Browsing by Subject "supermassive"
Now showing 1 - 3 of 3
- Results Per Page
- Sort Options
Item Connecting Transitions In Galaxy Properties To Refueling(2013-11) Kannappan, Sheila J.; Stark, David V.; Eckert, Kathleen D.; Moffett, Amanda J.; Wei, Lisa H.; Pisano, D. J.; Baker, Andrew J.; Vogel, Stuart N.; Fabricant, Daniel G.; Laine, Seppo; Norris, Mark A.; Jogee, Shardha; Lepore, Natasha; Hough, Loren E.; Weinberg-Wolf, Jennifer; Jogee, ShardhaWe relate transitions in galaxy structure and gas content to refueling, here defined to include both the external gas accretion and the internal gas processing needed to renew reservoirs for star formation. We analyze two z = 0 data sets: a high-quality similar to 200 galaxy sample (the Nearby Field Galaxy Survey, data release herein) and a volume-limited similar to 3000 galaxy sample with reprocessed archival data. Both reach down to baryonic masses similar to 10(9) M and span void-to-cluster environments. Two mass-dependent transitions are evident: (1) below the "gas-richness threshold" scale (V similar to 125 km s(-1)), gas-dominated quasi-bulgeless Sd-Im galaxies become numerically dominant; while (2) above the "bimodality" scale (V similar to 200 km s(-1)), gas-starved E/S0s become the norm. Notwithstanding these transitions, galaxy mass (or V as its proxy) is a poor predictor of gas-to-stellar mass ratio M-gas/M-*. Instead, M-gas/M-* correlates well with the ratio of a galaxy's stellar mass formed in the last Gyr to its preexisting stellar mass, such that the two ratios have numerically similar values. This striking correspondence between past-averaged star formation and current gas richness implies routine refueling of star-forming galaxies on Gyr timescales. We argue that this refueling underlies the tight M-gas/M-* versus color correlations often used to measure "photometric gas fractions." Furthermore, the threshold and bimodality scale transitions reflect mass-dependent demographic shifts between three refueling regimes-accretion-dominated, processing-dominated, and quenched. In this picture, gas-dominated dwarfs are explained not by inefficient star formation but by overwhelming gas accretion, which fuels stellar mass doubling in less than or similar to 1 Gyr. Moreover, moderately gas-rich bulged disks such as the Milky Way are transitional, becoming abundant only in the narrow range between the threshold and bimodality scales.Item Feedback In Luminous Obscured Quasars(2011-05) Greene, Jenny E.; Zakamska, Nadia L.; Ho, Luis C.; Barth, Aaron J.; Greene, Jenny E.We use spatially resolved long-slit spectroscopy from Magellan to investigate the extent, kinematics, and ionization structure in the narrow-line regions of 15 luminous, obscured quasars with z < 0.5. Increasing the dynamic range in luminosity by an order of magnitude, as well as improving the depth of existing observations by a similar factor, we revisit relations between narrow-line region size and the luminosity and linewidth of the narrow emission lines. We find a slope of 0.22 +/- 0.04 for the power-law relationship between size and luminosity, suggesting that the nebulae are limited by availability of gas to ionize at these luminosities. In fact, we find that the active galactic nucleus is effectively ionizing the interstellar medium over the full extent of the host galaxy. Broad (similar to 300-1000 km s(-1)) linewidths across the galaxies reveal that the gas is kinematically disturbed. Furthermore, the rotation curves and velocity dispersions of the ionized gas remain constant out to large distances, in striking contrast to normal and starburst galaxies. We argue that the gas in the entire host galaxy is significantly disturbed by the central active galactic nucleus. While only similar to 10(7)-10(8) M-circle dot worth of gas are directly observed to be leaving the host galaxies at or above their escape velocities, these estimates are likely lower limits because of the biases in both mass and outflow velocity measurements and may in fact be in accord with expectations of recent feedback models. Additionally, we report the discovery of two dual obscured quasars, one of which is blowing a large-scale (similar to 10 kpc) bubble of ionized gas into the intergalactic medium.Item Nuclear Star Clusters From Clustered Star Formation(2011-03) Agarwal, Meghann; Milosavljevic, Milos; Agarwal, MeghannPhotometrically distinct nuclear star clusters (NSCs) are common in late-type disk and spheroidal galaxies. The formation of NSCs is inevitable in the context of normal star formation in which a majority of stars form in clusters. A young, mass-losing cluster embedded in an isolated star-forming galaxy remains gravitationally bound over a period determined by its initial mass and the galactic tidal field. The cluster migrates radially toward the center of the galaxy and becomes integrated in the NSC if it reaches the center. The rate at which the NSC grows by accreting young clusters can be estimated from empirical cluster formation rates and dissolution times. We model cluster migration and dissolution and find that the NSCs in late-type disks and in spheroidals could have assembled from migrating clusters. The resulting stellar nucleus contains a small fraction of the stellar mass of the galaxy; this fraction is sensitive to the high-mass truncation of the initial cluster mass function (ICMF). The resulting NSC masses are consistent with the observed values, but generically, the final NSCs are surrounded by a spatially more extended excess over the inward-extrapolated exponential (or Sersic) law of the outer galaxy. We suggest that the excess can be related to the pseudobulge phenomenon in disks, though perhaps not all of the pseudobulge mass assembles this way. Comparison with observed NSC masses can be used to constrain the truncation mass scale of the ICMF and the fraction of clusters suffering prompt dissolution. We infer truncation mass scales of less than or similar to 10(6) M-circle dot (greater than or similar to 10(5) M-circle dot) without (with 90%) prompt dissolution. Since the NSC assembly is collisionless and non-dissipative, no relation to the process responsible for central black hole assembly in more massive galaxies is expected.