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Register a new userOptical identification of ISO far-infrared sources in the Lockman Hole using a deep VLA 1.4 GHz continuum survey
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By exploiting the far-infrared(FIR) and radio correlation, we have performed the Likelihood-Ratio analysis to identify optical counterparts to the far-infrared sources in the Lockman Hole. Using the likelihood ratio analysis and the associated reliability, 44 FIR sources have been identified with radio sources. Redshifts have been obtained for 29 out of 44 identified sources. One hyper-luminous infrared galaxy (HyLIRG) with and four ultraluminous infrared galaxies (ULIRGs) are identified in our sample. The space density of the FIR sources at z = 0.3-0.6 is 4.6times 10^{-5}Mpc^{-3}, implying a rapid evolution of the ULIRG population. Most of ISO FIR sources have their FIR-radio ratios similar to star-forming galaxies ARP 220 and M82. At least seven of our FIR sources show evidence for the presence of an active galactic nucleus (AGN) in optical emission lines, radio continuum excess, or X-ray activity. Three out of five (60%) of the ULIRG/HyLIRGs are AGN galaxies. Five of the seven AGN galaxies are within the ROSAT X-ray survey field, and two are within the XMM-Newton survey fields. X-ray emission has been detected in only one source, 1EX030, which is optically classified as a quasar. The non-detection in the XMM-Newton 2-10 keV band suggests a very thick absorption obscuring the central source of the two AGN galaxies. Several sources have an extreme FIR luminosity relative to the optical R-band, L(90mumathrm{m})/L(R) > 500, which is rare even among the local ULIRG population. While source confusion or blending might offer an explanation in some cases, they may represent a new population of galaxies with an extreme activity of star formation in an undeveloped stellar system -- i.e., formation of bulges or young ellipticals.
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We aim to study the nature of the faint, polarised radio source population whose source composition and redshift dependence contain information about the strength, morphology, and evolution of magnetic fields over cosmic timescales. We use a 15 pointing radio continuum L-band mosaic of the Lockman Hole, observed in full polarisation, generated from archival data of the WSRT. The data were analysed using the RM-Synthesis technique. We achieved a noise of 7 {mu}Jy/beam in polarised intensity, with a resolution of 15. Using infrared and optical images and source catalogues, we were able to cross-identify and determine redshifts for one third of our detected polarised sources. We detected 150 polarised sources, most of which are weakly polarised with a mean fractional polarisation of 5.4 %. With a total area of 6.5 deg^2 and a detection threshold of 6.25 {sigma} we find 23 polarised sources per deg^2. Based on our multi wavelength analysis, we find that our sample consists of AGN only. We find a discrepancy between archival number counts and those present in our data, which we attribute to sample variance. Considering the absolute radio luminosty, to distinguish weak and strong sources, we find a general trend of increased probability to detect weak sources at low redshift and strong sources at high redshift. Further, we find an anti-correlation between fractional polarisation and redshift for our strong sources sample at z{geq}0.6. A decrease in the fractional polarisation of strong sources with increasing redshift cannot be explained by a constant magnetic field and electron density over cosmic scales, however the changing properties of cluster environments over the cosmic timemay play an important role. Disentangling these two effects requires deeper and wider polarisation observations, and better models of the morphology and strength of cosmic magnetic fields.
We investigate the characteristics of FIR brightness fluctuations at 90 micron and 170 micron in the Lockman Hole, which were surveyed with ISOPHOT aboard the Infrared Space Observatory(ISO). We first calculated the angular correlation function of each field and then its Fourier transform(the angular Power Spectral Density: PSD) over the spatial frequency range of f=0.05 - 1 arcmin^-1. The PSDs are found to be rather flat at low spatial frequencies (f <= 0.1 arcmin^-1), slowly decreasing toward higher frequencies. These spectra are unlike the power-law ones seen in the IR cirrus fluctuations, and are well explained by randomly distributed point sources. Furthermore, point-to-point comparison between 90 micron and 170 micron brightness shows a linear correlation between them, and the slope of the linear fit is much shallower than that expected from the IR cirrus color, and is consistent with the color of galaxies at low or moderate redshift(z < 1). We conclude that the brightness fluctuations in the Lockman Hole are not caused by the IR cirrus, but are most likely due to faint star-forming galaxies. We also give the constraints on the galaxy number counts down to 35 mJy at 90 micron and 60 mJy at 170 micron, which indicate the existence of a strong evolution down to these fluxes in the counts. The galaxies responsible for the fluctuations also significantly contribute to the cosmic infrared background radiation.
