Judith S. Young

The global rate and efficiency of star formation in spiral galaxies as a function of morphology and environment

CCD images of Ha: and R-band emission in 120 spiral galaxies were obtained using the now-retired No. 1-0.9 m telescope of Kitt Peak National Observatory. These images were used to derive the distribution and total flux of continuum-subtracted Ha: line emission, and therefore the Ha: surface brightnesses and high mass star formation rates in these galaxies. We find a small but significant variation in the mean Ha surface brightness for spiral galaxies along the Hubble sequence; the Sd-Ir galaxies exhibit a mean Ha surface brightness 1.4 times higher than the Sbc-Scd galaxies, and 2-3 times higher than the Sa-Sb galaxies. Estimates for the total formation rate for high mass stars have been compared with global molecular gas masses to determine the global efficiency of high mass star formation as a function of morphological type and environment. We find that the mean efficiency of high mass star formation in this sample of spiral galaxies shows little dependence on morphological type for galaxies of type Sa through Scd, although there is a wide range in star formation efficiencies within each type. Galaxies in disturbed environments (i.e., strongly interacting systems) are found to have a mean star formation efficiency ~4 times higher than in isolated spiral galaxies, uncorrected for extinction. This confirms previous findings (Young et al 1986a,b; Sanders et al 1986; Solomon & Sage 1988; Tinney et al 1990), based on the far-inffared luminosity rather than the Ha luminosity to trace the rate of high mass star formation, that the mean star formation efficiency among isolated galaxies is significantly lower than that among interacting systems. This result provides further confirmation that the rate of high mass star formation is reasonably well traced by both the Ha and the IR luminosity in spiral galaxies.

Global properties of infrared bright galaxies

Infrared flux densities of 182 galaxies, including 50 galaxies in the Virgo cluster, were analyzed using IRAS data for 12, 25, 60, and 100 microns, and the results were compared with data listed in the Point Source Catalog (PSC, 1985). In addition, IR luminosities, L(IRs), colors, and warm dust masses were derived for these galaxies and were compared with the interstellar gas masses and optical luminosities of the galaxies. It was found that, for galaxies whose optical diameter measures between 5 and 8 arcmin, the PSC flux densities are underestimated by a factor of 2 at 60 microns, and by a factor of 1.5 at 100 microns. It was also found that, for 49 galaxies, the mass of warm dust correlated well with the H2 mass, and that L(IR) correlated with L(H-alpha), demonstrating that the L(IR) measures the rate of star formation in these galaxies. 74 refs.

The Molecular Gas Distribution and Schmidt Law in M33

The relationship between the star formation rate and surface density of neutral gas within the disk of M33 is examined with new imaging observations of 12CO J = 1-0 emission gathered with the Five College Radio Astronomy Observatory (FCRAO) 14 m telescope and IRAS HiRes images of the 60 and 100 μm emission. The Schmidt law, ΣSFR ~ Σ, is constructed using radial profiles of the H I 21 cm, CO, and far-infrared emission. We identify a strong correlation between the star formation rate and molecular gas surface density. This suggests that the condensation of giant molecular clouds (GMCs) is the limiting step to star formation within the M33 disk. The corresponding molecular Schmidt index, nmol, is 1.36 ± 0.08. The star formation rate has a steep dependence on total mass gas surface density, (Σ + Σ), owing to the shallow radial profile of the atomic gas that dominates the total gas surface density for most radii. The disk pressure of the gas is shown to play a prominent role in regulating the molecular gas fraction in M33.

The effects of environment on the molecular and atomic gas properties of large Virgo cluster spirals

The effect of the Virgo environment on the ISM of Virgo disks is presently ascertained by comparing the molecular and atomic gas properties of 40 Virgo cluster spiral galaxies with their optical properties. Virgo Sc galaxies fainter than B(T)exp 0 = 12 are found to have significantly weaker CO emission/unit area and unit mass than brighter Sc galaxies, which have a fairly good correlation between CO luminosity and optical luminosity. An anticorrelation is noted between CO and H I emission in Virgo Sc galaxies with B(T)exp 0 = 11-12. Comparisons of the CO/H I flux ratios with the H I defficiency parameter indicate that there is no CO deficiency in H I-deficient Virgo spirals. 93 refs.

