The ratio of molecular to atomic gas in spiral galaxies as a function of morphological type

In order to gain an understanding of the global processes which influence cloud and star formation in disk galaxies, it is necessary to determine the relative amounts of atomic, molecular, and ionized gas both as a function of position in galaxies and from galaxy to galaxy. With observations of the CO distributions in over 200 galaxies now completed as part of the Five College Radio Astronomy Observatory (FCRAO) Extragalactic CO Survey (Young et al. 1989), researchers are finally in a position to determine the type dependence of the molecular content of spiral galaxies, along with the ratio of molecular to atomic gas as a function of type. Do late type spirals really have more gas than early types when the molecular gas content is included. Researchers conclude that there is more than an order of magnitude decrease in the ratio of molecular to atomic gas mass as a function of morphological type from Sa-Sd; an average Sa galaxy has more molecular than atomic gas, and an average Sc has less. Therefore, the total interstellar gas mass to blue luminosity ratio, M sub gas/L sub B, increases by less than a factor of two as a function of type from Sa-Sd. The dominant effect found is that the phase of the gas in the cool interstellar medium (ISM) varies along the Hubble sequence. Researchers suggest that the more massive and centrally concentrated galaxies are able to achieve a molecular-dominated ISM through the collection of more gas in the potential. That gas may then form molecular clouds when a critical density is exceeded. The picture which these observations support is one in which the conversion of atomic gas to molecular gas is a global process which depends on large scale dynamics (cf Wyse 1986). Among interacting and merging systems, researchers find considerable scatter in the M(H2)/M(HI) ratio, with the mean ratio similar to that in the early type galaxies. The high global ratio of molecular to atomic gas could result from the removal of HI gas, the enhanced conversion of HI into H2, or both

Massive low surface brightness galaxies

A multi-wavelength study of an extreme type of galaxy which will assist us in our attempts to understand the formation and evolution of galaxies was completed. In particular, a subset of low surface brightness (bar-mu(sub B) is approximately greater than 25 mag arcsec(sup -2)), giant galaxies (LSBG's) which contain large amounts of atomic gas (M(HI) is approximately greater than 10(exp 10) solar mass), have blue optical diameters similar to those of giant spiral galaxies, but which do not seem to have prodigious amounts of ongoing star formation were observed. Our sample was drawn from the first and second Palomar Sky Surveys. This population of galaxies has been largely ignored because of selection effects which make it difficult to detect optically. The question of how these massive systems differ from the higher surface brightness 'normal' spiral galaxies is addressed. Using B and R surface photometry, in conjunction with H-alpha, HI, CO-12, and far-infrared data, an attempt is made to determine if these galaxies had an early epoch of star formation that has since faded, have ongoing star formation with an unusual interplanetary magnetic field (IMF), or are perhaps galaxies which have never efficiently formed stars due to a lack of molecular clouds

Young Stellar Populations in the Collisional Ring Galaxy NGC 922

We studied the star cluster population properties in the nearby collisional ring galaxy NGC 922 using HST/WFPC2 photometry and population synthesis modeling. We found that 69% of the detected clusters are younger than 7 Myr, and that most of them are located in the ring or along the bar, consistent with the strong Halpha emission. The images also show a tidal plume pointing toward the companion. Its stellar age is consistent with pre-existing stars that were probably stripped off during the passage of the companion. We compared the star-forming complexes observed in NGC 922 with those of a distant ring galaxy from the GOODS field. It indicates very similar masses and sizes, suggesting similar origins. Finally, we found clusters that are excellent progenitor candidates for faint fuzzy clusters.Comment: To be published in the IAU Symposium 262 proceeding. 2 page

The Closest Damped Lyman Alpha System

A difficulty of studying damped Lyman alpha systems is that they are distant, so one knows little about the interstellar medium of the galaxy. Here we report upon a damped Lyman alpha system in the nearby galaxy NGC 4203, which is so close (v_helio = 1117 km/s) and bright (B_o = 11.62) that its HI disk has been mapped. The absorption lines are detected against Ton 1480, which lies only 1.9' (12 h_50 kpc) from the center of NGC 4203. Observations were obtained with the Faint Object Spectrograph on HST (G270H grating) over the 2222-3277 Angstrom region with 200 km/s resolution. Low ionization lines of Fe, Mn, and Mg were detected, leading to metallicities of -2.29, -2.4, which are typical of other damped Lyman alpha systems, but well below the stellar metallicity of this type of galaxy. Most notably, the velocity of the lines is 1160 +- 10 km/s, which is identical to the HI rotational velocity of 1170 km/s at that location in NGC 4203, supporting the view that these absorption line systems can be associated with the rotating disks of galaxies. In addition, the line widths of the Mg lines give an upper limit to the velocity dispersion of 167 km/s, to the 99% confidence level.Comment: 4 pages LaTeX, including 1 figure and 1 table, uses emulateapj.sty. Accepted for publication by Astrophysical Journal Letter

DDO 88: A Galaxy-Sized Hole in the Interstellar Medium

We present an HI and optical study of the gas-rich dwarf irregular galaxy DDO 88. Although DDO 88's global optical and HI parameters are normal for its morphological type, it hosts a large (3 kpc diameter) and unusually complete ring of enhanced HI emission. The gas ring is located at approximately one-third of the total HI radius and one-half the optically-defined Holmberg radius, and contains 30% of the total HI of the galaxy. The ring surrounds a central depression in the HI distribution, so it may be a shell formed by a starburst episode. However, the UBV colors in the HI hole are not bluer than the rest of the galaxy as would be expected if an unusual star-forming event had taken place there recently, but there is an old (~1-3 Gyr), red cluster near the center of the hole that is massive enough to have produced the hole in the HI. An age estimate for the ring, however, is uncertain because it is not observed to be expanding. An expansion model produces a lower estimate of 0.5 Gyr, but the presence of faint star formation regions associated with the ring indicate a much younger age. We also estimate that the ring could have dispersed by now if it is older than 0.5 Gyr. This implies that the ring is younger than 0.5 Gyr. A younger age would indicate that the red cluster did not produce the hole and ring. If this ring and the depression in the gas which it surrounds were not formed by stellar winds and supernovae, this would indicate that some other, currently unidentified, mechanism is operating.Comment: 44 pages; 16 figures. To appear in AJ, January 2005. Available from ftp.lowell.edu, cd pub/dah/papers/d88 and http://www.fiu.edu/~simpsonc/d8