44 research outputs found
Long Fading Mid-Infrared Emission in Transient Coronal Line Emitters: Dust Echo of Tidal Disruption Flare
The sporadic accretion following the tidal disruption of a star by a
super-massive black hole (TDE) leads to a bright UV and soft X-ray flare in the
galactic nucleus. The gas and dust surrounding the black hole responses to such
a flare with an echo in emission lines and infrared emission. In this paper, we
report the detection of long fading mid-IR emission lasting up to 14 years
after the flare in four TDE candidates with transient coronal lines using the
WISE public data release. We estimate that the reprocessed mid-IR luminosities
are in the range between and erg~s
and dust temperature in the range of 570-800K when WISE first detected these
sources three to five years after the flare. Both luminosity and dust
temperature decreases with time. We interpret the mid-IR emission as the
infrared echo of the tidal disruption flare. We estimate the UV luminosity at
the peak flare to be 1 to 30 times erg s and for warm dust
masses to be in the range of 0.05-1.3 Msun within a few parsecs. Our results
suggest that the mid-infrared echo is a general signature of TDE in the
gas-rich environment
Genome-wide copy number variant analysis for congenital ventricular septal defects in Chinese Han population
A Uniformly Selected Sample of Low-mass Black Holes in Seyfert 1 Galaxies
We have conducted a systematic search of low-mass black holes (BHs) in active
galactic nuclei (AGNs) with broad Halpha emission lines, aiming at building a
homogeneous sample that is more complete than previous ones for fainter, less
highly accreting sources. For this purpose, we developed a set of elaborate,
automated selection procedures and applied it uniformly to the Fourth Data
Release of the Sloan Digital Sky Survey. Special attention is given to
AGN--galaxy spectral decomposition and emission-line deblending. We define a
sample of 309 type 1 AGNs with BH masses in the range -- \msun (with a median of solar mass), using the
virial mass estimator based on the broad Halpha line. About half of our sample
of low-mass BHs differs from that of Greene & Ho, with 61 of them discovered
here for the first time. Our new sample picks up more AGNs with low accretion
rates: the Eddington ratios of the present sample range from to ~1,
with 30% below 0.1. This suggests that a significant fraction of low-mass BHs
in the local Universe are accreting at low rates. The host galaxies of the
low-mass BHs have luminosities similar to those of field galaxies,
optical colors of Sbc spirals, and stellar spectral features consistent with a
continuous star formation history with a mean stellar age of less than 1 Gyr.Comment: Accepted for publication in Ap
Estimating Black Hole Masses in Active Galactic Nuclei Using the MgII 2800 Emission Line
We investigate the relationship between the linewidths of broad Mg II
\lambda2800 and Hbeta in active galactic nuclei (AGNs) to refine them as tools
to estimate black hole (BH) masses. We perform a detailed spectral analysis of
a large sample of AGNs at intermediate redshifts selected from the Sloan
Digital Sky Survey, along with a smaller sample of archival ultraviolet spectra
for nearby sources monitored with reverberation mapping. Careful attention is
devoted to accurate spectral decomposition, especially in the treatment of
narrow-line blending and Fe II contamination. We show that, contrary to popular
belief, the velocity width of Mg II tends to be smaller than that of Hbeta,
suggesting that the two species are not cospatial in the broad-line region.
Using these findings and recently updated BH mass measurements from
reverberation mapping, we present a new calibration of the empirical
prescriptions for estimating virial BH masses for AGNs using the broad Mg II
and Hbeta lines. We show that the BH masses derived from our new formalisms
show subtle but important differences compared to some of the mass estimators
currently used in the literature.Comment: Matches version to appear in ApJ, preliminarily 706:1-13, 2009; a
minor error corrected (in Table 2 only
What controls the Fe II strength in active galactic nuclei?
We used a large, homogeneous sample of 4178 z <= 0.8 Seyfert 1 galaxies and
QSOs selected from the Sloan Digital Sky Survey to investigate the strength of
Fe II emission and its correlation with other emission lines and physical
parameters of active galactic nuclei. We find that the strongest correlations
of almost all the emission-line intensity ratios and equivalent widths (EWs)
are with the Eddington ratio (L/L_{Edd}), rather than with the continuum
luminosity at 5100\AA\ (L_{5100}) or black hole mass (M_{BH}); the only
exception is the EW of ultraviolet Fe II emission, which does not correlate at
all with broad-line width, L_{5100}, M_{BH}, or L/L_{Edd}. By contrast, the
intensity ratios of both the ultraviolet and optical Fe II emission to Mg II
\lambda 2800 correlate quite strongly with L/L_{Edd}. Interestingly, among all
the emission lines in the near-UV and optical, the EW of narrow optical Fe II
emission has the strongest correlation with L/L_{Edd}. We suggest that the
variation of the emission-line strength in active galaxies is regulated by
L/L_{Edd} because it governs the global distribution of the column density of
the clouds gravitationally bound in the line-emitting region, as well as its
overall gas supply. The systematic dependence on L/L_{Edd} must be corrected
when using the FeII/MgII intensity ratio as a measure of the Fe/Mg abundance
ratio to study the history of chemical evolution in QSO environments.Comment: V3: to match the ApJ version scheduled to publish in June 2011.
Electronic tables are also available at
http://staff.ustc.edu.cn/~xbdong/Data_Release/ell_effect