Stephen T. Gottesman

A Comparative Study of Star-forming and Quiescent Dwarf Galaxies

We present the results from a comparative study of the atomic hydrogen (H I) and optical properties of a sample of 16 dwarf galaxies, chosen to investigate the effects of star formation on the properties of low-mass systems. The violent star formation bursts believed to occur in these low-mass systems suggest a possible connection between the actively star-forming blue compact dwarfs (BCDs), and the quiescent low surface brightness dwarfs (LSBDs). It has been suggested that LSBDs, upon undergoing a burst of star formation, will evolve into BCDs and then back into LSBDs when the star formation slows or stops as the H I column density falls below the critical threshold necessary to support it. We have examined the location and kinematics of H I in eight BCDs and eight LSBDs of similar H I masses and a range of color indices to investigate this evolutionary sequence. The starburst episodes in these low-mass galaxies should lead to (1) a dispersal/depletion of the H I seen in the eight LSB dwarfs and (2) more centrally concentrated and agitated H I in the eight BCDs. The results of this project indicate that the quiescent LSBD galaxies have more diffuse H I distributions and often show a ringlike structure, while the active galaxies have more highly centrally concentrated H I reservoirs. The bluer, more recently active systems of both types also have higher internal H I velocity dispersions, indicating that energy has been pumped into the interstellar medium of these galaxies. These observations are consistent with an evolutionary scheme wherein the H I reservoirs in these galaxies take on different characteristics depending upon their star formation histories.

WARM MOLECULAR HYDROGEN EMISSION IN NORMAL EDGE-ON GALAXIES NGC 4565 AND NGC 5907

We have observed warm molecular hydrogen in two nearby edge-on disk galaxies, NGC 4565 and NGC 5907, using the Spitzer high-resolution infrared spectrograph. The 0-0 S(0) 28.2 μm and 0-0 S(1) 17.0 μm pure rotational lines were detected out to 10 kpc from the center of each galaxy on both sides of the major axis, and in NGC 4565 the S(0) line was detected at r = 15 kpc on one side. This location is beyond the transition zone where diffuse neutral atomic hydrogen starts to dominate over cold molecular gas and marks a transition from a disk dominated by high surface-brightness far-infrared (far-IR) emission to that of a more quiescent disk. It also lies beyond a steep drop in the radio continuum emission from cosmic rays (CRs) in the disk. Despite indications that star formation activity decreases with radius, the H_2 excitation temperature and the ratio of the H_2 line and the far-IR luminosity surface densities, Σ(L_H_2)/Σ(L_(TIR)), change very little as a function of radius, even into the diffuse outer region of the disk of NGC 4565. This suggests that the source of excitation of the H_2 operates over a large range of radii and is broadly independent of the strength and relative location of UV emission from young stars. Although excitation in photodissociation regions is the most common explanation for the widespread H_2 emission, CR heating or shocks cannot be ruled out. At r = 15 kpc in NGC 4565, outside the main UV- and radio-continuum-dominated disk, we derived a higher than normal H_2 to 7.7 μm polycyclic aromatic hydrocarbon (PAH) emission ratio, but this is likely due to a transition from mainly ionized PAH molecules in the inner disk to mainly neutral PAH molecules in the outer disk. The inferred mass surface densities of warm molecular hydrogen in both edge-on galaxies differ substantially, being 4(-60) M_⊙ pc^(–2) and 3(-50) M_⊙ pc^(–2) at r = 10 kpc for NGC 4565 and NGC 5907, respectively. The higher values represent very unlikely point-source upper limits. The point-source case is not supported by the observed emission distribution in the spectral slits. These mass surface densities cannot support the observed rotation velocities in excess of 200 km s^(–1). Therefore, warm molecular hydrogen cannot account for dark matter in these disk galaxies, contrary to what was implied by a previous Infrared Space Observatory study of the nearby edge-on galaxy NGC 891.

Neutral Hydrogen in Arp 158

We present 21 cm observations of Arp 158. We have performed a study of the neutral hydrogen (H I) to help us understand the overall formation and evolution of this system. This is a disturbed system with distinct optical knots connected by a linear structure embedded in luminous material. There is also a diffuse spray to the southeast. The H I seems to be made up of three distinct, kinematically separate systems. Arp 158 bears a certain optical resemblance to NGC 520 (Arp 157), which has been identified as a mid-stage merger. From our 21 cm observations of Arp 158, we also see a comparable H I content with NGC 520. These similarities suggest that Arp 158 is also an intermediate-stage merger.

Arp 158: A Study of the HI

We present here 21 cm observations of Arp 158. We have performed a study of the HI to help us understand the overall formation and evolution of this system and its role in galaxy evolution.

Neutral hydrogen absorption in the radio spectrum of PKS 0241 + 011

The detection (at about the 3 sigma level) of an absorption feature at a heliocentric velocity of 1172 km/s in the spectrum of PKS 0241 + 011 is presented. The line of sight to the quasar (z-PKS = 1.4) lies well within the optical and H I boundaries of the barred spiral galaxy NGC 1073 (z-gal = 0.004). Two other continuum sources in the field of NGC 1073 do not show any absorption features. 17 refs.