Astronomy
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Browsing Astronomy by Subject "galaxies: fundamental parameters"
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Item The dependence of quenching upon the inner structure of galaxies at 0.5 ≤ $z$ < 0.8 in the DEEP2/AEGIS survey(The American Astronomical Society, 2012) Cheung, E.; Faber, S.M.; Koo, D.C.; Dutton, A.A.; Simard, L.; McGrath, E.J.; Huang, J.-S.; Bell, E.F.; Dekel, A.; Fang, J.J.; Salim, S.; Barro, G.; Bundy, K.; Coil, A.L.; Cooper, M.C.; Conselice, C.J.; Davis, M.; Domínguez, A.; Kassin, S.A.; Kocevski, D.D.; Koekemoer, A.M.; Lin, L.; Lotz, J.M.; Newman, J.a.; Phillips, A.C.; Rosario, D.J.; Weiner, B.J.; Willmer, C.N.A.The shutdown of star formation in galaxies is generally termed "quenching." Quenching may occur through a variety of processes, e.g., active galactic nucleus (AGN) feedback, stellar feedback, or the shock heating of gas in the dark matter halo. However, which mechanism(s) is, in fact, responsible for quenching is still in question. This paper addresses quenching by searching for traces of possible quenching processes through their effects on galaxy structural parameters such as stellar mass ($M_{\ast }$), $M_{\ast }/r_{e}$, surface stellar mass density ($\sim M_{\ast }/r^{2}_{e}$), and Sérsic index ($n$). We analyze the rest-frame $U - B$ color correlations versus these structural parameters using a sample of galaxies in the redshift range $0.5 \lesssim z < 0.8$ from the DEEP2/AEGIS survey. In addition to global radii, stellar masses, and Sérsic parameters, we also use "bulge" and "disk" photometric measurements from GIM2D fits to $\textit{HST/ACS V}$ and $I$ images. We assess the tightness of the color relationships by measuring their "overlap regions," defined as the area in color-parameter space in which red and blue galaxies overlap; the parameter that minimizes these overlap regions is considered to be the most effective color discriminator. We find that Sérsic index ($n$) has the smallest overlap region among all tested parameters and resembles a step function with a threshold value of $n$ = 2.3. There exists, however, a significant population of outliers with blue colors yet high n values that seem to contradict this behavior; they make up $\approx$ 40% of $n$ > 2.3 galaxies. We hypothesize that their Sérsic values may be distorted by bursts of star formation, AGNs, and/or poor fits, leading us to consider central surface stellar mass density, $\sum^{\ast }_{1 \:\text{kpc}}$, as an alternative to Sérsic index. Not only does $\sum^{\ast }_{1 \:\text{kpc}}$ correct the outliers, but it also forms a tight relationship with color, suggesting that the $\textit{innermost structure of galaxies is most physically linked with quenching}$. Furthermore, at $z \sim 0.65$, the majority of the blue cloud galaxies cannot simply fade onto the red sequence since their GIM2D bulge masses are only half as large on average as the bulge masses of similar red sequence galaxies, thus demonstrating that stellar mass must absolutely increase at the centers of galaxies as they quench. We discuss a two-stage model for quenching in which galaxy star formation rates are controlled by their dark halos while they are still in the blue cloud and a second quenching process sets in later, associated with the central stellar mass buildup. The mass buildup is naturally explained by any non-axisymmetric features in the potential, such as those induced by mergers and/or disk instabilities. However, the identity of the second quenching agent is still unknown. We have placed our data catalog onlineItem Star formation rate distributions: Inadequacy of the Schechter function(The American Astronomical Society, 2012) Salim, S.; Lee, J.C.In this paper, we posit that galaxy luminosity functions (LFs) come in two fundamentally different types depending on whether the luminosity traces galaxy stellar mass or its current star formation rate (SFR). $\textit{Mass function types}$ reflect the older stars and therefore the stellar mass distribution, while $\textit{SFR function types}$ arise from the young stars and hence the distribution of SFRs. Optical and near-infrared LFs are of the mass function type and are well fit by a Schechter function (power law with an exponential cutoff at the bright end). In contrast, LFs of the SFR function type are of a different form, one that cannot be adequately described by a Schechter function. We demonstrate this difference by generating SFR distributions for mock samples of galaxies drawn from a Schechter stellar mass distribution along with established empirical relations between the SFR and stellar mass. Compared with the Schechter function, SFR distributions have a shallower decline at the bright end, which can be traced to the large intrinsic scatter of SFRs at any given stellar mass. A superior description of SFR distributions is given by the "Saunders" function, which combines a power law with a Gaussian at the high end. We show that the Schechter-like appearance of UV and Hα LFs, although they are LFs of SFR function type, results when luminosities are not corrected for dust, or when average statistical corrections are used because individual attenuation measurements are not available. We thus infer that the non-Schechter form of the far-IR LFs is a true reflection of the underlying SFR distribution, rather than the purported artifact of active galactic nucleus contamination.