VLACOS324M - VLA-COSMOS Survey 324-MHz Continuum Source Catalog

Overview

This table contains a source catalog based on 90-cm (324-MHz) Very Large Array (VLA) imaging of the COSMOS field, comprising a circular area of 3.14 square degrees centered on 10^h 00^m 28.6^s, _02^o 12' 21" (J2000.0 RA and Dec). The image from the merger of 3 nights of observations using all 27 VLA antennas had an effective total integration time of ~ 12 hours, an 8.0 arcsecond x 6.0 arcsecond angular resolution, and an average rms of 0.5 mJy beam^-1. The extracted catalog contains 182 sources (down to 5.5 sigma), 30 of which are multi-component sources. Using Monte Carlo artificial source simulations, the authors derive the completeness of the catalog, and show that their 90-cm source counts agree very well with those from previous studies. In their paper, the authors use X-ray, NUV-NIR and radio COSMOS data to investigate the population mix of this 90-cm radio sample, and find that the sample is dominated by active galactic nuclei. The average 90-20 cm spectral index (S_nu_~ nu^alpha, where S_nu is the flux density at frequency nu and alpha the spectral index) of the 90-cm selected sources is -0.70, with an interquartile range from -0.90 to -0.53. Only a few ultra-steep-spectrum sources are present in this sample, consistent with results in the literature for similar fields. These data do not show clear steepening of the spectral index with redshift. Nevertheless, this sample suggests that sources with spectral indices steeper than -1 all lie at z >~ 1, in agreement with the idea that ultra-steep-spectrum radio sources may trace intermediate-redshift galaxies (z >~ 1).

Using both the signal and rms maps (see Figs. 1 and 2 in the reference paper) as input data, the authors ran the AIPS task SAD to obtain a catalog of candidate components above a given local signal-to-noise ratio (S/N) threshold. The task SAD was run four times with search S/N levels of 10, 8, 6 and 5, using the resulting residual image each time. They recovered all the radio components with a local S/N > 5.00. Subsequently, all the selected components were visually inspected, in order to check their reliability, especially for the components near strong side-lobes. After a careful analysis, a S/N threshold of 5.50 was adopted as the best compromise between a deep and a reliable catalog. The procedure yielded a total of 246 components with a local S/N > 5.50. More than one component, identified in the 90-cm map sometimes belongs to a single radio source (e.g. large radio galaxies consist of multiple components). Using the 90-cm COSMOS radio map, the authors combined the various components into single sources based on visual inspection. The final catalog (contained in this HEASARC table) lists 182 radio sources, 30 of which have been classified as multiple, i.e. they are better described by more than a single component. Moreover, in order to ensure a more precise classification, all sources identified as multi-component sources have been also double-checked using the 20-cm radio map. The authors found that all the 26 multiple 90-cm radio sources within the 20-cm map have 20-cm counterpart sources already classified as multiple.

The authors have made use of the VLA-COSMOS Large and Deep Projects over 2 square degrees, reaching down to an rms of ~15 µJy beam¹ ^ at 1.4 GHz and 1.5 arcsec resolution (Schinnerer et al. 2007, ApJS, 172, 46: the VLACOSMOS table in the HEASARC database). The 90-cm COSMOS radio catalog has, however, been extracted from a larger region of 3.14 square degrees (see Fig. 1 and Section 3.1 of the reference paper). This implies that a certain number of 90-cm sources (48) lie outside the area of the 20-cm COSMOS map used to select the radio catalog. Thus, to identify the 20-cm counterparts of the 90-cm radio sources, the authors used the joint VLA-COSMOS catalog (Schinnerer et al. 2010, ApJS, 188, 384: the VLACOSMJSC table in the HEASARC database) for the 134 sources within the 20-cm VLA-COSMOS area and the VLA- FIRST survey (White et al. 1997, ApJ, 475, 479: the FIRST table in the HEASARC database) for the remaining 48 sources. The 90-cm sources were cross-matched with the 20-cm VLA-COSMOS sources using a search radius of 2.5 arcseconds, while the cross-match with the VLA-FIRST sources has been done using a search radius of 4 arcseconds in order to take into account the larger synthesized beam of the VLA-FIRST survey of ~5 arcseconds. Finally, all the 90 cm - 20 cm associations were visually inspected in order to ensure also the association of the multiple 90-cm radio sources for which the value of the search radius used during the cross-match could be too restrictive. In summary, out of the total of 182 sources in the 90-cm catalog, 168 have counterparts at 20 cm.

