Gas and Star Formation at the Peak of Cosmic Star Forming Activity

Gas and star formation in galaxies are intimately linked to one another. Molecular hydrogen gas is the material out of which stars form, while the process of forming stars, in turn, depletes the reservoirs of gas in galaxies and builds up their stellar mass. Observations of star formation in galaxies over time indicate that they must form stars for timescales longer than would be expected from their gas content and star formation rates, indicating that processes that replenish the star forming fuel must be present. The focus of this thesis is on two components of this qualitative picture: the molecular hydrogen gas content of galaxies over time, and the link between gas and star formation in galaxies resembling those observed at the epoch of most active star formation. First, I present a systematic search for serendipitous carbon monoxide emitting sources in the second Plateau de Bure High-z Blue-Sequence Survey (PHIBSS2). These observations presented an opportunity to quantify the mass density of molecular gas in galaxies as a function of time, and to link this to the star formation history of the Universe. I use a match-filter technique to systematically detect 67 serendipitous sources, after which I characterize their properties, creating a catalog of their redshifts, line widths, fluxes, estimations of the detection reliability, and completeness of the detection algorithm. I find that these serendipitous sources are unrelated to the primary sources that were targeted by PHIBSS2, and use the catalog to construct luminosity functions spanning a redshift range from $\sim 0.3-5$. From these luminosity functions, I place constraints on the molecular hydrogen content in galaxies over cosmic time. My work presents one of the first attempts to use existing observations for this measurement and yields results that are consistent with previous studies, while demonstrating the scientific power of large, targeted surveys. Next, I study a sample of rare, nearby galaxies that are most similar to those we observe at the peak of cosmic star forming activity that occured $\sim 10$ billion years ago. These galaxies are drawn from the DYnamics of Newly Assembled Massive Objects (DYNAMO) survey, and their proximity to us allows for very detailed studies of their massive star forming clumps. I use observations from the Hubble Space Telescope (HST) to measure colors that are sensitive to stellar population age and extinction. From these measurements, I find that clumps in DYNAMO galaxies have colors that are most consistent with very young centers and outskirts that appear systematically older, by as much as 150~Myr in some cases. I attribute this age difference to the presence of ongoing star formation in the centers of clumps that maintains the population of massive, short-lived stars and gives rise to colors consistent with young ages. Furthermore, I find that within the disks of their host galaxies, younger clumps are preferentially located far from galaxy centers, while older clumps are preferentially located closer to the centers. These results are consistent with hydrodynamic simulations of high-redshift clumpy galaxies that predict clumps form in the outskirts of galaxies via a violent disk instability, and as they age, migrate to the centers of galaxies where they merge and contribute to the growth of galactic bulges. Building on this study, I combine observations of DYNAMO galaxies from the HST and the Atacama Large Millimeter/sub-millimeter Array (ALMA) to trace molecular hydrogen gas and star formation. I link these observations to measurements of the molecular gas velocity dispersions to test theories of star formation. I find that compared to local samples of ``normal'' star forming galaxies, DYNAMO systems have consistently high velocity dispersions, molecular gas surface densities, and star formation rate surface densities. Indeed, throughout their disks, DYNAMO galaxies are comparable to the centers of local star forming galaxies. Stellar bar driven gas flows into the centers of galaxies in these local samples may give rise to the high observed velocity dispersions, and gas and star formation rate surface densities. For DYNAMO galaxies, the widespread elevated values of these parameters may be driven by galactic-scale gas inflows, which is predicted by theories. Finally, current theories of star formation, such as the feedback regulated model, assume that turbulence dissipates on timescales proportional to the angular velocity of a galaxy (eddy or crossing time). Yet, I find such models have difficulty reproducing the DYNAMO measurements, and thus conclude that the turbulent dissipation timescale in DYNAMO galaxies must scale with galactocentric radius

The Ultraviolet and Infrared Star Formation Rates of Compact Group Galaxies: An Expanded Sample

Compact groups of galaxies provide insight into the role of low-mass, dense environments in galaxy evolution because the low velocity dispersions and close proximity of galaxy members result in frequent interactions that take place over extended timescales. We expand the census of star formation in compact group galaxies by \citet{tzanavaris10} and collaborators with Swift UVOT, Spitzer IRAC and MIPS 24 \micron\ photometry of a sample of 183 galaxies in 46 compact groups. After correcting luminosities for the contribution from old stellar populations, we estimate the dust-unobscured star formation rate (SFR$_{\mathrm{UV}}$) using the UVOT uvw2photometry. Similarly, we use the MIPS 24 \micron\ photometry to estimate the component of the SFR that is obscured by dust (SFR$_{\mathrm{IR}}$). We find that galaxies which are MIR-active (MIR-"red"), also have bluer UV colours, higher specific star formation rates, and tend to lie in H~{\sc i}-rich groups, while galaxies that are MIR-inactive (MIR-"blue") have redder UV colours, lower specific star formation rates, and tend to lie in H~{\sc i}-poor groups. We find the SFRs to be continuously distributed with a peak at about 1 M$_{\odot}$ yr$^{-1}$, indicating this might be the most common value in compact groups. In contrast, the specific star formation rate distribution is bimodal, and there is a clear distinction between star-forming and quiescent galaxies. Overall, our results suggest that the specific star formation rate is the best tracer of gas depletion and galaxy evolution in compact groups.Comment: 19 pages, 17 figure

The Morpho-kinematic Architecture of Super Star Clusters in the Center of NGC 253

