Thesis (Ph. D.)--University of Washington, 2000. / Vita. Includes bibliographical references (leaves 148-155).

The abundance of satellite galaxies is a critical small-scale test of the standard cosmological model. From comparing the predictions of structure formation in simulations with observations of Local Group dwarf galaxies there is a clear mismatch in the abundance, leading to the so-called “missing satellites” problem. The comparison between simulation and observation have, however, suffered from a limited sample of satellite galaxies, with the only reasonably complete sample being from the most local galaxy groups. It is unknown whether the observed abundance of dwarf satellite galaxies of the nearest groups is statistically representative of the abundance of dwarf satellites in the greater universe. We construct a volume-limited sample of galaxies down to a well-defined stellar mass limit (M★ ≥ 6 × 109 Msun ) using the Atlas3D parent sample of spiral and ellipsoidal galaxies by Cappellari et al. 2011. In order to statistically identify bound satellites around galaxies in our primary catalogue, we apply cuts on the background based on the properties of known dwarf satellites of the Local Group using both the Sloan Digital Sky Survey Data Release 8 (SDSS DR8) (http://www.sdss3.org/dr8/) spectroscopic and photometric galaxy catalogues. We detect an over-density of faint objects at projected separations of < 500 kpc at S/N ∼ 8, corresponding to an average of 4.8 ± 0.65 satellite detections per primary after stacking these systems and subtracting the background. We further find that the over-density of faint objects strongly depends on primary morphology and magnitude. While the Milky Way seems to be unusual in its number of bright satellites, our faint end satellite abundances are in agreement from the ∆m luminosity function for primaries in our sample as bright as the Milky Way. Our work has extended the work of previous authors by several magnitudes further down the faint end of the luminosity function.

No description available.

An outstanding question in galaxy evolution research is whether the star formation histories of low mass systems are dominated by global starbursts or modes that are more quiescent and continuous. In this thesis, we quantify the prevalence of global starbursts in dwarf galaxies at the present epoch, and attempt to infer their characteristic durations, frequencies and amplitudes in the past. Our approach is to directly tally the number of bursting dwarfs in a complete local sample, and to compute the fraction of star formation that is concentrated in these systems. The resulting starburst number and mass fractions are then combined with B-V colors from the literature, the H-alpha EWs presented here, and stellar evolutionary synthesis models in order to place constraints on the average starburst duty cycle. The primary dataset used has been put together by the 11 Mpc H-alpha UV Galaxy Survey, who have collected data on an approximately volume-limited, statistical sample of star-forming galaxies within 11 Mpc of the Milky Way.Our main observational results, along with the accumulation of star formation studies of dwarf galaxies over the past three decades, paint a consistent picture where systems that are currently experiencing a massive global burst are just the 6% +/- 3% tip of a low-mass galaxy iceberg. Moreover, bursts are responsible for 22% +/- 10% of the total star formation in the overall dwarf galaxy population, so the majority of stars in low-mass systems do not appear to be formed in this mode today.Over their lifetimes, however, a greater fraction of the stellar mass of a dwarf may be formed in the burst mode. Synthesis modeling suggests that bursts cycles appear to be necessary in order to simultaneously explain the present-day observed blue B-V colors and modest H-alpha EWs of TYPICAL, CURRENTLY NON-BURSTING dwarf irregulars, unless non-standard assumptions concerning the IMF and the escape fractions of Lyman continuum photons are made. The starburst cycle that we converge upon involves burst durations of 50-100 Myrs, cycle frequencies of 1 to 3 per Gyr, and elevated burst SFRs that are a factor of 6-10 higher than the rate in the quiescent state. Galaxies characterized by such a SFH would spend ~10% of their lives in the burst state, and form ~50% of their stellar mass during this time.

The abundance of satellite galaxies is a critical small-scale test of the standard cosmological model. From comparing the predictions of structure formation in simulations with observations of Local Group dwarf galaxies there is a clear mismatch in the abundance, leading to the so-called “missing satellites” problem. The comparison between simulation and observation have, however, suffered from a limited sample of satellite galaxies, with the only reasonably complete sample being from the most local galaxy groups. It is unknown whether the observed abundance of dwarf satellite galaxies of the nearest groups is statistically representative of the abundance of dwarf satellites in the greater universe. We construct a volume-limited sample of galaxies down to a well-defined stellar mass limit (M★ ≥ 6 × 109 Msun ) using the Atlas3D parent sample of spiral and ellipsoidal galaxies by Cappellari et al. 2011. In order to statistically identify bound satellites around galaxies in our primary catalogue, we apply cuts on the background based on the properties of known dwarf satellites of the Local Group using both the Sloan Digital Sky Survey Data Release 8 (SDSS DR8) (http://www.sdss3.org/dr8/) spectroscopic and photometric galaxy catalogues. We detect an over-density of faint objects at projected separations of < 500 kpc at S/N ∼ 8, corresponding to an average of 4.8 ± 0.65 satellite detections per primary after stacking these systems and subtracting the background. We further find that the over-density of faint objects strongly depends on primary morphology and magnitude. While the Milky Way seems to be unusual in its number of bright satellites, our faint end satellite abundances are in agreement from the ∆m luminosity function for primaries in our sample as bright as the Milky Way. Our work has extended the work of previous authors by several magnitudes further down the faint end of the luminosity function.

