Global Star Formation Efficiency Research Articles

Gas and Dust in the Taffy Galaxies: UGC 12914/15

We present a comprehensive study of the dust and gas properties in the after-head-on-collision UGC 12914/15 galaxy system using multitransition CO data and SCUBA submillimeter continuum images at both 450 and 850 mu m. 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 450 mu m was detected for the first time in the two galactic disks and in the connecting bridge. Using a large velocity gradient 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-H-2 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 similar to 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; L-IR/M-H2) 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 H II region adjacent to the disk of UGC 12915.

The Role of Magnetic Fields in Spiral Galaxies

Interstellar magnetic fields are strong: up to 25 muG in spiral arms and 40 muG in nuclear regions. In the spiral galaxy NGC 6946 the average magnetic energy density exceeds that of the thermal gas. Magnetic fields control the evolution of dense clouds and possibly the global star formation efficiency in galaxies. Gas flows and shocks in spiral arms and bars are modified by magnetic fields. Magnetic forces in star-forming circumnuclear regions are able to drive mass inflow towards the active nucleus. Magnetic fields are essential for the propagation of cosmic rays and the formation of galactic winds and halos.

2MASS Observations of the Perseus, Orion A, Orion B, and Monoceros R2 Molecular Clouds

We use the 2MASS Second Incremental Release Point Source Catalog to investigate the spatial distribution of young stars in the Perseus, Orion A, Orion B, and MonR2 molecular clouds. After subtracting a semiempirical model of the field star contamination from the observed star counts, stellar surface density maps are used to identify compact clusters and any stellar population found more uniformly distributed over the molecular cloud. Each cloud contains between two and seven clusters, with at least half of the cluster population found in a single, rich cluster. In addition, a distributed stellar population is inferred in the Orion A and MonR2 molecular clouds within the uncertainties of the field star subtraction with a surface density between 0.013 and 0.083 arcmin-2. Sensitivity calculations suggest, however, that the number of stars in the distributed population may be underestimated by a factor of 2 or more if stars have been forming with a Miller-Scalo IMF at a constant star formation rate for longer than 10 Myr. After considering the possible evolutionary status of the distributed population, the global star formation efficiency implied by the sum of the distributed and cluster populations ranges between 1% and 9% among the four clouds. The fraction of the total stellar population contained in clusters for the nominal extinction model ranges from ≈50% to 100% if the distributed population is relatively young (<10 Myr), to ≈25%–70% if it is relatively old (≈100 Myr). The relatively high fraction of stars contained in clusters regardless of the age of the distributed population, in conjunction with the young ages generally inferred for embedded clusters in nearby molecular clouds, indicates that a substantial fraction of the total stellar population in these regions has formed within the past few million years in dense clusters. This suggests that either the star formation rate in each these clouds has recently peaked if one assumes clouds have ages greater than 10 Myr or molecular clouds are younger than typically thought if one assumes that the star formation rate has been approximately constant in time.

The Efficiency of Star Formation in Galaxies as a Function of Galaxy Size and Environment

The results of the FCRAO Extragalactic CO Survey are used to examine the trends in the global star formation efficiency among galaxies as a function of galaxy size and environment. For isolated spiral galaxies, Virgo cluster spiral galaxies, pairs, and field spiral galaxies, an intriguing new result indicates that the global star formation efficiency decreases as the galaxy size increases. The galaxies in these diverse environments all exhibit the same trend in that the mean global star formation efficiency is 4 times smaller in galaxies 100 kpc in diameter compared to those only 10 kpc across. This trend may result from a greater shear in the disks of large galaxies, in which flat rotation curves are found, in contrast to the disks of smaller galaxies, in which rotation curves are rising over most of the disk. This shear would be expected to increase the turbulent energy in molecular clouds and to possibly reduce the efficiency of star formation. In sharp contrast to this trend of decreasing star formation efficiency with increasing galaxy size, the galaxies that are independently recognized to be merger remnants all display a completely different behavior in the star formation efficiency variation with galaxy size. At every galaxy size, the merger remnants display the largest star formation efficiencies observed. Furthermore, there appears to be a maximum star formation efficiency that merger remnants can maintain.

