Galaxy formation and evolution

Abstract:

We report ∼1″ resolution K-band (2 μm) imaging spectroscopy of the central parsec of our Galaxy. The derived radial velocities for 223 early- and late-type stars probe the nuclear mass distribution to spatial scales of 0.1 pc. We find a statistically very significant increase of projected stellar velocity dispersion from about 55 km s-1 at p ∼ 5 pc to 180 km s-1 at p ∼ 0.1 pc. The stars are also rotating about the dynamic center. The late-type stars follow general Galactic rotation, while the early-type stars show counter-rotation. Fitting simultaneously the observed projected surface densities and velocity dispersions, we derive the intrinsic volume densities and radial velocity dispersions as a function of distance from the dynamic center for both types of stars. We then derive the mass distribution between 0.1 and 5 pc from the Jeans equation assuming an isotropic velocity field. Our analysis requires a compact central dark mass of 2.5-3.2 × 106 M⊙, at 6-8 σ significance. The dark mass has a density of 109 M⊙ pc-3 or greater and a mass to 2 μm luminosity of ≥ 100. The increase in mass-to-luminosity ratio can be reduced but not eliminated even if extreme anisotropic velocity destributions are considered. The dark mass cannot be a cluster of solar mass remnants (such as neutron stars). It is either a compact cluster of 10-20 M⊙ black holes or a single massive black hole. © 1996. The American Astronomical Society. All rights reserved.

Abstract:

We report the discovery of a group of galaxies at redshift 2.38. We imaged ∼ 10% of a claimed supercluster of QSO absorption lines at z = 2.38. In this small field (2′ radius), we detect two Lyα-emitting galaxies. The discovery of two such galaxies in our tiny field supports Francis & Hewett's interpretation of the absorption-line supercluster as a high-redshift Great Wall. One of the Lyα galaxies lies 22″ from a background QSO and may be associated with a multicomponent Lyα absorption complex seen in the QSO spectrum. This galaxy has an extended (∼50 kpc), lumpy Lyα morphology surrounding a compact, IR-bright nucleus. The nucleus shows a pronounced break in its optical-UV colors at ∼4000 Å (rest frame), consistent with a stellar population of mass ∼7 × 10 M , an age of greater than 500 Myr, and little ongoing star formation. C iv emission is detected, which suggests that a concealed active galactic nucleus is present. The Lyα emission is redshifted by ∼490 km s with respect to the C IV emission, probably because of absorption. Extended Hα emission is also detected; the ratio of Lyα flux to Hα is abnormally low (∼0.7), probable evidence for extended dust. This galaxy is surrounded by a number of very red (B -K > 5 mag) objects, some of which have colors that suggest that they too are at z = 2.38. We hypothesize that this galaxy, its neighbors, and a surrounding lumpy gas cloud may be a giant elliptical galaxy in the process of bottom-up formation. © 1996. The American Astronomical Society. All rights reserved.