The Second Version of the OCARS Catalog of Optical Characteristics of Astrometric Radio Sources

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1 ISSN , Astronomy Reports, 2016, Vol. 60, No. 11, pp c Pleiades Publishing, Ltd., Original Russian Text c Z.M. Malkin, 2016, published in Astronomicheskii Zhurnal, 2016, Vol. 93, No. 11, pp The Second Version of the OCARS Catalog of Optical Characteristics of Astrometric Radio Sources Z. M. Malkin * Central (Pulkovo) Astronomical Observatory, Russian Academy of Sciences, St. Petersburg, Russia St. Petersburg State University, St. Petersburg, Russia Kazan Federal University, Kazan, Russia Received March 23, 2016; in final form, May 5, 2016 Abstract A new version of the Optical Characteristics of Astrometric Radio Sources (OCARS) catalog is presented. The catalog includes a list of radio sources observed in astrometric and geodetic VLBI programs since 1979, their redshifts, photometric data in 13 bands in the visible and near infrared, and a table indicating identifications btween the OCARS objects and objects in other catalogs. The main sources of information for the OCARS catalog are the NED and SIMBAD databases, as well as a variety of publications. Targeted observing programs designed to supplement the optical data for the astrometric radio sources have also been organized. The catalog currently contains 9956 sources, of which 5449 have redshifts and 7473 have photometric data. The catalog is updated, on average, once every several weeks, and is continuously augmented with new sources and new optical data. DOI: /S INTRODUCTION The Optical Characteristics of Astrometric Radio Sources (OCARS) catalog contains the optical characteristics of astrometric and geodetic radio sources that have been observed in astrometric and geodetic VLBI programs since When available, the type of object, redshift (z), and magnitudes in the visual and near infrared are included. Since the publication of the first version of the OCARS catalog in December 2007 [1], the catalog has been continuously developed by increasing the number of sources, adding new data, and refining existing data. The current state of the OCARS catalog is described in the current paper. The first version of the OCARS catalog was created as a source of supplemental data for the second release of the International Celestial Reference Frame (ICRF2, [2]), for which work was completed in A more detailed description of the first version of the catalog is given in [3]. The optical characteristics were first taken predominantly from the NED 1 and SIMBAD 2 databases. NED contains detailed data for extragalactic sources, including original measurements and accompanying commentary. One disadvantage of NED is the delay of two or more years in incorporating * malkin@gao.spb.ru data from new literature sources. SIMBAD contains fewer extragalactic sources with less detailed data than NED, but the data in in the database are updated more regularly. It should be noted that only limited set of journals is used in the NED and SIMBAD databases. To encompass all possible sources of information and shorten the time scale for supplementing the OCARS catalog with new data appearing since 2011, all available journals, the arxiv online publication site, and other catalogs are checked regularly for new material. The most important of these catalogs is the Sloan Digital Sky Survey (SDSS) 3 [4], from which many redshifts and photometric data are available. Much useful information was also obtained in early stages of work on the OCARS catalog from the AT20G [5], RM-Redshift [6], DEEP2 [7], MASIV [8], LQAC [9], and Milliquas [10] catalogs. Although most of these data eventually appear in NED and SIMBAD, working directly with original sources as they are published makes it possible to substantially enhance regular updating of the OCARS catalog and enables refinement of cross-identifications of objects in different catalogs. In addition to making use of available data from the literature, targeted observing programs were also initiated. The first observations in a program aimed

