1,299 research outputs found
A radio Search for high redshift HI absorption
Ground based optical observations have yielded considerable information on
the statistics of damped-lyman alpha systems. In particular these systems are
known to be the dominant repository of the observed neutral gas at high
redshift. However, particularly at high redshift, there is the possibility that
optical observations could be biased due to the exclusion of damped-lyman alpha
systems that contain moderate to significant amounts of dust. Independent
observational constraints on the neutral hydrogen content at high redshifts and
the amount of dust in high redshift systems can be obtained from a radio search
against the bright lobes of distant radio galaxies (which is less affected by
the presence of dust in foreground damped-lyman alpha systems). We describe
here a pilot radio survey along the line of sight to a small sample of high
redshift radio galaxies, and also present some preliminary results. The survey
uses a novel observing mode at the WSRT which enables one to make sensitive
searches of a large redshift interval in a modest amount of telescope time.Comment: A version with figures is available at http://www.nfra.nl/~chengalu/
To appear in "Cold Gas at High Redshift", Eds. M.Bremer et al. (Kluwer,
Dordrecht
COLA II - Radio and Spectroscopic Diagnostics of Nuclear Activity in Galaxies
We present optical spectroscopic observations of 93 galaxies taken from the
infra-red selected COLA (Compact Objects in Low Power AGN) sample. The sample
spans the range of far-IR luminosities from normal galaxies to LIRGs. Of the
galaxies observed, 78 (84%) exhibit emission lines. Using a theoretically-based
optical emission-line scheme we classify 15% of the emission-line galaxies as
Seyferts, 77% as starbursts, and the rest are either borderline AGN/starburst
or show ambiguous characteristics. We find little evidence for an increase in
the fraction of AGN in the sample as a function of far-IR luminosity but our
sample covers only a small range in infrared luminosity and thus a weak trend
may be masked. As a whole the Seyfert galaxies exhibit a small, but
significant, radio excess on the radio-FIR correlation compared to the galaxies
classified as starbursts. Compact (<0.05'') radio cores are detected in 55% of
the Seyfert galaxies, and these galaxies exhibit a significantly larger radio
excess than the Seyfert galaxies in which cores were not detected. Our results
indicate that there may be two distinct populations of Seyferts,
``radio-excess'' Seyferts, which exhibit extended radio structures and compact
radio cores, and ``radio-quiet'' Seyferts, in which the majority of the radio
emission can be attributed to star-formation in the host galaxy. No significant
difference is seen between the IR and optical spectroscopic properties of
Seyferts with and without radio cores. (Abridged)Comment: 24 pages, 4 figures, 6 tables. Accepted for publication in ApJ,
February 200
Star Formation in Sculptor Group Dwarf Irregular Galaxies and the Nature of "Transition" Galaxies
We present new H-alpha narrow band imaging of the HII regions in eight
Sculptor Group dwarf irregular (dI) galaxies. Comparing the Sculptor Group dIs
to the Local Group dIs, we find that the Sculptor Group dIs have, on average,
lower values of SFR when normalized to either galaxy luminosity or gas mass
(although there is considerable overlap between the two samples). The
properties of ``transition'' (dSph/dIrr) galaxies in Sculptor and the Local
Group are also compared and found to be similar. The transition galaxies are
typically among the lowest luminosities of the gas rich dwarf galaxies.
Relative to the dwarf irregular galaxies, the transition galaxies are found
preferentially nearer to spiral galaxies, and are found nearer to the center of
the mass distribution in the local cloud. While most of these systems are
consistent with normal dI galaxies which currently exhibit temporarily
interrupted star formation, the observed density-morphology relationship (which
is weaker than that observed for the dwarf spheroidal galaxies) indicates that
environmental processes such as ``tidal stirring'' may play a role in causing
their lower SFRs.Comment: 35 pages, 10 figures, accepted for Feb 2003 AJ, companion to
astro-ph/021117
Theoretical Modeling of Starburst Galaxies
We have modeled a large sample of infrared starburst galaxies using both the
PEGASE v2.0 and STARBURST99 codes to generate the spectral energy distribution
of the young star clusters. PEGASE utilizes the Padova group tracks while
STARBURST99 uses the Geneva group tracks, allowing comparison between the two.
