112 research outputs found
The Extraordinary Infrared Spectrum of NGC 1222 (Mkn 603)
The infrared spectra of starburst galaxies are dominated by the
low-excitation lines of [NeII] and [SIII], and the stellar populations deduced
from these spectra appear to lack stars larger than about 35 Msun. The only
exceptions to this result until now were low metallicity dwarf galaxies. We
report our analysis of the mid-infrared spectra obtained with IRS on Spitzer of
the starburst galaxy NGC 1222 (Mkn 603). NGC 1222 is a large spheroidal galaxy
with a starburst nucleus that is a compact radio and infrared source, and its
infrared emission is dominated by the [NeIII] line. This is the first starburst
of solar or near-solar metallicity, known to us, which is dominated by the
high-excitation lines and which is a likely host of high mass stars. We model
the emission with several different assumptions as to the spatial distibution
of the high- and low-excitation lines and find that the upper mass cutoff in
this galaxy is 40-100 Msun.Comment: accepted, Astronomical Journal. 29 pp, 4 figures. In replacement
version an acknowledgment to NRAO is adde
Molecular Gas and Nuclear Activity in Radio Galaxies Detected by IRAS
This paper reports the latest results from a millimeter-wave (CO)
spectroscopic survey of IRAS-detected radio galaxies with L_1.4GHz ~ 10^23-28
W/Hz in the redshift range z ~ 0.02-0.15. The IRAS flux-limited sample contains
33 radio galaxies with different radio morphologies and a broad range of
infrared luminosities L_IR = 10^9-12 L_sun), allowing for an investigation of
(a) whether low-z radio-selected AGN reside in molecular gas-rich host galaxes,
and (b) whether the CO properties are correlated with the properties of the
host galaxy or the AGN. All of the radio galaxies in Mazzarella et al. (1993)
and Mirabel et al. (1989) have been reobserved. Three new CO detections have
been made, raising the total number of CO detections to nine and setting the
survey detection rate at ~ 25%. Many of the CO lines have double-peaked
profiles, and the CO line widths are broad (average Delta v_FWHM ~ 500+/-130
km/s), exceeding the average CO widths of both ultraluminous infrared galaxies
(300+/-90 km/s) and Palomar-Green QSOs (260+/-160 km/s), and thus being
indicative of massive host galaxies. The CO luminosities translate into
molecular gas masses of ~ 0.4-7x10^9 M_sun, however, the 3-sigma CO upper
limits for nondetections do not rule out a molecular gas mass as high as that
of the Milky Way (~ 3x10^9 M_sun). Optical images of eight out of nine
molecular gas-rich radio galaxies show evidence of close companions and/or
tidal features. Finally, there is no obvious correlation between radio power
and molecular gas mass. However, it is notable that only one F-R II galaxy out
of 12 is detected in this CO survey; the remaining detections are of galaxies
hosting F-R I and compact radio jets.Comment: LaTex, 33 pages, including 1 jpg figure and 14 postscript figures,
ApJS, in press (August 2005
The Low CO Content of the Extremely Metal Poor Galaxy I Zw 18
We present sensitive molecular line observations of the metal-poor blue
compact dwarf I Zw 18 obtained with the IRAM Plateau de Bure interferometer.
