ISOCAM Sources Research Articles

A multi-wavelength study of the IRAS Deep Survey galaxy sample

Context. The IRAS Deep Survey (IDS) of the North Ecliptic Pole region remains one of the deepest complete far-infrared-selected galaxy samples available. ISOCAM observations in the LW3 filter (12-18 μm) of 3.'2 x 3.'2 fields centered on the nominal positions of 94 out of the 98 IDS sources yielding 116 ISOCAM detections. The resulting sample has allowed us to assess the source reliability, identify confusion effects, and pinpoint optical counterparts. Aims. To fully exploit the potential of this sample for investigating galaxy evolution, we carried out optical observations of candidate identifications to 100 out of the 116 ISOCAM sources. Methods. Optical imaging was acquired for 88 galaxies in 54 IDS fields: 44 galaxies were imaged in both the B- and the R-band, 33 only in the R-band, and 11 only in the B-band. These observations aimed at extending the MIR/FIR spectral energy distribution to the optical region and achieving morphological information. In addition, spectra were obtained for 65 IDS fields, 20 of which had not been observed before. Redshifts were measured for the 85 galaxies found within the positional error circles of 77 ISOCAM sources detected in those IDS fields. Results. These observations allowed us to acquire spectroscopic redshifts of all counterparts that until now lacked them, so that, in combination with the results by Ashby et al. (1996), we now have redshifts for all IDS/ISOCAM galaxies in the field and, in particular, for the complete sub-sample of IDS galaxies defined by Mazzei et al. (2001). Conclusions. In several cases we find evidence of multiple galaxies with very close redshifts within the error circles around ISOCAM positions, hinting at an important role for interactions in triggering the star formation activity. Although the z distribution of the complete sub-sample has a tail extending up to z ≃ 0.37, our results are inconsistent with the extreme cosmological evolution advocated by some models.

The very low-mass population of the Corona Australis and Chamaeleon II star forming regions

We present the results of a deep optical survey in the Corona Australis and Chamaeleon II star forming regions. Our optical photometry is combined with available near- and mid-infrared photometry to identify very low-mass candidate members in these dark clouds. In our Chamaeleon II field, only one object exhibits clear Hα emission, but the discrepancy between its optical and near-infrared colours suggests that it might be a foreground star. We also identify two objects without Hα emission that could be planetary mass members of Chamaeleon II. In Corona Australis, we find ten stars and three brown dwarf candidates in the Coronet cluster. Five of our new members are identified with ISOCAM sources. Only two of them have a mid-infrared excess, indicating the presence of an accretion disk. On the other hand, one brown dwarf candidate has a faint close companion, seen only in our deepest I-band image. For many of the candidates in both clouds, membership could not be inferred from their Hα emission or near-infrared colours; these objects need spectroscopic confirmation of their status.

An ISOCAM survey through gravitationally lensing galaxy clusters

Observations of the core of the massive cluster Cl 0024+1654, at a redshift z=0.39, were obtained with the Infrared Space Observatory using ISOCAM at 6.7 mum and 14.3 mum (hereafter 15 mum). Thirty five sources were detected at 15 mum and thirteen of them are spectroscopically identified with cluster galaxies. The remaining sources consist of four stars, one quasar, one foreground galaxy, three background galaxies and thirteen sources with unknown redshift. The ISOCAM sources have best-fit SEDs typical of spiral or starburst models observed 1 Gyr after the main starburst event. The median infrared luminosity of the twelve cluster galaxies is 1.0x10^11 Lsun, with 10 having infrared luminosity above 9x10^10 Lsun, and so lying near or above the 1x10^11 Lsun threshold for identification as a luminous infrared galaxy (LIRG). The [OII] star formation rates obtained for 3 cluster galaxies are one to two orders of magnitude lower than the infrared values, implying that most of the star formation is missed in the optical because it is enshrouded by dust in the starburst galaxy. The counterparts of about half of the 15 mum cluster sources are blue, luminous, star-forming systems and the type of galaxy that is usually associated with the Butcher-Oemler effect. Dust obscuration may be a major cause of the 15 mum sources appearing on the cluster main sequence. The majority of the ISOCAM sources in the Butcher-Oemler region of the colour-magnitude diagram are best fit by spiral-type SEDs whereas post-starburst models are preferred on the main sequence, with the starburst event probably triggered by interaction with one or more galaxies. Finally, the mid-infrared results on Cl 0024+1654 are compared with four other clusters observed with ISOCAM.

Increasing the reliability of ISOCAM cross-identifications by use of a probability pattern

The built-in play at the wheels of the ISOCAM instrument affects the accuracy of the astrometry of sources from the ISOCAM Parallel Catalogue. We present the results of different methods for cross-identifying ISOCAM sources with reference catalogues in the optical (USNO-A2.0) and in the near infrared (2MASS). For a sample of 7197 ISOCAM sources, the fraction having a real counterpart in the USNO and 2MASS catalogues was estimated to be 95% and 91% respectively. Using a nearest neighbor method for cross-identification, only 74% of the possible associations with USNO (79% with 2MASS) are retrieved. With the use of a so-called probability pattern to perform the associations, these values increase to 77% for USNO and 83% for 2MASS. In addition, the overall reliability of associations increases from 77 to 87% with USNO, and from 81 to 90% with 2MASS.

