Browsing by Subject "Galaxy evolution"

Now showing 1 - 3 of 3
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    An analysis of star formation in M31 using resolved stars and ultraviolet flux
    (2014-10) Simones, Jacob Edward
    We have used optical observations of resolved stars from the Panchromatic Hubble Andromeda Treasury (PHAT) to measure the recent (< 500 Myr) star formation histories (SFHs) of 33 FUV-bright regions in M31. The region areas ranged from ~ 104 to 106 pc2, which allowed us to test the reliability of FUV flux as a tracer of recent star formation on sub-kpc scales. The star formation rates (SFRs) derived from the extinction-corrected observed FUV fluxes were, on average, consistent with the 100-Myr mean SFRs of the SFHs to within the 1 σ scatter. Overall, the scatter was larger than the uncertainties in the SFRs and particularly evident among the smallest regions. The scatter was consistent with an even combination of discrete sampling of the initial mass function and high variability in the SFHs. This result demonstrates the importance of satisfying both the full-IMF and the constant-SFR assumptions for obtaining precise SFR estimates from FUV flux. Assuming a robust FUV extinction correction, we estimate that a factor of 2.5 uncertainty can be expected in FUV-based SFRs for regions smaller than 105 pc2, or a few hundred pc. We also examined ages and masses derived from UV flux under the common assumption that the regions are simple stellar populations (SSPs). The SFHs showed that most of the regions are not SSPs, and the age and mass estimates were correspondingly discrepant from the SFHs. For those regions with SSP-like SFHs, we found mean discrepancies of 10 Myr in age and a factor of 3 to 4 in mass. It was not possible to distinguish the SSP-like regions from the others based on integrated FUV flux.Starting from SFHs derived from the full PHAT photometric dataset, we have used stellar population synthesis to create maps of synthetic far- and near-ultraviolet (FUV and NUV) flux at sub-kpc resolution for the northeast quadrant of M31. The synthetic maps reproduced all of the main morphological features found in corresponding maps of observed FUV and NUV flux, including rings and large star-forming complexes. Comparing the flux maps pixel-by-pixel, we found the median synthetic-to-observed flux ratios to be 1.02 +0.74/-0.43 in FUV and 0.79 +0.35/-0.24 in NUV. The synthetic fluxes were therefore consistent overall with the observed fluxes in both filters. We used the observed fluxes and standard flux calibrations to derive star formation rate (SFR) maps, which we compared with a map of the mean SFRs over the last 100 Myr of the star formation histories (SFHs). We determined a lower limit of SFR ~ 10-5 Msun yr-1 below which the commonly assumed linear relationship between UV flux and SFR appears to break down. Above this limit, we found the median ratios of the flux-based SFRs to the mean SFRs to be 0.57 +0.47/-0.26 in FUV and 1.24 +0.88/-0.52 in NUV. Both the FUV and NUV flux-based SFRs were therefore consistent overall with the mean SFRs derived from the SFHs. Integrating over the entire mean SFR map, we found a global SFR of 0.3 Msun yr-1. The corresponding measurements from the flux-based SFR maps were factors of 0.74 (FUV) and 1.45 (NUV) of the global mean SFR value. It is not yet understood why the SFR ratios in the global case are larger than the median pixel-wise ratios. The primary source of uncertainty in both the synthetic flux maps and the flux-based SFR maps was most likely incomplete IMF sampling due to the small pixel areas. With the exception of the faintest areas of the galaxy, we did not identify any trends for flux or SFR with environment.
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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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    Scouting for Clumps in our Cosmic Backyard: Detection and Analysis of the Local Clumpy Galaxy Population
    (2024-05) Adams, Dominic
    A key change in the galaxy population between z ~ 1 and z ~ 0 is that most high-redshift star-forming galaxies host “giant star-forming clumps” (or simply “clumps”), with luminosities ranging an order of magnitude above typical star-forming regions nearby. The origins, lifetimes, and evolutionary roles of these clumps all remain uncertain. To date, large populations of clumpy galaxies have only been observed at low resolution (1 kpc or larger). However, limited observations of strongly-lensed and low-redshift galaxies reveal that most clumps have significant 10-100 pc substructure, which can only be resolved by space telescopes at low redshift. Local clump detections are therefore needed to enable high-resolution follow-up. This thesis presents the first-ever large (N > 1,000), low-redshift (z < 0.1) catalog ofgiant star-forming clumps. It is based upon the results of the citizen science project Galaxy Zoo: Clump Scout, which recruited volunteers to detect clumps in nearly 60,000 Sloan Digital Sky Survey (SDSS) galaxy images. Volunteer detections were aggregated by a novel maximum-likelihood model designed specifically for citizen science. Finally, a Faster-RCNN object detection network was trained on this catalog and applied nearly 240,000 SDSS galaxy images. The final catalog contains thousands of clumps analogous to those in high-redshift galaxies, including over 100 that can be observed at sub-100 parsec resolutions with existing instrumentation. Using this catalog, I compute the “clumpy fraction” – the fraction of star-forming galaxies hosting at least one clump – at z ~ 0. I find an order-of-magnitude drop in this fraction between redshift 1.5 and 0 for a range of galaxy masses. This drop correlates most closely with the drop in star formation rate and galaxy turbulence over that time. Further, I find that the environmental density does not significantly impact the clumpy fraction when controlling for star formation rate. Together, this suggests that most clumps have formed in situ (from turbulent gas collapse) since at least z ∼ 1.5, and that clump formation slowed due to the reduction in gas fractions and accretion rates over that time.