14,601 research outputs found
Dense and Warm Molecular Gas and Warm Dust in Nearby Galaxies
We performed 12CO(1-0), 13CO(1-0), and HCN(1-0) single-dish observations
(beam size ~14"-18") toward nearby starburst and non-starburst galaxies using
the Nobeyama 45 m telescope. The 13CO(1-0) and HCN(1-0) emissions were detected
from all the seven starburst galaxies, with the intensities of both lines being
similar (i.e., the ratios are around unity). On the other hand, for case of the
non-starburst galaxies, the 13CO(1-0) emission was detected from all three
galaxies, while the HCN(1-0) emission was weakly or not detected in past
observations. This result indicates that the HCN/13CO intensity ratios are
significantly larger (~1.15+-0.32) in the starburst galaxy samples than the
non-starburst galaxy samples (<0.31+-0.14). The large-velocity-gradient model
suggests that the molecular gas in the starburst galaxies have warmer and
denser conditions than that in the non-starburst galaxies, and the
photon-dominated-region model suggests that the denser molecular gas is
irradiated by stronger interstellar radiation field in the starburst galaxies
than that in the non-starburst galaxies. In addition, HCN/13CO in our sample
galaxies exhibit strong correlations with the IRAS 25 micron flux ratios. It is
a well established fact that there exists a strong correlation between dense
molecular gas and star formation activities, but our results suggest that
molecular gas temperature is also an important parameter.Comment: 14 pages, 6 figures. Accepted for publication in PAS
Starburst galaxies
The infrared properties of star-forming galaxies, primarily as determined by the Infrared Astronomy Satellite (IRAS), are compared to X-ray, optical, and radio properties. Luminosity functions are reviewed and combined with those derived from optically discovered samples using 487 Markarian galaxies with redshifts and published IRAS 60 micron fluxes, and 1074 such galaxies in the Center for Astrophysics redshift survey. It is found that the majority of infrared galaxies which could be detected are low luminosity sources already known from the optical samples, but non-infrared surveys have found only a very small fraction of the highest luminosity sources. Distributions of infrared to optical fluxes and available spectra indicate that the majority of IRAS-selected galaxies are starburst galaxies. Having a census of starburst galaxies and associated dust allow severl important global calculations. The source counts are predicted as a function of flux limits for both infrared and radio fluxes. These galaxies are found to be important radio sources at faint flux limits. Taking the integrated flux to z = 3 indicates that such galaxies are a significant component of the diffuse X-ray background, and could be the the dominant component depending on the nature of the X-ray spectra and source evolution
What triggers galaxy transformations? The environments of post-starburst galaxies
(abridged) There are good observational reasons to believe that the
progenitors of red galaxies have undergone starbursts, followed by a
post-starburst phase. We investigate the environments of post-starburst
galaxies by measuring \textsl{(1)} number densities in
radius comoving spheres, \textsl{(2)} transverse distances to nearest
Virgo-like galaxy clusters, and \textsl{(3)} transverse distances to nearest
luminous-galaxy neighbors. We compare the post-starburst galaxies to currently
star-forming galaxies identified solely by A-star excess or \Halpha emission.
