2 research outputs found
Extremely Red Objects in Two Quasar Fields at z ~ 1.5
We present an investigation of the properties and environments of bright
extremely red objects (EROs) found in the fields of the quasars TXS 0145+386
and 4C 15.55, both at z ~ 1.4. There is marginal evidence from Chandra ACIS
imaging for hot cluster gas with a luminosity of a few 10^44 ergs/s in the
field of 4C 15.55. The TXS 0145+386 field has an upper limit at a similar
value, but it also clearly shows an overdensity of faint galaxies. None of the
EROs are detected as X-ray sources. For two of the EROs that have
spectral-energy distributions and rest-frame near-UV spectra that show that
they are strongly dominated by old stellar populations, we determine
radial-surface-brightness profiles from adaptive-optics images. Both of these
galaxies are best fit by profiles close to exponentials, plus a compact nucleus
comprising ~30% of the total light in one case and 8% in the other. Neither is
well fit by an r^1/4-law profile. This apparent evidence for the formation of
massive ~2 X 10^11 disks of old stars in the early universe indicates that at
least some galaxies formed essentially monolithically, with high star-formation
rates sustained over a few 10^8 years, and without the aid of major mergers.Comment: 25 pages, 13 figures, accepted to Ap
A Disk Galaxy of Old Stars at z ~ 2.5
We describe observations of a galaxy in the field of the radio
galaxy 4C 23.56, photometrically selected to have a spectral-energy
distribution consistent with an old stellar population at the redshift of the
radio galaxy. Exploration of redshift--stellar-population-reddening constraints
from the photometry indicates that the galaxy is indeed at a redshift close to
that of 4C23.56, that the age of the most recent significant star formation is
roughly >~2 Gyr, and that reddening is fairly modest, with more reddening
required for the younger end of stellar age range. From analysis of a deep
adaptive-optics image of the galaxy, we find that an r^1/4-law profile, common
for local spheroidal galaxies, can be excluded quite strongly. On the other
hand, a pure exponential profile fits remarkably well, while the best fit is
given by a Sersic profile with index n=1.49. Reconstruction of the
two-dimensional form of the galaxy from the best-fit model is consistent with a
disk galaxy with neither a significant bulge component nor gross azimuthal
structure. The assembly of roughly 2L* of old stars into such a configuration
this early in the history of the universe is not easily explainable by any of
the currently popular scenarios for galaxy formation. A galaxy with these
properties would seem to require smooth but rapid infall of the large mass of
gas involved, followed by a burst of extremely vigorous and efficient star
formation in the resulting disk.Comment: 8 pages, 6 figures, emulateapj.sty, accepted for publication in The
Astrophysical Journa