We used our MAPPINGS III code to compute photoionization models which include a
self-consistent treatment of dust physics and chemical depletion. We use the
standard optical diagnostic diagrams as indicators of the hardness of the EUV
radiation field in these galaxies. These diagnostic diagrams are most sensitive
to the spectral index of the ionizing radiation field in the 1-4 Rydberg
region. We find that warm infrared starburst galaxies contain a relatively hard
EUV field in this region. The PEGASE ionizing stellar continuum is harder in
the 1-4 Rydberg range than that of STARBURST99. As the spectrum in this regime
is dominated by emission from Wolf-Rayet (W-R) stars, this difference is most
likely due to the differences in stellar atmosphere models used for the W-R
stars. We believe that the stellar atmospheres in STARBURST99 are more
applicable to the starburst galaxies in our sample, however they do not produce
the hard EUV field in the 1-4 Rydberg region required by our observations. The
inclusion of continuum metal blanketing in the models may be one solution.
Supernova remnant (SNR) shock modeling shows that the contribution by
mechanical energy from SNRs to the photoionization models is << 20%. The models
presented here are used to derive a new theoretical classification scheme for
starbursts and AGN galaxies based on the optical diagnostic diagrams.Comment: 36 pages, 16 figures, to be published in ApJ, July 20, 200
Merging of globular clusters within inner galactic regions. I. Do they survive the tidal interaction?
The main topic of this paper is the investigation of the modes of interaction
of globular clusters (GCs) moving in the inner part of a galaxy. This is
tackled by means of high-resolution N-body simulations, whose first results are
presented in this article. Our simulations dealt with primordial very massive
(order of 10^7 solar masses) GCs that were able to decay, because of dynamical
friction, into the inner regions of triaxial galaxies on a time much shorter
than their internal relaxation time. To check the disruptive role of both tidal
forces and GC-GC collisions, we maximised the tidal interaction considering GCs
on quasi-radial orbits.
The available CPU resources allowed us to follow 8 oscillations of the GCs
along their orbits and the main findings are: i) clusters with an initial high
enough King concentration parameter (c>=1.2), preserve up to 50% of their
initial mass; ii) the inner density distribution of the survived clusters keep
a King model profile; iii) GC-GC collisions have a negligible effect with
respect to that caused by the passage through the galactic center; iv) the
orbital energy dissipation due to the tidal interaction is of the same order of
that caused by dynamical friction; v) complex sub-structures like "ripples" and
"clumps" formed, as observed around real clusters.
These findings support the validity of the hypothesis of merging of GCs in
the galactic central region, with modes that deserve further careful
investigations.Comment: LaTeX 2e, AASTeX v5.x, 23 pages with 14 figures. Accepted for
publication on the Astrophysical Journal. Final version with major change
Large-Scale Structure at z~2.5
We have made a statistically complete, unbiased survey of C IV systems toward
a region of high QSO density near the South Galactic Pole using 25 lines of
sight spanning . Such a survey makes an excellent probe of
large-scale structure at early epochs. We find evidence for structure on the
proper Mpc scale ( km Mpc) as
determined by the two point C IV - C IV absorber correlation function, and
reject the null hypothesis that C IV systems are distributed randomly on such
scales at the level. The structure likely reflects the
distance between two groups of absorbers subtending and Mpc at and respectively. There is also a marginal trend for the association of
high rest equivalent width C IV absorbers and QSOs at similar redshifts but
along different lines of sight. The total number of C IV systems detected is
consistent with that which would be expected based on a survey using many
widely separated lines of sight. Using the same data, we also find 11 Mg II
absorbers in a complete survey toward 24 lines of sight; there is no evidence
for Mg II - Mg II or Mg II - QSO clustering, though the sample size is likely
still small to detect such structure if it exists.Comment: 56 pages including 32 of figures, in gzip-ed uuencoded postscript
format, 1 long table not included, aastex4 package. Accepted for publication
in ApJ Supplement
Radio continuum and far-infrared emission from the galaxies in the Eridanus group
The Eridanus galaxies follow the well-known radio-FIR correlation. Majority
(70%) of these galaxies have their star formation rates below that of the Milky
Way. The galaxies having a significant excess of radio emission are identified
as low luminosity AGNs based on their radio morphologies obtained from the GMRT
observations. There are no powerful AGNs (L{20cm} > 10^{23} W Hz^{-1}) in the
group. The two most far-infrared and radio luminous galaxies in the group have
optical and HI morphologies suggestive of recent tidal interactions. The
Eridanus group also has two far-infrared luminous but radio-deficient galaxies.
