No description available.

No description available.

We present new optical and radio spectroscopic observations of the remarkable galaxy Malin 1. This galaxy has unique features that include an extremely low surface brightness disk with an enormous mass of neutral hydrogen, and a low luminosity Seyfert nucleus. Malin 1 is exceptional in its values of MHO, LB, and MHI /Ln, and modest in its surface mass density of gas and stars. Spirals with large Min /LB tend to have low mean column densities of HI, and are close to the threshold for star formation due to instabilities in a rotating gas disk. In these terms, Malin 1 has a disk with extremely inefficient star formation. The bulge spectrum is dominated by the absorption features of an old, metal rich stellar population, although there is some evidence for hot (young) stars. The emission line excitations and widths in the nucleus are typical of a Seyfert galaxy; but Malin 1 is in the lowest 5% of the luminosity function of Seyferts, despite a copious fuel supply. Malin 1 is in a low density region of the universe. We propose it as an unevolving disk galaxy, where the surface mass density is so low that the chemical composition and mass fraction in gas change very slowly over a Hubble time. Its properties are similar to those of the damped Lyman -a absorption systems seen in the spectra of high redshift quasars. We emphasize that there are strong observational selection effects against finding gas -rich galaxies that are both massive and diffuse. Finally, we suggest that large and massive HI disks may have formed as early as z - 2, and remained quiescent to the present day. Subject headings : individual (Malin 1) - galaxies : photometry - galaxies : Seyfert - galaxies : stellar content - radio sources : 21 cm radiation - stars : formation

A study of the Galactic Center stellar population is continuing with a sensitive 2μm CCD camera. Using a 64 x 64 detector array, background limited images are recorded with modest amounts of observing time (tob, 20 sec to reach K =13). Magnitudes have been extracted using DAOPHOT from repeated imaging of the central 5' x 5' to search among approximately 1500 stars for long period variables (LPV's, P > 200d), particularily Miras. Miras have a well defined period - luminosity relationship as well as one in period -mass. This program investigates the nature of highly luminous stars at the Galactic Center. Presently 12 variables have been found and have several characteristics consistant with Miras. They have a maximum bolometric luminosity of -4.4 mag which supports the case that high luminosity stars in the central 6 pc are young supergiants.

No description available.

We analyze the resolved stellar populations of the faint stellar system, Crater, based on deep optical imaging taken with the Advanced Camera for Surveys on the Hubble Space Telescope. Crater's color-magnitude diagram (CMD) extends similar to 4 mag below the oldest main-sequence (MS) turnoff. Structurally, we find that Crater has a half-light radius of similar to 20 pc and no evidence for tidal distortions. We model. Crater's CMD as a simple stellar population (SSP) and alternatively by solving for its full star formation history. In both cases, Crater is well. described by an SSP with an age of similar to 7.5 Gyr, a metallicity of [ M / H] similar to 1.65, a total stellar mass of M-star similar to 1e4 M-circle dot, and. a luminosity of M-V similar to - 5.3, located at a distance of d similar to 145 kpc, with modest uncertainties due to differences in the underlying stellar evolution models. We argue that the sparse sampling of stars above the turnoff and subgiant branch are likely to be 1.0-1.4 M-circle dot blue stragglers and their evolved descendants, as opposed to intermediate- age MS stars. We find that. Crater is an unusually young cluster given its location in the Galaxy's outer halo. We discuss scenarios for Crater's origin, including the possibility of being stripped from the SMC or the accretion from lower- mass dwarfs such as Leo I or Carina. Despite uncertainty over its progenitor system, Crater appears to have been incorporated into the Galaxy more recently than z similar to 1 (8 Gyr ago), providing an important new constraint on the accretion history of the Galaxy.

