20,181 research outputs found
The X-ray Evolution of Merging Galaxies
We present here the first study of the X-ray properties of an evolutionary
sample of merging galaxies. Both ROSAT PSPC and HRI data are presented for a
sample of eight interacting galaxy systems, each believed to involve a similar
encounter between two spiral discs of approximately equal size. The mergers
span a large range in age, from completely detached to fully merged systems. A
great deal of interesting X-ray structure is seen, and the X-ray properties of
each individual system are discussed in detail. Along the merging sequence,
several trends are evident: in the case of several of the infrared bright
systems, the diffuse emission is very extended, and appears to arise from
material ejected from the galaxies. The onset of this process seems to occur
very soon after the galaxies first encounter one another, and these ejections
soon evolve into distorted flows. More massive extensions (perhaps involving up
to 1e10 solar masses of hot gas) are seen at the `ultraluminous' peak of the
interaction, as the galactic nuclei coalesce. The amplitude of the evolution of
the X-ray emission through a merger is markedly different from that of the
infrared and radio emission however, and this, we believe, may well be linked
with the large extensions of hot gas observed. The late, relaxed remnants,
appear relatively devoid of gas, and possess an X-ray halo very different from
that of typical ellipticals, a problem for the `merger hypothesis', whereby the
merger of two disc galaxies results in an elliptical galaxy. However, these
systems are still relatively young in terms of total merger lifetime, and they
may still have a few Gyr of evolution to go through, before they resemble
typical elliptical galaxies.Comment: 30 pages, 15 figures, accepted by MNRA
Cross-Calibration of the XMM-Newton EPIC pn & MOS On-Axis Effective Areas Using 2XMM Sources
We aim to examine the relative cross-calibration accuracy of the on-axis
effective areas of the XMM-Newton EPIC pn and MOS instruments. Spectra from a
sample of 46 bright, high-count, non-piled-up isolated on-axis point sources
are stacked together, and model residuals are examined to characterize the EPIC
MOS-to-pn inter-calibration. The MOS1-to-pn and MOS2-to-pn results are broadly
very similar. The cameras show the closest agreement below 1 keV, with MOS
excesses over pn of 0-2% (MOS1/pn) and 0-3% (MOS2/pn). Above 3 keV, the MOS/pn
ratio is consistent with energy-independent (or only mildly increasing)
excesses of 7-8% (MOS1/pn) and 5-8% (MOS2/pn). In addition, between 1-2 keV
there is a `silicon bump' - an enhancement at a level of 2-4% (MOS1/pn) and
3-5% (MOS2/pn). Tests suggest that the methods employed here are stable and
robust. The results presented here provide the most accurate cross-calibration
of the effective areas of the XMM-Newton EPIC pn and MOS instruments to date.
They suggest areas of further research where causes of the MOS-to-pn
differences might be found, and allow the potential for corrections to and
possible rectification of the EPIC cameras to be made in the future.Comment: 8 Pages, 2 figures (3 panels), 1 table. Accepted for publication in
A&
The XMM-Newton EPIC Background and the production of Background Blank Sky Event Files
We describe in detail the nature of XMM-Newton EPIC background and its
various complex components, summarising the new findings of the XMM-Newton EPIC
background working group, and provide XMM-Newton background blank sky event
files for use in the data analysis of diffuse and extended sources. Blank sky
event file data sets are produced from the stacking of data, taken from 189
observations resulting from the Second XMM-Newton Serendipitous Source
Catalogue (2XMMp) reprocessing. The data underwent several filtering steps,
using a revised and improved method over previous work, which we describe in
detail. We investigate several properties of the final blank sky data sets. The
user is directed to the location of the final data sets. There is a final data
set for each EPIC instrument-filter-mode combination.Comment: Paper accepted by A&A 22 December 2006. 14 pages, 8 figures. Paper
can also be found at http://www.star.le.ac.uk/~jac48/publications
XMM-Newton Slew Survey observations of the gravitational wave event GW150914
The detection of the first gravitational wave (GW) transient GW150914
prompted an extensive campaign of follow-up observations at all wavelengths.
