195 research outputs found
The Dwarf Irregular Galaxy UGC 7636 Exposed: Stripping At Work In The Virgo Cluster
We present the results of optical spectroscopy of a newly discovered H II
region residing in the H I gas cloud located between the dwarf irregular galaxy
UGC 7636 and the giant elliptical galaxy NGC 4472 in the Virgo Cluster. By
comparing UGC 7636 with dwarf irregular galaxies in the field, we show that the
H I cloud must have originated from UGC 7636 because (1) the oxygen abundance
of the cloud agrees with that expected for a galaxy with the blue luminosity of
UGC 7636, and (2) M_{H I}/L_B for UGC 7636 becomes consistent with the measured
oxygen abundance of the cloud if the H I mass of the cloud is added back into
UGC 7636. It is likely that tides from NGC 4472 first loosened the H I gas,
after which ram-pressure stripping removed the gas from UGC 7636.Comment: 12 pages, 2 eps figures (AASTeX 5.0); accepted for publication in ApJ
Letter
Discovery of a Powerful >1061 erg AGN Outburst in the Distant Galaxy Cluster SPT-CLJ0528-5300
We present ~103 ks of Chandra observations of the galaxy cluster SPT-CLJ0528-5300 (SPT0528, z = 0.768). This cluster harbors the most radio-loud (L 1.4GHz = 1.01 Ă 1033 erg sâ1 Hzâ1) central active galactic nucleus (AGN) of any cluster in the South Pole Telescope (SPT) SunyaevâZeldovich survey with available X-ray data. We find evidence of AGN-inflated cavities in the X-ray emission, which are consistent with the orientation of the jet direction revealed by Australia Telescope Compact Array radio data. The combined probability that two such depressionsâeach at ~1.4â1.8Ï significance, oriented ~180° apart and aligned with the jet axisâwould occur by chance is 0.1%. At gsim1061 erg, the outburst in SPT0528 is among the most energetic known in the universe, and certainly the most powerful known at z > 0.25. This work demonstrates that such powerful outbursts can be detected even in shallow X-ray exposures out to relatively high redshifts (z ~ 0.8), providing an avenue for studying the evolution of extreme AGN feedback. The ratio of the cavity power ( erg sâ1) to the cooling luminosity (L cool = (1.5 ± 0.5) Ă 1044 erg sâ1) for SPT0528 is among the highest measured to date. If, in the future, additional systems are discovered at similar redshifts with equally high P cav/L cool ratios, it would imply that the feedback/cooling cycle was not as gentle at high redshifts as in the low-redshift universe
The X-ray Properties of Nearby Abell Clusters from the ROSAT All-Sky Survey: The Sample and Correlations with Optical Properties
We present an analysis of the X-ray emission for a complete sample of 288
Abell clusters spanning the redshift range 0.016<= z <= 0.09 from the ROSAT
All-Sky Survey. This sample is based on our 20cm VLA survey of nearby Abell
clusters. We find an X-ray detection rate of 83%. We report cluster X-ray
fluxes and luminosities and two different flux ratios indicative of the
concentration and extent of the emission. We examine correlations between the
X-ray luminosity, Abell Richness, and Bautz-Morgan and Rood-Sastry cluster
morphologies. We find a strong correlation between Lx and cluster richness
coupled to a dependence on the optical morphological type. These results are
consistent with the observed scatter between X-ray luminosity and temperature
and a large fraction of cooling flows. For each cluster field we also report
the positions, peak X-ray fluxes, and flux-ratios of all X-ray peaks above
3-sigma significance within a box of 2x2 Mpc centered on Abell's position.Comment: 59 pages including 3 tables, + 10 figures. To appear in AJ, Dec 200
Cosmological Simulations with Scale-Free Initial Conditions I: Adiabatic Hydrodynamics
We analyze hierarchical structure formation based on scale-free initial
conditions in an Einstein-de Sitter universe, including a baryonic component.
We present three independent, smoothed particle hydrodynamics (SPH)
simulations, performed with two different SPH codes (TreeSPH and P3MSPH) at two
resolutions. Each simulation is based upon identical initial conditions, which
consist of Gaussian distributed initial density fluctuations that have an n=-1
power spectrum. The baryonic material is modeled as an ideal gas subject only
to shock heating and adiabatic heating and cooling. The evolution is expected
to be self-similar in time, and under certain restrictions we identify the
expected scalings for many properties of the distribution of collapsed objects
in all three realizations. The distributions of dark matter masses, baryon
masses, and mass and emission weighted temperatures scale quite reliably.
