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ATLGDS2P1G - Australia Telescope Local Group Dwarf Spheroidals 2.1-GHz Components Catalog

 HEASARC
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Overview

Dwarf spheroidal (dSph) galaxies are key objects in near-field cosmology, especially in connection to the study of galaxy formation and evolution at small scales. In addition, dSphs are optimal targets to investigate the nature of dark matter. However, while we begin to have deep optical photometric observations of the stellar population in these objects, little is known so far about their diffuse emission at any observing frequency, and hence on thermal and non-thermal plasma possibly residing within dSphs. In this paper, the authors present deep radio observations of six local dSphs performed with the Australia Telescope Compact Array (ATCA) at 16 cm wavelength (2100 MHz frequency). They mosaicked a region of radius of about 1 degree around three 'classical' dSphs (CDS), Carina, Fornax, and Sculptor, and of about half of degree around three 'ultrafaint' dSphs (UDS), Bootes II, Segue 2, and Hercules. The rms noise level is below 0.05 mJy for all the maps. The restoring beams full width at half-maximum (FWHM) ranged from (4.2 arcseconds by 2.5 arcseconds) to (30.0 arcseconds by 2.1 arcseconds) in the most elongated case. A catalog, including the 1392 sources detected in the six dSph fields, is presented here. In the reference paper, the main properties of the background sources are discussed, with the positions and fluxes of the brightest objects compared with the FIRST, NVSS, and SUMSS observations of the same fields. The observed population of radio emitters in these fields is dominated by synchrotron sources.The authors have computed the associated source number counts at 2 GHz down to fluxes of 0.25 mJy, which prove to be in agreement with AGN count models.

The observations presented in this paper were performed during 2011 July. The project was allocated a total of 123 h of ATCA observing time. The spectral setup included the simultaneous observation of a 2-GHz-wide band centered at 2100 MHz with a 1 MHz spectral resolution for continuum observations (recording all four polarization signals). The mapping of the three CDS required a 19 field-mosaic with a total on-source integration time of about 1 hour per field. For Bootes II and Hercules, a 7 field-mosaic with an on-source integration time of about 2 hours per field was chosen, while Segue 2, due to its smaller size,was imaged with a 3 field-mosaic with about 4 hours per field of integration time (with the purpose of maximizing the sensitivity). More precisely, a total of 16.5, 15.0, 17.0, 13.0, 10.9, and 9.6 hours were spent on-source for Carina, Fornax, Sculptor, Bootes II, Hercules, and Segue 2, respectively. The nominal rms sensitivity in each panel for the actual observing time was 36, 38, 35, 25, 28, and 20 µJy for Carina, Fornax, Sculptor, Bootes II, Hercules, and Segue 2, respectively. See Table 1 of the reference paper for the details of the average restoring beam parameters across all mosaic panels for each field of view (FoV).

The authors used two automated routines for source extraction and cataloging, which are provided by the SEXTRACTOR package (Bertin & Arnouts 1996, A&AS, 117, 393) and the task SFIND in MIRIAD. In these maps, SFIND and SEXTRACTOR give nearly identical results for astrometry (number of sources and positions), once the threshold parameters in SEXTRACTOR are tuned (the authors found a threshold typically slightly above 5 sigma). The mismatch in positions is random, and about 1 arcsecond on average for all FoVs. This value can be taken as an estimate of the positional accuracy. Photometry on the other hand, gave quite different results for some sources: in the catalog, the authors used the results from SFIND since this was specifically written to analyze radio images, accounting for artifacts and sidelobes. The number of sources in each dSph FoV is reported in Table 2 of the reference paper.

Radio sources can be made up of different components. To decide whether nearby sources are separated sources or components of a single source, the authors visually inspected all the fields where either the angular distance, theta, between sources was < 1 arcminute, or the criterion of Magliocchetti et al. (1998, MNRAS, 300, 257: theta < 100 arcseconds x sqrt[Speak/10 mJy]), was satisfied. A more detailed study of the 178 possible multiple sources will be reported in a future paper by these authors.