We find that post-starburst galaxies are in the same kinds of environments as
star-forming galaxies; this is our ``null hypothesis''. More importantly, we
find that at each value of the A-star excess, the star-forming and
post-starburst galaxies lie in very similar distributions of environment. The
only deviations from our null hypothesis are barely significant: a slight
deficit of post-starburst galaxies (relative to the star-forming population) in
very low-density regions, a small excess inside the virial radii of clusters,
and a slight excess with nearby neighbors. None of these effects is strong
enough to make the post-starburst galaxies a high-density phenomenon, or to
argue that the starburst events are primarily triggered by external tidal
impulses (e.g., from close passages of massive galaxies). The small excess
inside cluster virial radii suggests that some post-starbursts are triggered by
interactions with the intracluster medium, but this represents a very small
fraction of all post-starburst galaxies.Comment: ApJ in pres
The Evolution of Post-Starburst Galaxies from to the Present
Post-starburst galaxies are in the transitional stage between blue,
star-forming galaxies and red, quiescent galaxies, and therefore hold important
clues for our understanding of galaxy evolution. In this paper, we
systematically searched for and identified a large sample of post-starburst
galaxies from the spectroscopic dataset of the Sloan Digital Sky Survey (SDSS)
Data Release 9. In total, we found more than 6000 objects with redshifts
between and , making this the largest sample of
post-starburst galaxies in the literature. We calculated the luminosity
function of the post-starburst galaxies using two uniformly selected
subsamples: the SDSS Main Galaxy Sample and the Baryon Oscillation
Spectroscopic Survey CMASS Sample. The luminosity functions are reasonably fit
by half-Gaussian functions. The peak magnitudes shift as a function of redshift
from at to at . This is
consistent with the downsizing trend, whereby more massive galaxies form
earlier than low-mass galaxies. We compared the mass of the post-starburst
stellar population found in our sample to the decline of the global
star-formation rate and found that only a small amount () of all
star-formation quenching in the redshift range results in
post-starburst galaxies in the luminosity range our sample is sensitive to.
Therefore, luminous post-starburst galaxies are not the place where most of the
decline in star-formation rate of the universe is happening.Comment: 26 pages, 24 figures, 8 tables. Accepted for publication in The
Astrophysical Journa
A Multiwavelength Study on the Fate of Ionizing Radiation in Local Starbursts
The fate of ionizing radiation is vital for understanding cosmic ionization,
energy budgets in the interstellar and intergalactic medium, and star formation
rate indicators. The low observed escape fractions of ionizing radiation have
not been adequately explained, and there is evidence that some starbursts have
high escape fractions. We examine the spectral energy distributions of a sample
of local star-forming galaxies, containing thirteen local starburst galaxies
and ten of their ordinary star-forming counterparts, to determine if there
exist significant differences in the fate of ionizing radiation in these
galaxies. We find that the galaxy-to-galaxy variations in the SEDs is much
larger than any systematic differences between starbursts and non-starbursts.
For example, we find no significant differences in the total absorption of
ionizing radiation by dust, traced by the 24um, 70um, and 160um MIPS bands of
the Spitzer Space Telescope, although the dust in starburst galaxies appears to
be hotter than that of non-starburst galaxies. We also observe no excess
ultraviolet flux in the GALEX bands that could indicate a high escape fraction
of ionizing photons in starburst galaxies. The small H-alpha fractions of the
diffuse, warm ionized medium in starburst galaxies are apparently due to
temporarily boosted H-alpha luminosity within the star-forming regions
themselves, with an independent, constant WIM luminosity. This independence of
the WIM and starburst luminosities contrasts with WIM behavior in non-starburst
galaxies and underscores our poor understanding of radiation transfer in both
ordinary and starburst galaxies.Comment: 10 pages, 8 figures, accepted to ApJ 10/11/1
Star formation and dust extinction in nearby star forming and starburst galaxies
We study the star formation rate and dust extinction properties of a sample
of nearby star forming galaxies as derived from Halpha and UV (2000 A)
observations and we compare them to those of a sample of starburst galaxies.
The dust extinction in Halpha is estimated from the Balmer decrement and the
extinction in UV using the FIR to UV flux ratio or the attenuation law for
starburst galaxies of Calzetti et al. The Halpha and UV emissions are strongly
correlated with a very low scatter for the star forming objects and with a much
higher scatter for the starburst galaxies. The Halpha to UV flux ratio is found
larger by a factor ~ 2 for the starburst galaxies. We compare both samples with
a purely UV selected sample of galaxies and we conclude that the mean Halpha
and UV properties of nearby star forming galaxies are more representative of UV
selected galaxies than starburst galaxies.
We emphasize that the Halpha to UV flux ratio is strongly dependent on the
dust extinction: the positive correlation found between F{Halpha}/F{UV}$ and
F{FIR}/F{UV} vanishes when the Halpha and UV flux are corrected for dust
extinction. The Halpha to UV flux ratios converted into star formation rate and
combined with the Balmer decrement measurements are tentatively used to
estimate the dust extinction in UV.Comment: accepted for publication in Astronomy & Astrophysic
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