Catalog Bibcode

2014MNRAS.443.2590S

References

The VLA-COSMOS Survey - V. 324 MHz Continuum Observations

   Smolcic V., Ciliegi P., Jelic Vibor, Bondi M., Schinnerer E., Carilli C.L.,
   Riechers D.A., Salvato M., Brkovic A., Capak P., Ilbert O., Karim A.,
   McCracken H., Scoville N.Z.
   <Mon. Not. Royal Astr. Soc. 443, 2590 (2014)>
   =2014MNRAS.443.2590S

Provenance

This table was created by the HEASARC in October 2014 based on an electronic version of Table 1 from the reference paper which was obtained from the COSMOS web site at IRSA, specifically the file vla-cosmos_327_sources_published_version.tbl at http://irsa.ipac.caltech.edu/data/COSMOS/tables/vla/.

Parameters

Name
The full VLA-COSMOS source name. This form is 'COSMOSVLA324 JHHMMSS.ss+DDMMSS.sC', where COSMOSVLA324 is the survey acronym, 'J' specifies the J2000.0 coordinate equinox, 'HHMMSS.ss' are the hours, minutes and seconds of Right Ascension, '+' or '-' is the sign of the Declination, and 'DDMMSS.s' are the degrees, arcminutes and arcseconds of the Declination. Note that that the HEASARC has removed the '_' character that was included in these names and corrected the numerical part of the prefix (purportedly based on the average frequency in MHz of the two IFs which were used in the survey) from the value that was given by the authors (327) to 324 (MHz).

RA
The Right Ascension of the 90-cm radio source in the selected equinox. This was given in J2000.0 decimal degrees to a precision of 10^-7 degrees in the original table. In this 90-cm catalog, multiple component sources are identified by the multi_component_flag value of 1. Since the radio core of these multiple sources is not always detected, following the same procedure adopted for the 20-cm catalog, their virtual radio position has been calculated as the mean value of the position of all components weighted by their total radio flux. Moreover, in order to avoid slightly different virtual positions, for the 26 multiple 90-cm sources that have been visually associated with a 20-cm multiple source, the position has been assumed to be coincident with the 20-cm position. Finally, for all the 90-cm single-component sources, we report the position estimated on the 90 cm-map, even for those with a 20-cm counterpart.

Dec
The Declination of the 90-cm radio source in the selected equinox. This was given in J2000.0 decimal degrees to a precision of 10^-7 degrees in the original table. In this 90-cm catalog, multiple component sources are identified by the multi_component_flag value of 1. Since the radio core of these multiple sources is not always detected, following the same procedure adopted for the 20-cm catalog, their virtual radio position has been calculated as the mean value of the position of all components weighted by their total radio flux. Moreover, in order to avoid slightly different virtual positions, for the 26 multiple 90-cm sources that have been visually associated with a 20-cm multiple source, the position has been assumed to be coincident with the 20-cm position. Finally, for all the 90-cm single-component sources, we report the position estimated on the 90 cm-map, even for those with a 20-cm counterpart.

LII
The Galactic Longitude of the 90-cm radio source.

BII
The Galactic Latitude of the 90-cm radio source.

RA_Error
The rms uncertainty in the Right Ascension of the 90-cm source position, in arcseconds. No value is specified for multi-component sources, i.e., sources with multi_component_flag = 1.

Dec_Error
The rms uncertainty in the Declination of the 90-cm source position, in arcseconds. No value is specified for multi-component sources, i.e., sources with multi_component_flag = 1.

Flux_324_MHz
The peak 324-MHz flux (surface brightness) of the radio source, in mJy/beam. For multiple sources, the peak flux (at the listed position) is undetermined and therefore it and its error have been set to null values.

