144 research outputs found
HI-deficient galaxies in intermediate density environments
Observations show that spiral galaxies in galaxy clusters tend to have on
average less neutral hydrogen (HI) than galaxies of the same type and size in
the field. There is accumulating evidence that such HI-deficient galaxies are
also relatively frequent in galaxy groups. An important question is, which
mechanisms are responsible for the gas deficiency in galaxy groups. To gain a
better understanding of how environment affects the gas content of galaxies, we
identified a sample of six HI-deficient galaxies from the HI Parkes All Sky
Survey (HIPASS) using HI-optical scaling relations. One of the galaxies is
located in the outskirts of the Fornax cluster, four are in loose galaxy groups
and one is in a galaxy triplet. We present new high resolution HI observations
with the Australia Telescope Compact Array (ATCA) of these galaxies. We discuss
the possible cause of HI-deficiency in these galaxies based on HI observations
and various multi-wavelength data. We find that the galaxies have truncated HI
disks, lopsided gas distribution and some show asymmetries in their stellar
disks. We conclude that both ram pressure stripping and tidal interactions are
important gas removal mechanisms in low density environments.Comment: Accepted to MNRAS, 17 pages, 10 figures, 7 table
Interferometric Image Reconstruction using Closure Invariants and Machine Learning
Closure invariants in interferometry carry calibration-independent
information about the morphology of an observed object. Excepting simple cases,
a mapping between closure invariants and morphologies is not well established.
We aim to demonstrate that closure invariants can be used to classify the
morphology and estimate the morphological parameters using simple Machine
Learning models. We consider 6 morphological classes -- point-like, uniform
circular disc, crescent, dual disc, crescent with elliptical accretion disc,
and crescent with double jet lobes -- described by phenomenological parameters.
Using simple logistic regression, multi-layer perceptron (MLP), convolutional
neural network, and random forest models on closure invariants obtained from a
sparse aperture coverage, we find that all models except logistic regression
are able to classify the morphology with an score . The
classification accuracy notably improves with greater aperture coverage. We
also estimate morphological parameters of uniform circular disc, crescent, and
dual disc using simple MLP models, and perform a parametric image
reconstruction. The reconstructed images do not retain information about
absolute position or intensity scale. The estimated parameters and
reconstructed images are found to correspond well with the inputs. However, the
prediction accuracy worsens with increasing morphological complexity. This
proof-of-concept method opens an independent approach to interferometric
imaging under challenging observing conditions such as that faced by the Event
Horizon Telescope and Very Long Baseline Interferometry in general, and can
complement other methods to robustly constrain an object's morphology.Comment: Submitted to RAS Techniques and Instruments (RASTI) special edition
"Next-Generation Interferometric Image Reconstruction". 13 pages including
appendix, 35 figures grouped into 8 captioned figure
The HI - Star Formation Connection: Open Questions
We show data from the Survey of Ionization in Neutral Gas Galaxies (SINGG)
and Survey of Ultraviolet emission in Neutral Gas Galaxies (SUNGG) which survey
the star formation properties of HI selected galaxies as traced by H-alpha and
ultraviolet emission, respectively. The correlations found demonstrate a strong
relationship between the neutral ISM, young massive stars, and the evolved
stellar populations. For example the correlation between R band surface
brightness and the HI cycling time is tighter than the Kennicutt-Schmidt Star
Formation Law. Other scaling relations from SINGG give strong direct
confirmation of the downsizing scenario: low mass galaxies are more gaseous and
less evolved into stars than high mass galaxies. There are strong variations in
the H-alpha to UV flux ratios within and between galaxies. The only plausible
explanations for this result are that either the escape fraction of ionizing
photons or the upper end of the IMF varies with galaxy mass. We argue for the
latter interpretation, although either result has major implications for
astrophysics. A detailed dissection of the massive star content in the extended
HI disk of NGC2915 provides a consistent picture of continuing star formation
with a truncated or steep IMF, while other GALEX results indicate that star
formation edges seen in Halpha are not always apparent in the UV. These and
other recent results settle some old questions but open many new questions
about star formation and its relation to the ISM.Comment: To appear in AIP Conference Proceedings, "The Evolution of Galaxies
through the Neutral Hydrogen Window", Feb 1-3 2008, Arecibo, Puerto Rico,
eds. R. Minchin & E. Momjian. 7 page
AGN and their host galaxies in the local Universe: two mass independent Eddington ratio distribution functions characterize black hole growth
We use a phenomenological model to show that black hole growth in the local
Universe (z < 0.1) can be described by two separate, mass independent Eddington
ratio distribution functions (ERDFs). We assume that black holes can be divided
into two independent groups: those with radiatively efficient accretion,
primarily hosted by optically blue and green galaxies, and those with
radiatively inefficient accretion, which are mainly found in red galaxies. With
observed galaxy stellar mass functions as input, we show that the observed AGN
luminosity functions can be reproduced by using mass independent, broken power
law shaped ERDFs. We use the observed hard X-ray and 1.4 GHz radio luminosity
functions to constrain the ERDF for radiatively efficient and inefficient AGN,
respectively. We also test alternative ERDF shapes and mass dependent models.
