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 5705. 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 A 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 c (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 http://www.sdss.org/. 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.