725 research outputs found
CO observations of southern mergers
There are good reasons to believe that the formation of some elliptical galaxies result from the merging of two disk galaxies, as Toomre and Toomre first suggested (1972, Ap. J. 178, 623). Such a process strongly enhances the star-formation activity of the system, thus consuming its molecular gas. This might account for the low cold-gas content of elliptical galaxies compared to that of spirals. Researchers present here CO(1-0) and CO(2-1) observations of a sequence of three objects, NGC 1614, NGC 3256, and NGC 7252, that present characteristic features of merger remnants: single body and extended tidal tails. NGC 3256 and 7252 even exhibit the r(exp 1/4) radial light distribution that is the signature of elliptical galaxies, which indicates that their stellar bodies are in late stages of relaxation. Both NGC 1614 and NGC 3256 undergo extended bursts of star formation revealed by their large far-infrared luminosities, and by the presence in the near-infrared spectrum of the 3.28 microns feature (Morwood: 1986, A. A. 166, 4) attributed to polycyclic aromatic hydrocarbons. On the other hand, NGC 7252 has a milder activity of star formation, as suggested by a lower infrared luminosity, and thus seems to have gone past the starburst phase. The CO data were collected with the Swedish-ESO 15 m Submillimeter Telescope (SEST) (beamsize = 43 seconds at 115 GHz, 23 seconds at 230 GHz). For NGC 7252, researchers have only observed the central position in CO-12(1-0). The spectrum is displayed together with an HI spectrum obtained with the Nancay radiotelescope. Researchers mapped NGC 1614 and NGC 3256 in CO-12(1-0) and CO-12(2-1), and also observed the nucleus of NGC 3256 in CO-13(1-0). The various CO spectra obtained towards the nuclei of both galaxies are presented. Characteristics of the galaxies are gathered, with luminosities and masses in solar units and temperatures in Kelvins
The molecular spiral arms of NGC 6946
High resolution observations of molecules in external galaxies are essential to understanding physical processes leading to the formation of stars. One question is whether there is a spiral structure in the molecular gas, but it was not possible to resolve the spiral arms of external galaxies until the advent of large millimeter-wave telescopes. With the Institute for Radio Astronomy in the Millimeter Range (IRAM) 30 m telescope, researchers are carrying through the mapping of NGC 6946 in the CO-12(1-0) and (2-1) lines. This galaxy is a large, gas-rich Scd spiral with a strong star formation activity. NGC 6946 is well studied at radio and optical wavelengths, so that it is possible to compare the location of the spiral arms tracers: HI ridge, HII regions and molecular clouds. The disk CO emission is very contrasted (no lines for some positions, 1 K in CO(1-0) for some others) and correlated with the optical spiral arms: this clearly shows up in a figure which presents superimposed contours of CO(2-1) integrated emissivity and of H alpha line emission. The agreement is very good, and there is no displacement across the arm between the CO, HI and H alpha ridges of emission. The arms are barely resolved by the 23 inch beam and the molecular contrast averaged over the map is about 4. The CO(2-1) maxima are closer to the position of the HII regions than those of CO(1-0), which could be due to variations of excitation conditions. The CO excitation in the disk of NGC 6946 is low: when all data are convolved to the same resolution of 23 inches the CO(2-1) lines are about 0.45 times fainter than the CO(1-0) ones, while in the nucleus they have roughly the same intensity. This suggests that in the disk of NGC 6946 most of the CO emission comes from cold optically thick gas located in cloud envelopes rather than from cloud cores. The molecular and atomic component in the observed regions of NGC 6946 seems to be organized in large gaseous complexes
An Origin of the Huge Far-Infrared Luminosity of Starburst Mergers
Recently Taniguchi and Ohyama found that the higher CO to CO
integrated intensity ratios at a transition =1--0, CO)CO) , in a sample of starburst merging
galaxies such as Arp 220 are mainly attributed to the depression of CO
emission with respect to CO. Investigating the same sample of galaxies
analyzed by Taniguchi & Ohyama, we find that there is a tight, almost linear
correlation between the dust mass and CO luminosity. This implies that
dust grains are also depressed in the high- starburst mergers, leading to
the higher dust temperature () in them because of the relative
increase in the radiation density. Nevertheless, the average dust mass () of the high- starburst mergers is higher significantly than that of
