281 research outputs found
Dwarf Galaxies with Optical Signatures of Active Massive Black Holes
We present a sample of 151 dwarf galaxies (10^8.5 < M_stellar < 10^9.5 Msun)
that exhibit optical spectroscopic signatures of accreting massive black holes
(BHs), increasing the number of known active galaxies in this stellar mass
range by more than an order of magnitude. Utilizing data from the Sloan Digital
Sky Survey Data Release 8 and stellar masses from the NASA-Sloan Atlas, we have
systematically searched for active BHs in ~25,000 emission-line galaxies with
stellar masses comparable to the Magellanic Clouds and redshifts z<0.055. Using
the narrow-line [OIII]/H-beta versus [NII]/H-alpha diagnostic diagram, we find
photoionization signatures of BH accretion in 136 galaxies, a small fraction of
which also exhibit broad H-alpha emission. For these broad-line AGN candidates,
we estimate BH masses using standard virial techniques and find a range of 10^5
< M_BH < 10^6 Msun and a median of M_BH ~ 2 x 10^5 Msun. We also detect broad
H-alpha in 15 galaxies that have narrow-line ratios consistent with
star-forming galaxies. Follow-up observations are required to determine if
these are true type 1 AGN or if the broad H-alpha is from stellar processes.
The median absolute magnitude of the host galaxies in our active sample is Mg =
-18.1 mag, which is ~1-2 magnitudes fainter than previous samples of AGN hosts
with low-mass BHs. This work constrains the smallest galaxies that can form a
massive BH, with implications for BH feedback in low-mass galaxies and the
origin of the first supermassive BH seeds.Comment: 26 pages, 15 figures, 6 tables. Accepted for publication in The
Astrophysical Journa
X-ray and Ultraviolet Properties of AGN in Nearby Dwarf Galaxies
We present new Chandra X-ray Observatory and Hubble Space Telescope
observations of eight optically selected broad-line AGN candidates in nearby
dwarf galaxies (). Including archival Chandra observations of three
additional sources, our sample contains all ten galaxies from Reines et al.
(2013) with both broad H emission and narrow-line AGN ratios (6 AGNs, 4
Composites), as well as one low-metallicity dwarf galaxy with broad H
and narrow-line ratios characteristic of star formation. All eleven galaxies
are detected in X-rays. Nuclear X-ray luminosities range from to . In
all cases except for the star forming galaxy, the nuclear X-ray luminosities
are significantly higher than would be expected from X-ray binaries, providing
strong confirmation that AGN and composite dwarf galaxies do indeed host
actively accreting BHs. Using our estimated BH masses (which range from
), we find inferred Eddington
fractions ranging from , i.e. comparable to massive broad-line
quasars at higher redshift. We use the HST imaging to determine the ratio of
ultraviolet to X-ray emission for these AGN, finding that they appear to be
less X-ray luminous with respect to their UV emission than more massive quasars
(i.e. values an average of 0.36 lower than expected based on
the relation between and luminosity).
Finally, we discuss our results in the context of different accretion models
onto nuclear BHs.Comment: 15 pages, 15 figures, 4 tables. Submitted to Ap
Hubble Space Telescope Imaging of the Active Dwarf Galaxy RGG 118
RGG 118 (SDSS 1523+1145) is a nearby (), dwarf disk galaxy
() found to host an active
solar mass black hole at its core (Baldassare et al. 2015). RGG 118 is one of a
growing collective sample of dwarf galaxies known to contain active galactic
nuclei -- a group which, until recently, contained only a handful of objects.
Here, we report on new \textit{Hubble Space Telescope} Wide Field Camera 3 UVIS
and IR imaging of RGG 118, with the main goal of analyzing its structure. Using
2-D parametric modeling, we find that the morphology of RGG 118 is best
described by an outer spiral disk, inner component consistent with a
pseudobulge, and central PSF. The luminosity of the PSF is consistent with the
central point source being dominated by the AGN. We measure the luminosity and
mass of the "pseudobulge" and confirm that the central black hole in RGG 118 is
under-massive with respect to the and relations. This result is consistent with a picture in which black
holes in disk-dominated galaxies grow primarily through secular processes.Comment: Accepted to Astrophysical Journal. 11 pages, 8 figure
Mid-Infrared Colors of Dwarf Galaxies: Young Starbursts Mimicking Active Galactic Nuclei
Searching for active galactic nuclei (AGN) in dwarf galaxies is important for
our understanding of the seed black holes that formed in the early Universe.
Here, we test infrared selection methods for AGN activity at low galaxy masses.