Molecular Gas and Star Formation in Interacting and Isolated Galaxies

The results of the FCRAO Extragalactic CO Survey are used to examine the trends regarding the molecular gas distribution, the star formation efficiency, and the global gas surface densities (HI and H2) in galaxies as a function of environment. Relative to a sample of isolated Sbc-Scd galaxies, the strongly interacting galaxies have more compact gas distributions, a higher mean value for the global star formation efficiency, and a larger fraction of gas in molecular form. Not only is the molecular gas redistributed during interactions, but evidence is presented for an enhanced conversion of atomic to molecular gas as well.

The Global Rate and Efficiency of Star Formation in Spiral Galaxies as a Function of Morphology and Environment

The Global Rate and Efficiency of Star Formation in Spiral Galaxies as a Function of Morphology and Environment

Star formation in three nearby cloud complexes

We consider the distribution and nature of the gas and star formation in three nearby molecular cloud complexes: Taurus-Auriga, Ophiuchus, and Orion. Large-scale studies show that quite different distributions of dense gas and star formation exist in these regions, but that the global efficiency of star formation, so far, is about 1% in all of these complexes, very similar to the final efficiencies deduced for open clusters and associations. Furthermore, evidence for differences in the initial mass function among these regions is weak; all three regions may be compatible with the initial mass function for open clusters and field stars. Finally, we consider specific examples of star formation in each complex; the results indicate that the ideas of Adams, Lada, and Shu (1987) are reasonably consistent with the data on L1551 IRS 5, in the Taurus cloud, but that modifications are likely to be needed for IRAS 16293-2422 in Ophiuchus and for NGC2071 in Orion.

Star formation in three nearby cloud complexes

We consider the distribution and nature of the gas and star formation in three nearby molecular cloud complexes: Taurus-Auriga, Ophiuchus, and Orion. Large-scale studies show that quite different distributions of dense gas and star formation exist in these regions, but that the global efficiency of star formation, so far, is about 1% in all of these complexes, very similar to the final efficiencies deduced for open clusters and associations. Furthermore, evidence for differences in the initial mass function among these regions is weak; all three regions may be compatible with the initial mass function for open clusters and field stars. Finally, we consider specific examples of star formation in each complex; the results indicate that the ideas of Adams, Lada, and Shu (1987) are reasonably consistent with the data on L1551 IRS 5, in the Taurus cloud, but that modifications are likely to be needed for IRAS 16293-2422 in Ophiuchus and for NGC2071 in Orion.

Molecular clouds in the nuclear region of NGC 3079

CO emission in the central 1' of NGC 3079 has been mapped at 8" resolution using the Owens Valley Millimeter Interferometer. We find that more than 80% of the CO emission in this region arises from a rotating molecular disk 14" across, or 660 pc in radius. The location of this molecular disk coincides with the radio continuum peaks, but appears offset from the centroid of optical emission, probably due to optical obscuration in the highly inclined galaxy. The molecular mass of the rotating disk is 5.7 x 10^9^ M_sun_, which is 20% of the dynamical mass of the same region in the galaxy. We also present Hα imaging of NGC 3079 using the No. 1 0.9 m telescope of KPNO; we show that the CO disk is adjacent to the Hα shell recently reported by Ford et al. Finally, we have mapped the overall CO distribution in the disk at 45" resolution using the 14 m millimeter telescope of the Five College Radio Astronomy Observatory. There is a dynamic range of 3 orders of magnitude observed in the H_2_ surface densities in NGC 3079; relative to the central molecular surface density of 1000 M_sun_ pc^-2^, the values observed at radii of 5-10 kpc range from 65 to &lt;2 M_sun_ pc^-2^. An interaction of NGC 3079 with one of several small, nearby galaxies may have caused the clouds located in the disk to lose angular momentum, thus building up a significant molecular mass in the center of the galaxy. The similar L_IR_/M(H_2_) for NGC 3079 and the Milky Way suggests that the global star-formation efficiency is not exceptionally high. Thus, the high infrared luminosity is simply due to the large mass of star-forming clouds rather than a different triggering mechanism for star formation than in the Milky Way.