This paper provides an update of our previous scaling relations between galaxy-integrated molecular gas masses, stellar masses, and star formation rates (SFRs), in the framework of the star formation main sequence (MS), with the main goal of testing for possible systematic effects. For this purpose our new study combines three independent methods of determining molecular gas masses from CO line fluxes, far-infrared dust spectral energy distributions, and similar to 1 mm dust photometry, in a large sample of 1444 star-forming galaxies between z = 0 and 4. The sample covers the stellar mass range log(M-*/M-circle dot) = 9.0-11.8, and SFRs relative to that on the MS, delta MS = SFR/SFR (MS), from 10(-1.3) to 10(2.2). Our most important finding is that all data sets, despite the different techniques and analysis methods used, follow the same scaling trends, once method-to-method zero-point offsets are minimized and uncertainties are properly taken into account. The molecular gas depletion time t(depl), defined as the ratio of molecular gas mass to SFR, scales as (1 + z)(-0.6) x (delta MS)(-0.44) and is only weakly dependent on stellar mass. The ratio of molecular to stellar mass mu(gas) depends on (1+ z)(2.5) x (delta MS)(0.52) x (M-*)(-0.36), which tracks the evolution of the specific SFR. The redshift dependence of mu(gas) requires a curvature term, as may the mass dependences of t(depl) and mu(gas). We find no or only weak correlations of t(depl) and mu(gas) with optical size R or surface density once one removes the above scalings, but we caution that optical sizes may not be appropriate for the high gas and dust columns at high z.

We investigate the relationship between the black hole accretion rate (BHAR) and star formation rate (SFR) for Milky Way (MW) and Andromeda (M31)-mass progenitors from z = 0.2 to 2.5. We source galaxies from the K-s-band-selected ZFOURGE survey, which includes multiwavelength data spanning 0.3-160 mu m. We use decomposition software to split the observed spectral energy distributions (SEDs) of our galaxies into their active galactic nuclei (AGNs) and star-forming components, which allows us to estimate BHARs and SFRs from the infrared (IR). We perform tests to check the robustness of these estimates, including a comparison with BHARs and SFRs derived from X-ray stacking and far-IR analysis, respectively. We find that, as the progenitors evolve their relative black hole-galaxy growth (i.e. their BHAR/SFR ratio) increases from low to high redshift. The MW-mass progenitors exhibit a log-log slope of 0.64 +/- 0.11, while the M31-mass progenitors are 0.39 +/- 0.08. This result contrasts with previous studies that find an almost flat slope when adopting X-ray-/AGN-selected or mass-limited samples and is likely due to their use of a broad mixture of galaxies with different evolutionary histories. Our use of progenitor-matched samples highlights the potential importance of carefully selecting progenitors when searching for evolutionary relationships between BHAR/SFRs. Additionally, our finding that BHAR/SFR ratios do not track the rate at which progenitors quench casts doubts over the idea that the suppression of star formation is predominantly driven by luminous AGN feedback (i.e. high BHARs).

Context. The reionization of the Universe is one of the most important topics of present-day astrophysical research. The most plausible candidates for the reionization process are star-forming galaxies, which according to the predictions of the majority of the theoretical and semi-analytical models should dominate the H I ionizing background at z greater than or similar to 3. Aims. We measure the Lyman continuum escape fraction, which is one of the key parameters used to compute the contribution of star-forming galaxies to the UV background. It provides the ratio between the photons produced at lambda <= 912 angstrom rest-frame and those that are able to reach the inter-galactic medium, i.e. that are not absorbed by the neutral hydrogen or by the dust of the galaxy's inter-stellar medium. Methods. We used ultra-deep U-band imaging (U = 30.2 mag at 1 sigma) from Large Binocular Camera at the Large Binocular Telescope (LBC/LBT) in the CANDELS/GOODS-North field and deep imaging in the COSMOS and EGS fields in order to estimate the Lyman continuum escape fraction of 69 star-forming galaxies with secure spectroscopic redshifts at 3.27 <= z <= 3.40 to faint magnitude limits (L = 0.2L*, or equivalently M-1500 similar to -19). The narrow redshift range implies that the LBC U-band filter exclusively samples the lambda <= 912 angstrom rest-frame wavelengths. Results. We measured through stacks a stringent upper limit (<1.7% at 1 sigma) for the relative escape fraction of H I ionizing photons from bright galaxies (L > L*), while for the faint population (L = 0.2L*) the limit to the escape fraction is less than or similar to 10%. We computed the contribution of star-forming galaxies to the observed UV background at z similar to 3 and find that it is not sufficient to keep the Universe ionized at these redshifts unless their escape fraction increases significantly (>= 10%) at low luminosities (M-1500 >= -19). Conclusions. We compare our results on the Lyman continuum escape fraction of high-z galaxies with recent estimates in the literature, and discuss future prospects to shed light on the end of the Dark Ages. In the future, strong gravitational lensing will be fundamental in order to measure the Lyman continuum escape fraction down to faint magnitudes (M-1500 similar to -16) that are inaccessible with the present instrumentation on blank fields. These results will be important in order to quantify the role of faint galaxies to the reionization budget.