Catalog Bibcode

2015MNRAS.448.3731R

References

Local Group dSph radio survey with ATCA (I): Observations and background
sources
    Regis M., Richter L., Colafrancesco S., Massardi M., de Blok W.J.G.,
    Profumo S., Orford N.
   <Mon. Not. R. Astron. Soc., 448, 3731-3746 (2015)>
   =2015MNRAS.448.3731R    (SIMBAD/NED BibCode)

Provenance

This table was created by the HEASARC in June 2015 based on an electronic version of the source components catalog (Table 4 of the reference paper) which was obtained from the MNRAS web site. In Section 4 of the reference paper, the authors state that they "found 1835 entries in the catalog corresponding to a total of 1392 extracted sources with 178 cases being (possibly) multiple component sources". We note that the table downloaded from the MNRAS web site and used as the basis for this current HEASARC table actually contained only 1834 entries, The reason for this small discrepancy is not known.

Parameters

Name
The catalog source designation, using the prefix 'ATLGDS' (for Australia Telescope Local Group Dwarf Spheroidal' and the J2000.0 equatorial source coordinates, e.g., 'ATLGDS JHHMMSS.s-DDMMSS.s'. This parameter, which was not present in the original table as published, was created by the HEASARC in the style recommended by the Dictionary Of Nomenclature of Celestial Objects.

RA
The Right Ascension of the 2100-MHz radio source in the selected equinox. This was given in J2000.0 equatorial coordinates to a precision of 0.1 seconds of time in the original table.

Dec
The Declination of the 2100-MHz radio source in the selected equinox. This was given in J2000.0 equatorial coordinates to a precision of 0.1 arcseconds in the original table.

LII
The Galactic Longitude of the 2100-MHz radio source.

BII
The Galactic Latitude of the 2100-MHz radio source.

Flux_2p1_GHz
The peak flux density, Speak, of the 2100-MHz radio source in the untapered map, in mJy/beam. The values are not corrected for possible systematic effects, which are estimated to be negligible (see discussion in the reference paper).

Int_Flux_2p1_GHz
The integrated flux density, Stot, of the 2100-MHz radio source in the untapered map, in mJy.

Int_Flux_2p1_GHz_Error
The error in the integrated flux density of the 2100-MHz radio source in the untapered map, in mJy. The errors are obtained by summing in quadrature the local rms, the fit error, and 5% of the flux density, in order to account for possible inaccuracy in the calibration model and process, especially due to radio frequency interference (RFI).

Major_Axis
The FWHM major axis of the source, in arcminutes. In the catalog, the authors always quote the fitted sizes of source axes from the untapered map, with the caveat that, when the total flux density estimate is significantly below the total flux density from the tapered image, they underestimate the real size of the source, since do not account for the diffuse components. For sources with Stot/Speak < 1.3, the source is not successfully deconvolved and these values should not be considered reliable.

Minor_Axis
The FWHM minor axis of the source, in arcminutes. In the catalog, the authors always quote the fitted sizes of source axes from the untapered map, with the caveat that, when the total flux density estimate is significantly below the total flux density from the tapered image, they underestimate the real size of the source, since do not account for the diffuse components. For sources with Stot/Speak < 1.3, the source is not successfully deconvolved and these values should not be considered reliable.

Position_Angle
The position angle of the source, in degrees, measured North to East. For sources with Stot/Speak < 1.3, the source is not successfully deconvolved and these values should not be considered reliable.

Tprd_Int_Flux_2p1_GHz
The integrated flux density, Stot, of the 2100-MHz radio source in the tapered map, in mJy. Whether or not this estimate is more reliable than the corresponding value from the untapered map is discussed in Section 4 of the reference paper.

Source_Flags
This parameter contains flags differentiating single- from multiple-component sources, as follows: 

      S = single component source;
      M = multiple component source, followed by a number, identifying the
          multiple source to which the component belongs, and a letter
          identifying the field (a = Carina, b = Fornax, c = Sculptor, d =
          Bootes II, e = Hercules, f = Segue 2);

      T = Source present in the tapered map, but not in the untapered map: in
          this case, all the properties of the other parameters are derived
          from the tapered map.

Contact Person

Questions regarding the ATLGDS2P1G database table can be addressed to the HEASARC Help Desk.
Page Author: Browse Software Development Team
Last Modified: Monday, 16-Sep-2024 17:24:51 EDT