880 research outputs found
A Study of Active Galactic Nuclei in Low Surface Brightness Galaxies with Sloan Digital Sky Survey Spectroscopy
Active galactic nuclei (AGN) in low surface brightness galaxies (LSBGs) have
received little attention in previous studies. In this paper, we present
detailed spectral analysis of 194 LSBGs from the Impey et al. (1996) APM LSBG
sample which have been observed spectroscopically by the Sloan Digital Sky
Survey Data Release 5 (SDSS DR5). Our elaborate spectral analysis enables us to
carry out, for the first time, reliable spectral classification of nuclear
activities in LSBGs based on the standard emission line diagnostic diagrams in
a rigorous way. Star-forming galaxies are common, as found in about 52% LSBGs.
We find, contrary to some of the previous claims, that the fraction of galaxies
containing an AGN is significantly lower than that found in nearby normal
galaxies of high surface brightness. This is qualitatively in line with the
finding of Impey et al. (2001). This result holds true even within each
morphological type from Sa to Sc. LSBGs having larger central stellar velocity
dispersions, or larger physical sizes, tend to have a higher chance to harbor
an AGN. For three AGNs with broad emission lines, the black hole masses
estimated from the emission lines are broadly consistent with the well known
M- relation established for normal galaxies and AGNs.Comment: 5 tables and 14 figure
The Gamma-Ray Detected Narrow-Line Seyfert 1 Galaxy 1H 0323+342: Swift Monitoring and Suzaku Spectroscopy
As a radio-loud narrow-line Seyfert 1 galaxy (NLS1) detected by Fermi/LAT in
GeV -rays, 1H 0323+342 is a remarkable Active Galactic Nucleus (AGN)
showing properties characteristic of both NLS1s and blazars. Here we present
results of simultaneous X-ray and UV/optical monitoring observations on 1H
0323+342 taken with the UV/Optical Telescope (UVOT) and X-ray Telescope (XRT)
onboard the Swift satellite over six years from 2006. Overall, the object
showed statistically correlated variations in both the UV and X-ray bands on
timescales of years as well as on timescales of days. A deep Suzaku observation
reveals X-ray variability on timescales as short as a few tens of thousand
seconds, and an X-ray spectrum typical of Seyfert galaxies. The broad-band
spectral energy distribution, for which the data of UV and X-ray observations
taken on 2009 July 26-27 were used, can be well modeled with a simple one-zone
leptonic jet model plus accretion disk/corona emission. The latter is
predominantly responsible for the UV/optical and X-ray (0.3-10 keV) emission
and their observed variations. The correlated UV-X-ray variability on the
timescale of days is consistent with reprocessing of the X-ray radiation by the
accretion disk. The shortest timescale and large normalized excess variance of
the X-ray variability detected with Suzaku suggest a relatively small black
hole mass of the order of , consistent with the estimation based
on the broad H line in the optical.Comment: 38 pages, 12 figures, in press, accepted by AJ (first submitted on
2014 July 7th
Long Term Spectral Evolution of Tidal Disruption Candidates Selected by Strong Coronal Lines
We present results of follow-up optical spectroscopic observations of seven
rare, extreme coronal line emitting galaxies reported by Wang et al. (2012)
with Multi-Mirror Telescope (MMT). Large variations in coronal lines are found
in four objects, making them strong candidates of tidal disruption events
(TDE). For the four TDE candidates, all the coronal lines with ionization
status higher than [Fe VII] disappear within 5-9 years. The [Fe VII] faded by a
factor of about five in one object (J0952+2143) within 4 years, whereas emerged
in other two without them previously. A strong increment in the [O III] flux is
observed, shifting the line ratios towards the loci of active galactic nucleus
on the BPT diagrams. Surprisingly, we detect a non-canonical [O III]5007/[O
III]4959 2 in two objects, indicating a large column density of O and
thus probably optical thick gas. This also requires a very large ionization
parameter and relatively soft ionizing spectral energy distribution (e.g.
blackbody with K). Our observations can be explained as
echoing of a strong ultraviolet to soft X-ray flare caused by tidal disruption
events, on molecular clouds in the inner parsecs of the galactic nuclei.
Re-analyzing the SDSS spectra reveals double-peaked or strongly blue-shouldered
broad lines in three of the objects, which disappeared in the MMT spectra in
two objects, and faded by a factor of ten in 8 years in the remaining object
with a decrease in both the line width and centroid offset. We interpret these
broad lines as arising from decelerating biconical outflows. Our results
demonstrate that the signatures of echoing can persist for as long as ten
years, and can be used to probe the gas environment in the quiescent galactic
nuclei.Comment: 30 Pages, 10 Figures, 2 Tables, Accepted for Publication in Ap
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