Near-infrared spectra of ISO selected Chamaeleon I young stellar objects

We present 0.95{2.5 mm oderate (R 500) resolution spectra of 19 ISOCAM detected sources in the Chamaeleon I dark cloud. Thirteen of these stars are candidate very low mass members of the cloud proposed by Persi et al. (2000) on basis of the mid-IR color excess. The sample also includes a bona-de young brown dwarf (Cha H 1), a transition { stellar/sub-stellar { object (Cha H 2), one previously known T Tauri star (Sz 33) and three ISOCAM sources with no mid-IR excess. The spectra of the mid-IR color excess sources are relatively flat and featureless in this wavelength range. Both atomic and molecular lines (when in absorption) are partially veiled suggesting the presence of continuum emission from circumstellar dust. In addition some of the sources show Paschen and Brackett lines in emission. We apply the 2 m water vapor index dened by Wilking et al. (1999) to estimate spectral types. These stars have spectral types M0{8. We use Persi et al.'s stellar luminosity determinations, in combination with D'Antona & Mazzitelli latest pre-main sequence evolutionary tracks, to estimate masses and ages. The ISOCAM detected mid-IR excess sources have sub-solar masses down to the H-burning limit and a median age of few10 6 yr, in good agreement with the higher mass members of this cloud.

Sub-arcsec resolution near-infrared images of the Cederblad 110 region

We present the results of deep () sub-arcsec resolution (0.3´´) imaging observations of the Cederblad 110 region in the Chamaeleon I dark cloud. This region (roughly ) is characterized by the presence of six ISOCAM-detected young stellar objects (YSOs). Our images have recovered all these sources at near-infrared (near-IR) wavelengths. Ced 110 IRS4, the brightest object in the region, is associated with a remarkable near-IR bipolar nebulosity. Ced 110 IRS6 is resolved in a double system IRS6a and IRS6b with a separation of AU at the distance of the Cha I cloud). We have combined 1.3 mm, far and mid-IR fluxes from the literature with our JHK data and obtained the spectral energy distributions (SEDs) for three (IRS4, IRS6a and ISO-ChaI86) of the six ISOCAM sources in the region. We modelled the SEDs of IRS6a and ISO-ChaI86 with a spherically symmetric dusty envelope, using the DUSTY code. These objects are clear Class I sources of the cloud. Finally, we report the detection of three new objects in this region (NIR 72, 84 and 89) with significant near-IR excess. If these sources are associated with the cloud, the derived luminosities and masses suggest that they are candidate young brown dwarfs of Chamaeleon I.

The European Large‐AreaInfrared Space ObservatorySurvey V: ABeppoSAXHard X‐Ray Survey of the S1 Region

We present BeppoSAX observations of the southern S1 region in the European Large-Area Infrared Space Observatory (ISO) Survey (ELAIS). These observations cover an area of ~1.7 deg2 and reach an on-axis (~0.7 deg2) 2-10 keV (hard X-ray, or HX) sensitivity of ~10-13 ergs s-1 cm-2. This is the first HX analysis of an ISOCAM survey. We detect nine sources with a signal-to-noise ratio SNRHX > 3, four additional sources with a 1.3-10 keV (total X-ray, or T) SNRT > 3, and two additional sources that seem to be associated with QSOs having SNRT > 2.9. The number densities of the SNRHX > 3 sources are consistent with the ASCA and BeppoSAX log N-log S functions. Six BeppoSAX sources have reliable ISOCAM 15 μm counterparts within ~60''. All these ISOCAM sources have optical counterparts of R < 20 mag. Five of these sources have been previously optically classified, four as QSOs and one as a broad absorption line (BAL) QSO at z = 2.2. The remaining unclassified source has X-ray and photometric properties consistent with those of a nearby Seyfert galaxy. One further HX source has a 2.6 σ ISOCAM counterpart associated with a galaxy at z = 0.325. If this ISOCAM source is real, the HX/MIR properties suggest either an unusual QSO or a cD cluster galaxy. We have constructed MIR and HX spectral energy distributions to compute the expected HX/MIR ratios for these classes of objects up to z = 3.2 and assess the HX/MIR survey depth. The BAL QSO has an observed X-ray softness ratio and HX/MIR flux ratio similar to those of QSOs but different from those found for low-redshift BAL QSOs. This difference can be explained in terms of absorption, and it suggests that high-redshift BAL QSOs should be comparatively easy to detect in the HX band, allowing their true fraction in the high-redshift QSO population to be determined. The QSOs cover a wide redshift range (0.4 < z < 2.6) and have HX/MIR flux ratios consistent with those found for nearby IRAS and optically selected Palomar-Green QSOs. This suggests that MIR-selected QSOs of R < 20 mag come from the same population as optically selected QSOs. We confirm this with a comparison of the B/MIR flux ratios of MIR and blue-band-selected QSOs.

A Population of Distant Luminous Infrared Galaxies Revealed by 15 μm ISOCAM Deep Surveys

The counts derived from the five mid-IR 15 μm (12–18μm LW3 band) ISOCAM Guaranteed Time Extragalactic Surveys performed in the regions of the Lockman Hole and Marano Field, the HDF-North and South (plus flanking fields), together with those of the lensing cluster A2390 at low fluxes and those of IRAS at high fluxes, cover four decades in flux from 50 μJy to ~0.3 Jy. The roughly 1000 sources detected with ISOCAM, 600 of which have a flux above the 80 % completeness limit, guarantee a very high statistical significance for the integral and differential source counts from 0.1 mJy up to ~5 mJy. The slope of the differential counts is very steep (α = −3.0) in the flux range 0.4-4 mJy, hence much above the Euclidean expectation of α = −2.5. When compared with no-evolution models based on IRAS, our counts show a factor ~ 10 excess at 400 μJy, and a fast convergence, with α = −1.6 at lower fluxes.Multiwavelength studies of a subsample of the ISOCAM sources in HDF and CFRS fields indicate that they are intrinsically bright galaxies (Luminous Infrared Galaxies, LIRGs), with median redshift 0.7. These galaxies, despite their low surface density, are responsible for a large part of star formation at z &lt; 1 and contribute substantially to the cosmic infrared background at 140 μm.