We have obtained a deep radio image with the Very Large Array at 6 cm in the Lockman Hole. The noise level in the central part of the field is about 11 microJy. From these data we have extracted a catalogue of 63 radio sources. The analysis of the radio spectral index suggests a flattening of the average radio spectra and an increase of the population of flat spectrum radio sources in the faintest flux bin. Cross correlation with the ROSAT/XMM X-ray sources list yields 13 reliable radio/X-ray associations, corresponding to about 21 per cent of the radio sample. Most of these associations (8 out of 13) are classified as Type II AGN. Using optical CCD (V and I) and K band data we found an optical identification for 58 of the 63 radio sources. This corresponds to an identification rate of about 92 per cent, one of the highest percentages so far available. From the analysis of the colour-colour diagram and of the radio flux - optical magnitude diagram we have been able to select a subsample of radio sources whose optical counterparts are likely to be high redshift (z>0.5) early-type galaxies, hosting an Active Galactic Nucleus responsible of the radio activity. We also find evidence that at these faint radio limits a large fraction (about 60 per cent) of the faintest optical counterparts (i.e. sources in the magnitude range 22.5<I<24.5 mag) of the radio sources are Extremely Red Objects (EROs) with I-K>4.
The ROSAT Deep Survey in the Lockman Hole contains a complete sample of 50 X-ray sources with fluxes in the 0.5-2.0 keV band larger than 5.5E-15 erg/cm2/s. Previous work has provided optical identification of 46 of the 50 X-ray sources; over 75% of the sources are AGNs (Schmidt et al. 1998). We present now the atlas of optical finding charts and the full description of the spectra, including emission line properties of the optical counterparts, which are important for the object classification. New optical/infrared observations of three of the four unidentified sources show that one source is an AGN and two sources with an unusually large ratio of X-ray to optical flux have counterparts in the K-band suggesting that they are obscured AGNs. Furthermore, we found evidence from radio emission that the remaining unidentified source is a powerful radio galaxy (AGN). We thus obtain a 100% completeness. During the course of our optical identification work, we obtained optical spectr a of 83 field galaxies, of which 67 were narrow-emission line galaxies (NELG). We demonstrate that it is highly unlikely that a significant number of NELG are physically associated with X-ray sources.
We present the optical identification of mid-IR and radio sources detected in the European Large Area ISO Survey (ELAIS) areas N1 and N2. Using the r band optical data from the Wide Field Survey we apply a likelihood ratio method to search for the counterparts of the 1056 and 691 sources detected at 15 micron and 1.4 GHz respectively, down to flux limits of S_{15}=0.5 mJy and S_{1.4 GHz}=0.135 mJy. We find that ~92% of the 15 micron ELAIS sources have an optical counterpart down to the magnitude limit of the optical data, r=24. All mid-IR sources with fluxes S_{15} >= 3 mJy have an optical counterpart. The magnitude distribution of the sources shows a well defined peak at relatively bright magnitudes r~18. About 20% of the identified sources show a point-like morphology; its magnitude distribution has a peak at fainter magnitudes than those of galaxies. The mid-IR-to-optical and radio-to-optical flux diagrams are presented and discussed in terms of actual galaxy models. Objects with mid-IR-to-optical fluxes larger than 1000 are found that can only be explained as highly obscured star forming galaxies or AGNs. Blank fields being 8% of the 15 micron sample have even larger ratios suggesting that they may be associated with higher redshift and higher obscured objects.
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