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.

CO luminosity functions for far-infrared- and B-band-selected galaxies and the first estimate for Omega(H I+H2)

We derive a nonparametric CO luminosity function using an FIR- and an optical B-band-selected sample of the galaxies included in the FCRAO Extragalactic CO Survey. The FIR-selected sample is defined using the IRAS bright galaxy samples (BGS; IRAS 60 μm flux density ≥5.24 Jy). Although our CO sample is not complete, the normalization using the BGS reproduces the IRAS 60 μm luminosity function in excellent agreement with those found in the literature. Similarly, a B-band-selected sample defined using the Revised Shapley-Ames catalog is used to derive a CO luminosity function for a comparison. A Schechter function describes both the derived CO luminosity functions reasonably well. Adopting the standard CO-to-H2 conversion factor, we derive a molecular gas density of ρ = (3.1 ± 1.2) × 107 h M☉ Mpc-3 for the local volume. Combining with the measurements of the local H I mass density and the helium contribution, we estimate that the total mass density of cold neutral gas in the local universe is Ωgas = (4.3 ± 1.1) × 10-4 h-1, which is about 20% of the total stellar mass density Ω*.

Twin peaks of CO emission in the central regions of barred galaxies

New high-resolution (∼2″) CO maps of the central regions of three barred galaxies show that the strongest CO emission arises from twin peaks, which are oriented perpendicular to the large-scale stellar bars and located where dust lanes intersect nuclear rings of H II regions. These twin gas concentrations can be explained by the crowding of gas streamlines near inner Lindblad resonances. In a fourth barred system, the nuclear starburst galaxy NGC 3504, a large concentration of molecular gas is centered on the nucleus, apparently inside an inner Lindblad resonance

An Atlas of Hα and R Images and Radial Profiles of 63 Bright Virgo Cluster Spiral Galaxies

Narrowband Hα and broadband R images and radial profiles are presented for 63 bright spiral galaxies in the Virgo Cluster. The sample is complete for Sb-Scd galaxies with B ≤ 12 and inclination ≤75°. Isophotal radii, disk scale lengths, concentration parameters, and integrated fluxes are derived for the sample galaxies.

MOLECULAR GAS IN HIGH-LUMINOSITY IRAS GALAXIES

The paper reports observations of CO(J = 1-0) emission from an unbiased sample of the highest-luminosity IRAS galaxies with the aim of measuring their molecular gas content and determining whether star formation is a viable energy source for these high luminosities. All of the observed galaxies are rich in molecular gas with H2 masses in the range (4 x 10 to the 9th)-(4 x 10 to the 10th) solar masses. Their primary luminosity source appears to be star formation in molecular clouds. The majority, if not all, of the most luminous IRAS galaxies (L-FIR greater than 10 to the 11th solar luminosities) appear to be strongly interacting systems; those with the highest L-FIR/M(H2) ratios are mergers or close contact pairs. 14 references.

MOLECULAR GASDYNAMICS OF THE YOUNG NUCLEAR STARBURST IN THE BARRED GALAXY NGC 3504

We present CO (J=1→0) interferometry at 2″.5 resolution and Hα CCD observations of the circumnuclear starburst region of the barred spiral galaxy NGC 3504. The CO emission is centrally peaked, extends over a region 16″ (1.6 kpc) in diameter, and is relatively azimuthally symmetric. The CO radial distribution is well fitted by an exponential with a scale length of 2″.3 (220 pc). This simple distribution is surprisingly unusual for the center of a galaxy. The velocity field is consistent with purely circular motions. Gas comprises ∼40% of the dynamical mass within a radius of 100 pc (1″), if the standard CO-H 2 relationship is assumed