We analyze the optical counterpart to the ultra-compact high velocity cloud AGC 226067, utilizing imaging taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. The color-magnitude diagram of the main body of AGC 226067 reveals an exclusively young stellar population, with an age of similar to 7-50 Myr, and is consistent with a metallicity of [Fe/H] similar to -0.3 as previous work has measured via H II region spectroscopy. Additionally, the color-magnitude diagram is consistent with a distance of D approximate to 17 Mpc, suggesting an association with the Virgo cluster. A secondary stellar system located similar to 1.'6 (similar to 8 kpc) away in projection has a similar stellar population. The lack of an old red giant branch ((sic)5 Gyr) is contrasted with a serendipitously discovered Virgo dwarf in the ACS field of view (Dw J122147+132853), and the total diffuse light from AGC 226067 is consistent with the luminosity function of the resolved similar to 7-50 Myr stellar population. The main body of AGC 226067 has a M-V = -11.3 +/- 0.3, or M-stars = 5.4 +/- 1.3 x 10(4) M-circle dot given the stellar population. We searched 20 deg(2) of imaging data adjacent to AGC. 226067 in the Virgo Cluster, and found two similar stellar systems dominated by a blue stellar population, far from any massive galaxy counterpart-if this population has star-formation properties that are similar to those of. AGC 226067, it implies similar to 0.1Me(circle dot) yr(-1) in Virgo intracluster star formation. Given its unusual stellar population, AGC 226067 is likely a stripped remnant and is plausibly the result of compressed gas from the ram pressure stripped M86 subgroup (similar to 350 kpc away in projection) as it falls into the Virgo Cluster.

We investigate the optical and Wide-field Survey Explorer (WISE) colors of "E+A" identified post-starburst galaxies, including a deep analysis of 190 post-starbursts detected in the 2 mu m All Sky Survey Extended Source Catalog. The post-starburst galaxies appear in both the optical green valley and the WISE Infrared Transition Zone. Furthermore, we find that post-starbursts occupy a distinct region of [3.4]-[4.6] versus [4.6]-[12] WISE colors, enabling the identification of this class of transitioning galaxies through the use of broadband photometric criteria alone. We have investigated possible causes for the WISE colors of post-starbursts by constructing a composite spectral energy distribution (SED), finding that the mid-infrared (4-12 mu m) properties of post-starbursts are consistent with either 11.3 mu m polycyclic aromatic hydrocarbon emission, or thermally pulsating asymptotic giant branch (TP-AGB) and post-AGB stars. The composite SED of extended post-starburst galaxies with 22 mu m emission detected with signal-to-noise ratio >= 3 requires a hot dust component to produce their observed rising mid-infrared SED between 12 and 22 mu m. The composite SED of WISE. 22 mu m non-detections (S/N < 3), created by stacking 22 mu m images, is also flat, requiring a hot dust component. The most likely source of the mid-infrared emission of these E+A galaxies is a buried active galactic nucleus (AGN). The inferred upper limits to the Eddington ratios of post-starbursts are 10(-2)-10(-4), with an average of 10(-3). This suggests that AGNs are not radiatively dominant in these systems. This could mean that including selections capable of identifying AGNs as part of a search for transitioning and post-starburst galaxies would create a more complete census of the transition pathways taken as a galaxy quenches its star formation.

We have measured spectral indices for ~ 2000 galaxies in the CNOC1 redshift survey of 15 X-ray luminous clusters at 0.2 < ≈ < 0.55. A detailed comparison is made between the star formation histories of galaxies in these clusters with an identically selected sample of galaxies in the lower density field population, to establish the effects these cluster environments have on galaxy evolution. We find that the mean star formation rate, as determined from the [OII]λ3727 emission line, is suppressed in all cluster galaxies, out to and even beyond the virial radius. The number of actively star forming galaxies, and the mean star formation rate among cluster galaxies, increases with increasing distance from the cluster centre. This correlation is not completely due to the morphology-radius relation, as cluster galaxies of a given physical size, fractional bulge luminosity and redshift have lower star formation rates than similar galaxies in the field environment. We find no evidence that the cluster environment induces star formation in its constituent galaxies. Galaxies with positive W₀(OII), of any strength, are more common in the field than they are in the clusters. In particular, the A+em galaxies, which have spectra that may reflect dust obscured starburst activity, make up only 6.3 ± 2.1% of the field population, and are twice as common there as they are in the cluster sample. If star formation is terminated in a galaxy after a short starburst, the spectrum will show strong Balmer absorption lines without [OII] emission; we find that less than ~5% of all galaxies have such a spectrum, and there is no evidence that they are preferentially found within the cluster sample. Spectrophotometric model results suggest that many of these galaxies may have had their star formation abruptly truncated without such a starburst. Alternatively, Hα observations of Abell 2390 cluster galaxies suggest that the lack of [OII] emission in some such galaxies may be due to dust obscuration, and not necessarily indicative of the absence of star formation activity. These results suggest that star formation is terminated in galaxies that are incorporated into these clusters. This termination need not be abrupt, and may take place over a period of several Gyr. Thus, the differential evolution of cluster galaxies may result because field galaxies are able to refuel their stellar disk with gas from an extended halo, thus perpetuating star formation, while such a halo would be disrupted within rich clusters, and star formation would gradually cease. / Graduate

