3,576 research outputs found
Cold gas in group-dominant elliptical galaxies
We present IRAM 30m telescope observations of the CO(1-0) and (2-1) lines in
a sample of 11 group-dominant elliptical galaxies selected from the CLoGS
nearby groups sample. Our observations confirm the presence of molecular gas in
4 of the 11 galaxies at >4 sigma significance, and combining these with data
from the literature we find a detection rate of 43+-14%, comparable to the
detection rate for nearby radio galaxies, suggesting that group-dominant
ellipticals may be more likely to contain molecular gas than their non-central
counterparts. Those group-dominant galaxies which are detected typically
contain ~2x10^8 Msol of molecular gas, and although most have low star
formation rates (<1 Msol/yr) they have short depletion times, indicating that
the gas must be replenished on timescales ~100 Myr. Almost all of the galaxies
contain active nuclei, and we note while the data suggest that CO may be more
common in the most radio-loud galaxies, the mass of molecular gas required to
power the active nuclei through accretion is small compared to the masses
observed. We consider possible origin mechanisms for the gas, through cooling
of stellar ejecta within the galaxies, group-scale cooling flows, and gas-rich
mergers, and find probable examples of each type within our sample, confirming
that a variety of processes act to drive the build up of molecular gas in
group-dominant ellipticals.Comment: 9 pages, 5 postscript figures, 4 tables, accepted by A&A. Revised
throughout in response to referee's comments, including updates to Table 1
and Figure 4, and addition of Figure
AGN Feedback in groups and clusters of galaxies
The lack of very cool gas at the cores of groups and clusters of galaxies,
even where the cooling time is significantly shorter than the Hubble time, has
been interpreted as evidence of sources that re-heat the intergalactic medium.
Most studies of rich clusters adopt AGN feedback to be this source of heating.
From ongoing GMRT projects involving clusters and groups, we demonstrate how
low-frequency GMRT radio observations, together with Chandra/XMM-Newton X-ray
data, present a unique insight into the nature of feedback, and of the energy
transfer between the AGN and the IGM.Comment: 5 pages, 3 figures, To appear in ASP Conference Series, Vol. 407, The
Low-Frequency Radio Universe, Eds: D. J. Saikia, D. A. Green, Y. Gupta and T.
Venturi (Invited talk, conference held at NCRA-TIFR, Pune, INDIA, 8-12
December, 2008
Interacting Large-Scale Magnetic Fields and Ionised Gas in the W50/SS433 System
The W50/SS433 system is an unusual Galactic outflow-driven object of
debatable origin. We have used the Australia Telescope Compact Array (ATCA) to
observe a new 198 pointing mosaic, covering , and
present the highest-sensitivity full-Stokes data of W50 to date using
wide-field, wide-band imaging over a 2 GHz bandwidth centred at 2.1 GHz. We
also present a complementary H mosaic created using the Isaac Newton
Telescope Photometric H Survey of the Northern Galactic Plane (IPHAS).