Thesis (Ph. D.)--Texas Christian University, 2007. / Title from dissertation title page (viewed Dec. 10, 2007). Includes abstract. Includes bibliographical references.

Thesis (Ph.D.)--McMaster University (Canada), 1996. / Source: Dissertation Abstracts International, Volume: 58-06, Section: B, page: 3088. Adviser: W. E. Harris.

Thesis (Ph. D.)--University of Chicago, Department of Astronomy & Astrophysics, June 2002. / Includes bibliographical references. Also available on the Internet.

According to the current cosmological paradigm, ``Lambda Cold Dark Matter'' (LambdaCDM), only ~20% of the gravitating matter in the universe is made up of ordinary (i.e. baryonic) matter, while the rest consists of invisible dark matter (DM) particles, which existence can be inferred from their gravitational influence on baryonic matter and light. Despite the large success of the LambdaCDM model in explaining the large scale structure of the Universe and the conditions of the early Universe, there has been debate on whether this model can fully explain the observations of low mass (dwarf) galaxies. The Local Group (LG), which hosts most of the known dwarf galaxies, is a unique laboratory to test the predictions of the LambdaCDM model on small scales. I analyze the kinematics of LG members, including the Milky~Way-Andromeda (MW-M31) pair and dwarf galaxies, in order to constrain the mass of the LG. I construct samples of LG analogs from large cosmological N-body simulations, according to the following kinematics constraints: (a) the separation and relative velocity of the MW-M31 pair; (b) the receding velocity of dwarf galaxies in the outskirts of the LG. I find that these constraints yield a median total mass of 2*10^{12} solar masses for the MW and M31, but with a large uncertainty. Based on the mass and the kinematics constraints, I select twelve LG candidates for the APOSTLE simulations project. The APOSTLE project consists of high-resolution cosmological hydrodynamical simulations of the LG candidates, using the EAGLE galaxy formation model. I show that dwarf satellites of MW and M31 analogs in APOSTLE are in good agreement with observations, in terms of number, luminosity and kinematics. There have been tensions between the observed masses of LG dwarf spheroidals and the predictions of N-body simulations within the LambdaCDM framework; simulations tend to over-predict the mass of dwarfs. This problem is known as the ``too-big-to-fail'' problem. I find that the enclosed mass within the half-light radii of Galactic classical dwarf spheroidals, is in excellent agreement with the simulated satellites in APOSTLE, and that there is no too-big-to-fail problem in APOSTLE simulations. A few factors contribute in solving the problem: (a) the mass of haloes in hydrodynamical simulations are lower compared to their N-body counterparts; (b) stellar mass-halo mass relation in APOSTLE is different than the ones used to argue for the too-big-to-fail problem; (c) number of massive satellites correlates with the virial mass of the host, i.e. MW analogs with virial masses above ~ 3*10^{12} solar masses would have faced too-big-to-fail problems; (d) uncertainties in observations were underestimated in previous works. Stellar mass-halo mass relation in APOSTLE predicts that all isolated dwarf galaxies should live in haloes with maximum circular velocity (V_max) above 20 km/s. Satellite galaxies, however, can inhabit lower mass haloes due to tidal stripping which removes mass from the inner regions of satellites as they orbit their hosts. I examine all satellites of the MW and M31, and find that many of them live in haloes less massive than V_max=20 km/s. I additionally show that the low mass population is following a different trend in stellar mass-size relation compared to the rest of the satellites or field dwarfs. I use stellar mass-halo mass relation of APOSTLE field galaxies, along with tidal stripping trajectories derived in Penarrubia et al., in order to predict the properties of the progenitors of the LG satellites. According to this prediction, some satellites have lost a significant amount of dark matter as well as stellar mass. Cra~II, And~XIX, XXI, and XXV have lost 99 per-cent of their stellar mass in the past. I show that the mass discrepancy-acceleration relation of dwarf galaxies in the LG is at odds with MOdified Newtonian Dynamics (MOND) predictions, whereas tidal stripping can explain the observations very well. I compare observed velocity dispersion of LG satellites with the predicted values by MOND. The observations and MOND predictions are inconsistent, in particular in the regime of ultra faint dwarf galaxies. / Graduate

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. Johannesburg, February 6, 2015. / We explore indirect detections of Dark Matter, focusing on deep radio observations of six dwarf spheroidal galaxies (dSph), Carina, Fornax, BootesII, Hercules, Segue2, Sculptor. We discuss the WIMP Dark Matter particle annihilation process and describe brie y the particles produced in this process. We consider the emissions, which can result from electrons and positrons produced. We describe why dSph are the best observational targets for indirect Dark Matter detection at radio frequencies. We describe the theoretical framework for predicting Dark Matter synchrotron emissions and make some predictions for the six dSph of interest to us. We discuss ATCA observations of these dSph and explore the background source subtraction process in detail. We obtain an upper limit on the WIMP mass and compare our results to various other experiments. We discuss prospects for this work towards attaining an indirect Dark Matter detection.