We present deep optical images of the Large and Small Magellanic Clouds (LMC and SMC) using a low cost telephoto lens with a wide field of view to explore stellar substructure in the outskirts of the stellar disk of the LMC (< 10 degrees from the LMC center). These data have higher resolution than existing star count maps, and highlight the existence of stellar arcs and multiple spiral arms in the northern periphery, with no comparable counterparts in the south. We compare these data to detailed simulations of the LMC disk outskirts, following interactions with its low mass companion, the SMC. We consider interaction in isolation and with the inclusion of the Milky Way tidal field. The simulations are used to assess the origin of the northern structures, including also the low density stellar arc recently identified in the Dark Energy Survey data by Mackey et al. at similar to 15 degrees. We conclude that repeated close interactions with the SMC are primarily responsible for the asymmetric stellar structures seen in the periphery of the LMC. The orientation and density of these arcs can be used to constrain the LMC's interaction history with and impact parameter of the SMC. More generally, we find that such asymmetric structures should be ubiquitous about pairs of dwarfs and can persist for 1-2 Gyr even after the secondary merges entirely with the primary. As such, the lack of a companion around a Magellanic Irregular does not disprove the hypothesis that their asymmetric structures are driven by dwarf-dwarf interactions.

Variable emission from Sgr A*, the luminous counterpart to the super-massive black hole at the center of our Galaxy, arises from the innermost portions of the accretion flow. Better characterization of the variability is important for constraining models of the low-luminosity accretion mode powering Sgr A*, and could further our ability to use variable emission as a probe of the strong gravitational potential in the vicinity of the 4 x 10(6) M-circle dot black hole. We use the Herschel Spectral and Photometric Imaging Receiver (SPIRE) to monitor Sgr. A* at wavelengths that are difficult or impossible to observe from the ground. We find highly significant variations at 0.25, 0.35, and 0.5 mm, with temporal structure that is highly correlated across these wavelengths. While the variations correspond to < 1% changes in the total intensity in the Herschel beam containing Sgr. A*, comparison to independent, simultaneous observations at 0.85 mm strongly supports the reality of the variations. The lowest point in the light curves, similar to 0.5 Jy below the time-averaged flux density, places a lower bound on the emission of Sgr. A* at 0.25 mm, the first such constraint on the THz portion of the spectral energy distribution. The variability on few hour timescales in the SPIRE light curves is similar to that seen in historical 1.3 mm data, where the longest time series is available, but the distribution of variations in the sub-mm do not show a tail of large-amplitude variations seen at 1.3 mm. Simultaneous X-ray photometry from XMM-Newton shows no significant variation within our observing period, which may explain the lack of very large submillimeter variations in our data if X-ray and submillimeter flares are correlated.

This thesis presents results from two simulation studies of galaxy formation. In the first project, a dark-matter-only HORIZON simulation is used to investigate the environment and redshift dependence of mass accretion onto haloes and subhaloes. It is found that the halo accretion rate varies less strongly with redshift than predicted by the Extended Press--Schechter formalism, and that low accretion events may drive the radio-mode feedback hypothesized for recent galaxy formation models. The subhaloes at $z<0.5$ in the simulation accrete at higher rates than haloes, on average, and it is argued that this is due to their enhanced clustering at small scales. There is no dependence of accretion rate on environment at $zsim2$, but a weak correlation emerges at $zleq0.5$. The results further support previous suggestions that at $z>1$, dark matter haloes and their associated black holes grew coevally, but imply that haloes could be accreting at fractional rates that are up to a factor of 3--4 higher than their associated black holes by the present day. In the second project, outputs from one of the Adaptive Mesh Refinement NUT simulations are analyzed in order to test whether filamentary flows of cold gas are responsible for the build-up of angular momentum within a Milky Way type disk at $zgeq3$. A set of algorithms are presented that use the resolved physical scale of $12,mathrm{pc}$ to identify: (i) the central gas disk and its plane of orientation; (ii) the complex individual filament trajectories that connect to the disk, and; (iii) the infalling satellites. The results suggest that two filaments at $zgtrsim 5.5$, which later merge to form a single filament at $zlesssim 4$, drive the angular momentum and mass budget of the disk between $3lesssim zlesssim 8$, whereas luminous satellite mergers make negligible fractional contributions. These findings hence provide strong quantitative evidence that the growth of thin disks in low mass haloes at high redshift is supported via inflowing streams of cold gas.

