Quenching or Bursting: Star Formation Acceleration--A New Methodology for Tracing Galaxy Evolution

We introduce a new methodology for the direct extraction of galaxy physical parameters from multi-wavelength photometry and spectroscopy. We use semi-analytic models that describe galaxy evolution in the context of large scale cosmological simulation to provide a catalog of galaxies, star formation histories, and physical parameters. We then apply stellar population synthesis models and a simple extinction model to calculate the observable broad-band fluxes and spectral indices for these galaxies. We use a linear regression analysis to relate physical parameters to observed colors and spectral indices. The result is a set of coefficients that can be used to translate observed colors and indices into stellar mass, star formation rate, and many other parameters, including the instantaneous time derivative of the star formation rate which we denote the {\it Star Formation Acceleration (SFA)}, We apply the method to a test sample of galaxies with GALEX photometry and SDSS spectroscopy, deriving relationships between stellar mass, specific star formation rate, and star formation acceleration. We find evidence for a mass-dependent SFA in the green valley, with low mass galaxies showing greater quenching and higher mass galaxies greater bursting. We also find evidence for an increase in average quenching in galaxies hosting AGN. A simple scenario in which lower mass galaxies accrete and become satellite galaxies, having their star forming gas tidally and/or ram-pressure stripped, while higher mass galaxies receive this gas and react with new star formation can qualitatively explain our results.Comment: 33 pages, 31 figures, ApJ accepte

The Carnegie-Chicago Hubble Program: Discovery of the Most Distant Ultra-faint Dwarf Galaxy in the Local Universe

Ultra-faint dwarf galaxies (UFDs) are the faintest known galaxies and due to their incredibly low surface brightness, it is difficult to find them beyond the Local Group. We report a serendipitous discovery of an UFD, Fornax UFD1, in the outskirts of NGC 1316, a giant galaxy in the Fornax cluster. The new galaxy is located at a projected radius of 55 kpc in the south-east of NGC 1316. This UFD is found as a small group of resolved stars in the Hubble Space Telescope images of a halo field of NGC 1316, obtained as part of the Carnegie-Chicago Hubble Program. Resolved stars in this galaxy are consistent with being mostly metal-poor red giant branch (RGB) stars. Applying the tip of the RGB method to the mean magnitude of the two brightest RGB stars, we estimate the distance to this galaxy, 19.0 +- 1.3 Mpc. Fornax UFD1 is probably a member of the Fornax cluster. The color-magnitude diagram of these stars is matched by a 12 Gyr isochrone with low metallicity ([Fe/H] ~ -2.4). Total magnitude and effective radius of Fornax UFD1 are Mv ~ -7.6 +- 0.2 mag and r_eff = 146 +- 9 pc, which are similar to those of Virgo UFD1 that was discovered recently in the intracluster field of Virgo by Jang & Lee (2014).Fornax UFD1 is the most distant known UFD that is confirmed by resolved stars. This indicates that UFDs are ubiquitous and that more UFDs remain to be discovered in the Fornax cluster.Comment: 6 pages, 4 figures, 1 table, accepted for publication in ApJ Letter

The Starburst Nature of Lyman-Break Galaxies: Testing UV Extinction with X-rays

We derive the bolometric to X-ray correlation for a local sample of normal and starburst galaxies and use it, in combination with several UV reddening schemes, to predict the 2--8 keV X-ray luminosity for a sample of 24 Lyman-break galaxies in the HDF/CDF-N. We find that the mean X-ray luminosity, as predicted from the Meurer UV reddening relation for starburst galaxies, agrees extremely well with the Brandt stacking analysis. This provides additional evidence that Lyman-break galaxies can be considered as scaled-up local starbursts and that the locally derived starburst UV reddening relation may be a reasonable tool for estimating the UV extinction at high redshift. Our analysis shows that the Lyman-break sample can not have far-IR to far-UV flux ratios similar to nearby ULIGs, as this would predict a mean X-ray luminosity 100 times larger than observed, as well as far-IR luminosities large enough to be detected in the sub-mm. We calculate the UV reddening expected from the Calzetti effective starburst attenuation curve and the radiative transfer models of Witt & Gordon for low metallicity dust in a shell geometry with homogeneous or clumpy dust distributions and find that all are consistent with the observed X-ray emission. Finally, we show that the mean X-ray luminosity of the sample would be under predicted by a factor of 6 if the the far-UV is unattenuated by dust.Comment: 7 pages, 3 figures. Accepted for publication in A

GALEX and Optical Light Curves of EF Eridanus During a Low State: the Puzzling Source of UV Light