non-high galaxies. This is naturally understood because the galaxy mergers
could accumulate a lot of dust grains from their progenitor galaxies together
with supply of dust grains formed newly in the star forming regions. Since
(FIR) given the dust emissivity law, , the increases in both and
explain well why the starburst mergers are so bright in the FIR. We discuss
that the superwind activity plays an important role in destroying dust grains
as well as dense gas clouds in the central region of mergers.Comment: 10 pages (aaspp4.sty), 3 postscript figures (embedded). Accepted for
publication in Astrophysical Journal Letter
Depression in Luminous Starburst Mergers
It is known that the class of luminous starburst galaxies tends to have
higher integrated line intensity ratios
() than normal spiral galaxies (). Since most previous studies
investigated only , it remains uncertain whether the luminous starburst
galaxies are overabundant in CO or underabundant in CO. Here we
propose a new observational test to examine this problem. Our new test is to
compare far-infrared luminosities [(FIR)] with those of CO and
and , respectively]. It is shown that there
is a very tight correlation between and L(FIR), as found in many
previous studies. However, we find that the CO luminosities of the
high-R galaxies are lower by a factor of three on the average than those
expected from the correlation for the remaining galaxies with ordinary
values. Therefore, we conclude that the observed high values for the
luminous starburst galaxies are attributed to their lower CO line
intensities.Comment: 9 pages (aaspp4.sty), 3 postscript figures (embedded). Accepted for
publication in Astrophysical Journal Letter
Gas and Dust in the Taffy Galaxies: Ugc12914/15
We present a comprehensive study of the dust and gas properties in the
after-head-on-collision UGC12914/15 galaxy system using multi-transition CO
data and SCUBA sub-mm continuum images at both 450 and 850m. CO(3-2) line
emission was detected in the disks of UGC 12914 and UGC 12915 as well as in a
bridge connecting the two galaxies. Dust emission at 450m was detected for
the first time in the two galactic disks and in the connecting bridge. Using an
LVG excitation analysis model we have obtained good estimates of the physical
parameters in different regions of this system and the amount of molecular gas
was found to be 3-4 times lower than that estimated by other investigators
using the standard Galactic CO-to-H2 conversion factor. Comparing with the dust
mass derived from the SCUBA data, we found that the gas-to-dust ratio was
comparable to the Galactic value in the two galaxy disks but a factor of ~3
higher in the bridge. The physical condition of the molecular gas in the bridge
is comparable to that in the diffuse clouds in our Galaxy. Our result is
consistent with the scenario that the bridge molecular gas originated from the
disk molecular clouds and has been drawn out of the galactic disks due to
direct cloud-cloud collision.
Our data indicate that the global star formation efficiency (SFE) in UGC
12915 is comparable to that of normal spiral galaxies, and the SFE is 40% lower
in UGC 12914 than in UGC 12915. Little star formation activity was found in the
bridge except in an HII region adjacent to the disk of UGC 12915.Comment: Accepted by AJ. 45 pages, 10 figures (Fig 1-5 and Fig 7 in gif
format
Systematic Uncertainties in Stellar Mass Estimation for Distinct Galaxy Populations
We show that different stellar-mass estimation methods yield overall mass
scales that disagree by factors up to ~2 for the z=0 galaxy population, and
more importantly, relative mass scales that sometimes disagree by factors >~3
between distinct classes of galaxies (spiral/irregular types, classical E/S0s,
and E/S0s whose colors reflect recent star formation). This comparison
considers stellar mass estimates based on (a) two different calibrations of the
correlation between K-band mass-to-light ratio and B-R color (Bell et al.,
Portinari et al.) and (b) detailed fitting of UBRJHK photometry and optical
spectrophotometry using two different population synthesis models
(Bruzual-Charlot, Maraston), with the same initial mass function in all cases.
We also compare stellar+gas masses with dynamical masses. This analysis offers
only weak arguments for preferring a particular stellar-mass estimation method,
given the plausibility of real variations in dynamical properties and dark
matter content. These results help to calibrate the systematic uncertainties
inherent in mass-based evolutionary studies of galaxies, including comparisons
of low and high redshift galaxies.Comment: 5 pages including 2 enlarged figures, ApJ Letters, accepte
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