UNC PHYSICS AND ASTRONOMY THESIS DEFENSE
Haw Cheng, UNC-Chapel Hill
“A Photometric Study of the DwarfSpheroidal Galaxies Leo IV and Bootes II.”
I present a photometric study of the ultra-faint dwarf (UFD) galaxies Leo IV and Boötes II in the V and IC filters. I fit Dartmouth isochrones to the color-magnitude diagrams (CMD’s) of both objects and derive their ages relative to the old metal poor globular cluster M92. Variable stars were observed in both objects and photometric variability plots obtained for them. Two such stars, RR Lyraes previously observed by Moretti et al. 2009, were observed in Leo IV. No further variable stars were uncovered in Leo IV. Comparison of the horizontal branch’s observed V magnitude to the absolute magnitudes of the RR Lyraes as determined via Cacciari & Clemeniti (2003)’s MV -metallicity relation yields a distance modulus of 21.01 ± 0.07, or a distance of 159 ± 5 kpc, for Leo IV, in good agreement with previous studies. One of the stars, V1, is observed to exhibit the Blazhko effect. Leo IV is found to be predominantly old and metal poor. It is well fit by isochrones of [Fe/H] = −2.46 and [α/Fe] = 0.2 and 0.4. Isochrone fits and the ΔVTOHB method of VandenBerg et al. 2013 suggest that TO it is approximately 2 Gyr younger than M92, while a “standard” ΔVTOHB method suggests TO that it is approximately 2.5 Gyr younger. A spread in age with a plausible value of ∼ 2 Gyr cannot be ruled out. A 10 Gyr old synthetic horizontal branch with [Fe/H] = −1.70 and [α/Fe] of 0.2 is fit to Leo IV’s red horizontal branch (RHB). The good fit of this synthetic horizontal branch to the RHB and its close match to the metallicity of a known radial velocity member of Leo IV on the RHB, as well as the good fit of the matching isochrone to the other evolutionary sequences of Leo IV’s CMD, lends support to the idea that Leo IV’s RHB is in fact real and possibly represents a metal rich subpopulation, rather than being merely a chance accumulation of foreground stars as proposed by Okamoto et al. 2012. Leo IV’s possible population of blue stragglers is also examined. While this study’s sample and spatial coverage is too small for any kind of detailed spatial distribution study, it is found that the stars in the blue straggler region of the CMD show no signs of central concentration. Boötes II’s CMD is found to be consistent with that of a single age, mono-metallicity system. It is well fit by isochrones of [Fe/H] = −1.79 and [α/Fe] = 0.2 and 0.4. Boötes II is found to be between 0.5 to 1.5 Gyr younger than M92. Distance was left as a free parameter in the fits. Boötes II is found to have distance modulus of between 18.02 to 18.15 (corresponding to distances of 40.2 kpc and 42.7 kpc, respectively), in good agreement with previous studies. A single variable star is discovered in Boötes II. Its photometric variability plot clearly shows it to be an ab type RR Lyrae.