A deep VLA survey at 6 cm in the Lockman Hole

P. Ciliegi, G. Zamorani, G. Hasinger, I. Lehmann, G. Szokoly, G. Wilson

Research output: Contribution to journalArticle

101 Citations (Scopus)

Abstract

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 ∼11/μJy. From these data we have extracted a catalogue of 63 radio sources with a maximum distance of 10 arcmin from the field center and with peak flux density greater than 4.5 times the local rms noise. The differential source counts are in good agreement with those obtained by other surveys. 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 ∼21 % 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 with approximate limits of V ∼ 25.5 mag, I ∼ 24.5 mag and K′ ∼ 20.2 mag, we found an optical identification for 58 of the 63 radio sources. This corresponds to an identification rate of ∼92%, 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. This class of objects, rather than a population of star-forming galaxies, appears to be the dominant population (≳50%) in a 5 GHz selected sample with a flux limit as low as 50 μJy. We also find evidence that at these faint radio limits a large fraction (∼60%) 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 and combining our radio data with existing ISO data we conclude that these EROs sources are probably associated with high redshift, passively evolving elliptical galaxies. The six radio selected EROs represent only ∼2% of the optically selected EROs present in the field. If their luminosity is indeed a sign of AGN activity, the small number of radio detections suggests that a small fraction of the EROS population contains an active nucleus.

Original languageEnglish
Pages (from-to)901-918
Number of pages18
JournalAstronomy and Astrophysics
Volume398
Issue number3
Publication statusPublished - Feb 2003

Fingerprint

radio
EROS (satellites)
galaxies
color-color diagram
radio spectra
Very Large Array (VLA)
elliptical galaxies
flattening
XMM-Newton telescope
extremely high frequencies
radioactivity
active galactic nuclei
lists
cross correlation
catalogs
charge coupled devices
x rays
flux density
diagrams
luminosity

Keywords

  • Cosmology: observations
  • Galaxies: general: starburst
  • Quasar: general

ASJC Scopus subject areas

  • Space and Planetary Science

Cite this

Ciliegi, P., Zamorani, G., Hasinger, G., Lehmann, I., Szokoly, G., & Wilson, G. (2003). A deep VLA survey at 6 cm in the Lockman Hole. Astronomy and Astrophysics, 398(3), 901-918.

A deep VLA survey at 6 cm in the Lockman Hole. / Ciliegi, P.; Zamorani, G.; Hasinger, G.; Lehmann, I.; Szokoly, G.; Wilson, G.

In: Astronomy and Astrophysics, Vol. 398, No. 3, 02.2003, p. 901-918.

Research output: Contribution to journalArticle

Ciliegi, P, Zamorani, G, Hasinger, G, Lehmann, I, Szokoly, G & Wilson, G 2003, 'A deep VLA survey at 6 cm in the Lockman Hole', Astronomy and Astrophysics, vol. 398, no. 3, pp. 901-918.
Ciliegi P, Zamorani G, Hasinger G, Lehmann I, Szokoly G, Wilson G. A deep VLA survey at 6 cm in the Lockman Hole. Astronomy and Astrophysics. 2003 Feb;398(3):901-918.
Ciliegi, P. ; Zamorani, G. ; Hasinger, G. ; Lehmann, I. ; Szokoly, G. ; Wilson, G. / A deep VLA survey at 6 cm in the Lockman Hole. In: Astronomy and Astrophysics. 2003 ; Vol. 398, No. 3. pp. 901-918.
@article{95753c77f49945dcb0b74ec83157a427,
title = "A deep VLA survey at 6 cm in the Lockman Hole",
abstract = "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 ∼11/μJy. From these data we have extracted a catalogue of 63 radio sources with a maximum distance of 10 arcmin from the field center and with peak flux density greater than 4.5 times the local rms noise. The differential source counts are in good agreement with those obtained by other surveys. 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 ∼21 {\%} 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 with approximate limits of V ∼ 25.5 mag, I ∼ 24.5 mag and K′ ∼ 20.2 mag, we found an optical identification for 58 of the 63 radio sources. This corresponds to an identification rate of ∼92{\%}, 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. This class of objects, rather than a population of star-forming galaxies, appears to be the dominant population (≳50{\%}) in a 5 GHz selected sample with a flux limit as low as 50 μJy. We also find evidence that at these faint radio limits a large fraction (∼60{\%}) 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 and combining our radio data with existing ISO data we conclude that these EROs sources are probably associated with high redshift, passively evolving elliptical galaxies. The six radio selected EROs represent only ∼2{\%} of the optically selected EROs present in the field. If their luminosity is indeed a sign of AGN activity, the small number of radio detections suggests that a small fraction of the EROS population contains an active nucleus.",
keywords = "Cosmology: observations, Galaxies: general: starburst, Quasar: general",
author = "P. Ciliegi and G. Zamorani and G. Hasinger and I. Lehmann and G. Szokoly and G. Wilson",
year = "2003",
month = "2",
language = "English",
volume = "398",
pages = "901--918",
journal = "Astronomy and Astrophysics",
issn = "0004-6361",
publisher = "EDP Sciences",
number = "3",

