This thesis investigates the impact of photoionisation on gas clouds of various scales in our Galaxy. On kiloparsec scales, the origin of the diffuse ionised gas (DIG) has been studied using Monte Carlo photoionisation simulations in static simulations of the interstellar medium (ISM). Low density pathways through the gas allow photons to propagate, producing the DIG. The emission in this gas indicates that the temperature increases as the density of the gas decreases, suggesting a heating mechanism in the gas that dominates over photoionisation. This has been investigated with the inclusion of an approximation to cosmic ray heating, which is able to reproduce the observed line ratios of [NII]/Hα and [SII]/Hα. It has been suggested that the Hα emission in this gas may not be a result of in situ emission, rather, it results from dust scattering of Hα photons from HII regions. Dust scattering simulations find that 20% of the Hα intensity observed in the diffuse gas is a result of scattering, leaving in situ ionisation as the primary producer of the DIG. The dynamical impact of photoionisation in small-scale gas clouds, as well as larger scale diffuse gas, is now recognised as important in our understanding of star forming regions. A new code has been developed using a combination of hydrodynamical and Monte Carlo photoionisation methods. Tests of the validity of this code are presented, showing that this method accurately reproduces benchmark tests. This method has been used to investigate the lifecycle and x-ray observations of ultra-compact HII (UCHII) regions with the inclusion of stellar wind and photoionisation feedback. These simulations find that x-ray emission is not produced in high enough intensity to be observed until after the UCHII phase, when stellar winds begin to drive the expansion.

Early quiescent galaxies at z similar to 2 are known to be remarkably compact compared to their nearby counterparts. Possible progenitors of these systems include galaxies that are structurally similar, but are still rapidly forming stars. Here, we present Karl G. Jansky Very Large Array (VLA) observations of the CO(1-0) line toward three such compact, star-forming galaxies (SFGs) at z similar to 2.3, significantly detecting one. The VLA observations indicate baryonic gas fractions. greater than or similar to 5 times lower and gas depletion timescales. greater than or similar to 10 times shorter than normal, extended massive SFGs at these redshifts. At their current star formation rates, all three objects will deplete their gas reservoirs within 100 Myr. These objects are among the most gas-poor objects observed at z > 2, and are outliers from standard gas scaling relations, a result that remains true regardless of assumptions about the CO-H-2 conversion factor. Our observations are consistent with the idea that compact, SFGs are in a rapid state of transition to quiescence in tandem with the buildup of the z similar to 2 quenched population. In the detected compact galaxy, we see no evidence of rotation or that the CO-emitting gas is spatially extended relative to the stellar light. This casts doubt on recent suggestions that the gas in these compact galaxies is rotating and significantly extended compared to the stars. Instead, we suggest that, at least for this object, the gas is centrally concentrated, and only traces a small fraction of the total galaxy dynamical mass.

A major question in galaxy formation is how the gas supply that fuels activity in galaxies is modulated by their environment. We use spectroscopy of a set of well-characterized clusters and groups at 0.4 < z < 0.8 from the ESO Distant Cluster Survey and compare it to identically selected field galaxies. Our spectroscopy allows us to isolate galaxies that are dominated by old stellar populations. Here we study a stellar-mass-limited sample (log(M*/M-circle dot) > 10.4) of these old galaxies with weak [O II] emission. We use line ratios and compare to studies of local early-type galaxies to conclude that this gas is likely excited by post-AGB stars and hence represents a diffuse gas component in the galaxies. For cluster and group galaxies the fraction with EW([O II]) > 5 angstrom is f([O II]) = 0.08(-0.03)(+0.02) and f([O II]) = 0.06(-0.04)(+0.07), respectively. For field galaxies we find f([O II]) = 0.2 (+0.07)(-0.06), representing a 2.8 sigma difference between the [O II] fractions for old galaxies between the different environments. We conclude that a population of old galaxies in all environments has ionized gas that likely stems from stellar mass loss. In the field galaxies also experience gas accretion from the cosmic web, and in groups and clusters these galaxies have had their gas accretion shut off by their environment. Additionally, galaxies with emission preferentially avoid the virialized region of the cluster in position-velocity space. We discuss the implications of our results, among which is that gas accretion shutoff is likely effective at group halo masses (log M/M-circle dot > 12.8) and that there are likely multiple gas removal processes happening in dense environments.