Flux_324_MHz_Error
The rms uncertainty in the peak 324-MHz flux (surface brightness) of the radio source, in mJy/beam. Following the procedure adopted for the VLA-COSMOS 20-cm survey, the uncertainties in the peak flux density and integrated flux have been calculated using the equation given by Condon (1997, PASP, 109, 166; see also Hopkins et al. 2003 AJ, 125, 465 and Schinnerer et al. 2004,AJ, 128, 1974). For multiple sources, the peak flux (at the listed position) is undetermined and therefore it and its error have been set to null values.

Int_Flux_324_MHz
The total (integrated) 324-MHz flux (surface brightness) of the radio source, in mJy. For all the multiple sources, the total flux was calculated using the AIPS task TVSTAT, which allows the integration of the map values over irregular areas. For the unresolved sources, the total flux density is set equal to the peak brightness and the angular size is undetermined and set equal to zero in the catalog.

Int_Flux_324_MHz_Error
The rms uncertainty in the total (integrated) 324-MHz flux (surface brightness) of the radio source, in mJy. This is not given multi-component sources. Following the procedure adopted for the VLA-COSMOS 20-cm survey, the uncertainties in the peak flux density and integrated flux have been calculated using the equation given by Condon (1997, PASP, 109, 166; see also Hopkins et al. 2003 AJ, 125, 465 and Schinnerer et al. 2004,AJ, 128, 1974).

RMS_Flux_324_MHz
The measured local rms noise at the 90-cm source position, in mJy/beam.

Major_Axis
The major axis of the deconvolved 90-cm source, in arcseconds. For the unresolved sources, the total flux density is set equal to the peak brightness and the angular size is undetermined and set equal to zero in the catalog.

Minor_Axis
The minor axis of the deconvolved 90-cm source, in arcseconds. For the unresolved sources, the total flux density is set equal to the peak brightness and the angular size is undetermined and set equal to zero in the catalog.

Position_Angle
The position angle of the deconvolved 90-cm source, in degrees.

Resolved_Flag
This flag parameter is set to 1 if the 90-cm source is resolved (or multi-component), else is set to 0. In order to determine whether the identified single-component sources are resolved (i.e. extended, larger than the clean beam), the authors make use of the ratio between the total (S_tot) and peak (S_peak) fluxes (both calculated with the AIPS Gaussian fitting algorithm JMFIT within AIPS/SAD) as this is a direct measure of the extension of a radio source. In Fig. 5 of the reference paper, the authors plot the ratio between the total and the peak flux density as a function of the signal-to-noise ratio S/N (= S_peak / rms) for all the 152 single-component sources in the catalog. To select the resolved sources, they determined the lower envelope of the points in Fig. 5, which contains 90% of the sources with S_tot < S_peak and mirrored it above the S_tot/S_peak = 1 line (the upper envelope in Fig. 5). They consider the 56 sources lying above the upper envelope resolved which, in addition to the 30 multi-component sources, gives a total of 86 resolved sources.

Multi_Component_Flag
This flag parameter is set to 1 if the 90-cm source is multi-component in nature, else is set to 0.

Name_20_cm
The name of the 20-cm counterpart source, e.g., 'FIRST VLA J095646.93+020746.1' or 'COSMOSVLADP J095737.21+024604.7'. See the Overview above for more details. Note that the FIRST names are not in the style recommended by the Dictionary of Nomenclature of Celestial Objects, which suggests the style 'FIRST JHHMMSS.s+DDMMSS'.

Flux_1p4_GHz
The peak surface brightness of the 20-cm counterpart to the 90-cm source, in mJy/beam.

Int_Flux_1p4_GHz
The total (integrated) flux density of the 20-cm counterpart to the 90-cm source, in mJy.

Offset
The angular separation between the 20-cm and 90-cm source positions, in arcseconds. For the 26 multiple 90-cm sources that have been visually associated with a 20-cm multiple source, the position has been assumed to be coincident with the 20-cm position, and thus the offset is 0.00 arcseconds by definition.

Contact Person

Questions regarding the VLACOS324M database table can be addressed to the HEASARC Help Desk.

Page Author: Browse Software Development Team
Last Modified: Monday, 16-Sep-2024 17:36:09 EDT