431 research outputs found
The Role of Stars in the Energetics of LINERs
Imaging studies have shown that about 25% of LINER galaxies display a compact
nuclear UV source. I compare the HST ultraviolet spectra now available for
seven such ``UV-bright'' LINERs. The spectra of NGC 404, NGC 4569, and NGC 5055
show clear absorption-line signatures of massive stars, indicating a stellar
origin for the UV continuum. Similar features are probably present in NGC 6500.
The same stellar signatures may be present but undetectable in NGC 4594, due to
the low signal-to-noise ratio of the spectrum, and in M81 and NGC 4579, due to
superposed strong, broad emission lines. The compact central UV continuum
source that is observed in these galaxies is a nuclear star cluster rather than
a low-luminosity active galactic nucleus (AGN), at least in some cases. At
least four of the LINERs suffer from an ionizing photon deficit, in the sense
that the ionizing photon flux inferred from the observed far-UV continuum is
insufficient to drive the optical H I recombination lines. Examination of the
nuclear X-ray flux of each galaxy shows a high X-ray/UV ratio in the four
``UV-photon starved'' LINERs. In these four objects, a separate component,
emitting predominantly in the extreme-UV, is the likely ionizing agent, and is
perhaps unrelated to the observed nuclear UV emission. Future observations can
determine whether the UV continuum in LINERs is always dominated by a starburst
or, alternatively, that there are two types of UV-bright LINERs:
starburst-dominated and AGN-dominated. Interestingly, recent results show that
starbursts dominate the nuclear energetics in many Seyfert 2s as well.Comment: LaTex, 10 pages, invited review to appear in Proc. of the 32nd COSPAR
Meeting, The AGN-Galaxy Connection, ed. H. R. Schmitt, A. L. Kinney, and L.
C. H
FGF signaling regulates Wnt ligand expression to control vulval cell lineage polarity in C. elegans
The interpretation of extracellular cues leading to the polarization of intracellular components and asymmetric cell divisions is a
fundamental part of metazoan organogenesis. The Caenorhabditis elegans vulva, with its invariant cell lineage and interaction of
multiple cell signaling pathways, provides an excellent model for the study of cell polarity within an organized epithelial tissue. Here,
we show that the fibroblast growth factor (FGF) pathway acts in concert with the Frizzled homolog LIN-17 to influence the localization
of SYS-1, a component of the Wnt/β-catenin asymmetry pathway, indirectly through the regulation of cwn-1. The source of the FGF
ligand is the primary vulval precursor cell (VPC) P6.p, which controls the orientation of the neighboring secondary VPC P7.p by
signaling through the sex myoblasts (SMs), activating the FGF pathway. The Wnt CWN-1 is expressed in the posterior body wall muscle
of the worm as well as in the SMs, making it the only Wnt expressed on the posterior and anterior sides of P7.p at the time of the
polarity decision. Both sources of cwn-1 act instructively to influence P7.p polarity in the direction of the highest Wnt signal. Using
single molecule fluorescence in situ hybridization, we show that the FGF pathway regulates the expression of cwn-1 in the SMs. These
results demonstrate an interaction between FGF and Wnt in C. elegans development and vulval cell lineage polarity, and highlight
the promiscuous nature of Wnts and the importance of Wnt gradient directionality within C. elegans
Dynamical Masses of Young Star Clusters in NGC 4038/4039
In order to estimate the masses of the compact, young star clusters in the
merging galaxy pair, NGC 4038/4039 (``the Antennae''), we have obtained medium
and high resolution spectroscopy using ISAAC on VLT-UT1 and UVES on VLT-UT2 of
five such clusters. The velocity dispersions were estimated using the stellar
absorption features of CO at 2.29 microns and metal absorption lines at around
8500 \AA, including lines of the Calcium Triplet. The size scales and light
profiles were measured from HST images. From these data and assuming Virial
equilibrium, we estimated the masses of five clusters. The resulting masses
range from 6.5 x 10^5 to 4.7 x 10^6 M_sun. These masses are large, factor of a
few to more than 10 larger than the typical mass of a globular cluster in the
Milky Way. The mass-to-light ratios for these clusters in the V- and K-bands in
comparison with stellar synthesis models suggest that to first order the IMF
slopes are approximately consistent with Salpeter for a mass range of 0.1 to
100 M_sun. However, the clusters show a significant range of possible IMF
slopes or lower mass cut-offs and that these variations may correlate with the
interstellar environment of the cluster. Comparison with the results of
Fokker-Planck simulations of compact clusters with properties similar to the
clusters studied here, suggest that they are likely to be long-lived and may
lose a substantial fraction of their total mass. This mass loss would make the
star clusters obtain masses which are comparable to the typical mass of a
globular cluster.Comment: 16 pages, 12 figures, A&A accepte
The Super Star Cluster NGC 1569-A Resolved on Sub-Parsec Scales with Hubble Space Telescope Spectroscopy
We present 3000--10000 Ang HST/STIS long-slit spectroscopy of the bright
super star cluster A (SSC-A) in the dwarf starburst galaxy NGC 1569. The 0.05"
HST angular resolution allows, for the first time, to probe for spatial
variations in the stellar population of a ~ 10^6 M_sun SSC. Integrated
ground-based spectra of SSC-A have previously revealed young Wolf-Rayet (WR)
signatures that coexist with features from supposedly older, red supergiant
(RSG), populations. We find that the WR emission complexes come solely from the
subcluster A2, identified in previous HST imaging, and are absent from the main
cluster A1, thus resolving the question of whether the WR and RSG features
arise in a single or distinct clusters. The equivalent widths of the WR
