Optical Identification of Flat Spectrum Radio Sources in the SDSS

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					                                Optical Identification of Flat-Spectrum Radio Sources in the SDSS
                              Jared A. Crooks, Min-Su Shin, Yen-Ting Lin, Gillian R. Knapp, Michael A. Strauss (Princeton University)

    We have searched the Data Release 6 catalogues of the Sloan Digital
Sky Survey for optical counterparts to flat-spectrum (α > −0.5 between 4.8
and 8 GHz, where fν ∝ ν α ) radio sources in the 4.8 GHz– selected CRATES
catalogue. Of the 3590 radio sources falling within the DR6 footprint, about
65% are detected in the SDSS imaging to a limiting AB magnitude of gri ∼
22.5, and about 42% of these have SDSS spectra. The large majority of the
optical counterparts are quasars with redshifts ranging to greater than 5.
Repeat observations of quasars in the SDSS southern stripe show variabil-
ity on months to years time scales. About 130 BL Lac objects are found.
33 of these have composite BL Lac/galaxy spectra allowing redshifts to be
found. These redshifts show that the BL Lac objects are less luminous as
a class than are quasars. Other flat–spectrum radio sources are identified
with galactic nuclei, in many cases showing “AGN” optical emission spec-
tra, and with heavily reddened quasars and galaxies. Some bright elliptical
galaxies have flat-spectrum radio nuclei with no other observational indica-
tors of nuclear activity. Among the small number of “oddball” objects are
a possible flaring radio star, several known gravitationally lensed quasars,
and a probable supernova in a bright HII region in the nearby galaxy NGC