2 OCARS CATALOG 997 at determining redshifts of astrometric radio sources were organized at the Pulkovo Observatory [11]. Time for several series of observations on the 6-m telescope of the Special Astrophysical Observatory (SAO) of the Russian Academy of Sciences was granted in For various reasons, primarily unfavorable weather conditions, these observations have led to new redshift determinations for only about ten objects [11]. Observations on various non- Russian telescopes (the NTT in Chile, Gemini in Hawaii and Chile, and NOT in the Canary Islands) organized by Titov et al. [12, 13] proved much more effective, leading to the determination of redshifts for 270 OCARS radio sources. It is especially valuable that a large number of the observed objects were located in the southern hemisphere, where the fraction of radio sources with known optical charateristics is much lower than in the northern hemisphere (see Table 1). In addition to spectral observations aimed at determining redshifts, the Pulkovo Observatory and Paris Observatory jointly began a program of photometric observations of several hundred OCARS objects in 2015, for which no reliable optical brightness measurements were available (this project is led by F. Taris of the Paris Observatory). The main goal in establishing the OCARS catalog was to determine redshifts and correct erroneous redshifts of astrometric radio sources that were included in supplementary data for the ICRF catalog, since these data were of considerable interest for various scientific studies [3]. Information about magnitudes was initially considered to be of secondary interest for providing more complete information and planning spectral observations. Work on a new version of the ICRF ICRF3 was begun in 2012 [14]. It is planned to compare this catalog of radio-source coordinates with highprecision optical coordinates from the Gaia Celestial Reference Frame (GCRF) catalog obtained through the Gaia mission [15]. To enhanced the quality of this comparison, it is necessary to have as many of the ICRF3 sources as possible included in the Gaia observational program. With this aim, radio sources that were sufficiently bright in the optical were selected for observations with Gaia [16] together with intense VLBI observations to refine their coordinates in the ICRF. Planning of these observations requires knowledge of the optical brightnesses of the radio sources, which provide key information for this project. The larger the number of radio sources with reliable optical brightnesses, the larger the number of common objects for establishing a tie between the radio and optical reference frames. Therefore, the need to obtain full information about the visual or infrared magnitudes of OCARS objects became a topical task. Whereas the first version of the OCARs catalog included only one magnitude in the optical or infrared, the second version includes magnitudes in the 13 SIMBAD bands. These detailed photometric data are presented in a separate file that has been included in the second version of the OCARS catalog since August Another useful function of the OCARS catalog is establishing correspondences between objects in various astrometric and astrophysical catalogs, which are not always obvious and unambiguous. To address this task, a third file with a table of cross-identifications of objects in various catalogs and databases was added to the OCARS catalog in August The current version of this file includes identifications for sources in OCARS, ICRF2, and LQAC [9], as well as the International VLBI Service for Geodesy and Astrometry (IVS) [17]. The second version of the OCARS catalog was officially announced at an all-russian astrometric conference in Pulkovo in 2015 [18]. In this current paper, we present a description of the current status of the catalog. 2. GENERAL DESCRIPTION OF THE OCARS CATALOG Unlike the first version, supported in , the second version of the OCARS catalog includes three files: ocars.txt is the main file containing the source coordinates, source types, redshifts, and approximate magnitudes, togther with commentary; this file corresponds to the first version of the OCARS catalog; ocars_m.txt contains photometric data in the 13 uu BgV rriizjhk bands; ocars_n.txt contains a table of corresponding source names in various catalogs; currently, only cross-identifications with IVS programs 4 and the LQAC catalog [9] are included; The list of objects included in the OCARS catalog is formed from various astrometric and geodetic VLBI programs and catalogs in the following order: sources in the ICRF2 [2]; other sources observed in the framework of IVS programs; sources from the NASA Goddard VLBI group catalog 5 ; sources from the RFC catalog, 6 which is the most complete astrometric catalog of radio sources, is updated each quarter, and contributed more than half

3 998 MALKIN Table 1. Statistics of the OCARS Catalog (all sources) Number of sources (30.4%) (52.8%) (16.8%) With known types 5783 (58.1%) AGNs 4292 (74.2%) Quasars 2933 (68.3%) BL Lacs 688 (16.0%) Seyferts 400 (9.3%) Radio galaxies 1491 (25.8%) With redshifts (z) 5449 (54.7%) (32.0%) (55.6%) (12.4%) Unreliable 719 (13.2%) With photometry 7473 (75.1%) (30.5%) (53.6%) (15.9%) With z and photometry 5410 (54.3%) (32.0%) (55.7%) (12.3%) the OCARS objects; the latest version of OCARS used the RFC-2016a catalog based on observations obtained in as part of IVS and other radio astrometric programs [19 31]; sources from the literature. The approximate magnitudes listed in the file ocars.txt are determined as follows. If photometric information for a source is available in the file ocars_m.txt, the magnitude is taken from that file, with the order of preference being VRrgBiIzuUJHK. Otherwise (about a hundred sources), we used a tentative magnitude from the NED database without identifying the band to which it corresponds. Experience shows that these NED tentative magnitudes usually correspond to the B or Vbands. Table 1 presents overall statistics for the OCARS catalog, including the distribution of the objects over three 60 declination zones: northern, equatorial, and southern. For comparison, for a uniform distribution over the celestial sphere, 50% of the sources should lie in the equatorial zone, and 25% in each of the northern and southern zones. The statistics in Table 1 clearly show that the distributions of both the radio sources and the subsets of sources with entries for various optical characteristics are not uniform, with adeficit of sources and optical data in the southern hemisphere. The distribution of the OCARS sources over the celestial sphere presented in Fig. 1 also shows a deficit of sources near the Galactic equator. A variety of redshifts in Table 1 can be considered unreliable; these are photometric redshifts noted as unreliable in NED and in the literature, cases when there are appreciably different redshifts for the same object in the literature, and redshifts given as lower or upper limits. For objects with ambiguous redshifts z, comments are provided in the OCARS catalog about the various values found in the literature, to enable users to apply their own judgement about the reliability of individual redshift measurements for a given source. Only slightly more than half of the sources have both photometric and redshift data. Only 12% of sources in the first version of the catalog published in 2007 had such data, since the inclusion of photometric data was not assigned sufficient importance at that time, as was noted above. However, already in 2009, the catalog included photometric and redshift data for 42.0% of the sources. The expanded photometric data that have been added to the second version of the OCARS catalog are described in detail in the following section. Table 2 presents statistics for the OCARS sources that are included in the ICRF2 [2]. We can see that the OCARS data for these objects are more complete in terms of the fraction of the sources for which there is information about their optical characteristics. The redshift and magnitude distributions of the OCARS objects are presented in Figs. 2 and 3. These figures show that the mode of the magnitude distribution of the radio sources is 19 m. The optically weakest OCARS sources have magnitudes of about 25 m, but we must bear in mind that this refers only to optically identified sources with measured optical brightnesses. Figure 4 shows a plot of redshift versus magnitude. Sources with redshifts z< 0.3display a correlation between these quantities, while there is no obvious relationship between redshift and magnitude for more distant objects. The growth with time of the total number of sources in the OSCARS catalog and the number of sources with known optical characteristics is illustrated in Fig. 5.