However, the density estimates in the central regions of these structures are
determined by the degree of numerical resolution. As a result, mean gas
densities and luminosities obey the expected scalings only when calculated
within a limited dynamic range in density contrast. The temperatures and
luminosities of the groups show tight correlations with the baryon masses,
which can be well-represented by power-laws. The Press-Schechter (PS)
approximation predicts the distribution of group dark matter and baryon masses
fairly well, though it tends to overestimate the baryon masses. Combining the
PS mass distribution with the measured relations for T(M) and L(M) predicts the
temperature and luminosity distributions reasonably, though there are some
discrepancies at high temperatures/luminosities. The three simulations agree
well for the properties of groups that are resolved by 32 or more particles.Comment: 40 pages, 16 embedded postscript figures, uses AASTEX 4.0 style.
Minor wording changes, to appear in ApJ. Abridged abstrac
Far Ultraviolet Morphology of Star Forming Filaments in Cool Core Brightest Cluster Galaxies
We present a multiwavelength morphological analysis of star forming clouds and filaments in the central (<50 kpc) regions of 16 low redshift ( \Msol) stars reveals filamentary and clumpy morphologies, which we quantify by means of structural indices. The FUV data are compared with X-ray, Ly, narrowband H, broadband optical/IR, and radio maps, providing a high spatial resolution atlas of star formation locales relative to the ambient hot ( K) and warm ionised ( K) gas phases, as well as the old stellar population and radio-bright AGN outflows. Nearly half of the sample possesses kpc-scale filaments that, in projection, extend toward and around radio lobes and/or X-ray cavities. These filaments may have been uplifted by the propagating jet or buoyant X-ray bubble, or may have formed {\it in situ} by cloud collapse at the interface of a radio lobe or rapid cooling in a cavity's compressed shell. The morphological diversity of nearly the entire FUV sample is reproduced by recent hydrodynamical simulations in which the AGN powers a self-regulating rain of thermally unstable star forming clouds that precipitate from the hot atmosphere. In this model, precipitation triggers where the cooling-to- freefall time ratio is . This condition is roughly met at the maxmial projected FUV radius for more than half of our sample, and clustering about this ratio is stronger for sources with higher star formation rates
Deep \u3cem\u3eChandra\u3c/em\u3e, \u3cem\u3eHST\u3c/em\u3e-Cos, and MegaCam Observations of the Phoenix Cluster: Extreme Star Formation and AGN Feedback on Hundred Kiloparsec Scales
We present new ultraviolet, optical, and X-ray data on the Phoenix galaxy cluster (SPT-CLJ2344-4243). Deep optical imaging reveals previously undetected filaments of star formation, extending to radii of ~50â100 kpc in multiple directions. Combined UV-optical spectroscopy of the central galaxy reveals a massive (2 x 109 Mâ), young (~4.5 Myr) population of stars, consistent with a time-averaged star formation rate of 610 ± 50 Mâ yrâ1. We report a strong detection of O ᎠÉȘ λλ1032,1038, which appears to originate primarily in shock-heated gas, but may contain a substantial contribution (\u3e1000 Mâ yrâ1) from the cooling intracluster medium (ICM). We confirm the presence of deep X-ray cavities in the inner ~10 kpc, which are among the most extreme examples of radio-mode feedback detected to date, implying jet powers of 2â7 x 1045 erg sâ1. We provide evidence that the active galactic nucleus inflating these cavities may have only recently transitioned from quasar-mode to radio-mode, and may currently be insufficient to completely offset cooling. A model-subtracted residual X-ray image reveals evidence for prior episodes of strong radio-mode feedback at radii of ~100 kpc, with extended ghost cavities indicating a prior epoch of feedback roughly 100 Myr ago. This residual image also exhibits significant asymmetry in the inner ~200 kpc (0.15R500), reminiscent of infalling cool clouds, either due to minor mergers or fragmentation of the cooling ICM. Taken together, these data reveal a rapidly evolving cool core which is rich with structure (both spatially and in temperature), is subject to a variety of highly energetic processes, and yet is cooling rapidly and forming stars along thin, narrow filaments
First-Year Spectroscopy for the SDSS-II Supernova Survey
This paper presents spectroscopy of supernovae discovered in the first season
of the Sloan Digital Sky Survey-II Supernova Survey. This program searches for
and measures multi-band light curves of supernovae in the redshift range z =
0.05 - 0.4, complementing existing surveys at lower and higher redshifts. Our
goal is to better characterize the supernova population, with a particular
focus on SNe Ia, improving their utility as cosmological distance indicators
and as probes of dark energy. Our supernova spectroscopy program features
rapid-response observations using telescopes of a range of apertures, and
provides confirmation of the supernova and host-galaxy types as well as precise
redshifts. We describe here the target identification and prioritization, data
reduction, redshift measurement, and classification of 129 SNe Ia, 16
spectroscopically probable SNe Ia, 7 SNe Ib/c, and 11 SNe II from the first
