1,073 research outputs found
Hydrogen Two-Photon Continuum Emission from the Horseshoe Filament in NGC 1275
Far ultraviolet emission has been detected from a knot of Halpha emission in
the Horseshoe filament, far out in the NGC 1275 nebula. The flux detected
relative to the brightness of the Halpha line in the same spatial region is
very close to that expected from Hydrogen two-photon continuum emission in the
particle heating model of Ferland et al. (2009) if reddening internal to the
filaments is taken into account. We find no need to invoke other sources of far
ultraviolet emission such as hot stars or emission lines from CIV in
intermediate temperature gas to explain these data.Comment: 9 pages, 8 figures. Accepted for publication in MNRA
Collisional excitation of [C II], [O I] and CO in Massive Galaxies
Many massive galaxies at the centres of relaxed galaxy clusters and groups
have vast reservoirs of cool (~10,000 K) and cold (~100 K) gas. In many low
redshift brightest group and cluster galaxies this gas is lifted into the hot
ISM in filamentary structures, which are long lived and are typically not
forming stars. Two important questions are how far do these reservoirs cool and
if cold gas is abundant what is the cause of the low star formation efficiency?
Heating and excitation of the filaments from collisions and mixing of hot
particles in the surrounding X-ray gas describes well the optical and near
infra-red line ratios observed in the filaments. In this paper we examine the
theoretical properties of dense, cold clouds emitting in the far infra-red and
submillimeter through the bright lines of [C II]157 \mu m , [O I]63 \mu m and
CO, exposed to these energetic ionising particles. While some emission lines
may be optically thick we find this is not sufficient to model the emission
line ratios. Models where the filaments are supported by thermal pressure
support alone also cannot account for the cold gas line ratios but a very
modest additional pressure support, either from turbulence or magnetic fields
can fit the observed [O I]/[C II] line ratios by decreasing the density of the
gas. This may also help stabilise the filaments against collapse leading to the
low rates of star formation. Finally we make predictions for the line ratios
expected from cold gas under these conditions and present diagnostic diagrams
for comparison with further observations. We provide our code as an Appendix.Comment: 17 pages, submitted to MNRA
X-ray bright active galactic nuclei in massive galaxy clusters III: New insights into the triggering mechanisms of cluster AGN
We present the results of a new analysis of the X-ray selected Active
Galactic Nuclei (AGN) population in the vicinity of 135 of the most massive
galaxy clusters in the redshift range of 0.2 < z < 0.9 observed with Chandra.
With a sample of more than 11,000 X-ray point sources, we are able to measure,
for the first time, evidence for evolution in the cluster AGN population beyond
the expected evolution of field AGN. Our analysis shows that overall number
density of cluster AGN scales with the cluster mass as .
There is no evidence for the overall number density of cluster member X-ray AGN
depending on the cluster redshift in a manner different than field AGN, nor
there is any evidence that the spatial distribution of cluster AGN (given in
units of the cluster overdensity radius r_500) strongly depends on the cluster
mass or redshift. The scaling relation we measure is
consistent with theoretical predictions of the galaxy merger rate in clusters,
which is expected to scale with the cluster velocity dispersion, , as or . This consistency suggests that AGN in
clusters may be predominantly triggered by galaxy mergers, a result that is
further corroborated by visual inspection of Hubble images for 23
spectroscopically confirmed cluster member AGN in our sample. A merger-driven
scenario for the triggering of X-ray AGN is not strongly favored by studies of
field galaxies, however, suggesting that different mechanisms may be primarily
responsible for the triggering of cluster and field X-ray AGN.Comment: 21 Pages, 8 figures, 5 tables. Submitted to MNRAS. Comments are
welcome, and please request Steven Ehlert for higher resolution figure
Filamentary Star Formation in NGC 1275
We examine the star formation in the outer halo of NGC~1275, the central
galaxy in the Perseus cluster (Abell 426), using far ultraviolet and optical
images obtained with the Hubble Space Telescope. We have identified a
population of very young, compact star clusters with typical ages of a few Myr.
The star clusters are organised on multiple-kiloparsec scales. Many of these
star clusters are associated with "streaks" of young stars, the combination of
which has a cometary appearance. We perform photometry on the star clusters and
diffuse stellar streaks, and fit their spectral energy distributions to obtain
ages and masses. These young stellar populations appear to be normal in terms
of their masses, luminosities and cluster formation efficiency; <10% of the
young stellar mass is located in star clusters. Our data suggest star formation
is associated with the evolution of some of the giant gas filaments in NGC~1275
that become gravitationally unstable on reaching and possibly stalling in the
outer galaxy. The stellar streaks then could represent stars moving on
ballistic orbits in the potential well of the galaxy cluster. We propose a
model where star-forming filaments, switched on ~50~Myr ago and are currently
feeding the growth of the NGC~1275 stellar halo at a rate of ~2-3 solar masses
per year. This type of process may also build stellar halos and form isolated
star clusters in the outskirts of youthful galaxies.Comment: 15 pages, 10 figures, accepted for publication in MNRA
Resolving the cosmic X-ray background with a next-generation high-energy X-ray observatory
The cosmic X-ray background (CXB), which peaks at an energy of ~30 keV, is
produced primarily by emission from accreting supermassive black holes (SMBHs).