The center of the nearby galaxy NGC 253 hosts a population of more than a dozen super star clusters (SSCs) that are still in the process of forming. The majority of the star formation of the burst is concentrated in these SSCs, and the starburst is powering a multiphase outflow from the galaxy. In this work, we measure the 350 GHz dust continuum emission toward the center of NGC 253 at 47 mas (0.8 pc) resolution using data from the Atacama Large Millimeter/submillimeter Array. We report the detection of 350 GHz (dust) continuum emission in the outflow for the first time, associated with the prominent South-West streamer. In this feature, the dust emission has a width of approximate to 8 pc, is located at the outer edge of the CO emission, and corresponds to a molecular gas mass of similar to(8-17)x10(6) M (circle dot). In the starburst nucleus, we measure the resolved radial profiles, sizes, and molecular gas masses of the SSCs. Compared to previous work at the somewhat lower spatial resolution, the SSCs here break apart into smaller substructures with radii 0.4-0.7 pc. In projection, the SSCs, dust, and dense molecular gas appear to be arranged as a thin, almost linear, structure roughly 155 pc in length. The morphology and kinematics of this structure can be well explained as gas following x (2) orbits at the center of a barred potential. We constrain the morpho-kinematic arrangement of the SSCs themselves, finding that an elliptical, angular-momentum-conserving ring is a good description of both the morphology and kinematics of the SSCs

Outflows from Super Star Clusters in the Central Starburst of NGC253

Young massive clusters play an important role in the evolution of their host galaxies, and feedback from the high-mass stars in these clusters can have profound effects on the surrounding interstellar medium. The nuclear starburst in the nearby galaxy NGC253 at a distance of 3.5 Mpc is a key laboratory in which to study star formation in an extreme environment. Previous high resolution (1.9 pc) dust continuum observations from ALMA discovered 14 compact, massive super star clusters (SSCs) still in formation. We present here ALMA data at 350 GHz with 28 milliarcsecond (0.5 pc) resolution. We detect blueshifted absorption and redshifted emission (P-Cygni profiles) towards three of these SSCs in multiple lines, including CS 7$-$6 and H$^{13}$CN 4$-$3, which represents direct evidence for previously unobserved outflows. The mass contained in these outflows is a significant fraction of the cluster gas masses, which suggests we are witnessing a short but important phase. Further evidence of this is the finding of a molecular shell around the only SSC visible at near-IR wavelengths. We model the P-Cygni line profiles to constrain the outflow geometry, finding that the outflows must be nearly spherical. Through a comparison of the outflow properties with predictions from simulations, we find that none of the available mechanisms completely explains the observations, although dust-reprocessed radiation pressure and O star stellar winds are the most likely candidates. The observed outflows will have a very substantial effect on the clusters' evolution and star formation efficiency.Comment: Accepted to Ap

ALMA Imaging of a Galactic Molecular Outflow in NGC 4945

We present the ALMA detection of molecular outflowing gas in the central regions of NGC 4945, one of the nearest starbursts and also one of the nearest hosts of an active galactic nucleus (AGN). We detect four outflow plumes in CO J= 3 - 2 at similar to 0.3 resolution that appear to correspond to molecular gas located near the edges of the known ionized outflow cone and its (unobserved) counterpart behind the disk. The fastest and brightest of these plumes has emission reaching observed line-of-sight projected velocities of over 450 km s(-1) beyond systemic, equivalent to an estimated physical outflow velocity v greater than or similar to 600 km s(-1) for the fastest emission. Most of these plumes have corresponding emission in HCN or HCO + J= 4 - 3. We discuss a kinematic model for the outflow emission where the molecular gas has the geometry of the ionized gas cone and shares the rotation velocity of the galaxy when ejected. We use this model to explain the velocities we observe, constrain the physical speed of the ejected material, and account for the fraction of outflowing gas that is not detected due to confusion with the galaxy disk. We estimate a total molecular mass outflow rate (M) over dot(mol) similar to 20 M-circle dot yr(-1) flowing through a surface within 100 pc of the disk midplane, likely driven by a combination of the central starburst and AGN

Outflows from Super Star Clusters in the Central Starburst of NGC 253

Young massive clusters play an important role in the evolution of their host galaxies, and feedback from the high-mass stars in these clusters can have profound effects on the surrounding interstellar medium. The nuclear starburst in the nearby galaxy NGC 253 at a distance of 3.5 Mpc is a key laboratory in which to study star formation in an extreme environment. Previous high-resolution (1.9 pc) dust continuum observations from the Atacama Large Millimeter/submillimeter Array (ALMA) discovered 14 compact, massive super star clusters (SSCs) still in formation. We present here ALMA data at 350 GHz with 28 mas (0.5 pc) resolution. We detect blueshifted absorption and redshifted emission (P-Cygni profiles) toward three of these SSCs in multiple lines, including CS 7-6 and (HCN)-C-13 4-3, which represent direct evidence for previously unobserved outflows. The mass contained in these outflows is a significant fraction of the cluster gas masses, which suggests we are witnessing a short but important phase. Further evidence of this is the finding of a molecular shell around the only SSC visible at near-IR wavelengths. We model the P-Cygni line profiles to constrain the outflow geometry, finding that the outflows must be nearly spherical. Through a comparison of the outflow properties with predictions from simulations, we find that none of the available mechanisms completely explains the observations, although dust-reprocessed radiation pressure and O star stellar winds are the most likely candidates. The observed outflows will have a very substantial effect on the clusters\u27 evolution and star formation efficiency