Prof. Dimitra Rigopoulou

Abstract:

We investigate extremely red objects (EROs) using near- and mid-infrared observations in five passbands (3.6 to 24 μm) obtained from the Spitzer Space Telescope, and deep ground-based R and K imaging. The great sensitivity of the Infrared Array Camera (IRAC) camera allows us to detect 64 EROs (a surface density of 2.90 ± 0.36 arcmin-2; [3.6]AB < 23.7) 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 μm to be EROs. These percentages rise to about 40% at 5.8 μm, and about 60% at 8.0 μm. We utilize the spectral bump at 1.6 μm to divide the EROs into broad redshift slices using only near-infrared colors (2.2/3.6/4.5 μm). We conclude that two-thirds of all EROs lie at redshift z > 1.3. Detections at 24 μm 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.

Abstract:

IRAC imaging of a 4′.7 x 4′.7 area in the Lockman Hole detected over 400 galaxies in the IRAC 3.6 and 4.5 μm bands, 120 in the 5.8 μm band, and 80 in the 8.0 μm band in 30 minutes of observing time. Color-color diagrams suggest that about half of these galaxies are at redshifts 0.6 < z < 1.3 with about a quarter at higher redshifts (z > 1.3). We also detect IRAC counterparts for six of the seven SCUBA sources and all nine XMM-Newton sources in this area. The detection of the counterparts of the SCUBA sources and galaxies at z > 1.3 demonstrates the ability of IRAC to probe the universe at very high redshifts.

Abstract:

We investigate the luminous X-ray sources in the Lockman Hole (LH) and the extended Groth strip (EGS) detected at 24 μm using the Multiband Imaging Photometer (MIPS) and also with the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope. We assemble optical/infrared spectral energy distributions (SEDs) for 45 X-ray/24 μm sources in the EGS and LH. Only about one-quarter of the hard X-ray/24 μm sources show pure type 1 active galactic nucleus (AGN) SEDs. More than half of the X-ray/24 μm sources have stellar emission-dominated or obscured SEDs, similar to those of local type 2 AGN and spiral/starburst galaxies. One-third of the sources detected in hard X-rays do not have a 24 μm counterpart. Two such sources in the LH have SEDs resembling those of S0/elliptical galaxies. The broad variety of SEDs in the optical-to-Spitzer bands of X-ray-selected AGNs means that AGNs selected according to the behavior in the optical/infrared will have to be supplemented by other kinds of data (e.g., X-ray) to produce unbiased samples of AGNs.

Abstract:

We present the final band-merged European Large-Area ISO Survey (ELAIS) Catalogue at 6.7, 15, 90 and 175 μm, and the associated data at U, g′, r′, i′, Z, J, H, K and 20 cm. The origin of the survey, infrared and radio observations, data-reduction and optical identifications are briefly reviewed, and a summary of the area covered and the completeness limit for each infrared band is given. A detailed discussion of the band-merging and optical association strategy is given. The total Catalogue consists of 3762 sources. 23 per cent of the 15-μm sources and 75 per cent of the 6.7-μm sources are stars. For extragalactic sources observed in three or more infrared bands, colour-colour diagrams are presented and discussed in terms of the contributing infrared populations. Spectral energy distributions (SEDs) are shown for selected sources and compared with cirrus, M82 and Arp220 starburst, and active galactic nuclei (AGN) dust torus models. Spectroscopic redshifts are tabulated, where available. For the N1 and N2 areas, the Isaac Newton Telescope ugriz Wide Field Survey permits photometric redshifts to be estimated for galaxies and quasars. These agree well with the spectroscopic redshifts, within the uncertainty of the photometric method [∼ 10 per cent in (1 + z) for galaxies]. The redshift distribution is given for selected ELAIS bands and colour-redshift diagrams are discussed. There is a high proportion of ultraluminous infrared galaxies (log10 of 1-1000 μm luminosity Lir > 12.22) in the ELAIS Catalogue (14 per cent of 15-μm galaxies with known z), many with Arp220-like SEDs. 10 per cent of the 15-μm sources are genuine optically blank fields to r′ = 24: these must have very high infrared-to-optical ratios and probably have z > 0.6, so are high-luminosity dusty starbursts or Type 2 AGN. Nine hyperluminous infrared galaxies (Lir > 13.22) and nine extremely red objects (EROs) (r - K > 6) are found in the survey. The latter are interpreted as ultraluminous dusty infrared galaxies at z ∼ 1. The large numbers of ultraluminous galaxies imply very strong evolution in the star formation rate between z = 0 and 1. There is also a surprisingly large population of luminous (Lir > 11.5), cool (cirrus-type SEDs) galaxies, with Lir - L opt > 0, implying Av > 1.