The CXB therefore serves as a constraint on the integrated SMBH growth in the
Universe and the accretion physics and obscuration in active galactic nuclei
(AGNs). This paper gives an overview of recent progress in understanding the
high-energy (>~10 keV) X-ray emission from AGNs and the synthesis of the CXB,
with an emphasis on results from NASA's NuSTAR hard X-ray mission. We then
discuss remaining challenges and open questions regarding the nature of AGN
obscuration and AGN physics. Finally, we highlight the exciting opportunities
for a next-generation, high-resolution hard X-ray mission to achieve the
long-standing goal of resolving and characterizing the vast majority of the
accreting SMBHs that produce the CXB.Comment: Science White paper submitted to Astro2020 Decadal Survey; 5 pages, 3
figures, plus references and cover pag
Developing an On-Line Interactive Health Psychology Module.
On-line teaching material in health psychology was developed which ensured a range of students could access appropriate material for their course and level of study. This material has been developed around the concept of smaller 'content chunks' which can be combined into whole units of learning (topics), and ultimately, a module. On the basis of the underlying philosophy that the medium is part of the message, we considered interactivity to be a key element in engaging the student with the material. Consequently, the key aim of this development was to stimulate and engage students, promoting better involvement with the academic material, and hence better learning. It was hoped that this was achieved through the development of material including linked programmes and supporting material, small Java Scripts and basic email, forms and HTML additions. This material is outlined as are some of the interactive activities introduced, and the preliminary student and tutor experience described
ALMA observation of the disruption of molecular gas in M87
We present the results from Atacama Large Millimeter Array (ALMA) observations centred 40 arcsec (3 kpc in projection) south-east of the nucleus of M87. We report the detection of extended CO (2-1) line emission with a total flux of (5.5 ± 0.6) × 10-18 erg s-1 cm-2 and corresponding molecular gas mass M_{H_2}=(4.7 ± 0.4) × 10^5 M_{⊙}, assuming a Galactic CO to H2 conversion factor. ALMA data indicate a line-of-sight velocity of -129 ± 3 km s-1, in good agreement with measurements based on the [C II] and H α+[N II] lines, and a velocity dispersion of σ = 27 ± 3 km s-1. The CO (2-1) emission originates only outside the radio lobe of the active galactic nucleus (AGN) seen in the 6 cm Very Large Array image, while the filament prolongs further inwards at other wavelengths. The molecular gas in M87 appears to be destroyed or excited by AGN activity, either by direct interaction with the radio plasma, or by the shock driven by the lobe into the X-ray emitting atmosphere. This is an important piece of the puzzle in understanding the impact of the central AGN on the amount of the coldest gas from which star formation can proceed
Probing the extreme realm of AGN feedback in the massive galaxy cluster, RX J1532.9+3021
We present a detailed Chandra, XMM-Newton, VLA and HST analysis of one of the
strongest cool core clusters known, RX J1532.9+3021 (z=0.3613). Using new, deep
90 ks Chandra observations, we confirm the presence of a western X-ray cavity
or bubble, and report on a newly discovered eastern X-ray cavity. The total
mechanical power associated with these AGN-driven outflows is (22+/-9)*10^44
erg/s, and is sufficient to offset the cooling, indicating that AGN feedback
still provides a viable solution to the cooling flow problem even in the
strongest cool core clusters. Based on the distribution of the optical
filaments, as well as a jet-like structure seen in the 325 MHz VLA radio map,
we suggest that the cluster harbours older outflows along the north to south
direction. The jet of the central AGN is therefore either precessing or
sloshing-induced motions have caused the outflows to change directions. There
are also hints of an X-ray depression to the north aligned with the 325 MHz
jet-like structure, which might represent the highest redshift ghost cavity
discovered to date. We further find evidence of a cold front (r=65kpc) that
coincides with the outermost edge of the western X-ray cavity and the edge of
the radio mini-halo. The common location of the cold front with the edge of the
radio mini-halo supports the idea that the latter originates from electrons
being reaccelerated due to sloshing induced turbulence. Alternatively, its
coexistence with the edge of the X-ray cavity may be due to cool gas being
dragged out by the outburst. We confirm that the central AGN is highly
sub-Eddington and conclude that a >10^10M_Sun or a rapidly spinning black hole
is favoured to explain both the radiative-inefficiency of the AGN and the
powerful X-ray cavities.Comment: Accepted for publication to ApJ (minor corrections), 16 pages, 16
figures, 5 tables. Full resolution at http://www.stanford.edu/~juliehl/M1532
- …