37 research outputs found
The hot stars in orbit around the M31 central supermassive black hole: are they young or old?
AIMS: The cluster of hot stars observed in orbit around the central black
hole of M31 has been interpreted as a 200 Myr starburst. The formation of a
population of young stars in close proximity to a massive black hole presents a
difficult challenge to star formation theory. We point out that in a high
stellar density environment, the course of stellar evolution is modified by
frequent collisions and mergers. METHODS: Blue stragglers, which are the
results of mergers in globular clusters, occupy the same position in the
color-magnitude diagram as the observed hot stars in M31. For confirmation, the
integrated spectrum of P3 is shown to be compatible with the spectral energy
distribution of a blue horizontal branch field star. RESULTS: We suggest an old
stellar population of evolved blue horizontal-branch stars and of merger
products cannot be ruled out on the basis of the available data. Observations
are suggested that would help distinguish between a ``young'' and ``old''
stellar population interpretation of the observations.Comment: 8 pages, 2 figures, accepted by A&A for publication, in pres
The Chandra X-Ray Observatory's Radiation Environment and the AP-8/AE-8 Model
The Chandra X-ray Observatory (CXO) was launched on July 23, 1999 and reached
its final orbit on August 7, 1999. The CXO is in a highly elliptical orbit,
approximately 140,000 km x 10,000 km, and has a period of approximately 63.5
hours (~ 2.65 days). It transits the Earth's Van Allen belts once per orbit
during which no science observations can be performed due to the high radiation
environment. The Chandra X-ray Observatory Center (CXC) currently uses the
National Space Science Data Center's ``near Earth'' AP-8/AE-8 radiation belt
model to predict the start and end times of passage through the radiation
belts. However, our scheduling software uses only a simple dipole model of the
Earth's magnetic field. The resulting B, L magnetic coordinates, do not always
give sufficiently accurate predictions of the start and end times of transit of
the Van Allen belts. We show this by comparing to the data from Chandra's
on-board radiation monitor, the EPHIN (Electron, Proton, Helium Instrument
particle detector) instrument. We present evidence that demonstrates this
mis-timing of the outer electron radiation belt as well as data that also
demonstrate the significant variablity of one radiation belt transit to the
next as experienced by the CXO. We also present an explanation for why the
dipole implementation of the AP-8/AE-8 model is not ideally suited for the CXO.
Lastly, we provide a brief discussion of our on-going efforts to identify a
model that accounts for radiation belt variability, geometry, and one that can
be used for observation scheduling purposes.Comment: 12 pgs, 6 figs, for SPIE 4012 (Paper 76
Clustering of Intermediate Luminosity X-ray selected AGN at z~3
We present the first clustering results of X-ray selected AGN at z~3. Using
Chandra X-ray imaging and UVR optical colors from MUSYC photometry in the
ECDF-S field, we selected a sample of 58 z~3 AGN candidates. From the optical
data we also selected 1385 LBG at 2.8<z< 3.8 with R<25.5. We performed
auto-correlation and cross-correlation analyses, and here we present results
for the clustering amplitudes and dark matter halo masses of each sample. For
the LBG we find a correlation length of r_0,LBG = 6.7 +/- 0.5 Mpc, implying a
bias value of 3.5 +/- 0.3 and dark matter (DM) halo masses of log(Mmin/Msun) =
11.8 +/- 0.1. The AGN-LBG cross-correlation yields r_0,AGN-LBG = 8.7 +/- 1.9
Mpc, implying for AGN at 2.8<z<3.8 a bias value of 5.5 +/- 2.0 and DM halo
masses of log(Mmin/Msun) = 12.6 +0.5/-0.8. Evolution of dark matter halos in
the Lambda CDM cosmology implies that today these z~3 AGN are found in high
mass galaxies with a typical luminosity of 7+4/-2 L*.Comment: Accepted for publication in ApJ Letters. 4 pages, 4 figures (1 in
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Optical Spectroscopy Of X-Ray Sources In The Extended Chandra Deep Field South
We present the first results of our optical spectroscopy program aimed to provide redshifts and identifications for the X-ray sources in the Extended Chandra Deep Field South. A total of 339 sources were targeted using the IMACS spectrograph at the Magellan telescopes and the VIMOS spectrograph at the VLT. We measured redshifts for 186 X-ray sources, including archival data and a literature search. We find that the active galactic nucleus (AGN) host galaxies have on average redder rest-frame optical colors than nonactive galaxies, and that they live mostly in the "green valley." The dependence of the fraction of AGNs that are obscured on both luminosity and redshift is confirmed at high significance and the observed AGN spatial density is compared with the expectations from existing luminosity functions. These AGNs show a significant difference in the mid-IR to X-ray flux ratio for obscured and unobscured AGNs, which can be explained by the effects of dust self-absorption on the former. This difference is larger for lower luminosity sources, which is consistent with the dust opening angle depending on AGN luminosity.National Aeronautics and Space Administration PF8-90055, NAS8-03060NSF AST0407295Spitzer JPL RSA1288440Natural Science and Engineering Research Council of Canada (NSERC)National Academy of SciencesNASA/INTEGRAL NNG05GM79GAstronom
Mid-infrared Properties and Color Selection for X-ray Detected AGN in the MUSYC ECDF-S field
We present the mid-infrared colors of X-ray-detected AGN and explore
mid-infrared selection criteria. Using a statistical matching technique, the
likelihood ratio, over 900 IRAC counterparts were identified with a new MUSYC
X-ray source catalog that includes ~1000 published X-ray sources in the Chandra
Deep Field-South and Extended Chandra Deep Field-South. Most X-ray-selected AGN
have IRAC spectral shapes consistent with power-law slopes, f_{nu} ~
nu^{alpha}, and display a wide range of colors, -2 < alpha < 2. Although X-ray
sources typically fit to redder (more negative alpha) power-laws than non-X-ray
detected galaxies, more than 50% do have flat or blue (galaxy-like) spectral
shapes in the observed 3-8 micron band. Only a quarter of the X-ray selected
AGN detected at 24 micron are well fit by featureless red power laws in the
observed 3.6-24 micron, likely the subset of our sample whose infrared spectra
are dominated by emission from the central AGN region. Most IRAC
color-selection criteria fail to identify the majority of X-ray-selected AGN,
finding only the more luminous AGN, the majority of which have broad emission
lines. In deep surveys, these color-selection criteria select 10-20% of the
entire galaxy population and miss many moderate luminosity AGN.Comment: 29 pages, including 7 figures. Accepted for publication in the
Astrophysical Journa
Black hole growth and host galaxy morphology
We use data from large surveys of the local Universe (SDSS+Galaxy Zoo) to
show that the galaxy-black hole connection is linked to host morphology at a
fundamental level. The fraction of early-type galaxies with actively growing
black holes, and therefore the AGN duty cycle, declines significantly with
increasing black hole mass. Late-type galaxies exhibit the opposite trend: the
fraction of actively growing black holes increases with black hole mass.Comment: 4 pages, 2 figures. Proceedings of the IAU Symposium no. 267,
"Co-Evolution of Central Black Holes and Galaxies: Feeding and Feedback",
eds. B.M. Peterson, R.S. Somerville and T. Storchi-Bergman