The magnetic structure of W50 is found to be consistent with the prevailing
hypothesis that the nebula is a reanimated shell-like supernova remnant (SNR),
that has been re-energised by the jets from SS433. We observe strong
depolarization effects that correlate with diffuse H emission, likely
due to spatially-varying Faraday rotation measure (RM) fluctuations of
to 61 rad m on scales to 6 pc. We also report the discovery of
numerous, faint, H filaments that are unambiguously associated with the
central region of W50. These thin filaments are suggestive of a SNR's shock
emission, and almost all have a radio counterpart. Furthermore, an RM-gradient
is detected across the central region of W50, which we interpret as a loop
magnetic field with a symmetry axis offset by to the
east-west jet-alignment axis, and implying that the evolutionary processes of
both the jets and the SNR must be coupled. A separate RM-gradient is associated
with the termination shock in the Eastern ear, which we interpret as a
ring-like field located where the shock of the jet interacts with the
circumstellar medium. Future optical observations will be able to use the new
H filaments to probe the kinematics of the shell of W50, potentially
allowing for a definitive experiment on W50's formation history.Comment: Submitted to MNRA
Deep Chandra Observations of HCG 16 - II. The Development of the Intra-group Medium in a Spiral-Rich Group
We use a combination of deep Chandra X-ray observations and radio continuum
imaging to investigate the origin and current state of the intra-group medium
in the spiral-rich compact group HCG 16. We confirm the presence of a faint
(=1.8710 erg/s), low
temperature (0.30 keV) intra-group medium (IGM) extending
throughout the ACIS-S3 field of view, with a ridge linking the four original
group members and extending to the southeast, as suggested by previous Rosat
and XMM-Newton observations. This ridge contains
6.610 solar masses of hot gas and is at least
partly coincident with a large-scale HI tidal filament, indicating that the IGM
in the inner part of the group is highly multi-phase. We present evidence that
the group is not yet virialised, and show that gas has probably been
transported from the starburst winds of NGC 838 and NGC 839 into the
surrounding IGM. Considering the possible origin of the IGM, we argue that
material ejected by galactic winds may have played a significant role,
contributing 20-40% of the observed hot gas in the system.Comment: 11 pages, 6 figures, 1 table, accepted for publication in ApJ;
updated references and fixed typos identified at proof stag
Deep Chandra Observations of HCG 16 - I. Active Nuclei, Star formation and Galactic Winds
We present new, deep Chandra X-ray and Giant Metrewave Radio Telescope
610~MHz observations of the spiral-galaxy-rich compact group HCG 16, which we
use to examine nuclear activity, star formation and the high luminosity X-ray
binary populations in the major galaxies. We confirm the presence of obscured
active nuclei in NGC 833 and NGC 835, and identify a previously unrecognized
nuclear source in NGC 838. All three nuclei are variable on timescales of
months to years, and for NGC 833 and NGC 835 this is most likely caused by
changes in accretion rate. The deep Chandra observations allow us to detect for
the first time an Fe-K emission line in the spectrum of the Seyfert 2
nucleus of NGC 835. We find that NGC 838 and NGC 839 are both
starburst-dominated systems, with only weak nuclear activity, in agreement with
previous optical studies. We estimate the star formation rates in the two
galaxies from their X-ray and radio emission, and compare these results with
estimates from the infra-red and ultra-violet bands to confirm that star
formation in both galaxies is probably declining after galaxy-wide starbursts
were triggered ~400-500 Myr ago. We examine the physical properties of their
galactic superwinds, and find that both have temperatures of ~0.8 keV. We also
examine the X-ray and radio properties of NGC 848, the fifth largest galaxy in
the group, and show that it is dominated by emission from its starburst.Comment: 18 pages, 11 figures, 11 tables, accepted for publication in ApJ;
updated references and fixed typos identified at proof stag
Evidence for cospatial optical and radio polarized emission in active galactic nuclei
We investigate the relationship between the optical and radio emission of active galactic nuclei (AGN) by analysing optical and 15+22+43 GHz Very Long Baseline Array (VLBA) polarization observations simultaneous to within a day for 11 BL Lacertae (BL Lac) objects and the blazar 3C279. We have determined and corrected for the Faraday rotation measures in the very long baseline interferometry (VLBI) cores, enabling us to compare the intrinsic (zero-wavelength) VLBI-core polarization angles and the optical polarization angles chi(opt). A clear alignment between these two angles emerges in the transition toward higher radio frequencies, and a prominent peak at 0 degrees is visible in the distribution of |chi(opt) - chi(43 GHz)|. This correlation implies that the magnetic-field orientations in the regions giving rise to the optical and radio polarization are the same, and can be easily understood if the radio and optical polarization are roughly cospatial. It is difficult to rule out the possibility that they arise in different regions in a straight jet with a uniform magnetic-field structure, but this seems less likely, since the VLBI jets of AGN are often bent on parsec-scales. This may suggest that much or all of the strong optical polarization in these sources arises in the inner radio jets, possibly associated with the formation and emergence of compact new VLBI components
