[Paper Review] The ATESP 5 GHz radio survey. II. Physical properties of the faint radio population

A. Mignano, I. Prandoni+6 more|ArXiv.org|Oct 30, 2007

Galaxies: Formation, Evolution, Phenomena49 references16 citations

TL;DR

This study analyzes the physical properties of 131 sub-mJy radio sources from the ATESP 5 GHz survey using multi-wavelength data, combining 1.4 and 5 GHz radio data with deep optical and near-infrared imaging to determine redshifts, morphologies, and spectral indices. It finds that 78% of the sources are powered by active galactic nuclei—either quasars or low-luminosity AGNs in early-type galaxies—while flat-spectrum sources with high radio-to-optical ratios are predominantly core-dominated AGNs.

ABSTRACT

One of the most debated issues about sub-mJy radio sources, which are responsible for the steepening of the 1.4 GHz source counts, is the origin of their radio emission. Particularly interesting is the possibility of combining radio spectral index information with other observational properties to assess whether the sources are triggered by star formation or nuclear activity. The aim of this work is to study the optical and near infrared properties of a complete sample of 131 radio sources with S>0.4 mJy, observed at both 1.4 and 5 GHz as part of the ATESP radio survey. We use deep multi-colour (UBVRIJK) images, mostly taken in the framework of the ESO Deep Public Survey, to optically identify and derive photometric redshifts for the ATESP radio sources. Deep optical coverage and extensive colour information are available for 3/4 of the region covered by the radio sample. Typical depths of the images are U~25, B~26, V~25.4, R~25.5, I~24.3, 19.51000), typical of classical powerful radio galaxies and quasars. Flat-spectrum sources with low R values are preferentially identified with early type galaxies, where the radio emission is most probably triggered by low-luminosity active galactic nuclei. Considering both early type galaxies and quasars as sources with an active nucleus, such sources largely dominate our sample (78%). Flat-spectrum sources associated with early type galaxies are quite compact (d<10-30 kpc), suggesting core-dominated radio emission.

Motivation & Objective

• To determine the physical origins of radio emission in sub-mJy radio sources, particularly distinguishing between star formation and active galactic nuclei (AGN) activity.
• To assess the role of radio spectral index, optical morphology, and photometric redshifts in classifying faint radio sources.
• To investigate the dominance of AGN versus star-forming galaxies in the faint radio population using a complete, flux-limited sample.
• To examine the structural and spectral properties of flat-spectrum sources to infer the nature of their radio emission mechanisms.

Proposed method

• Utilized a complete sample of 131 radio sources with S > 0.4 mJy from the ATESP 5 GHz survey, observed at both 1.4 GHz and 5 GHz to derive radio spectral indices.
• Combined deep multi-color optical (UBVRI) and near-infrared (J, Ks) imaging from the ESO Deep Public Survey and other archival data to identify optical and near-IR counterparts.
• Applied photometric redshift techniques to estimate redshifts for 56 sources with reliable optical and near-IR data.
• Classified sources based on morphology (e.g., elliptical, spiral, burst-like), radio spectral index, and radio-to-optical flux ratios (R parameter).
• Used radio spectral index analysis to distinguish between steep-spectrum sources (likely star-forming galaxies) and flat-spectrum sources (likely AGNs).
• Correlated radio structure and compactness (d < 10–30 kpc) with core-dominated emission in flat-spectrum sources to infer AGN dominance.

Experimental results

Research questions

• RQ1 What is the dominant physical mechanism powering the radio emission in sub-mJy radio sources?
• RQ2 How do radio spectral indices and radio-to-optical flux ratios help distinguish between AGN and star-forming galaxy contributions?
• RQ3 To what extent do early-type galaxies and quasars dominate the faint radio source population?
• RQ4 What is the structural and morphological signature of core-dominated radio emission in flat-spectrum sources?
• RQ5 How do photometric redshifts and multi-wavelength data improve the classification of faint radio sources?

Key findings

• Optical or near-infrared counterparts were identified for 78% (66 out of 85) of the radio sources in the deep imaging region.
• Fifty-six sources had reliable photometric redshifts and morphological classifications derived from multi-wavelength data.
• Flat-spectrum sources with high radio-to-optical ratios (R > 1000) are predominantly classical quasars or powerful radio galaxies.
• Flat-spectrum sources with low R values are preferentially associated with early-type galaxies, indicating low-luminosity AGN activity.
• 78% of the sample are classified as AGN-dominated, including both quasars and low-luminosity AGNs in early-type galaxies.
• Flat-spectrum sources associated with early-type galaxies are compact (d < 10–30 kpc), consistent with core-dominated radio emission.