55 research outputs found
The Environmental Impact of Intra-Cluster Medium on the Interstellar Medium in Early Type Galaxies
Draft versions of three articles submitted for publication are presented. The first two articles address high resolution X-ray images of early type galaxies observed with the ROSAT HRI and PSPC. Data for NGC 1553 and NGC 5846 indicate that the emission is highly irregular, with interesting features at different scales. The gas temperatures also vary both with the galactocentric radius and in correspondence to regions of higher emission and denser material. Strikingly similar features are observed in the X-ray and H-alpha morphologies of NGC 1553 and NGC 5846, while smooth, regular isophotes are observed in NGC 4649 at both wavelengths. The third article addresses ROSAT PSPC observations of 5 X-ray bright early type galaxies
The Environmental Impact of Intra-Cluster Medium on the Interstellar Medium in Early Type Galaxies
High resolution X-ray images of three early type galaxies observed with the ROSAT HRI are presented. Data for NGC 1553 and NGC 5846 indicate that the emission is highly irregular, with interesting features on scales from a few arcsec to a few arcmin. The gas temperatures also vary both with the galactocentric radius and in correspondence to regions of higher emission and denser material. Strikingly similar features are observed in the X-ray and H(alpha) morphologies of NGC 1553 and NGC 5846, while smoother, more regular isophotes are observed in NGC 4649 at both wavelengths. A connection between these two kinds of emission therefore seems likely. In the light of our observations we discuss possible scenarios that can and account for the connection between X-ray and H(alpha) emissions
The diverse X-ray properties of four truly isolated elliptical galaxies: NGC 2954, NGC 6172, NGC 7052, and NGC 7785
We investigate the X-ray properties of four isolated elliptical galaxies,
selected from the Updated Zwicky Catalog according to strict isolation
criteria. Isolated galaxies are not influenced by the group/cluster
environment, and their X-ray emission can be studied independently of the often
overwhelming contribution of the hot intergalactic medium. They are therefore
suited to studying the X-ray characteristics relative to their intrinsic
properties. We analyzed our own XMM-Newton and archival Chandra data in detail
for three objects, and derived, when possible, the spatial and spectral
characteristics of each source. An upper limit for the fourth one was obtained
from archival ASCA data. We compared their characteristics with those of other
23 isolated objects for which X-ray and optical data are available in the
literature. We explored possible theoretical explanations to interpret our
results. In spite of our attempt to select very homogeneous objects, both in
terms of optical properties and environmental characteristics, we find a wide
range in X-ray luminosities and LX/LB ratios for the four objects: two of them
show a hot gaseous halo, whereas no gas is detected in the other two, to a
factor >10 in luminosity. In fact, we find a large spread in the LX/LB for all
galaxies considered, suggesting that the presence of hot gas is not easily
related to the optical luminosity or to the mass, even in isolated systems.
Younger objects tend to be less luminous in X-rays than older systems. However,
it appears that older objects could span a wide range in luminosities.Comment: [email protected], 12 pages, 14 PostScript figures, 3
tables, LaTeX. The last sentence of Section 2 has been modified. The
published version of the manuscript is also available at
http://moby.mib.infn.it/~memola/publications.htm
Interaction between the intergalactic medium and central radio source in the NGC 4261 group of galaxies
Using observations from the Chandra and XMM-Newton X-ray observatories, we
examine the interaction between the intra-group medium and central radio source
in the nearby NGC 4261 galaxy group. We confirm the presence of cavities
associated with the radio lobes and estimate their enthalpy to be ~2.4x10^58
erg. The mechanical power output of the jets is >=10^43 erg/s, at least a
factor of 60 greater than the cooling luminosity in the region the lobes
inhabit. We identify rims of compressed gas enclosing the lobes, but find no
statistically significant temperature difference between them and their
surroundings, suggesting that the lobe expansion velocity is approximately
sonic (Mach<=1.05). The apparent pressure of the radio lobes, based on the
synchrotron minimum energy density argument, is a factor of 5 lower than that
of the intra-group medium. Pressure balance could be achieved if entrainment of
thermal gas provided additional non-radiating particles in the lobe plasma, but
the energy required to heat these particles would be ~20 per cent. of the
mechanical energy output of the radio source. NGC 4261 has a relatively compact
cool core, which should probably be categorised as a galactic corona. The
corona is capable of fuelling the active nucleus for considerably longer than
the inferred source lifetime, but can be only inefficiently heated by the AGN
or conduction. The expansion of the radio lobes has affected the structure of
the gas in the galaxy, compressing and moving the material of the corona
without causing significant shock heating, and expelling gas from the immediate
neighbourhood of the jets. We discuss the possible implications of this
environment for the duration of the AGN outburst, and consider mechanisms which
might lead to the cessation of nuclear activity.Comment: Accepted for publication in MNRAS, 17 pages, 6 figure
Broad Band X-ray Spectra of M31 Sources with BeppoSAX
We present the first spectral study of the X-ray emitting stellar sources in
M31 in the energy band from 0.1 to 10 keV. We find that the globular cluster
sources have spectral characteristics consistent with those of the Milky Way
object, namely that the spectrum can be described by a thermal model with 6-20
keV from 2 to 10 keV. Evidence of high absorption in some of these sources is
most likely an indication that they lie in or behind the HI ring in the disk of
the galaxy.
We also find one peculiar globular cluster source, with spectral
characteristics more typically associated with either High Mass X-ray Binaries
or black hole candidates. We therefore suggest that either the source has been
wrongly identified with a globular cluster or that the system contains a Black
Hole. We confirm earlier report that the spectrum of the bulge of M31 is
consistent with the superposition of many LMXB spectra. It is likely that a
large fraction of the 15-30 keV detection obtained from the PDS instrument is
associated with the bulge, thus extending the spectral data for this complex of
sources up to 30 keV. The high energy part of the spectrum can be parameterized
with typical LMXB spectra, while at low energies an additional component is
required.Comment: Accepted for publication in A&A, main journa
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
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