On the nature of the ISO-selected sources in the ELAIS S2 region

Abstract

We have studied the optical, near-IR and radio properties of a complete sample of 43 sources detected at 15-micron in one of the deeper ELAIS repeatedly observed region. The extragalactic objects in this sample have 15-micron flux densities in the range 0.4-10 mJy, where the source counts start diverging from no evolution models. About 90% of the sources (39 out of 43) have optical counterparts brighter than I=21 mag. Eight of these 39 sources have been identified with stars on the basis of imaging data, while for another 22 sources we have obtained optical spectroscopy, reaching a high identification percentage (30/43, ~70%). All but one of the 28 sources with flux density > 0.7 mJy are identified. Most of the extragalactic objects are normal spiral or starburst galaxies at moderate redshift (z_med~0.2); four objects are Active Galactic Nuclei. We have used the 15-micron, H_alpha and 1.4-GHz luminosities as indicators of star-formation rate and we have compared the results obtained in these three bands. While 1.4-GHz and 15-micron estimates are in good agreement, showing that our galaxies are forming stars at a median rate of ~40 Mo/yr, the raw H_alpha-based estimates are a factor ~5-10 lower and need a mean correction of ~2 mag to be brought on the same scale as the other two indicators. A correction of ~2 mag is consistent with what suggested by the Balmer decrements H_alpha/H_beta and by the optical colours. Moreover, it is intermediate between the correction found locally for normal spirals and the correction needed for high-luminosity 15-micron objects, suggesting that the average extinction suffered by galaxies increases with infrared luminosity.

arXiv comment19 pages, 11 figures (3 in JPEG format), MNRAS, accepted

References in corpus (29)

• Star Formation in Galaxies Along the Hubble Sequence
• The Dust Content and Opacity of Actively Star-Forming Galaxies
• Radio Properties of Infrared Selected Galaxies in the IRAS 2 Jy Sample
• The Star Formation Histories of Galaxies in Distant Clusters
• The Bulk of the Cosmic Infrared Background Resolved by ISOCAM
• Towards a resolution of the discrepancy between different estimators of star formation rate
• The European Large Area ISO Survey I: Goals, Definition and Observation
• A Long-Wavelength View on Galaxy Evolution from Deep Surveys by the Infrared Space Observatory
• The H-alpha and Infrared Star Formation Rates for the Nearby Field Galaxy Survey
• Optical Spectral Signatures of Dusty Starburst Galaxies
• The VLA-VIRMOS Deep Field I. Radio observations probing the microJy source population
• The star-formation history of the universe - an infrared perspective
• Star Formation Rates in Faint Radio Galaxies
• The FIR/Radio correlation of high redshift galaxies in the HDF-N
• A deep VLA survey at 6 cm in the Lockman Hole
• The AGN contribution to mid-infrared surveys
• [O II] as a tracer of current star formation
• Pure Luminosity Evolution models for faint field galaxy samples
• New Method for ISOCAM Data Reduction - II. Mid-Infrared Extragalactic Source Counts in the Southern ELAIS Field
• Star Formation, Metallicity and Dust Properties Derived from the SAPM Galaxy Survey Spectra
• The Chandra Deep Field North Survey. IX. Extended X-ray Sources
• The Chandra Deep Field North Survey. XI. X-ray emission from luminous infrared starburst galaxies
• Local Luminosity Function at 15$\mu m$ and Galaxy Evolution Seen by ISOCAM 15$\mu m$ Surveys
• The Radio/Mid-Infrared Correlation and the Contribution of 15-micron Galaxies to the 1.4-GHz Source Counts
• Observations of the Hubble Deep Field South with the Infrared Space Observatory - I. Observations, data reduction and mid-infrared source counts
• A New Method for ISOCAM Data Reduction - I. Application to the European Large Area ISO Survey Southern Field: Method and Results
• VLT-ISAAC near-IR Spectroscopy of ISO selected Hubble Deep Field South Galaxies
• Observations of the Hubble Deep Field South with the Infrared Space Observatory - II. Associations and star formation rates
• The Evolution of type 1 AGN in the IR (15mu). The view from ELAIS-S1

Cited by in corpus (15)

• Infrared luminosity functions from the Chandra Deep Field South : the Spitzer view on the history of dusty star formation at 0<z<1
• The European Large Area ISO Survey (ELAIS): The Final Band-merged Catalogue
• The Mid-IR luminosity function of galaxies in the ELAIS Southern fields
• Active Galactic Nuclei in the mid-IR. Evolution and Contribution to the Cosmic Infrared Background
• The nature of the mid-infrared population from optical identifications of the ELAIS-S1 sample
• Spitzer and ISO Galaxy Counts in the Mid-Infrared
• The European Large Area ISO Survey IX: the 90 micron luminosity function from the Final Analysis sample
• Source Counts at 15 microns from the AKARI NEP Survey
• ISOCAM observations in the Lockman Hole - II The 14.3 micron deep survey: data reduction, catalogue and source counts
• Optical and Infrared Diagnostics of SDSS galaxies in the SWIRE Survey
• Final Analysis of ELAIS 15 $\mu$m Observations : Method, Reduction and Catalogue
• The 12um ISO-ESO-Sculptor and 24um Spitzer faint counts reveal a population of ULIRG/AGN/dusty massive ellipticals Evolution by types and cosmic star formation
• The European Large Area ISO Survey (ELAIS): Optical Identifications of 15 micron and 1.4 GHz sources in N1 and N2
• A population of extreme mid-to-near-infrared sources: obscured AGN and dusty starbursts
• A study of the 15 micron quasars in the ELAIS N1 and N2 fields