It is believed that these galaxies are observed within a few Myr of the onset
of an intense star formation episode after being quiescent for at least a 100
Myr. The upper end of the radio luminosity distribution of the Eridanus
galaxies (L_{20cm} ~ 10^{22} W Hz^{-1}) is consistent with that of the field
galaxies, other groups, and late-type galaxies in nearby clusters.Comment: 16 pages; Accepted for publication in Journal of Astroph. & Astron.
March, 200
Improving Heart Failure Self-Management Support by Actively Engaging Out-of-Home Caregivers: Results of a Feasibility Study
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72336/1/j.1751-7133.2008.07474.x.pd
The Unified Model & Evolution of Active Galaxies: Implications from a Spectropolarimetric Study
We extend the analysis presented in Tran (2001) of a spectropolarimetric
survey of the CfA and 12micron samples of Seyfert 2 galaxies (S2s). We confirm
that S2s with hidden broad line regions (HBLRs) tend to have hotter
circumnuclear dust temperatures, show mid-IR spectra more characteristic of S1
galaxies, and are intrinsically more luminous than non-HBLR S2s. The level of
obscuration and circumnuclear star formation, however, appear to be similar
between HBLR and non-HBLR S2 galaxies, based on an examination of various
observational indicators. HBLR S2s, on average, share many similar large-scale,
presumably isotropic, characteristics with Seyfert 1 galaxies (S1s), as would
be expected if the unified model is correct, while non-HBLR S2s generally do
not. The active nuclear engines of non-HBLR S2s then, appear to be truly weaker
than HBLR S2s, which in turn, are fully consistent with being S1s viewed from
another direction. There is also evidence that the fraction of detected HBLR
increases with radio power of the AGN. Thus, not all Seyfert 2 galaxies may be
intrinsically similar in nature, and we speculate that evolutionary processes
may be at work.Comment: 15 pages with embedded figs, ApJ in press, vol. 583, 2003 Feb. 1. v2:
minor corrections to text, some typos removed; updated reference list: some
added, some remove
Full characterization of vibrational coherence in a porphyrin chromophore by two-dimensional electronic spectroscopy
In this work we present experimental and calculated two-dimensional electronic spectra for a 5,15-bisalkynyl porphyrin chromophore. The lowest energy electronic Qy transition couples mainly to a single 380 cm–1 vibrational mode. The two-dimensional electronic spectra reveal diagonal and cross peaks which oscillate as a function of population time. We analyze both the amplitude and phase distribution of this main vibronic transition as a function of excitation and detection frequencies. Even though Feynman diagrams provide a good indication of where the amplitude of the oscillating components are located in the excitation-detection plane, other factors also affect this distribution. Specifically, the oscillation corresponding to each Feynman diagram is expected to have a phase that is a function of excitation and detection frequencies. Therefore, the overall phase of the experimentally observed oscillation will reflect this phase dependence. Another consequence is that the overall oscillation amplitude can show interference patterns resulting from overlapping contributions from neighboring Feynman diagrams. These observations are consistently reproduced through simulations based on third order perturbation theory coupled to a spectral density described by a Brownian oscillator model
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