These data constrain the CO J=1-0 luminosity within our 300 pc (FWHM) beam to
be L_CO < 1 \times 10^5 K km s^-1 pc^2 (I_CO < 1 K km s^-1), an order of
magnitude lower than previous limits. Although I Zw 18 is starbursting, it has
a CO luminosity similar to or less than nearby low-mass irregulars (e.g. NGC
1569, the SMC, and NGC 6822). There is less CO in I Zw 18 relative to its
B-band luminosity, HI mass, or star formation rate than in spiral or dwarf
starburst galaxies (including the nearby dwarf starburst IC 10). Comparing the
star formation rate to our CO upper limit reveals that unless molecular gas
forms stars much more efficiently in I Zw 18 than in our own galaxy, it must
have a very low CO-to-H_2 ratio, \sim 10^-2 times the Galactic value. We detect
3mm continuum emission, presumably due to thermal dust and free-free emission,
towards the radio peak.Comment: 5 pages in emulateapj style, accepted by the Astrophysical Journa
The Molecular ISM of Dwarf Galaxies on Kiloparsec Scales: A New Survey for CO in Northern, IRAS-detected Dwarf Galaxies
We present a new survey for CO in dwarf galaxies using the Kitt Peak 12m
telescope. We observed the central regions of 121 northern dwarfs with IRAS
detections and no known CO emission. We detect CO in 28 of these galaxies and
marginally detect another 16, increasing by about 50% the number of such
galaxies known to have significant CO emission. The galaxies we detect are
comparable in mass to the LMC, although somewhat brighter in CO and fainter in
the FIR. Within dwarfs, we find that the CO luminosity, L_CO, is most strongly
correlated with the K-band and the far infrared luminosities. There are also
strong correlations with the radio continuum and B-band luminosities, and
linear diameter. We suggest that L_CO and L_K correlate well because the
stellar component of a galaxy dominates the midplane gravitational field and
thus sets the pressure of the atomic gas, which controls the formation of H_2
from HI. We compare our sample with more massive galaxies and find that dwarfs
and large galaxies obey the same relationship between CO and the 1.4 GHz radio
continuum (RC) surface brightness. This relationship is well described by a
Schmidt Law with Sigma_RC proportional to Sigma_CO^1.3. Therefore, dwarf
galaxies and large spirals exhibit the same relationship between molecular gas
and star formation rate (SFR). We find that this result is robust to moderate
changes in the RC-to-SFR and CO-to-H_2 conversion factors. Our data appear to
be inconsistent with large (order of magnitude) variations in the CO-to-H_2
conversion factor in the star forming molecular gas. [abridged]Comment: 28 pages, 14 figures, 5 tables, ApJ accepte
Effect of Dust Extinction on Estimating Star Formation Rate of Galaxies: Lyman Continuum Extinction
We re-examine the effect of Lyman continuum ( \AA)
extinction (LCE) by dust in H {\sc ii} regions in detail and discuss how it
affects the estimation of the global star formation rate (SFR) of galaxies. To
clarify the first issue, we establish two independent methods for estimating a
parameter of LCE (), which is defined as the fraction of Lyman continuum
photons contributing to hydrogen ionization in an H {\sc ii} region. One of
those methods determines from the set of Lyman continuum flux, electron
density and metallicity. In the framework of this method, as the metallicity
and/or the Lyman photon flux increase, is found to decrease. The other
method determines from the ratio of infrared flux to Lyman continuum flux.
Importantly, we show that f \la 0.5 via both methods in many H {\sc ii}
regions of the Galaxy. Thus, it establishes that dust in such H {\sc ii}
regions absorbs significant amount of Lyman continuum photons directly. To
examine the second issue, we approximate to a function of only the
dust-to-gas mass ratio (i.e., metallicity), assuming a parameter fit for the
Galactic H {\sc ii} regions. We find that a characteristic , which is
defined as averaged over a galaxy-wide scale, is 0.3 for the nearby spiral
galaxies. This relatively small indicates that a typical increment
factor due to LCE for estimating the global SFR () is large () for the nearby spiral galaxies. Therefore, we conclude that the effect of
LCE is not negligible relative to other uncertainties of estimating the SFR of
galaxies.Comment: 18 papges, 11 figures, accepted by Ap
Absorption-Line Probes of Gas and Dust in Galactic Superwinds
We discuss moderate resolution spectra of the NaD absorption-line in a sample
of 32 far-IR-bright starburst galaxies. In 18 cases, the line is produced
primarily by interstellar gas, and in 12 of these it is blueshifted by over 100
km/s relative to the galaxy systemic velocity. The absorption-line profiles in
these outflow sources span the range from near the galaxy systemic velocity to
a maximum blueshift of 400 to 600 km/s. The outflows occur in galaxies
systematically viewed more nearly face-on than the others. We therefore argue
that the absorbing material consists of ambient interstellar gas accelerated
along the minor axis of the galaxy by a hot starburst-driven superwind. The NaD
lines are optically-thick, but indirect arguments imply total Hydrogen column
densities of N_H = few X 10^{21} cm^{-2}. This implies that the superwind is