Our parent sample consists of ~18,000 nearby dwarf galaxies (M*< 3 x 10^9 Msun,
) in the Sloan Digital Sky Survey with significant detections in the
first three bands of the AllWISE data release from the Wide-field Infrared
Survey Explorer (WISE). First, we demonstrate that the majority of
optically-selected AGNs in dwarf galaxies are not selected as AGNs using WISE
infrared color diagnostics and that the infrared emission is dominated by the
host galaxies. We then investigate the infrared properties of
optically-selected star-forming dwarf galaxies, finding that the galaxies with
the reddest infrared colors are the most compact, with blue optical colors,
young stellar ages and large specific star formation rates. These results
indicate that great care must be taken when selecting AGNs in dwarf galaxies
using infrared colors, as star-forming dwarf galaxies are capable of heating
dust in such a way that mimics the infrared colors of more luminous AGNs. In
particular, a simple color cut alone should not be
used to select AGNs in dwarf galaxies. With these complications in mind, we
present a sample of 41 dwarf galaxies worthy of follow-up observations that
fall in WISE infrared color space typically occupied by more luminous AGNs.Comment: 17 pages, 12 figures, accepted by the Astrophysical Journa
Simulating X-ray Reverberation in the UV-Emitting Regions of Active Galactic Nuclei Accretion Disks with 3D Multi-Frequency Magnetohydrodynamic Simulations
Active galactic nuclei (AGN) light curves observed with different wavebands
show that the variability in longer wavelength bands lags the variability in
shorter wavelength bands. Measuring these lags, or reverberation mapping, is
used to measure the radial temperature profile and extent of AGN disks,
typically with a reprocessing model that assumes X-rays are the main driver of
the variability in other wavelength bands. To demonstrate how this reprocessing
works with realistic accretion disk structures, we use 3D local shearing box
multi-frequency radiation magnetohydrodynamic (MHD) simulations to model the
UV-emitting region of an AGN disk, which is unstable to the magnetorotational
instability (MRI) and convection. At the same time, we inject hard X-rays
(~keV) into the simulation box to study the effects of X-ray irradiation on
the local properties of the turbulence and the resulting variability of the
emitted UV light curve. We find that disk turbulence is sufficient to drive
intrinsic variability in emitted UV light curves and that a damped random walk
(DRW) model is a good fit to this UV light curve for timescales ~days.
Meanwhile, the injected X-rays have almost no impact on the power spectrum of
the emitted UV light curve. In addition, the injected X-ray and emitted UV
light curves are only correlated if there is X-ray variability on timescales
~day, in which case we find a correlation coefficient . These
results suggest that hard X-rays with scattering dominated opacity are likely
not the main driver of the reverberation signals.Comment: 9 pages, 3 figures, submitted to ApJ
Variable Hard X-ray Emission from the Candidate Accreting Black Hole in Dwarf Galaxy Henize 2-10
We present an analysis of the X-ray spectrum and long-term variability of the
nearby dwarf starburst galaxy Henize 2-10. Recent observations suggest that
this galaxy hosts an actively accreting black hole with mass ~10^6 M_sun. The
presence of an AGN in a low-mass starburst galaxy marks a new environment for
active galactic nuclei (AGNs), with implications for the processes by which
"seed" black holes may form in the early Universe. In this paper, we analyze
four epochs of X-ray observations of Henize 2-10, to characterize the long-term
behavior of its hard nuclear emission. We analyze observations with Chandra
from 2001 and XMM-Newton from 2004 and 2011, as well as an earlier, less
sensitive observation with ASCA from 1997. Based on detailed analysis of the
source and background, we find that the hard (2-10 keV) flux of the putative
AGN has decreased by approximately an order of magnitude between the 2001
Chandra observation and exposures with XMM-Newton in 2004 and 2011. The
observed variability confirms that the emission is due to a single source. It
is unlikely that the variable flux is due to a supernova or ultraluminous X-ray
source, based on the observed long-term behavior of the X-ray and radio
emission, while the observed X-ray variability is consistent with the behavior
of well-studied AGNs.Comment: 7 pages, 4 figures, 2 tables; accepted for publication in Ap
Variable Hard-X-Ray Emission from the Candidate Accreting Black Hole in Dwarf Galaxy Henize 2–10
We present an analysis of the X-ray spectrum and long-term variability of the nearby dwarf starburst galaxy Henize 2–10. Recent observations suggest that this galaxy hosts an actively accreting black hole (BH) with mass ~106 . The presence of an active galactic nucleus (AGN) in a low-mass starburst galaxy marks a new environment for AGNs, with implications for the processes by which seed BHs may form in the early universe. In this paper, we analyze four epochs of X-ray observations of Henize 2–10, to characterize the long-term behavior of its hard nuclear emission. We analyze observations with Chandra from 2001 and XMM-Newton from 2004 and 2011, as well as an earlier, less sensitive observation with ASCA from 1997. Based on a detailed analysis of the source and background, we find that the hard (2–10 keV) flux of the putative AGN has decreased by approximately an order of magnitude between the 2001 Chandra observation and exposures with XMM-Newton in 2004 and 2011. The observed variability confirms that the emission is due to a single source. It is unlikely that the variable flux is due to a supernova or ultraluminous X-ray source, based on the observed long-term behavior of the X-ray and radio emission, while the observed X-ray variability is consistent with the behavior of well-studied AGNs
The Effect of AGN on the Global HI Content of Isolated Low-Mass Galaxies
We investigate the global neutral hydrogen (HI) content of isolated galaxies
selected from the SDSS spectroscopic survey with optical evidence of Active
Galactic Nuclei (AGN). Our sample includes galaxies with unresolved HI
observations from the ALFALFA 70% data release combined with deeper HI
observations of low-mass galaxies with 7.0 < log(M*) < 9.5. We examine the HI
masses of this sample using the distance from the star-forming sequence on the
OIII\Hb and NII\Ha Baldwin Phillips Terlevich (BPT) diagram as a measurement of
AGN activity. In agreement with previous studies, we find that, for galaxies
with log(M*) > 9.5, AGN activity does not correlate with the global HI content.
However, for galaxies with 9.2 < log(M*) < 9.5, we identify a set of objects at
large distances from the BPT star-forming sequence and lower than expected HI
masses. This gas-depleted sample is red in both g-r and NUV-r colors and
compact without distinguishable signs of star formation. This is surprising
because the vast majority of isolated galaxies in this stellar mass regime are
both star-forming and gas-rich. These galaxies are greater than 1.5 Mpc from
any massive galaxy, ruling out environmental processes as a source of the
gas-depletion. We suggest that either black hole feedback or shocks from
extremely bursty star formation cause the emission lines and have destroyed or
otherwise consumed the cold gas.Comment: Accepted to ApJ, 14 pages, 7 figure
- …