS), a Blue Cloud (BC) and
the so-called Green Valley (GV). Loose, less evolved groups of galaxies likely
not virialized yet may lack a well defined RS. This is actually explained in
the framework of galaxy evolution. We are focussing on understanding galaxy
migration towards the RS, checking for signatures of such a transition in their
photometric and morphological properties. We report on the UV properties of a
sample of ETGs galaxies inhabiting the RS. The analysis of their structures, as
derived by fitting a Sersic law to their UV luminosity profiles, suggests the
presence of an underlying disk. This is the hallmark of dissipation processes
that still must have a role in the evolution of this class of galaxies. SPH
simulations with chemo-photometric implementations able to match the global
properties of our targets are used to derive their evolutionary paths through
UV-optical CDM, providing some fundamental information such as the crossing
time through the GV, which depends on their luminosity. The transition from the
BC to the RS takes several Gyrs, being about 3-5 Gyr for the the brightest
galaxies and more long for fainter ones, if it occurs. The photometric study of
nearby galaxy structures in UV is seriously hampered by either the limited FoV
of the cameras (e.g in HST) or by the low spatial resolution of the images (e.g
in the GALEX). Current missions equipped with telescopes and cameras sensitive
to UV wavelengths, such as Swift-UVOT and Astrosat-UVIT, provide a relatively
large FoV and better resolution than the GALEX. More powerful UV instruments
(size, resolution and FoV) are obviously bound to yield fundamental advances in
the accuracy and depth of the surface photometry and in the characterisation of
the galaxy environment.Comment: 12 pages, 6 figures: accepted for publication in Astrophysics & Space
Science as contributions to the workshop: "UV astronomy, the needs and the
means
Galaxy Halo Masses from Galaxy-Galaxy Lensing
We present measurements of the extended dark halo profiles of bright early
type galaxies at redshifts 0.1 to 0.9 obtained via galaxy-galaxy lensing
analysis of images taken at the CFHT using the UH8K CCD mosaic camera. Six half
degree fields were observed for a total of 2 hours each in I and V, resulting
in catalogs containing ~20 000 galaxies per field. We used V-I color and I
magnitude to select bright early type galaxies as the lens galaxies, yielding a
sample of massive lenses with fairly well determined redshifts and absolute
magnitudes M ~ M_* \pm 1. We paired these with faint galaxies lying at angular
distances 20" to 60", corresponding to physical radii of 26 to 77 kpc (z = 0.1)
and 105 to 315 kpc (z = 0.9), and computed the mean tangential shear of the
faint galaxies. The shear falls off with radius roughly as expected for flat
rotation curve halos. The shear values were weighted in proportion to the
square root of the luminosity of the lens galaxy. Our results give a value for
the average mean rotation velocity of an L_* galaxy halo at r~50-200 kpc of v_*
= 238^{+27}_{-30} km per sec for a flat lambda (Omega_m0 = 0.3, Omega_l0 = 0.7)
cosmology (v_* = 269^{+34}_{-39} km per sec for Einstein-de Sitter), and with
little evidence for evolution with redshift. We compare to halo masses measured
by other groups/techniques. We find a mass-to-light ratio of ~121\pm28h(r/100
kpc) and these halos constitute Omega ~0.04 \pm 0.01(r/100 kpc) of closure
density. (abridged)Comment: Accepted for publication in ApJ (minor modifications) - 32 pages, 11
figs, 5 table
Thermoâeconomic analysis of a hybrid ejector refrigerating system based on a low grade heat source
The rising of the global energy demand requires the use of alternative energy conversion systems employing renewable sources. In the refrigeration and air conditioning fields, heat driven ejector systems represent a promising way to produce the cooling effect by using available low-grade temperature sources. In this paper, a thermoâeconomic analysis of a waste heat recovery hybrid ejector cycle (WHRHEC) was carried out. A thermodynamic model was firstly developed to simulate a WHRHEC able to obtain chilled water with a cooling load of 20 kW, by varying the working fluids and the pinch point values in the heat exchangers. Specific singleâ and twoâphase heat transfer correlations were used to estimate the heat transfer surface and therefore the investment costs. The operative ranges that provide a reasonable compromise between the setâup costs and the cycle performances were then defined and compared to the current waste heatâdriven technologies, such as absorption chillers and organic Rankine cycles (ORCs) coupled with vapor compression cycles (VCCs). The last part of the paper presents an economic analysis providing the map of the design (plant size) and contingent (specific cost of energy, waste heat availability) variables that lead to the economic convenience of a WHRHEC system when integrated to a conventional VCC plant
Thermodynamic Analysis of a Multi-Ejector, CO2, Air-To-Water Heat Pump System
Abstract Nowadays, air conditioning systems for residential and office buildings, contribute largely to the energy consumptions and to the direct and indirect emissions of greenhouse gases. Carbon dioxide (CO2) could be an interesting option to replace traditional HFCs in space heating applications, due to its environmentally friendly characteristics: zero ODP and extremely low GWP, but, in order to spread its use, improvements in performances are needed. In fact, CO2 requires transcritical cycles with high expansion losses. The use of an ejector can reduce these losses and improve the performances up to 30% (depending on the performances of the ejector itself and on the operating conditions). In the a/c applications, characterized by variable operating conditions, multi-ejector systems could be used, where some ejectors work in parallel, in different combination, varying the operating conditions. Currently, a project of DTE-PCU-SPCT Department of ENEA and Industrial Engineering Department of Federico II University of Naples, is in progress, in order to evaluate experimentally the effect of several ejectors geometries on the global performance of a CO2 heat pump working with a transcritical cycle. As a part of this project, a complete heat pump system for production of hot water for sanitary use and for space heating is tested to investigate the effect of the ejector size on the balancing of the global performance of the whole system
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