}

TY - JOUR

T1 - A deep VLA survey at 6 cm in the Lockman Hole

AU - Ciliegi, P.

AU - Zamorani, G.

AU - Hasinger, G.

AU - Lehmann, I.

AU - Szokoly, G.

AU - Wilson, G.

PY - 2003/2

Y1 - 2003/2

N2 - 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 ∼11/μJy. From these data we have extracted a catalogue of 63 radio sources with a maximum distance of 10 arcmin from the field center and with peak flux density greater than 4.5 times the local rms noise. The differential source counts are in good agreement with those obtained by other surveys. 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 ∼21 % 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 with approximate limits of V ∼ 25.5 mag, I ∼ 24.5 mag and K′ ∼ 20.2 mag, we found an optical identification for 58 of the 63 radio sources. This corresponds to an identification rate of ∼92%, 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. This class of objects, rather than a population of star-forming galaxies, appears to be the dominant population (≳50%) in a 5 GHz selected sample with a flux limit as low as 50 μJy. We also find evidence that at these faint radio limits a large fraction (∼60%) 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 and combining our radio data with existing ISO data we conclude that these EROs sources are probably associated with high redshift, passively evolving elliptical galaxies. The six radio selected EROs represent only ∼2% of the optically selected EROs present in the field. If their luminosity is indeed a sign of AGN activity, the small number of radio detections suggests that a small fraction of the EROS population contains an active nucleus.

AB - 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 ∼11/μJy. From these data we have extracted a catalogue of 63 radio sources with a maximum distance of 10 arcmin from the field center and with peak flux density greater than 4.5 times the local rms noise. The differential source counts are in good agreement with those obtained by other surveys. 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 ∼21 % 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 with approximate limits of V ∼ 25.5 mag, I ∼ 24.5 mag and K′ ∼ 20.2 mag, we found an optical identification for 58 of the 63 radio sources. This corresponds to an identification rate of ∼92%, 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. This class of objects, rather than a population of star-forming galaxies, appears to be the dominant population (≳50%) in a 5 GHz selected sample with a flux limit as low as 50 μJy. We also find evidence that at these faint radio limits a large fraction (∼60%) 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 and combining our radio data with existing ISO data we conclude that these EROs sources are probably associated with high redshift, passively evolving elliptical galaxies. The six radio selected EROs represent only ∼2% of the optically selected EROs present in the field. If their luminosity is indeed a sign of AGN activity, the small number of radio detections suggests that a small fraction of the EROS population contains an active nucleus.

KW - Cosmology: observations

KW - Galaxies: general: starburst

KW - Quasar: general

UR - http://www.scopus.com/inward/record.url?scp=0037321623&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0037321623&partnerID=8YFLogxK

M3 - Article

AN - SCOPUS:0037321623

VL - 398

SP - 901

EP - 918

JO - Astronomy and Astrophysics

JF - Astronomy and Astrophysics

SN - 0004-6361

IS - 3

ER -