In this thesis, I present the results of a new search for, and study of, luminous galaxies in the first billion years of cosmic time. This work is primarily based on a new selection of bright (L≫L*) Lyman-break galaxies (LBGs) at z ≅6 and z≅7 in the UltraVISTA first and second data releases (DR1, DR2) and the UKIDSS (UKIRT Infrared Deep Survey) UDS DR10 (Ultra Deep Survey). The UltraVISTA survey provides deep Y, J,H andKs near-imaging over 1.5 deg² of the Cosmic Evolution Survey (COSMOS) field and the UKIDSS UDS provides J,H and K band data overlapping with the Subaru XMM-Newton Deep Survey (SXDS), with both fields also containing deep optical and mid-infrared imaging essential for the clean detection of z > 5 galaxies. The fields combined provide an unprecedented 1.65 deg² of deep multiwavelength data with which to securely select LBGs using a photometric redshift fitting technique, which can additionally remove probable low-redshift galaxy interlopers and galactic dwarf stars that can contaminate ground-based samples. At z ≅7, the DR1 of the UltraVISTA survey was used to select a sample of ten high-redshift galaxy candidates, which extended to a 5σ limiting magnitude of Y + J ~ 25 (AB magnitude, 2-arcsec diameter circular aperture) over 1 deg². A stack of the four most robust objects from the sample indicated that they were massive (M*≅5 × 109M ʘ), had blue rest-frame UV slopes (β ≅−2.0±0.2) and were highly star-forming (SFR ≅25–50Mʘ yr−1) when compared to previous, fainter, samples of galaxies at z = 7. The number counts of z≅7 galaxies selected within the UltraVISTA DR1 survey was higher than that expected from extrapolations of the rest-frame UltraViolet (UV) luminosity function (LF) from fainter data, a result that was strongly confirmed with an improved search for z ≅7 galaxies using the UltraVISTA DR2 imaging and the UDS field. A total of 34 galaxies at 6.5 < z < 7.5 were found in the combined fields, which included the previously identified robust galaxies from the DR1 imaging. This expanded sample allowed the first determination of the rest-frame UV LF in the range −23.0 < MUV < −21.5 at z ≅7, and the results reveal a power-law decline to bright magnitudes in contrast to the commonly assumed exponentially declining Schechter function extrapolated from fainter data. The excess of galaxies observed at bright magnitudes cannot be accounted for by gravitational lensing or by significant contamination of the sample by Active Galactic Nuclei (AGN) . The observed LF is well described by a double power law, which at the bright end follows the form of the underlying dark matter halo mass function, suggesting that the physical mechanism that inhibits star formation activity in massive galaxies (e.g. AGN feedback or some other form of ‘mass quenching’) has yet become efficient at z ≅7. The deeper imaging data confirm that the z ≅7 LBGs show blue rest-frame UV slopes (median β = −2.0) and are massive (up to M*≅1010M ʘ). Furthermore, an analysis of the ground-based imaging shows that the majority are resolved consistent with larger sizes (r1/2 ≅1–1.5 kpc) than displayed by less massive galaxies. Finally, a new search for z ≅6 galaxies within the UltraVISTA and UDS datasets was undertaken, resulting in a sample of 266 LBGs (−22.7 < MUV < −20.5) galaxies with which to investigate the rest-frame UV LF. The potential contamination by galactic brown dwarfs was investigated quantitatively using a simple model of the Galaxy, showing that the expected contamination rate of the sample was < 3 per cent, and that the stars can be effectively removed by fitting standard stellar spectra to the observed photometry. The galaxy surface density in the UltraVISTA/COSMOS field exceeds that in the UDS/SXDS by a factor of ≅1.4, indicating strong cosmic variance between the two fields. The number counts of galaxies we find are a factor of 2 lower than predicted by the recent LF determination by Bouwens et al., and the derived rest-frame UV LF at z ≅6 revealed that an under dense UDS field can account for some of the observed differences between previous analyses. An evolution in the characteristic magnitude between z ≅5 and z≅7 of ∆M*~ 0.5 was found in contrast to other smaller area surveys, and a double power law was shown to equally well describe the LF at z = 6 as compared to the commonly assumed Schechter function. The bright-end of the LF at z ≅6 tentatively shows a steeper decline than found at z ≅7, which could indicate the onset of mass quenching of the most massive galaxies or the rise of dust obscuration. Comparison with the predictions of the latest theoretical models and simulations of galaxies reveals that most models require substantial (A1500 ~ 1.5–2) average dust extinction at the bright end to reproduce the shape of the galaxy UV LF at z ≅7.