season. We also describe our efforts to measure and remove the substantial host
galaxy contamination existing in the majority of our SN spectra.Comment: Accepted for publication in The Astronomical Journal(47pages, 9
figures
Sunyaev-Zel'dovich observations of galaxy clusters out to the virial radius with the Arcminute Microkelvin Imager
We present observations using the Small Array of the Arcminute Microkelvin
Imager (AMI; 14-18 GHz) of four Abell and three MACS clusters spanning
0.171-0.686 in redshift. We detect Sunyaev-Zel'dovich (SZ) signals in five of
these without any attempt at source subtraction, although strong source
contamination is present. With radio-source measurements from high-resolution
observations, and under the assumptions of spherical -model,
isothermality and hydrostatic equilibrium, a Bayesian analysis of the data in
the visibility plane detects extended SZ decrements in all seven clusters over
and above receiver noise, radio sources and primary CMB imprints. Bayesian
evidence ratios range from 10^{11}:1 to 10^{43}:1 for six of the clusters and
3000:1 for one with substantially less data than the others. We present
posterior probability distributions for, e.g., total mass and gas fraction
averaged over radii internal to which the mean overdensity is 1000, 500 and
200, r_200 being the virial radius. Reaching r_200 involves some extrapolation
for the nearer clusters but not for the more-distant ones. We find that our
estimates of gas fraction are low (compared with most in the literature) and
decrease with increasing radius. These results appear to be consistent with the
notion that gas temperature in fact falls with distance (away from near the
cluster centre) out to the virial radius.Comment: 18 pages, 10 figures, submitted to MNRAS (updated authors and fixed
Figure 1
Cold, clumpy accretion onto an active supermassive black hole
Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales. The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas. Recent theory and simulations instead predict that accretion can be dominated by a stochastic, clumpy distribution of very cold molecular clouds - a departure from the "hot mode" accretion model - although unambiguous observational support for this prediction remains elusive. Here we report observations that reveal a cold, clumpy accretion flow towards a supermassive black hole fuel reservoir in the nucleus of the Abell 2597 Brightest Cluster Galaxy (BCG), a nearby (redshift z=0.0821) giant elliptical galaxy surrounded by a dense halo of hot plasma. Under the right conditions, thermal instabilities can precipitate from this hot gas, producing a rain of cold clouds that fall toward the galaxy's centre, sustaining star formation amid a kiloparsec-scale molecular nebula that inhabits its core. The observations show that these cold clouds also fuel black hole accretion, revealing "shadows" cast by the molecular clouds as they move inward at about 300 kilometres per second towards the active supermassive black hole in the galaxy centre, which serves as a bright backlight. Corroborating evidence from prior observations of warmer atomic gas at extremely high spatial resolution, along with simple arguments based on geometry and probability, indicate that these clouds are within the innermost hundred parsecs of the black hole, and falling closer towards it
An X-ray Spectral Survey of Radio-Loud AGN With ASCA
We present a uniform and systematic analysis of the 0.6-10 keV X-ray spectra
of radio-loud active galactic nuclei (AGN) observed by ASCA. The sample 10
BLRGs, 5 QSRs, 9 NLRGs, and 10 RGs. At soft X-rays, about half of the NLRGs and
all of the RGs exhibit a thermal plasma component, with a bimodal distribution
of temperatures and luminosities, suggesting an origin either in a surrounding
cluster or loose group or in a hot corona. At energies above 2 keV, a hard
power-law component is detected in 90% of cases. The power-law photon indices
and luminosities in BLRGs, QSRs, and NLRGs are similar, consistent with
orientation-based unification schemes. Significant excess cold absorption is
detected in most NLRGs, but also in some BLRGS and QSRs, which was somewhat
unexpected. In contrast to Seyfert galaxies, only one object showss the
signature of a warm absorber. The nuclear X-ray luminosity is correlated with
the luminosity of the [O III] emission line, the FIR emission at 12 microns,
and the lobe radio power at 5 GHz. The Fe K line is detected in 50% of BLRGs,
one QSR, and a handful of NLRGs. This sample also includes 6 Weak Line Radio
Galaxies (WLRGs). Their spectra WLRGs can be generally decomposed into a soft
thermal component with hard absrorbed power-law component, which is
significantly flatter than any other radio-loud AGNs. Their intrinsic
luminosities are two orders of magnitude lower than in other sources of the
sample. An interesting possibility is that WLRGs represent an extreme
population of radio galaxies in which the central black hole is accreting at a
rate well below the Eddington rate.Comment: To appear in the Astrophysical Journal. 72 pages, including many
tables and figures. Fig 1 is separate, in TIFF format. Postscript version of
fig 1 and postscript version of entire preprint can be obtained from
http://www.astro.psu.edu/users/mce/preprint_index.htm
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