Astronomy
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Item PHYSICAL CONDITIONS OF THE MULTI-PHASE INTERSTELLAR MEDIUM IN NEARBY GALAXIES FROM INFRARED AND MILLIMETER-WAVE SPECTROSCOPY(2022) Tarantino, Elizabeth; Bolatto, Alberto D; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Gas and dust in the interstellar medium (ISM) cools and condenses, gravitationally collapses, and forms stars. At the same time, stars can heat and ionize their surroundings, influencing the physical conditions of the nearby ISM. In this thesis, I take a multi-wavelength, spectroscopic approach to investigate the physical conditions of the multi-phase ISM in nearby galaxies. The [CII] fine-structure transition at 158 micron is frequently the brightest far-infrared line in galaxies and can trace the ionized, atomic, and molecular phases of the ISM. I present velocity-resolved [CII] observations from SOFIA in the nearby galaxies M101 and NGC 6946 and determine that [CII] emission is associated with the atomic and molecular gas about equally, with little contribution from the ionized gas. Using the [CII] cooling function, I calculate the thermal pressure of the cold neutral medium and find that the high star formation rates in our sample can drive large thermal pressures, consistent with predictions from analytical theory. Next, I investigate the properties of the ionized gas around one of the hottest and most luminous Wolf-Rayet (WR) stars in the Small Magellanic Cloud. I use spatially resolved mid-infrared Spitzer and far-infrared Herschel spectroscopy to establish the physical conditions of the ionized gas. Using the photoionization code Cloudy, I construct models with a range of constant densities between n_H = 4 - 12 cm^-3 and a stellar wind-blown cavity of 15 pc that reproduce the intensity and spatial distribution of most ionized gas emission lines. The higher ionization lines cannot be produced by the models --- however, I show that wind-driven shocks or a harder ionizing WR spectrum can explain their intensities. Lastly, I explore the properties of molecular clouds in a large (170x350 pc) map of an active star-forming region in the Large Magellanic Cloud. Using 12CO(2-1) and 13CO(2-1) observations from the ALMA ACA, I decompose the emission into individual cloud structures and determine their sizes, linewidths, mass surface densities, and virial parameters. Almost all of the clouds are gravitationally bound or marginally bound and share similar properties to molecular clouds in the Milky Way. I do not find evidence that the surrounding star formation significantly influences the kinematic properties of the clouds through stellar feedback.Item Probing the Multiphase Interstellar Medium and Star Formation in Nearby Galaxies through Far-infrared Spectroscopy(2015) Herrera Camus, Rodrigo; Bolatto, Alberto; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present a study of different aspects of the multi-phase interstellar medium (ISM) of nearby galaxies, including detailed analysis of the low-excitation ionized gas, the thermal pressure (Pth) of the neutral gas, the dust-to-gas mass ratio (DGR) in low-metallicity environments, and the use of far-infrared transitions as tracers of the star formation rate (SFR). We based our work on the largest sample to date of spatially-resolved, infrared observations of nearby galaxies drawn from the KINGFISH and ``Beyond the Peak'' surveys. We use deep infrared observations to study the DGR of the extremely metal-poor galaxy I Zw 18. We measure a DGR upper-limit of 8.1x10^{-5}. This value is a factor of ~8 lower than the expected DGR if a linear correlation between DGR and metallicity, as observed in spirals, were to hold. Based on the line ratio between the [NII] 122 and 205 um transitions, for 140 regions selected from 21 galaxies we measure electron densities of the singly-ionized gas in the ne~1-230 cm^{-3} range, with a median value of ne=30 cm^{-3}. We find that ne increases as a function of SFR and radiation field strength. We study the reliability of the [CII] and [NII] 122 and 205 um transitions as SFR tracers. In general, we find good correlations between the emission from these fine-structure lines and star formation activity. However, a decrease in the photoelectric heating efficiency in the case of the [CII] line, or collisional quenching of the [NII] lines, can cause calibrations based on these transitions to underestimate the SFR. Finally, for a sample of atomic-dominated regions selected from 31 galaxies, we use the [CII] and HI lines to measure the cooling rate per H atom and Pth of the cold, neutral gas. We find a \pt\ distribution that can be well described by a log-normal distribution with median Pth/k~5,500 K cm^{-3}. We find correlations of increasing Pth with radiation field intensity and SFR, which is consistent with the results from two-phase ISM models in pressure equilibrium.Item The Origins and Ionization Mechanisms of Halpha Filaments in the Cool Cores of Galaxy Groups and Clusters(2011) McDonald, Michael Adam; Veilleux, Sylvain; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present a survey of 10 galaxy groups and 23 galaxy clusters aimed at explaining the presence of warm, ionized filaments in the cool cores of galaxy clusters. By combining deep, high spatial resolution H-alpha, far-UV, and X-ray data from the Maryland Magellan Tunable Filter, Hubble Space Telescope, and Chandra X-ray Observatory, respectively, we have assembled the most complete picture of these mysterious filaments to date. This extensive database has allowed us to shed new light on two critical questions: i) Where does the cool gas in these filaments come from? ii) What process or processes are responsible for ionizing the cool filaments. As a pilot project, we obtained high-resolution H-alpha and far-UV data for Abell1795 and find that the previously-discovered H-alpha filament is, in fact, two very thin, intertwined filaments extending 50 kpc and with a width < 700 pc. The clumpy UV morphology and UV/H-alpha flux ratios of these filaments suggest that they may consist of chains of star-forming regions. Based on these data we conclude that the H-alpha emission is a result of photoionization by young stars and that the cool gas filaments are a byproduct of the intracluster medium cooling onto a fast-moving central galaxy. When we consider the full sample of 23 galaxy clusters, we find several strong correlations between the X-ray and H-alpha data. In general, complex, extended H-alpha filaments are found in clusters with cool, low-entropy cores. Furthermore, the morphology of the warm gas is correlated with the soft X-ray morphology and the filaments are found to occupy regions where, locally, the intracluster medium is cooling fastest. Finally, we find a strong correlation between the mass of of gas cooling below X-ray temperatures and the mass of gas in the warm filaments. These results provide strong evidence that the ionized filaments are a result of highly-asymmetric runaway cooling in the intracluster medium. By extending this sample to include 10 galaxy groups we are able to probe more than 2 orders of magnitude in mass and cooling rate. We find that there is only a weak correlation between the presence of ionized filaments and the total mass of the system. Instead, the presence of ionized filaments appears to depend almost entirely on the core properties, specifically whether there is a cool, low-entropy core. We find that groups are, in general, cooling more efficiently that clusters, due to their lower starting temperature. This can only be the case if cool-core groups are experiencing exclusively weak AGN feedback, which we show is the case. Finally, using new far-UV data from the Hubble Space Telescope, we find that 12/15 systems with H-alpha emission are consistent with being photoionized by young stars. The three remaining systems are under-luminous in the far-UV for their H-alpha luminosity, suggesting an alternative ionization source such as fast shocks or significant internal reddening. When we supplement this sample with UV data from GALEX and IR data from Spitzer we find a correlation between the star formation rate and the ICM cooling rate for 32 systems. The inferred efficiency of stars forming out of the cooling ICM is 14(+18/-8)%, which is consistent with the Universal fraction of baryons in stars. These results suggest that, for the majority of cases, young star formation provides sufficient UV flux to ionize the cool filaments.