Although no dedicated XMM-Newton observations have been performed, the
satellite passed through the GW150914 error box during normal operations. Here
we report the analysis of the data taken during these satellite slews performed
two hours and two weeks after the GW event. Our data cover 1.1 square degrees
and 4.8 square degrees of the final GW localization region. No credible X-ray
counterpart to GW150914 is found down to a sensitivity of 6E-13 erg/cm2/s in
the 0.2-2 keV band. Nevertheless, these observations show the great potential
of XMM-Newton slew observations for the search of the electromagnetic
counterparts of GW events. A series of adjacent slews performed in response to
a GW trigger would take <1.5 days to cover most of the typical GW credible
region. We discuss this scenario and its prospects for detecting the X-ray
counterpart of future GW detections.Comment: 6 pages, 3 figures, 2 tables. Accepted for publication in ApJ Letter
Development of a nickel cadmium storage cell immune to damage from overdischarge and overcharge
Nickel-cadmium battery immune to damage from overcharge and overdischarg
On the formation of dwarf galaxies and stellar halos
Using analytic arguments and a suite of very high resolution (10^3 Msun per
particle) cosmological hydro-dynamical simulations, we argue that high
redshift, z ~ 10, M ~ 10^8 Msun halos, form the smallest `baryonic building
block' (BBB) for galaxy formation. These halos are just massive enough to
efficiently form stars through atomic line cooling and to hold onto their gas
in the presence of supernovae winds and reionisation. These combined effects,
in particular that of the supernovae feedback, create a sharp transition: over
the mass range 3-10x10^7 Msun, the BBBs drop two orders ofmagnitude in stellar
mass. Below ~2x10^7 Msun, galaxies will be dark with almost no stars and no
gas. Above this scale is the smallest unit of galaxy formation: the BBB.
A small fraction (~100) of these gas rich BBBs fall in to a galaxy the size
of the Milky Way. Ten percent of these survive to become the observed LG dwarf
galaxies at the present epoch. Those in-falling halos on benign orbits which
keep them far away from the Milky Way or Andromeda manage to retain their gas
and slowly form stars - these become the smallest dwarf irregular galax ies;
those on more severe orbits lose their gas faster than they can form stars and
become the dwarf spheroidals. The remaining 90% of the BBBs will be accreted.
We show that this gives a metallicity and total stellar mass consistent with
the Milky Way old stellar halo (abridged).Comment: 15 pages, 7 figures, one figure added to match accepted version. Some
typos fixed. MNRAS in pres
The X-ray properties of the merging galaxy pair NGC 4038/9 - the Antennae
We report the results of an X-ray spectral imaging observation of the
Antennae with the ROSAT PSPC. 55% of the soft X-ray flux from the system is
resolved into discrete sources, including components identified with the
galactic nuclei and large HII regions, whilst the remainder appears to be
predominantly genuinely diffuse emission from gas at a temperature ~4x10^6 K.
The morphology of the emission is unusual, combining a halo which envelopes the
galactic discs, with what appears to be a distorted, but well-collimated
bipolar outflow. We derive physical parameters for the hot gas in both diffuse
components, which are of some interest, given that the Antennae probably
represents an elliptical galaxy in the making.Comment: 15 pages plus 9 figures, uuencoded encapsulated postscript file.
Accepted for publication in MNRA
The periodic sl(2|1) alternating spin chain and its continuum limit as a bulk Logarithmic Conformal Field Theory at c=0
The periodic sl(2|1) alternating spin chain encodes (some of) the properties
of hulls of percolation clusters, and is described in the continuum limit by a
logarithmic conformal field theory (LCFT) at central charge c=0. This theory
corresponds to the strong coupling regime of a sigma model on the complex
projective superspace , and the spectrum of critical exponents can be
obtained exactly. In this paper we push the analysis further, and determine the
main representation theoretic (logarithmic) features of this continuum limit by
extending to the periodic case the approach of [N. Read and H. Saleur, Nucl.
Phys. B 777 316 (2007)]. We first focus on determining the representation
theory of the finite size spin chain with respect to the algebra of local
energy densities provided by a representation of the affine Temperley-Lieb
algebra at fugacity one. We then analyze how these algebraic properties carry
over to the continuum limit to deduce the structure of the space of states as a
representation over the product of left and right Virasoro algebras. Our main
result is the full structure of the vacuum module of the theory, which exhibits
Jordan cells of arbitrary rank for the Hamiltonian.Comment: 69pp, 8 fig
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