Browsing by Subject "Dwarf galaxies"

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    On the nature of starbursts.
    (2010-05) McQuinn, Kristen Brookes W.
    Starbursts are a fascinating phenomenon that can significantly impact the host galaxy and the surrounding intergalactic medium. Understanding the nature of a starburst requires a detailed analysis of the resolved stellar populations and the recent star formation history (SFH) of the galaxy. Using a CMD fitting technique and stellar evolution models, we derive the SFHs of twenty nearby dwarf starburst galaxies from Hubble Space Telescope V and I band images. The star formation rates (SFRs) from this diverse sample of dwarf galaxies span three orders of magnitude but all show elevated levels of star formation (SF) in their recent past when viewed in the context of the host galaxy’s past SFH. Fifteen of the twenty galaxies show currently bursting SF and five galaxies show “fossil” bursts. From our reconstructed SFHs, it is evident that the elevated SFRs of a burst are sustained for hundreds of Myr. The SF migrates around the host galaxies in many cases as derived from the temporally and spatially resolved stellar populations and is a cumulation of SF not only in star clusters but also in field regions of low surface brightness in the galaxies. Contrary to the shorter time of 3-10 Myr often cited, the starburst durations we measure range from 450 − 600 Myr in fifteen of the dwarf galaxies and up to 1.3 Gyr in four galaxies; comparable to or longer than the dynamical timescales for each system. The same feedback loop from massive stars that may quench flickering SF does not disrupt the overall burst event in this sample of galaxies. In the fifteen galaxies that show ongoing bursts, the final durations may be longer than we report here. One galaxy shows a burst that has been ongoing for only 20 Myr; we are likely seeing the beginning of a burst event in this system. Using the duration of the starbursts, we calculate that the bursts deposited 1053.9 − 1057.2 erg of energy into the interstellar medium through stellar winds and supernovae and produced 3.2%−26% of the host galaxy’s mass. We also explore two other metrics for identifying starbursts: the gas consumption timescale and the strength of H#11; emission produced by the burst. Interestingly, four galaxies classified as starbursts in our most recent time bin of 4-10 Myr show non-starburst levels of H#11; emission from the last ∼5 Myr indicating that, while the bursts are long-lasting events, the SFR can change on timescales of only a few Myr.
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    Star formation histories of the large and small magellanic clouds
    (2014-05) Neary, Kyle Brian
    The star formation histories (SFHs) of galaxies can tell us a great deal about the formation and evolution of galactic systems. By creating color-magnitude diagrams (CMDs) of resolved stellar populations, we can derive the star formation rate (SFR) as a function of time and metallicity (i.e., the SFH) for the observed stellar population. In this thesis, I study the methods used to make these measurements and calculations, and apply them to conduct a study of the Magellanic Clouds. Throughout this study, we aim to address two fundamental questions: “How can we obtain deep, spatially comprehensive photometric coverage of dwarf galaxies and use those data to construct accurate and meaningful SFHs?” and “Can we constrain the possible evolutionary scenarios of the Magellanic Clouds from their ancient SFHs?”We present the results from a study of the ancient star formation histories (SFHs) of the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). At about 50 kpc and 60 kpc away, respectively, from the Milky Way (MW), the LMC and SMC provide us an excellent opportunity to make deep photometric measurement, allowing us to construct an accurate ancient SFH. Using archival data from the Hubble Space Telescope/Wide Field Planetary Camera 2 (HST /WFPC2), we construct CMDs for 56 LMC fields and 15 SMC fields. This data set provides diverse spatial coverage as well as photometric depth that reaches well below the oldest main sequence turn off (MSTO), allowing us to construct a global SFH of each galaxy with excellent temporal resolution, even in the oldest time bins. We derive the SFHs using the same maximum likelihood CMD fitting technique for both the LMC and the SMC to allow for the direct comparison of the results, without the introduction of unnecessary systematic offsets and uncertainties. At very early times, we find that the LMC experienced an initial burst of star formation activity that was absent in the SMC. After about 12 Gyr ago, the SFHs both galaxies tracked each other very well. After about 12 Gyr ago, they were both relatively quiescent until 4–6 Gyr ago, when the star formation rate (SFR) dramatically increased in both galaxies, and maintained that rate until the present. These findings have driven us to conclude that the MCs did not originally form together, and they are likely on their first passage through the MW. Finally, we discuss ongoing and upcoming studies of dwarf galaxies in the Andromeda system. Using ground-based and space-based data, we hope to derive spatially comprehensive SFHs with excellent temporal resolution. Accurate, quantitative SFHs of many of the Andromeda dwarfs will allow us to study a more significant portion of a whole galactic system than ever before. It will also have significant cosmological implications because we will be able to determine if the MW population is representative of galaxies in general or if the local environment plays a significant role in the evolution of a large galaxy.