1,174 research outputs found
The impact of stripped Nuclei on the Super-Massive Black Hole number density in the local Universe
The recent discovery of super-massive black holes (SMBHs) in the centers of
high-mass ultra compact dwarf galaxies (UCDs) suggests that at least some UCDs
are the stripped nuclear star clusters of lower mass galaxies. Tracing these
former nuclei of stripped galaxies provides a unique way to track the assembly
history of a galaxy or galaxy cluster. In this paper we present a new method to
estimate how many UCDs host an SMBH in their center and thus are stripped
galaxy nuclei. We revisit the dynamical mass measurements that suggest many
UCDs have more mass than expected from stellar population estimates, which
recent observations have shown is due to the presence of an SMBH. We revise the
stellar population mass estimates using a new empirical relation between the
mass-to-light ratio (M/L) and metallicity, and use this to predict which UCDs
are most likely to host an SMBH. This enables us to calculate the fraction of
UCDs that host SMBHs across their entire luminosity range for the first time.
We then apply the SMBH occupation fraction to the observed luminosity function
of UCDs and estimate that in the Fornax and Virgo cluster alone there should be
stripped nuclei with SMBHs. This analysis shows that stripped
nuclei with SMBHs are almost as common in clusters as present-day galaxy
nuclei. We estimate the local SMBH number density in stripped nuclei to
, which represents a significant fraction (10-40\%)
of the SMBH density in the local Universe. These SMBHs hidden in stripped
nuclei will increase expected event rates for tidal disruption events and
SMBH-SMBH and SMBH-BH mergers. The existence of numerous stripped nuclei with
SMBHs are a direct consequence of hierarchical galaxy formation, but until now
their impact on the SMBH density had not been quantified.Comment: 15 pages, 8 Figures, accepted for publication in Ap
Circumnuclear Structures in Megamaser Host Galaxies
Using HST, we identify circumnuclear (- pc scale) structures in
nine new HO megamaser host galaxies to understand the flow of matter from
kpc-scale galactic structures down to the supermassive black holes (SMBHs) at
galactic centers. We double the sample analyzed in a similar way by Greene et
al. (2013) and consider the properties of the combined sample of 18 sources. We
find that disk-like structure is virtually ubiquitous when we can resolve
pc scales, in support of the notion that non-axisymmetries on these
scales are a necessary condition for SMBH fueling. We perform an analysis of
the orientation of our identified nuclear regions and compare it with the
orientation of megamaser disks and the kpc-scale disks of the hosts. We find
marginal evidence that the disk-like nuclear structures show increasing
misalignment from the kpc-scale host galaxy disk as the scale of the structure
decreases. In turn, we find that the orientation of both the pc scale
nuclear structures and their host galaxy large-scale disks is consistent with
random with respect to the orientation of their respective megamaser disks.Comment: 24 pages, 16 figures, 4 tables; Resubmitted to ApJ after referee's
comment
The Structure of Nuclear Star Clusters in Nearby Late-type Spiral Galaxies from Hubble Space Telescope Wide Field Camera 3 Imaging
We obtained Hubble Space Telescope/Wide Field Camera 3 imaging of a sample of
ten of the nearest and brightest nuclear clusters residing in late-type spiral
galaxies, in seven bands that span the near-ultraviolet to the near-infrared.
Structural properties of the clusters were measured by fitting two-dimensional
surface brightness profiles to the images using GALFIT. The clusters exhibit a
wide range of structural properties. For six of the ten clusters in our sample,
we find changes in the effective radius with wavelength, suggesting radially
varying stellar populations. In four of the objects, the effective radius
increases with wavelength, indicating the presence of a younger population
which is more concentrated than the bulk of the stars in the cluster. However,
we find a general decrease in effective radius with wavelength in two of the
objects in our sample, which may indicate extended, circumnuclear star
formation. We also find a general trend of increasing roundness of the clusters
at longer wavelengths, as well as a correlation between the axis ratios of the
NCs and their host galaxies. These observations indicate that blue disks
aligned with the host galaxy plane are a common feature of nuclear clusters in
late-type galaxies, but are difficult to detect in galaxies that are close to
face-on. In color-color diagrams spanning the near-UV through the near-IR, most
of the clusters lie far from single-burst evolutionary tracks, showing evidence
for multi-age populations. Most of the clusters have integrated colors
consistent with a mix of an old population (> 1 Gyr) and a young population
(~100-300 Myr). The wide wavelength coverage of our data provides a sensitivity
to populations with a mix of ages that would not be possible to achieve with
imaging in optical bands only.Comment: Corrected a typo in author name and affiliation for MC and corrected
a typo in the conclusio
Visual perceptual echo reflects learning of regularities in rapid luminance sequences
A novel neural signature of active visual processing has recently been described in the form of the ‘perceptual echo’, in which the cross-correlation between a sequence of randomly fluctuating luminance values and occipital electrophysiological (EEG) signals exhibits a long-lasting periodic (~100ms cycle) reverberation of the input stimulus (VanRullen & Macdonald, 2012). As yet, however, the mechanisms underlying the perceptual echo and its function remain unknown. Reasoning that natural visual signals often contain temporally predictable, though non-periodic features, we hypothesized that the perceptual echo may reflect a periodic process associated with regularity learning. To test this hypothesis, we presented subjects with successive repetitions of a rapid non-periodic luminance sequence, and examined the effects on the perceptual echo, finding that echo amplitude linearly increased with the number of presentations of a given luminance sequence. These data suggest that the perceptual echo reflects a neural signature of regularity learning.
Furthermore, when a set of repeated sequences was followed by a sequence with inverted luminance polarities, the echo amplitude decreased to the same level evoked by a novel stimulus sequence. Crucially, when the original stimulus sequence was re-presented, the echo amplitude returned to a level consistent with the number of presentations of this sequence, indicating that the visual system retained sequence specific information, for many seconds, even in the presence of intervening visual input.
Altogether, our results reveal a previously undiscovered regularity learning mechanism within the human visual system, reflected by the perceptual echo
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