In this paper we use ASPECS, the ALMA Spectroscopic Survey in the Hubble Ultra Deep Field in band. 3 and band. 6, to place blind constraints on the CO luminosity function and the evolution of the cosmic molecular gas density as a function of redshift up to z similar to 4.5. This study is based on galaxies that have been selected solely through their CO emission and not through any other property. In all of the redshift bins the ASPECS measurements reach the predicted "knee" of the CO luminosity function (around 5 x 10(9) K km s(-1) pc(2)). We find clear evidence of an evolution in the CO luminosity function with respect to z similar to 0, with more CO-luminous galaxies present at z similar to 2. The observed galaxies at z similar to 2 also appear more gas-rich than predicted by recent semi-analytical models. The comoving cosmic molecular gas density within galaxies as a function of redshift shows a drop by a factor of 3-10 from z similar to 2 to z similar to 0 (with significant error bars), and possibly a decline at z > 3. This trend is similar to the observed evolution of the cosmic star formation rate density. The latter therefore appears to be at least partly driven by the increased availability of molecular gas reservoirs at the peak of cosmic star formation (z similar to 2).

We present a (CO)-C-12 (2-1) mosaic map of the spiral galaxy NGC 6946 by combining data from the Submillimeter Array and the IRAM 30m telescope. We identify 390 giant molecular clouds (GMCs) from the nucleus to 4.5 kpc in the disk. GMCs in the inner 1 kpc are generally more luminous and turbulent, some of which have luminosities > 10(6)K. km. s(-1) pc(2) and velocity dispersions > 10. km s(-1). Large-scale bar-driven dynamics likely regulate GMC properties in the nuclear region. Similar to the MilkyWay and other disk galaxies, GMC mass function of NGC 6946 has a shallower slope (index > -2) in the inner region, and a steeper slope (index < -2) in the outer region. This difference in mass spectra may be indicative of different cloud formation pathways: gravitational instabilities might play a major role in the nuclear region, while cloud coalescence might be dominant in the outer disk. Finally, the NGC 6946 clouds are similar to those inM33 in terms of statistical properties, but they are generally less luminous and turbulent than the M51 clouds.

The South Pole Telescope has discovered 100 gravitationally lensed, high-redshift, dusty, star-forming galaxies (DSFGs). We present 0."5 resolution 870 mu m. Atacama Large Millimeter/submillimeter Array imaging of a sample of 47 DSFGs spanning z = 1.9-5.7, and construct gravitational lens models of these sources. Our visibility-based lens modeling incorporates several sources of residual interferometric calibration uncertainty, allowing us to properly account for noise in the observations. At least 70% of the sources are strongly lensed by foreground galaxies (mu(870) (mu m) > 2), with a median magnification of mu(870) (mu m) = 6.3, extending to mu(870) (mu m) > 30. We compare the intrinsic size distribution of the strongly lensed sources to a similar number of unlensed DSFGs and find no significant differences in spite of a bias between the magnification and intrinsic source size. This may indicate that the true size distribution of DSFGs is relatively narrow. We use the source sizes to constrain the wavelength at which the dust optical depth is unity and find this wavelength to be correlated with the dust temperature. This correlation leads to discrepancies in dust mass estimates of a factor of two compared to estimates using a single value for this wavelength. We investigate the relationship between the [C II] line and the far-infrared luminosity and find that the same correlation between the [C II]/L-FIR. ratio and Sigma(FIR). found for low-redshift star-forming galaxies applies to high-redshift galaxies and extends at least two orders of magnitude higher in SFIR. This lends further credence to the claim that the compactness of the IR-emitting region is the controlling parameter in establishing the "[C II] deficit."

We present and analyze the possibility of using optical u-band luminosities to estimate star-formation rates (SFRs) of galaxies based on the data from the South Galactic Cap u. band Sky Survey (SCUSS), which provides a deep u-band photometric survey covering about 5000 deg(2) of the South Galactic Cap. Based on two samples of normal star-forming galaxies selected by the. BPT diagram, we explore the correlations between u-band, H alpha, and IR luminosities by combing SCUSS data with the Sloan Digital Sky Survey and Wide-field Infrared Survey Explorer (WISE). The attenuation-corrected u-band luminosities are tightly correlated with the Balmer decrement-corrected Ha luminosities with an rms scatter of similar to 0.17 dex. The IR-corrected u luminosities are derived based on the correlations between the attenuation of u-band luminosities and WISE. 12 (or 22) mu m luminosities, and then calibrated with the Balmer-corrected Ha luminosities. The systematic residuals of these calibrations are tested against the physical properties over the ranges covered by our sample objects. We find that the best-fitting nonlinear relations are better than the linear ones and recommended to be applied in the measurement of SFRs. The systematic deviations mainly come from the pollution of old stellar population and the effect of dust extinction; therefore,. a. more detailed analysis is needed in future work.