Diversity in the stellar velocity dispersion profiles of a large sample of Brightest Cluster Galaxies
We analyse spatially-resolved deep optical spectroscopy of Brightest Cluster
Galaxies (BCGs) located in 32 massive clusters with redshifts of 0.05 0.30, to investigate their velocity dispersion profiles. We compare these
measurements to those of other massive early-type galaxies, as well as central
group galaxies, where relevant. This unique, large sample extends to the most
extreme of massive galaxies, spanning M between -25.7 to -27.8 mag, and
host cluster halo mass M up to 1.7 10 M. To
compare the kinematic properties between brightest group and cluster members,
we analyse similar spatially-resolved long-slit spectroscopy for 23 nearby
Brightest Group Galaxies (BGGs) from the Complete Local-Volume Groups Sample
(CLoGS). We find a surprisingly large variety in velocity dispersion slopes for
BCGs, with a significantly larger fraction of positive slopes, unique compared
to other (non-central) early-type galaxies as well as the majority of the
brightest members of the groups. We find that the velocity dispersion slopes of
the BCGs and BGGs correlate with the luminosity of the galaxies, and we
quantify this correlation. It is not clear whether the full diversity in
velocity dispersion slopes that we see is reproduced in simulations.Comment: 25 pages, 17 figures, accepted for publication in MNRA
Increasing concentrations of dichloromethane, CH2Cl2, inferred from CARIBIC air samples collected 1998–2012
Atmospheric concentrations of dichloromethane, CH2Cl2, a regulated toxic air pollutant and minor contributor to stratospheric ozone depletion, were reported to have peaked around 1990 and to be declining in the early part of the 21st century. Recent observations suggest this trend has reversed and that CH2Cl2 is once again increasing in the atmosphere. Despite the importance of ongoing monitoring and reporting of atmospheric CH2Cl2, no time series has been discussed in detail since 2006. The CARIBIC project (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) has analysed the halocarbon content of whole-air samples collected at altitudes of between ~10–12 km via a custom-built container installed on commercial passenger aircraft since 1998, providing a long-term record of CH2Cl2 observations. In this paper we present this unique CH2Cl2 time series, discussing key flight routes which have been used at various times over the past 15 years. Between 1998 and 2012 increases were seen in all northern hemispheric regions and at different altitudes, ranging from ~7–10 ppt in background air to ~13–15 ppt in regions with stronger emissions (equating to a 38–69% increase). Of particular interest is the rising importance of India as a source of atmospheric CH2Cl2: based on CARIBIC data we provide regional emission estimates for the Indian subcontinent and show that regional emissions have increased from 3–14 Gg yr^-1 (1998–2000) to 16–25 Gg yr^-1 (2008). Potential causes of the increasing atmospheric burden of CH2Cl2 are discussed. One possible source is the increased use of CH2Cl2 as a feedstock for the production of HFC-32, a chemical used predominantly as a replacement for ozone-depleting substances in a variety of applications including air conditioners and refrigeration
From neurons to epidemics: How trophic coherence affects spreading processes
Trophic coherence, a measure of the extent to which the nodes of a directed
network are organised in levels, has recently been shown to be closely related
to many structural and dynamical aspects of complex systems, including graph
eigenspectra, the prevalence or absence of feed-back cycles, and linear
stability. Furthermore, non-trivial trophic structures have been observed in
networks of neurons, species, genes, metabolites, cellular signalling,
concatenated words, P2P users, and world trade. Here we consider two simple yet
apparently quite different dynamical models -- one a
Susceptible-Infected-Susceptible (SIS) epidemic model adapted to include
complex contagion, the other an Amari-Hopfield neural network -- and show that
in both cases the related spreading processes are modulated in similar ways by
the trophic coherence of the underlying networks. To do this, we propose a
network assembly model which can generate structures with tunable trophic
coherence, limiting in either perfectly stratified networks or random graphs.
We find that trophic coherence can exert a qualitative change in spreading
behaviour, determining whether a pulse of activity will percolate through the
entire network or remain confined to a subset of nodes, and whether such
activity will quickly die out or endure indefinitely. These results could be
important for our understanding of phenomena such as epidemics, rumours, shocks
to ecosystems, neuronal avalanches, and many other spreading processes
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