Column Density Of Molecular Gas Research Articles

Optical Classification of Megamaser Galaxies

We have obtained spectroscopic observations of the nuclear regions of 42 galaxies known to harbor strong OH masers. These megamaser galaxies represent a subsample of FIR (ultra)luminous galaxies, which typically have FIR luminosities in excess of 1011 L☉. The primary goal of this study is to investigate the nuclear activity sources of OH megamaser galaxies. We are able to classify the nuclear emission-line of all but one of our sample, and we find that this class of galaxies is dominated by active galactic nuclei (AGNs), although starburst galaxies do make up an appreciable fraction of the megamasers as well. Fully 45% of the megamasers exhibit Seyfert or LINER spectra, predominantly of Seyfert 2 type, although two galaxies with broad lines are observed. This observation is consistent with the current unification models for the two types of Seyfert activity, since the assumed geometry necessary to detect a megamaser places the Seyfert nucleus behind a high column density of molecular gas (i.e., the molecular torus). Starburst-nucleus galaxies comprise 32.5% of our sample, while 22.5% are classified as composite nuclear spectra (CSN) sources, showing evidence of both AGN and starburst activity. A number of objects show unusual emission-line ratios, not surprising for a group of galaxies that are known a priori to possess substantial absorbing material along the line of sight to their nuclei. Our results are compared to previous studies of FIR-selected galaxy samples, as well as to radio-continuum observations of these galaxies. The activity classification obtained from the radio data disagrees with the optical classifications in roughly 25% of the sources; we discuss possible explanations for these discrepancies.

OH Satellite-Line Masers in the Nucleus of NGC 253

We report the detection of 1720 and 1612 MHz OH ground-state satellite-line maser regions in the starburst galaxy NGC 253. We find 10 compact maser features, all of which are located along the narrow ridge of bright radio continuum emission within the central 10 arcsec2 of the nucleus. The compact OH features appear physically associated with regions of dense molecular gas, supernova remnants, and H II regions. The data for several of the features are consistent with collisional inversion from the interaction of supernova remnant shocks with molecular gas, similar to that found for Galactic supernova remnants by Frail et al. For many other features near the nucleus of NGC 253, we find conjugate behavior in the 1720 and 1612 MHz lines, with one line showing stimulated emission and the other showing stimulated absorption. These results suggest the importance of infrared pumping associated with the active star formation in NGC 253 and are consistent with large column densities of molecular gas [N(H2) 1022 cm-2] .

ROSATPSPC Observations of Cool Rich Clusters

We present the results of ROSAT PSPC observations of three cool (kT ≤ 3 keV) Abell clusters of galaxies (A262, A539, and A2589). Spatially resolved X-ray spectroscopy shows that the gas in these clusters is essentially isothermal beyond the central cooling flow region. The PSPC spectra of A262 also show excess soft X-ray absorption above that expected from the galactic hydrogen column density. The excess absorption is not confined to the central cooling flow region but is distributed across the entire field of view with a slight increase in absorption at large radii. We show that this excess absorption coincides with a region of galactic cirrus observed by IRAS. Using the 100 μm flux density toward A262, we derive a column density of molecular gas that is comparable to the column density of atomic gas and can account for the excess absorption. The spatial distribution of the molecular gas is also consistent with the observed variation in soft X-ray absorption across the PSPC field of view. Using the observed density and temperature profiles of the hot gas in these clusters, we determine their gravitating masses from the equation of hydrostatic equilibrium. The anisotropy parameter of the member galaxies is then calculated from the observed line-of-sight velocity dispersion and the X-ray—determined mass. For A262, we find that the early-type galaxies are virialized with a nearly isotropic velocity dispersion, while most of the spiral galaxies cannot be virialized for any value of the anisotropy parameter. There is also a strong alignment between the optical isophotes of the central dominant galaxy, the cluster X-ray isophotes in both A262 and A2589, and the large-scale orientation of the host supercluster. These results are consistent with a formation scenario in which clusters form through the accretion of gas and galaxies along large-scale filaments. We also calculate the entropy profile of the hot gas for a sample of groups, cool clusters, and hot clusters. We find that the gas entropy in cool clusters increases very gradually with radius while rich clusters show a much steeper gradient. The entropy of the core gas (beyond the cooling flow region) is essentially a constant in groups and rich clusters. These results suggest that the diffuse gas in groups and clusters was preheated before, or during, gravitational collapse. The marginal radial increase in the gas entropy in groups indicates that gravitational heating was almost negligible and the gas passed through weak shocks only as it was accreted. The steeper entropy gradient in rich clusters is consistent with the stronger shock heating expected in more massive systems.

Distance and total column density to the periodic radio star LSI + 61 deg 303

New observations toward the periodic radio star LSI + 61 deg 303 in the lines of H I at 21 cm and CO-18 at 2.7 mm are reported. Using the kinematic method, H I observations are interpreted in terms of the two-armed spiral shock model of Roberts (1972) to derive a distance to LSI + 61 deg 303 of 2.0 + or {minus} 0.2 kpc. The results clearly show the presence of the Perseus arm shock and locate LSI + 61 deg 303 between this shock and the more distant postshock gas. In addition, by using the H I and CO-18 data, the total neutral and molecular gas column density is derived along the line of sight toward LSI + 61 deg 303. 32 refs.

Cosmic rays and the masses of giant molecular clouds

The use of cosmic gamma rays in elucidating the spatial density of cosmic rays in the Galaxy depends crucially on knowledge of the distribution of the target gas. There has been argument as to the latter quantity and a new analysis is timely. In the present work we make a critical analysis of the various methods of determining the masses of local giant molecular clouds with particular emphasis being placed on the use of far-infrared data in examining the Taurus and Orion GMCS. Consistent results are found for the conversion factor, X, in coverting the 12CO signal ( integral T(12CO)d nu ) to the column density of molecular gas (in 1020 H molecules cm-2): X approximately=1.5. A notable result is that the far-infrared method applied to the nearest and coldest GMC (Taurus) reveals no evidence for the presence of significant quantities of very cold dust (and associated gas). There is thus support for the validity of the authors use of this method in the Inner Galaxy and their previous work in this field (Broadbent et al. 1989); in that work they found a slightly lower value of X ( approximately=1.0) in the Inner Galaxy. The reasons for other workers having found higher values of X in the Inner Galaxy are examined and it is concluded that problems of cloud definition and confusion in this crowded region are responsible. Returning to the question of the cosmic-ray distribution, the confirmation of their previous gas estimates indicates that their claim (e.g. in Bhat et al. 1986) that there is a significant dependence of cosmic-ray intensity on Galactocentric distance is well founded. The significance of the result is that the majority of the observed cosmic rays at Earth are of Galactic origin.

1300 micron continuum and C18O line mapping of the giant molecular cloud cores in Orion, W49, and W51.

We present observations of the 1300 micron continuum emission and the C18O spectral-line emission from three well-studied giant molecular cloud cores: Orion, W49, and W51. The observations were obtained at the Five College Radio Astronomy Observatory, and they provide a means to examine the consistency of these two methods to trace the column density structure of molecular clouds. We find a good general correlation between the 1300 micron continuum, which traces the column density of dust, and the C18O J = 2 --> 1 line emission, which traces the column density of molecular gas, when the effects of source temperature are taken into consideration. Moreover, nominal values for the gas and dust abundances and the dust properties reproduce the observed continuum-to-line ratios. Thus, no strong C18O abundance gradients within sources has been found, and it appears that either the C18O emission lines or the submillimeter dust emission may be used to derive the mass column density within molecular clouds accurately.