This thesis examines properties of galaxies in and around compact groups. Astrometry, photometry and morphological classifications are derived from CCD images for all 463 galaxies in Hickson's sample of 100 compact groups. Some minor revisions to the membership of the original catalog are made. At high galactic latitude (b > 30°), the catalog is estimated to be 90% complete for groups with total B[formula omitted] magnitude 13.0 or less. 49% of all the catalogued galaxies, and 48% of first-ranked galaxies are spiral. No significant difference is found between the distribution of morphological types of first-ranked galaxies and all group galaxies. Morphological concordance occurs among galaxies within a group: more galaxies are the same type (spiral or nonspiral), than would be expected by chance. Galaxy morphological type correlates with group optical luminosity and, more strongly, with velocity dispersion, but not with galaxy space density. These results imply that the morphological types of galaxies in compact groups are strongly influenced by the environment, and that this influence occurs mostly at the time of galaxy formation. Fields surrounding 97 compact groups with known redshifts were examined on the Palomar Observatory Sky Survey prints. 3889 galaxies were identified within 1.125h⁻¹ Mpc of the centre of each group. Coordinates, magnitudes, diameters, and Hubble types are derived for these galaxies. 78% of the groups show no significant excess of field galaxies within 0.5h⁻¹ Mpc. This indicates that most compact groups are truly isolated. 59% of these field galaxies were classified as spiral, a higher fraction that for the group galaxies. This difference is more pronounced for groups which do show a significant field galaxy excess. These results indicate that most groups are dense dynamical entities. Monte-Carlo calculations indicate that 35% of galaxy quintets are predicted to contain a single discordant redshift due to the chance alignment of an unrelated field galaxy. This is in agreement with the observed number of four discordant quintets in 10. These results are consistent with the cosmological interpretation of galaxy redshifts. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

We have examined the late-type stellar content of 3 disk fields of NGC 2403 using VRI CCD photometry. The AGB luminosity function has been constructed and differs strongly with that of the LMC. Notably, even fewer bright AGB stars are seen in NGC 2403 than in the LMC. The shape of the AGB luminosity function for the innermost field, however, indicates that a recent burst of star formation may have occurred. Red and blue supergiants and HII regions are also identified in the field to support this. The relative distance modulus between NGC 2403 and the LMC is derived to be 8.90 and the true distance modulus to NGC 2403 is (m - M)₀ = 27.40 ± 0.24. The carbon to M star number for the innermost field is 8/17 which corresponds to [Fe/H] = -0.35. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Includes bibliography. / Chapter I gives an overview of the current well-known theoretical models of large-scale structure formation and explains how the predictions of these models can be tested by the investigation of voids in the distribution of galaxies. Chapter II reviews work that has already been done on the statistics and properties of voids. In particular, attention is focussed on the establishment of a new statistic, the void probability function, which is sensitive to the presence of voids and is intimately linked to the hierarchy of N-point correlation functions derived by Peebles (1980). Chapter II also contains a brief review of the work done by Gott on point-smoothing analysis and the use of the topological genus parameter to distinguish between various theoretical models of structure formation. Chapters III and IV present the original work done by the author in establishing a method of · surveying all important voids in the available data. A systematic search for all voids out to a redshift of about 15000 km/s is described. Methods of assessing the significance of these voids are discussed. Upon applying these methods, a list of "statistically significant" voids is constructed and this list is used to derive a spectrum of void sizes. Finally, these chapters illustrate how this new technique of studying voids might serve as an important tool in determining the origins of the large-scale structure observed in the universe today.

When darkmatter haloes are accreted by massive host clusters, strong gravitational tidal forces begin stripping mass from the accreted subhaloes. This stripping eventually removes all mass beyond a subhalo's tidal radius, with unbound mass remaining in the vicinity of the satellite for at most a dynamical time t(dyn). The N-body subhalo study of Chamberlain et al. verified this picture and pointed out a useful observational consequence: correlations between subhaloes beyond the tidal radius are sensitive to the infall time, t(infall), of the subhalo on to its host. We perform this correlation using similar to 160 000 red satellite galaxies in Sloan Digital Sky Survey redMaPPer clusters and find evidence that subhalo correlations do persist well beyond the tidal radius, suggesting that many of the observed satellites fell into their current host less than a dynamical time ago, t(infall) < t(dyn). Combined with estimated dynamical times t(dyn) similar to 3-5 Gyr and SED fitting results for the time at which satellites stopped forming stars, t(quench) similar to 6 Gyr, we infer that for a significant fraction of the satellites, star formation quenched before those satellites entered their current hosts. The result holds for red satellites over a large range of cluster-centric distances 0.1-0.6 Mpc h(-1). We discuss the implications of this result formodels of galaxy formation.