J/A+A/569/A52 Ultra steep spectrum radio sources (Singh+, 2014)
Multiwavelength characterization of faint ultra steep spectrum radio sources:
A search for high-redshift radio galaxies.
Singh V., Beelen A., Wadadekar Y., Sirothia S., Ishwara-Chandra C.H.,
Basu A., Omont A., McAlpine K., Ivison R.J., Oliver S., Farrah D., Lacy M.
<Astron. Astrophys., 569, A52-52 (2014)>
=2014A&A...569A..52S 2014A&A...569A..52S
ADC_Keywords: Surveys ; Galaxies, radio ; Redshifts
Keywords: galaxies: nuclei - galaxies: active - radio continuum: galaxies -
galaxies: high-redshift - galaxies: general - galaxies: evolution
Abstract:
Ultra steep spectrum (USS) radio sources are one of the efficient
tracers of powerful high-z radio galaxies (HzRGs). In contrast to
searches for powerful HzRGs from radio surveys of moderate depths,
fainter USS samples derived from deeper radio surveys can be useful in
finding HzRGs at even higher redshifts and in unveiling a population
of obscured weaker radio-loud AGN at moderate redshifts. Using our
325MHz GMRT observations (5σ∼800µJy) and 1.4GHz VLA
observations (5σ∼80-100µJy) available in two subfields
(VLA-VIMOS VLT Deep Survey (VLA-VVDS) and Subaru X-ray Deep Field
(SXDF)) of the XMM-LSS field, we derive a large sample of 160 faint
USS radio sources and characterize their nature. The optical and IR
counterparts of our USS sample sources are searched using existing
deep surveys, at respective wavelengths. We attempt to unveil the
nature of our faint USS sources using diagnostic techniques based on
mid-IR colors, flux ratios of radio to mid-IR, and radio luminosities.
Redshift estimates are available for 86/116 (∼74%) USS sources in the
VLA-VVDS field and for 39/44 (∼87%) USS sources in the SXDF fields
with median values (zmedian)∼1.18 and ∼1.57, respectively, which are
higher than estimates for non-USS radio sources (z_median non-USS_
∼0.99 and ∼0.96), in the two subfields. The MIR color-color diagnostic
and radio luminosities are consistent with most of our USS sample
sources at higher redshifts (z>0.5) being AGN. The flux ratio of radio
to mid-IR (S1.4GHz/S3.6µm) versus redshift diagnostic plot
suggests that more than half of our USS sample sources distributed
over z∼0.5 to 3.8 are likely to be hosted in obscured environments. A
significant fraction (∼26% in the VLA-VVDS and ∼13% in the SXDF) of
our USS sources without redshift estimates mostly remain unidentified
in the existing optical, IR surveys, and exhibit high radio to mid-IR
flux ratio limits similar to HzRGs, and so, can be considered as
potential HzRG candidates. Our study shows that the criterion of ultra
steep spectral index remains a reasonably efficient method to select
high-z sources even at sub-mJy flux densities. In addition to powerful
HzRG candidates, our faint USS sample also contains populations of
weaker radio-loud AGNs potentially hosted in obscured environments.
Description:
We obtained 325 MHz GMRT observations of the XMM-LSS field over sky
area of ∼12deg2 with synthesized beamsize ∼10.2"x7.9". In the
mosaiced 325MHz GMRT image the average noise rms is ∼160uJy, while in
the central region the average noise-rms reaches to ∼120uJy. Our
325MHz observations are one of the deepest low-frequency surveys over
such a wide sky area and detect ∼2553/3304 radio sources at
≥5.0σ with noise rms cut-off ≤200/300uJy.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 103 161 325MHz GMRT observations of the Ultra Steep
Spectrum (USS) sample
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See also:
III/334 : VIDEO: VISTA Deep Extragalactic Observations (Jarvis+, 2013)
J/A+A/403/857 : VLA-VIRMOS Deep Field (Bondi+, 2003)
J/MNRAS/372/741 : SXDF 100µJy catalogue (Simpson+, 2006)
J/MNRAS/421/3060 : Subaru/XMM Deep Field radio imaging. III. (Simpson+, 2012)
Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 3 A3 --- Sample Sample (B03 or S06) (1)
5 A1 --- n_GMRT [*+] low signal or double lobe source (2)
6- 9 A4 --- --- [GMRT]
10- 22 A13 --- GMRT Source name as given in our 325MHz GMRT catalog
(Sirothia et al., in prep.) (HHMMSS+DDMMSS)
24- 25 I2 h RAh Right ascension (J2000) (3)
27- 28 I2 min RAm Right ascension (J2000) (3)
30- 34 F5.2 s RAs Right ascension (J2000) (3)
36 A1 --- DE- Declination sign (J2000) (3)
37- 38 I2 deg DEd Declination (J2000) (3)
40- 41 I2 arcmin DEm Declination (J2000) (3)
43- 47 F5.2 arcsec DEs Declination (J2000) (3)
49- 55 F7.3 mJy S.325 325MHz flux density from our GMRT
observations
57- 61 F5.3 mJy e_S.325 rms uncertainty on S325MHz
63- 68 F6.3 mJy S1.4 1.4GHz flux density from B03 or S06
70- 74 F5.3 mJy e_S1.4 rms uncertainty on S1.4
76- 80 F5.2 --- alpha [-2.6/-1] Radio spectral index α
between 325MHz to 1.4GHz
82- 85 F4.2 --- e_alpha rms uncertainty on alpha
87- 91 F5.3 --- zph [0.03/3.5]?=- Photometric redshift estimate (4)
93- 97 F5.3 --- zsp [0.03/3.9]?=- Spectroscopic redshift (5)
99-103 F5.2 [W/Hz] logL1.4 ?=- 1.4GHz radio luminosity (6)
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Note (1): The two samples are:
B03 = VLA-VVDS; Bondi et al. (2003A&A...403..857B 2003A&A...403..857B, Cat. J/A+A/403/857)
S06 = SXDF; Simpson et al. (2006MNRAS.372..741S 2006MNRAS.372..741S, Cat. J/MNRAS/372/741)
Note (2): Notes as follows:
* = indicates sources detected at <5σ in 1.4GHz
+ = indicates sources with clear double lobe radio morphology
Note (3): Positions from 1.4GHz VLA observations presented in Bondi et al.
(2003A&A...403..857B 2003A&A...403..857B) and Simpson et al. (2006MNRAS.372..741S 2006MNRAS.372..741S).
Note (4): Photometric redshift estimates taken from McAlpine et al.
(2013MNRAS.436.1084M 2013MNRAS.436.1084M) and Simpson et al. (2012MNRAS.421.3060S 2012MNRAS.421.3060S,
Cat. J/MNRAS/421/3060) for the sources in the B03 and S06 fields, respectively.
Note (5): Spectroscopic redshifts taken from VVDS catalog (Le Fevre et al.,
2013A&A...559A..14L 2013A&A...559A..14L) and Simpson et al. (2012MNRAS.421.3060S 2012MNRAS.421.3060S,
Cat. J/MNRAS/421/3060).
Note (6): Spectroscopic redshifts, when available, are given preference over
photometric redshifts to estimate the radio luminosities.
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 17-Nov-2014