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Register a new userExtremely Red Objects in Two Quasar Fields at z ~ 1.5
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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.
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We report on a study of the surface density of Extremely Red Objects (EROs) in the fields of 13 radio-loud quasars at 1.8 < z < 3.0 covering a total area of 61.7 sqr arcmin. There is a large variation in the ERO surface density from field to field, and as many as 30--40 % of the fields have roughly 4--5 times more EROs than what is expected from a random distribution. The average surface density exceeds the value found in large random-field surveys by a factor of 2--3, a result which is significant at the >3 sigma level. Hence, it appears that the quasar lines of sight are biassed towards regions of high ERO density. This might be caused by clusters or groups of galaxies physically associated with the quasars. However, an equally likely possibility is that the observed ERO excess is part of overdensities in the ERO population along the line of sight to the quasars. In this case, the non-randomness of quasar fields with respect to EROs may be explained in terms of gravitational lensing.
We have discovered a concentration of extremely red objects (EROs; R-K>6) in the field of the z=2.69 quasar QSO 1213-0017 (UM 485), which is significantly overabundant compared to the field ERO surface density. The optical/near-IR colors of the EROs and numerous other red galaxies in this field are consistent with elliptical galaxies at z=1-2. HST optical images for a subset of galaxies show regular morphologies, most of them being disky or diffuse and without any obvious evidence for interactions. Ground-based IR images show similar morphologies, indicating any dust reddening in these objects is spatially uniform. Optical spectroscopy with the W. M. Keck Telescope has found that four of the red galaxies lie at <z>=1.31, and a fifth lies in the foreground at z=1.20. Of the <z>=1.31 galaxies, one is a reddened AGN while the remaining three have rest-frame UV absorption-line spectra characteristic of old (few Gyr) stellar populations, similar to the old red galaxy LBDS 53W091 at z=1.55. Including the MgII absorber seen in the QSO spectrum, we find five galaxies at <z>=1.31 spread over 1.5 h_50^{-1} Mpc on the sky. These results suggest we have discovered a coherent structure of old galaxies at high-redshift, possibly associated with a massive galaxy cluster.
We have discovered an excess of extremely red objects (EROs) surrounding the z=2.69 quasar QSO 1213-0017 (UM 485). Optical/IR colors for these galaxies are consistent with z=1-2 ellipticals, and there are at least 5 galaxies with spectroscopic redshifts at <z>=1.31. Keck optical spectra for 3 of the red galaxies show rest-frame UV breaks resembling local elliptical galaxies. Our initial results suggest a coherent structure in redshift, possibly arising from a massive galaxy cluster.
We investigate Extremely Red Objects (EROs) using near- and mid-infrared observations in five passbands (3.6 to 24 micron) obtained from the Spitzer Space Telescope, and deep ground-based R and K imaging. The great sensitivity of the IRAC camera allows us to detect 64 EROs in only 12 minutes of IRAC exposure time, by means of an R-[3.6] color cut (analogous to the traditional red R-K cut). A pure infrared K-[3.6] red cut detects a somewhat different population and may be more effective at selecting z > 1.3 EROs. We find 17% of all galaxies detected by IRAC at 3.6 or 4.5 micron to be EROs. These percentages rise to about 40% at 5.8 micron, and about 60% at 8.0 micron. We utilize the spectral bump at 1.6 micron to divide the EROs into broad redshift slices using only near-infrared colors (2.2/3.6/4.5 micron). We conclude that two-thirds of all EROs lie at redshift z > 1.3. Detections at 24 micron imply that at least 11% of 0.6 < z < 1.3 EROs and at least 22% of z > 1.3 EROs are dusty star-forming galaxies.
We have examined the occurrence of Extremely Red Objects (EROs) in the fields of 13 luminous quasars (11 radio-loud and two radio-quiet) at 1.8 < z < 3.0. The average surface density of K_s<=19 mag EROs is two-three times higher than in large, random-field surveys, and the excess is significant at the $approx 3$ sigma level even after taking into account that the ERO distribution is highly inhomogeneous. This is the first systematic investigation of the surface density of EROs in the fields of radio-loud quasars above z=2, and shows that a large number of the fields contain clumps of EROs, similar to what is seen only in the densest areas in random-field surveys. The high surface densities and angular distribution of EROs suggest that the excess originates in high-z galaxy concentrations, possibly young clusters of galaxies. The fainter EROs at K_s>19 mag show some evidence of being more clustered in the immediate 20 arcsec region around the quasars, suggesting an association with the quasars.Comparing with predictions from spectral synthesis models, we find that if the $K_sapprox19$ mag ERO excess is associated with the quasars at $zapprox2$, their magnitudes are typical of >~ L* passively evolving galaxies formed at z~3.5 (Omega_m=0.3, Omega_l=0.7, and H0=70 km/s/Mpc). Another interpretation of our results is that the excess originates in concentrations of galaxies at $zapprox1$ lying along the line of sight to the quasars. If this is the case, the EROs may be tracing massive structures responsible for a magnification bias of the quasars.
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