6,236 research outputs found
Search for pulsating PMS stars in NGC 6383
A search for pulsating pre-main sequence (PMS) stars was performed in the
young open cluster NGC 6383 using CCD time series photometry in Johnson B & V
filters. With an age of only ~1.7 million years all cluster members later than
spectral type A0 have not reached the ZAMS yet, hence being ideal candidates
for investigating PMS pulsation among A and F type stars. In total 286 stars
have been analyzed using classical Fourier techniques. From about a dozen of
stars within the boundaries of the classical instability strip, two stars were
found to pulsate: NGC 6383 #170, with five frequencies simultaneously, and NGC
6383 #198, with a single frequency. In addition, NGC 6383 #152 is a suspected
PMS variable star, but our data remain inconclusive. Linear, non-adiabatic
models assuming PMS evolutionary phase and purely radial pulsation were
calculated for the two new PMS pulsators. NGC 6383 #170 appears to pulsate
radially in third and fifth overtones, while the other three frequencies seem
to be of non-radial nature. NGC 6383 #198 pulsates monoperiodically, most
probably in the third radial overtone. Magnitudes and B-V colours were
available in the literature for only one third of all stars and we used them
for calibrating the remaining.Comment: 12 pages, 11 figures, accepted by MNRA
Two and Three Dimensional Extreme Ultraviolet Holographic Imaging with a Nanometer Spatial Resolution
The supermassive black hole mass - S\'ersic index relations for bulges and elliptical galaxies
Scaling relations between supermassive black hole mass, M_BH, and host galaxy
properties are a powerful instrument for studying their coevolution. A complete
picture involving all of the black hole scaling relations, in which each
relation is consistent with the others, is necessary to fully understand the
black hole-galaxy connection. The relation between M_BH and the central light
concentration of the surrounding bulge, quantified by the S\'ersic index n, may
be one of the simplest and strongest such relations, requiring only
uncalibrated galaxy images. We have conducted a census of literature S\'ersic
index measurements for a sample of 54 local galaxies with directly measured
M_BH values. We find a clear M_BH - n relation, despite an appreciable level of
scatter due to the heterogeneity of the data. Given the current M_BH - L_sph
and the L_sph - n relations, we have additionally derived the expected M_BH - n
relations, which are marginally consistent at the 2 sigma level with the
observed relations. Elliptical galaxies and the bulges of disc galaxies are
each expected to follow two distinct bent M_BH - n relations due to the
S\'ersic/core-S\'ersic divide. For the same central light concentration, we
predict that M_BH in the S\'ersic bulges of disc galaxies are an order
magnitude higher than in S\'ersic elliptical galaxies if they follow the same
M_BH - L_sph relation.Comment: 12 pages, 6 figures, 5 tables, accepted for publication in MNRA
The Black Hole Mass-Bulge Luminosity Relationship for Active Galactic Nuclei from Reverberation Mapping and Hubble Space Telescope Imaging
We investigate the relationship between black hole mass and bulge luminosity
for AGNs with reverberation-based black hole mass measurements and bulge
luminosities from two-dimensional decompositions of Hubble Space Telescope host
galaxy images. We find that the slope of the relationship for AGNs is 0.76-0.85
with an uncertainty of ~0.1, somewhat shallower than the M_BH \propto
L^{1.0+/-0.1} relationship that has been fit to nearby quiescent galaxies with
dynamical black hole mass measurements. This is somewhat perplexing, as the AGN
black hole masses include an overall scaling factor that brings the AGN
M_BH-sigma relationship into agreement with that of quiescent galaxies. We
discuss biases that may be inherent to the AGN and quiescent galaxy samples and
could cause the apparent inconsistency in the forms of their M_BH-L_bulge
relationships.Comment: 5 pages, 3 figures and 2 tables, submitted to ApJ Letter
Elusive Active Galactic Nuclei
A fraction of active galactic nuclei do not show the classical Seyfert-type
signatures in their optical spectra, i.e. they are optically "elusive". X-ray
observations are an optimal tool to identify this class of objects. We combine
new Chandra observations with archival X-ray data in order to obtain a first
estimate of the fraction of elusive AGN in local galaxies and to constrain
their nature. Our results suggest that elusive AGN have a local density
comparable to or even higher than optically classified Seyfert nuclei. Most
elusive AGN are heavily absorbed in the X-rays, with gas column densities
exceeding 10^24 cm^-2, suggesting that their peculiar nature is associated with
obscuration. It is likely that in elusive AGN, the nuclear UV source is
completely embedded and the ionizing photons cannot escape, which prevents the
formation of a classical Narrow Line Region. Elusive AGN may contribute
significantly to the 30 keV bump of the X-ray background.Comment: accepted for publication in MNRAS Letters, 6 pages, 3 figures, typos
and references correcte
The Nature of X-ray Bright Optically Normal Galaxies
Recent X-ray surveys by {\it Chandra} and {\it XMM-Newton} have revealed a
population of X-ray bright, optically normal galaxies (XBONGs) at moderate
redshifts. We propose that many XBONGs are powered by an inner radiatively
inefficient accretion flow (RIAF) plus an outer radiatively efficient thin
accretion disk. The absence of optical/UV activity in XBONGs is explained by
the truncation of the thin disk near the black hole, while the relatively
strong X-ray emission is explained as inverse Compton emission from the hot
RIAF. As an example, we show that the spectra of two XBONGs can be fit fairly
well with such a model. By comparing these two sources to other accreting black
holes, we argue that XBONGs are intermediate in their characteristics between
distant luminous active galactic nuclei and nearby low-luminosity nuclei.Comment: 15 pages, 3 figures, the final version accepted by ApJ; substantially
shortened but new material adde
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