features in A2 --- including the CIV 5808 complex which we detect in this
object for the first time --- are larger than previously observed in other WR
galaxies. Models with sub-solar metallicity, as inferred from the nebular
emission lines of this galaxy, predict much lower equivalent widths. On the
``clean'' side of A1, opposite to A2, we find no evidence for radial gradients
in the observed stellar population at 0.05"<R<0.40" (~0.5 to 5 pc), neither in
broad-band, low-resolution, spectra nor in medium-resolution spectra of the
infrared CaII triplet.Comment: 5 pages, accepted for publication in ApJ Le
Ortho-to-Para Ratio Studies of Shocked H2 Gas in the Two Supernova Remnants IC 443 and HB 21
We present near-infrared (2.5-5.0 {\mu}m) spectral studies of shocked H2 gas
in the two supernova remnants IC 443 and HB 21, which are well known for their
interactions with nearby molecular clouds. The observations were performed with
Infrared Camera (IRC) aboard the AKARI satellite. At the energy range 7000 K <=
E(v,J) <= 20000 K, the shocked H2 gas in IC 443 shows an ortho-to-para ratio
(OPR) of 2.4+0.3-0.2, which is significantly lower than the equilibrium value
3, suggesting the existence of non-equilibrium OPR. The shocked gas in HB 21
also indicates a potential non-equilibrium OPR in the range of 1.8-2.0. The
level populations are well described by the power-law thermal admixture model
with a single OPR, where the temperature integration range is 1000-4000 K. We
conclude that the obtained non-equilibrium OPR probably originates from the
reformed H2 gas of dissociative J-shocks, considering several factors such as
the shock combination requirement, the line ratios, and the possibility that H2
gas can form on grains with a non-equilibrium OPR. We also investigate C-shocks
and partially-dissociative J-shocks for the origin of the non-equilibrium OPR.
However, we find that they are incompatible with the observed ionic emission
lines for which dissociative J-shocks are required to explain. The difference
in the collision energy of H atoms on grain surfaces would make the observed
difference between the OPRs of IC 443 and HB 21, if dissociative J-shocks are
responsible for the H2 emission. Our study suggests that dissociative J-shocks
can make shocked H2 gas with a non-equilibrium OPR.Comment: aastex preprint 12pt, 32 pages, 9 figures, 7 tables, ApJ accepte
Dense Gas in Nearby Galaxies: XVII. The Distribution of Ammonia in NGC253, Maffei2 and IC342
The central few 100 pc of galaxies often contain large amounts of molecular
gas. The chemical and physical properties of these extragalactic star formation
regions differ from those in galactic disks, but are poorly constrained. This
study aims to develop a better knowledge of the spatial distribution and
kinetic temperature of the dense neutral gas associated with the nuclear
regions of three prototypical spiral galaxies, NGC253, IC342, and Maffei2. VLA
CnD and D configuration measurements have been made of three ammonia (NH3)
inversion transitions. The (J,K)=(1,1) and (2,2) transitions of NH3 were imaged
toward IC342 and Maffei2. The (3,3) transition was imaged toward NGC253. The
entire flux obtained from single-antenna measurements is recovered for all
three galaxies observed. Derived lower limits to the kinetic temperatures
determined for the giant molecular clouds in the centers of these galaxies are
between 25 and 50K. There is good agreement between the distributions of NH3
and other H2 tracers, such as rare CO isotopologues or HCN, suggesting that NH3
is representative of the distribution of dense gas. The "Western Peak" in IC342
is seen in the (6,6) line but not in lower transitions, suggesting maser
emission in the (6,6) transition.Comment: 13 pages, 8 figures, latex format, accepted by A&
The origin of variability of the intermediate-mass black-hole ULX system HLX-1 in ESO 243-49
The ultra-luminous intermediate-mass black-hole system HLX-1 in the ESO
243-49 galaxy exhibits variability with a possible recurrence time of a few
hundred days. Finding the origin of this variability would constrain the still
largely unknown properties of this extraordinary object. Since it exhibits an
intensity-hardness behavior characteristic of black-hole X-ray transients, we
have analyzed the variability of HLX-1 in the framework of the disk instability
model that explains outbursts of such systems. We find that the long-term
variability of HLX-1 is unlikely to be explained by a model in which outbursts
are triggered by thermal-viscous instabilities in an accretion disc. Possible
alternatives include the instability in a radiation-pressure dominated disk but
we argue that a more likely explanation is a modulated mass-transfer due to
tidal stripping of a star on an eccentric orbit around the intermediate-mass
black hole. We consider an evolutionary scenario leading to the creation of
such a system and estimate the probability of its observation. We conclude,
using a simplified dynamical model of the post-collapse cluster, that no more
than 1/100 to 1/10 of Mbh < 10^4 Msun IMBHs - formed by run-away stellar
mergers in the dense collapsed cores of young clusters - could have a few times
1 Msun Main-Sequence star evolve to an AGB on an orbit eccentric enough for
mass transfer at periapse, while avoiding collisional destruction or being
scattered into the IMBH by 2-body encounters. The finite but low probability of
this configuration is consistent with the uniqueness of HLX-1. We note,
however, that the actual response of a standard accretion disk to bursts of
mass transfer may be too slow to explain the observations unless the orbit is
close to parabolic (and hence even rarer) and/or additional heating, presumably
linked to the highly time-dependent gravitational potential, are invoked.Comment: 8 pages, 2 figures. Additional figure, extended discussion. To be
published in ApJ, June 10, 2011, v734 -
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