expelling matter at a rate comparable to the star-formation rate. This
outflowing material is very dusty: we find a strong correlation between the
depth of the NaD profile and the line-of-sight reddening (E(B-V) = 0.3 to 1
over regions several-to-ten kpc in size). The estimated terminal velocities of
superwinds inferred from these data and extant X-ray data are typically 400 to
800 km/s, are independent of the galaxy rotation speed, and are comparable to
(substantially exceed) the escape velocities for (dwarf) galaxies. The
resulting loss of metals can establish the mass-metallicity relation in
spheroids, produce the observed metallicity in the ICM, and enrich a general
IGM to 10 solar metallicity. If the outflowing dust grains survive their
journey into the IGM, their effect on observations of cosmologically-distant
objects is significant.Comment: 65 pages, including 16 figures. ApJ, in pres
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
H-alpha Kinematics of the SINGS Nearby Galaxies Survey. II
This is the second part of an H-alpha kinematics follow-up survey of the
Spitzer Infrared Nearby Galaxies Survey (SINGS) sample. The aim of this program
is to shed new light on the role of baryons and their kinematics and on the
dark/luminous matter relation in the star forming regions of galaxies, in
relation with studies at other wavelengths. The data for 37 galaxies are
presented. The observations were made using Fabry-Perot interferometry with the
photon-counting camera FaNTOmM on 4 different telescopes, namely the
Canada-France-Hawaii 3.6m, the ESO La Silla 3.6m, the William Herschel 4.2m,
and the Observatoire du mont Megantic 1.6m telescopes. The velocity fields are
computed using custom IDL routines designed for an optimal use of the data. The
kinematical parameters and rotation curves are derived using the GIPSY
software. It is shown that non-circular motions associated with galactic bars
affect the kinematical parameters fitting and the velocity gradient of the
rotation curves. This leads to incorrect determinations of the baryonic and
dark matter distributions in the mass models derived from those rotation
curves.Comment: 18 pages, 5 figures, 4 tables. Accepted for publication in MNRAS. All
high-res. figures are available at
http://www.astro.umontreal.ca/fantomm/singsII
Cold gas in elliptical galaxies
We explore the evolution of the cold gas (molecular and neutral hydrogen) of
elliptical galaxies and merger remnants ordered into a time sequence on the
basis of spectroscopic age estimates. We find that the fraction of cold gas in
early merger remnants decreases significantly for ~1-2 Gyr, but subsequent
evolution toward evolved elliptical systems sees very little change. This trend
can be attributed to an initial gas depletion by strong star-formation which
subsequently declines to quiescent rates. This explanation is consistent with
the merger picture for the formation of elliptical galaxies. We also explore
the relation between HI-to-H2 mass ratio and spectroscopic galaxy age, but find
no evidence for a statistically significant trend. This suggests little net HI
to H2 conversion for the systems in the present sample.Comment: 11 pages, 6 figures, accepted for publication by MNRA
Warm molecular hydrogen in the Spitzer SINGS galaxy sample
(simplified) Results on the properties of warm H2 in 57 normal galaxies are
derived from H2 rotational transitions, obtained as part of SINGS. This study
extends previous extragalactic surveys of H2, the most abundant constituent of
the molecular ISM, to more common systems (L_FIR = e7 to 6e10 L_sun) of all
morphological and nuclear types. The S(1) transition is securely detected in
the nuclear regions of 86% of SINGS galaxies with stellar masses above 10^9.5
M_sun. The derived column densities of warm H2 (T > ~100 K), even though
averaged over kiloparsec-scale areas, are commensurate with those of resolved
PDRs; the median of the sample is 3e20 cm-2. They amount to between 1% and >30%
of the total H2. The power emitted in the sum of the S(0) to S(2) transitions
is on average 30% of the [SiII] line power, and ~4e-4 of the total infrared
power (TIR) within the same area for star-forming galaxies, which is consistent
with excitation in PDRs. The fact that H2 emission scales tightly with PAH
emission, even though the average radiation field intensity varies by a factor
ten, can also be understood if both tracers originate predominantly in PDRs,
either dense or diffuse. A large fraction of the 25 LINER/Sy targets, however,
strongly depart from the rest of the sample, in having warmer H2 in the excited
states, and an excess of H2 emission with respect to PAHs, TIR and [SiII]. We
propose a threshold in H2 to PAH power ratios, allowing the identification of
low-luminosity AGNs by an excess H2 excitation. A dominant contribution from
shock heating is favored in these objects. Finally, we detect, in nearly half
the star-forming targets, non-equilibrium ortho to para ratios, consistent with
FUV pumping combined with incomplete ortho-para thermalization by collisions,
or possibly non-equilibrium PDR fronts advancing into cold gas.Comment: ApJS, in pres
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