We present an update to the ultraviolet-to-radio database of global broadband photometry for the 79 nearby galaxies that comprise the union of the KINGFISH (Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel) and SINGS (Spitzer Infrared Nearby Galaxies Survey) samples. The 34-band data set presented here includes contributions from observational work carried out with a variety of facilities including GALEX, SDSS, Pan-STARRS1, NOAO, 2MASS, Wide-Field Infrared Survey Explorer, Spitzer, Herschel, Planck, JCMT, and the VLA. Improvements of note include recalibrations of previously published SINGS BVRCIC and KINGFISH farinfrared/submillimeter photometry. Similar to previous results in the literature, an excess of submillimeter emission above model predictions is seen primarily for low-metallicity dwarf or irregular galaxies. This 33-band photometric data set for the combined KINGFISH+SINGS sample serves as an important multiwavelength reference for the variety of galaxies observed at low redshift. A thorough analysis of the observed spectral energy distributions is carried out in a companion paper.

The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) will map the power spectrum of 0.8 million blindly discovered Lyman-alpha Emitting Galaxies (LAE) using a revolutionary new array of massively replicated, fiber-fed spectrographs dubbed the Visible Integral-Field Replicable Unit Spectrograph (VIRUS). In the era of the Hobby-Eberly Telescope wide-field upgrade, the current Low Resolution Spectrograph (LRS) must be replaced with a fiber instrument. In this thesis, I discuss the development of the second generation LRS (LRS2), which is an R>1200 multi-channel instrument based on the VIRUS design and fed by a 287 fiber, 7” x 12” microlens coupled integral field unit. I focus on the blue optimized version of the instrument (3720<[lamda] (Angstroms)<7000), specifically on the opto-mechanical design of the VPH grisms. With the purpose of making the instrument ideal for the follow-up of LAE in the HETDEX survey, I discuss the science drivers for selecting the spectral resolution of the instrument. To test the utility of such an instrument, I present R~2400 spectra of two LAE that were originally discovered in the HETDEX Pilot Survey (Adams et al. 2011). These data were taken with the VIRUS prototype spectrograph in a high-resolution mode at the McDonald Observatory Harlan J. Smith 2.7 m telescope. The Lyman-alpha line profiles are constrained by near-infrared observations of rest-frame optical emission lines from Finkelstein et al. (2011), which set the systemic redshift of the galaxies. I discuss the velocity offsets of the Lyman-alpha line from the systemic line center and the implications for the HETDEX survey. I compare the line profiles to theory, specifically to those describing dust attenuation, outflows or inflows of neutral gas on the galactic scale, and attenuation in the intergalactic medium. This study provides an example of how LRS2 can be used to probe Lyman-alpha emission in 2<z<3 star-forming galaxies. / text

We present a survey of the [C II] 158 mu m line and underlying far-infrared (FIR) dust continuum emission in a sample of 27 greater than or similar to 6 quasars using the Atacama Large Millimeter Array (ALMA) at similar to 1 '' resolution. The [C II] line was significantly detected (at > 5-sigma) in 23 sources (85%). We find typical line luminosities of L-[C (II]) = 10(9-10) L-circle dot, and an average line width of similar to 385 km s(-1). The [C II]-to-far-infrared luminosity ratios ([C II]/FIR) in our sources span one order of magnitude, highlighting a variety of conditions in the star-forming medium. Four quasar host galaxies are clearly resolved in their [C II] emission on a few kpc scales. Basic estimates of the dynamical masses of the host galaxies give masses between 2 x 10(10) and 2 x 10(11) M-circle dot, i.e., more than an order of magnitude below what is expected from local scaling relations, given the available limits on the masses of the central black holes (> 3 x 10(8) M-circle dot, assuming Eddington-limited accretion). In stacked ALMA [C II] spectra of individual sources in our sample, we find no evidence of a deviation from a single Gaussian profile. The quasar luminosity does not strongly correlate with either the [C II] luminosity or equivalent width. This survey (with typical on-source integration times of 8 minutes) showcases the unparalleled sensitivity of ALMA at millimeter wavelengths, and offers a unique reference sample for the study of the first massive galaxies in the universe.