Our results are consistent with a mass independent AGN fraction and AGN hosts
being randomly drawn from the galaxy population. We argue that the ERDF is not
shaped by galaxy-scale effects, but by how efficiently material can be
transported from the inner few parsecs to the accretion disc. Our results are
incompatible with the simplest form of mass quenching where massive galaxies
host higher accretion rate AGN. Furthermore, if reaching a certain Eddington
ratio is a sufficient condition for maintenance mode, it can occur in all red
galaxies, not just the most massive ones.Comment: 33 pages, 15 figures, accepted for publication in ApJ, Fig. 6 shows
the main resul
HI properties and star formation history of a fly-by pair of blue compact dwarf galaxies
A fly-by interaction has been suggested to be one of the major explanations
for enhanced star formation in blue compact dwarf (BCD) galaxies, yet no direct
evidence for this scenario has been found to date. In the HI Parkes all-sky
survey (HIPASS), ESO 435-IG 020 and ESO 435- G 016, a BCD pair were found in a
common, extended gas envelope of atomic hydrogen, providing an ideal case to
test the hypothesis that the starburst in BCDs can be indeed triggered by a
fly-by interaction. Using high-resolution data from the Australia Telescope
Compact Array (ATCA), we investigated HI properties and the spectral energy
distribution (SED) of the BCD pair to study their interaction and star
formation histories. The high-resolution HI data of both BCDs reveal a number
of peculiarities, which are suggestive of tidal perturbation. Meanwhile, 40% of
the HIPASS flux is not accounted for in the ATCA observations with no HI gas
bridge found between the two BCDs. Intriguingly, in the residual of the HIPASS
and the ATCA data, 10% of the missing flux appears to be located between the
two BCDs. While the SED-based age of the most dominant young stellar population
is old enough to have originated from the interaction with any neighbors
(including the other of the two BCDs), the most recent star formation activity
traced by strong H emission in ESO 435-IG 020 and the shear motion of
gas in ESO 435- G 016, suggest a more recent or current tidal interaction.
Based on these and the residual emission between the HIPASS and the ATCA data,
we propose an interaction between the two BCDs as the origin of their recently
enhanced star formation activity. The shear motion on the gas disk, potentially
with re-accretion of the stripped gas, could be responsible for the active star
formation in this BCD pair.Comment: 10 pages, 8 figures, 3 tables, accepted for publication in A&
Young Stellar Populations in the Collisional Ring Galaxy NGC 922
We studied the star cluster population properties in the nearby collisional
ring galaxy NGC 922 using HST/WFPC2 photometry and population synthesis
modeling. We found that 69% of the detected clusters are younger than 7 Myr,
and that most of them are located in the ring or along the bar, consistent with
the strong Halpha emission. The images also show a tidal plume pointing toward
the companion. Its stellar age is consistent with pre-existing stars that were
probably stripped off during the passage of the companion. We compared the
star-forming complexes observed in NGC 922 with those of a distant ring galaxy
from the GOODS field. It indicates very similar masses and sizes, suggesting
similar origins. Finally, we found clusters that are excellent progenitor
candidates for faint fuzzy clusters.Comment: To be published in the IAU Symposium 262 proceeding. 2 page
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