                                                                                                                                                                      Figure 2 shows SDSS color-color diagrams for these various spectral
                                                                                                                                                                  types, compared with the colors of a representative sample of high-latitude
                                                                                                                                                                  stars (contours). Open black points are quasars, filled blue points BL Lacs;
                                                                                                                                                                  filled green points BG objects; and red points galaxies. In all cases, the
                                                                                                                                                                  SDSS model magnitude is used. The colors of BL Lac objects overlap those
                                                                                                                                                                  of low-redshift quasars, but the color distributions of the two object types
                                                                                                                                                                  differ. The galaxies are largely red-sequence luminous elliptical galaxies.
                                                                                                                                                                  Figure 3 shows the redshift distribution for the galaxies, BG objects and
                                                                                                                                                                  quasars. The quasar redshifts are shown by the heavy black histogram, the
                                                                                                                                                                  galaxies by the red histogram, and the BG objects by the blue histogram.
                                                                                                                                                                  The flat spectrum quasars have a redshift distribution very similar to that
                                                                                                                                                                  of the entire sample of optically identified quasars from SDSS DR5 (Schnei-
                                                                                                                                                                  der et al. 2007, AJ, 134, 102). The highest redshift for a CRATES quasar
                                                                                                                                                                  is z = 5.3. The sample properties of flat spectrum quasars appear to be
                                                                                                                                                                  indistinguishable from those of the much larger optical samples.
                                                                                                                                                                      The redshift distributions of the galaxies and BG objects are quite sim-
                                                                                                                                                                  ilar, and in particular there are more low-redshift BG objects than quasars
                                                                                                                                                                  despite the much smaller sample size. The results in Figures 2 and 3 rein-
                                                                                                                                                                  force the identification of BL Lacs with low-luminosity radio jet sources in
                                                                                                                                                                  elliptical galaxies (e.g. Browne 1983, MNRAS 204, 238) rather than with
    We have matched the flat-spectrum radio sources from the all-sky CRATES           The usual definition of a BL Lac object at optical wavelengths is that        quasars. The redshift distribution of the featureless BL Lac object, and of
catalogue (Healey et al. 2007, ApJS 171, 161) to SDSS photometry from            it has a continuous, power-law spectrum with no spectral lines. The quan-        the much larger samples identified by Collinge et al. (2005) and Plotkin et
Data Release 6 (Adelman-McCarthy et al. 2008, ApJS, in press) and spec-          titative version of this definition differentiates BL Lacs from quasars by         al.(2008) remains to be determined - limits can be found by searching for
troscopy as obtained up to Summer 2007. The large majority of the CRATES         requiring that the object have no emission lines with rest-frame equivalent      intergalactic absorption features, in particular MgII 2800˚ and Lyman-α
sources are compact and have position accuracies of ∼ 60 mas, similar to         widths greater than 5 ˚ (March˜ et al. 1996, MNRAS 281, 425; Collinge et
                                                                                                        A         a                                               absorption.
the accuracy of SDSS. Of the 11,000 CRATES sources, 3590 fall within the         al. 2005, AJ 129, 2542). We did not follow this strict definition but rather          The investigation also turned up several unusual objects, including in-
footprint of DR6. 2324 (65%) have counterparts in the SDSS photometric           used visual inspection. Galaxies are separated from BL Lacs by the strength      teracting galaxies, broad absorption line quasars, strongly variable quasars,
catalogues, i.e. are optically brighter than gri ∼ 22.3 (AB), and 995 have       of the CaII H&K break, given by                                                  and an HII region in the nearby galaxy NGC 5705. The SDSS image of this
spectra. The matching radius was 3 . The dispersion in the position differ-                                                                                        galaxy is shown, with the location of the HII region indicated. This bright
ences is ∼ 90 mas, consistent with the position accuracies of the individual                           C = 0.14 + 0.86(f2 − f1 )/f2                               region shows an intense emission spectrum characteristic of star formation.
surveys. The small number of objects with position difference > 0.5 are                                                                                            The CRATES source is positionally coincident with the HII region, but is
very nearby galaxies of large angular size, whose centroids are likely to be     where f2 is the mean flux density in the rest-frame wavelength region near        far too radio bright to be due to free-free emission and is likely to be a
less well determined.                                                            4125 ˚ and f1 the flux density near 3800 ˚ (March˜ et al. 1996; Collinge
                                                                                       A                                       A       a                          supernova, with the “flat” radio spectrum being an artifact of the different
    The spectra were classified by eye into four broad categories: 148 galaxies   et al. 2005; Plotkin et al. 2008, in preparation). Objects with C > 0.4 are      dates on which the radio observations were made.
(most of these have emission spectra typical of AGNs and absorption spectra      classified as galaxies, those with C < 0.4 as BL Lac objects. Some of these
of red elliptical galaxies; though there are some quiescent ellipticals and      latter have detectable emission from the galaxy and therefore a measured             Acknowledgments Funding for SDSS-I and SDSS-II has been provided
some starbursting/interacting galaxies such as Arp 220); 700 quasars; 97         redshift. We find 33 of these BG objects.                                         by the Alfred P. Sloan Foundation, the Participating Institutions, the Na-
BL Lac objects; and 33 objects with composite galaxy/BL Lac spectra.                 Example spectra are shown in Figure 1. The galaxy has a redshift z =         tional Science Foundation, the US Department of Energy, the National Aero-
These last, like the BL Lac objects themselves, are all point sources in the     0.106; the BG object z = 0.136; and the quasar z = 0.157. The horizontal         nautics and Space Administration, the Japanese Monbukagakusho, the Max
SDSS imaging. Thus, most of the flat-spectrum radio sources are quasars           line represents fλ = 0 for the galaxy and BG object; the spectra of the quasar   Planck Society, and the Higher Education Funding Council of the United
(cf. Caccianiga et al. 2002 MNRAS 329, 877; Kimball & Ivezi´ 2008, in            and BL Lac object have been scaled and lowered for clarity. Note that as         Kingdom. The SDSS web site is This research is
preparation).                                                                    well as strong continuum and starlight components, the BG object also has        supported by NASA via grant 1264472 to Princeton University.
                                                                                 AGN emission. Note also that the BL Lac object shows no intergalactic
                                                                                 absorption features. The lack of Lyman-α forest emission means that this
                                                                                 object’s redshift is likely to be less than 2.2.

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