Low state optical photometry of EF Eri during an extended low accretion state combined with GALEX near and far UV time-resolved photometry reveals a source of UV flux that is much larger than the underlying 9500K white dwarf, and that is highly modulated on the orbital period. The near UV and optical light curves can be modeled with a 20,000K spot but no spot model can explain both the large amplitude FUV variations and the SED. The limitations of limb darkening, cyclotron and magnetic white dwarf models in explaining the observations are discussed.Comment: 10 pages, 4 figures to be published in ApJ Letter

Identification of the Biological Function of Rab-GGT β-Subunits by Reverse Techniques

Protein prenylation is a post-translational process where lipids are added to carboxyl end groups, which allows proteins to function properly in the eukaryotic cell. The job of prenylation is to help in targeting certain proteins to specific membrane along with promoting protein-protein interactions. We use reverse genetics techniques to understand the function of prenylation in plant development by examining the phenotypic changes caused by specific gene disruption. One of the excellent model organisms Physcomitrella patens (moss) is used due to its simple structure, limited tissue and cells, sequenced genome, and its high gene targeting efficiency. Rab geranylgeranyl transferase-II (Rab-GGT) is one of three enzymes that can perform protein prenylation and the actual function of Rab-GGT is largely unknown. Moss has one copy of Rab-GGT α subunit (PpRGTA1) and two copies of β subunit (PpRGTB1 and PpRGTB2). This study focuses on the role of the Rab-GGT β subunit in the moss. It has been found that the knockout of either PpRGTB1 or PpRGTB2 results in no visible phenotype, which leads us to believe that these genes are functionally redundant. The knockout of both PpRGTB1 and PpRGTB2 genes has shown to be lethal, which means Rab-GGT is required for viability. To figure out the function of Rab-GGT, we use RNA interference approach to down-regulate the expression level of PpRGTB2 in the PpRGTB1 knockout background so we can observe the phenotypic consequences.https://ir.library.louisville.edu/uars/1049/thumbnail.jp

The Effects of Diffuse Ionized Gas and Spatial Resolution on Metallicity Gradients: TYPHOON Two-Dimensional Spectrophotometry of M83

We present a systematic study of the diffuse ionized gas (DIG) in M83 and its effects on the measurement of metallicity gradients at varying resolution scales. Using spectrophotometric data cubes of M83 obtained at the 2.5m duPont telescope at Las Campanas Observatory as part of the TYPHOON program, we separate the HII regions from the DIG using the [SII]/H$\alpha$ ratio, HIIphot (HII finding algorithm) and the H$\alpha$ surface brightness. We find that the contribution to the overall H$\alpha$ luminosity is approximately equal for the HII and DIG regions. The data is then rebinned to simulate low-resolution observations at varying resolution scales from 41 pc up to 1005 pc. Metallicity gradients are measured using five different metallicity diagnostics at each resolution. We find that all metallicity diagnostics used are affected by the inclusion of DIG to varying degrees. We discuss the reasons of why the metallicity gradients are significantly affected by DIG using the HII dominance and emission line ratio radial profiles. We find that applying the [SII]/H$\alpha$ cut will provide a closer estimate of the true metallicity gradient up to a resolution of 1005 pc for all metallicity diagnostics used in this study.Comment: 26 pages, 12 figures + Appendix/Supplementary Material, accepted for publication by MNRA

Characterization of the Function of PpRGTB2 from Mutant Phenotypes

Protein prenylation, a common lipid post-translational modification, is required for growth and development in eukaryotes. One type, , but its biological function is not well known. The moss Physcomitrella patens (P. patens) was used as a model organism due to its simple structure, limited cell types, sequenced genome, and its high gene targeting efficiency. P. patens has one copy of Rab-GGT α subunit (PpRGTA1) and two copies of β subunit (PpRGTB1 and PpRGTB2). It has been found that the knockout of either PpRGTB1 or PpRGTB2 results in no visible phenotype, which indicates that these genes must be functionally redundant. The knockout of both PpRGTB1 and PpRGTB2 genes has shown to be lethal, which means Rab-GGT is required for viability. To determine the function of Rab-GGT, we used the RNA interference approach to down-regulate the expression level of PpRGTB2 in the PpRGTB1 knockout background to observe these phenotypic changes. P. patens grows in long thread-like filaments made of cells, also called protonema. Protonema includes two different cell types, chloronema and caulonema. Each of these cell types has distinct features that can be observed and quantified. This study focuses on quantifying P. patens’ distinctive features in cell size, width, and the amount of protruding caulonema present in the wildtype vs PpRGTB2 knockdown lines. The observed defects indicate the RGTB2 gene plays a vital role in moss growth and development