2,064 research outputs found
Astrophysical bow shocks: An analytical solution for the hypersonic blunt body problem in the intergalactic medium
Aims: Bow shock waves are a common feature of groups and clusters of galaxies
since they are generated as a result of supersonic motion of galaxies through
the intergalactic medium. The goal of this work is to present an analytical
solution technique for such astrophysical hypersonic blunt body problems.
Methods: A method, developed by Schneider (1968, JFM, 31, 397) in the context
of aeronautics, allows calculation of the galaxy's shape as long as the shape
of the bow shock wave is known (so-called inverse method). In contrast to other
analytical models, the solution is valid in the whole flow region (from the
stagnation point up to the bow shock wings) and in particular takes into
account velocity gradients along the streamlines. We compare our analytical
results with two-dimensional hydrodynamical simulations carried out with an
extended version of the VH-1 hydrocode which is based on the piecewise
parabolic method with a Lagrangian remap. Results: It is shown that the applied
method accurately predicts the galaxy's shape and the fluid variables in the
post-shock flow, thus saving a tremendous amount of computing time for future
interpretations of similar objects. We also find that the method can be applied
to arbitrary angles between the direction of the incoming flow and the axis of
symmetry of the body. We emphasize that it is general enough to be applied to
other astrophysical bow shocks, such as those on stellar and galactic scales.Comment: 11 pages, 7 figures, accepted for publication in A&
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
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
Mass and Light in the Universe
We present a weak lensing and photometric study of six half by half degree
fields observed at the CFHT using the UH8K CCD mosaic camera. The fields were
observed for a total of 2 hours each in I and V, resulting in catalogs
containing ~ 20 000 galaxies per passband per field. We use V-I color and I
magnitude to select bright early type galaxies at redshifts 0.1 < z < 0.9. We
measure the gravitational shear from faint galaxies in the range 21 < m_I < 25
from a composite catalog and find a strong correlation with that predicted from
the early types if they trace the mass with mass-to-light ratio 300\pm75 h (in
solar units) for a flat (Omega_m0 = 0.3, Omega_l0 = 0.7) lambda cosmology and
400\pm100 h for Einstein-de Sitter. We make two-dimensional reconstructions of
the mass surface density. Cross-correlation of the measured mass surface
density with that predicted from the early type galaxy distribution shows a
strong peak at zero lag (significant at the 5.2-sigma level). We azimuthally
average the cross- and auto-correlation functions. We conclude that the
profiles are consistent with early type galaxies tracing mass on scales of > 45
arcsec (> 200 kpc at z = 0.5). We sub-divide our bright early type galaxies by
redshift and obtain similar conclusions. These mass-to-light ratios imply
\Omega_m0 = 0.10\pm0.02 (\Omega_m0 = 0.13\pm0.03 for Einstein-de Sitter) of
closure density.Comment: 27 pages, 19 figs (4 ps, 15 gif), 4 tables, accepted for publication
in Ap.J. (email Gillian for better resolution ps versions of gif greyscale
plots
Seyfert's Sextet: where is the gas?
Seyfert's Sextet (a.k.a HCG 79) is one of the most compact and isolated
galaxy groups in the local Universe. It shows a prominent diffuse light
component that accounts for ~50% of the total observed light. This likely
indicates that the group is in an advanced evolutionary phase, which would
predict a significant hot gaseous component. Previous X-ray observations had
suggested a low luminosity for this system, but with large uncertainties and
poor resolution. We present the results from a deep (70 ks), high resolution
Chandra observation of Seyfert's Sextet, requested with the aim of separating
the X-ray emission associated with the individual galaxies from that of a more
extended inter-galactic component. We discuss the spatial and spectral
characteristics of this group we derive with those of a few similar systems
also studied in the X-ray band. The high resolution X-ray image indicates that
the majority of the detected emission does not arise in the compact group but
is concentrated towards the NW and corresponds to what appears to be a
background galaxy cluster. The emission from the group alone has a total
luminosity of ~1x10^40 erg/s in the (0.5-5) keV band. Most of the luminosity
can be attributed to the individual sources in the galaxies, and only ~2x10^39
erg/s is due to a gaseous component. However, we find that this component is
also mostly associated with the individual galaxies of the Sextet, leaving
little or no residual in a truly IGM component. The extremely low luminosity of
the diffuse emission in Seyfert's Sextet might be related to its small total
mass.Comment: 8 pages, 7 figures. Accepted on A&
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