4 OCARS CATALOG (a) 45 Declination, deg Right ascension, h (b) 45 Declination, deg Right ascension, h Fig. 1. Distribution over the celestial sphere of OCARS sources for which photometric data (a) and redshifts (b) are available. The curve shows the Galactic equator. The coordinates of the OCARS objects were obtained from various sources, in the following order: the catalog of the NASA Goddard VLBI group ( 41% of sources); the RFC catalog ( 58% of sources); other data (0.6% of sources). The OCARS catalog is updated, on average, every several weeks; in all, 84 updates have been carried out sincd The history of these updates is shown in Fig PHOTOMETRIC DATA Photometric data in the OCARS catalog (the file ocars_m.txt) are presented for the 13 bands included in SIMBAD, uu BgV rriizjhk. Data for

5 1000 MALKIN Number of sources Redshift Fig. 2. Redshift distribution for the OCARS sources. Number of sources Magnitude Fig. 3. Magnitude distribution for the OCARS sources. close bands, such as uu, rr, and ii, are not merged, in order to preserve fuller information from the input sources, enable better control of the information included, and make it easier to identify potentially unreliable data. Only the very small number of measurements in the Z band have been merged with those in the z band. Statistics for the OCARS photometric data are

6 OCARS CATALOG (a) Magnitude z 25 (b) Magnitude z Fig. 4. Optical magnitude versus redshift z on linear (a) and logarithmic (b) scales. presented in Table 3. The first part of this table presents the number of sources with photometric data in each band. The largest number of sources have optical brightness measurements in the R band. This band is also close to the central photometric G band of the Gaia observatory, which facilitates the selection of OCARS sources to be used for the ICRF and GCRF frame tie. The second part of the table presents color indices (a given band minus R) both for all sources and separately for objects of various types. These data show that the color indices are nearly the same for all types of Active Galactic Nuclei (AGNs) and galaxies. Note that 47 OCARS objects have photometric data in only one band, 892 objects have photometric data in two bands, and only 31 objects have photometric data in all 13 bands. Photometric data only in the infrared (JHK) are available for 215 sources. The derived color indices can be used to reduce the infrared magnitudes to the R band, so that such sources can also be used in the selection of suitable objects for the radio optical frame tie.

7 1002 MALKIN Number of sources Total number of sources Sources with magnitude info Sources with redshift info Year Fig. 5. Growth in the number of OCARS sources with time Year Fig. 6. History of updating the OCARS catalog. Some comments are necessary to convey a better understanding of the photometric data in the OCARS catalog. The radiation of the extragalactic astrometric radio sources that form the basis for modern celestial reference frames is generated primarily by AGNs, and is usually characterized by appreciable variability in various wavebands. Monitoring of the optical brightness of astrometric extragalactic objects is being carried out by Taris et al. [32, 33]. The results of this monitoring show that the optical brightnesses of astrometric radio sources usually vary within several tenths of a magnitude, although the spread of the brightness variations fairly often exceeds a magnitude, and can reach up to three magnitudes, as in the case of the AGN Of course, this last case is not typical. Analysis of literature data for the OCARS sources in the NED database also shows that photometric brightness measurements for a given object within a given band can differ by a magnitude or more. Therefore, the optical magnitudes of the OCARS objects have a statistical character to some extent. A more complete description of the optical brightnesses would include the observational interval for magnitude measurements, but such information is available for only a small number of objects. Therefore, such an expansion of the OCARS catalog is currently only planned for the future. The current version of the catalog presents the mean magnitude when several different measured values within a single frequency band are present in the literature. Another source of uncertainty in the magnitudes is ambiguity in the SDSS data [4], from which the ugriz magnitudes for a large number of OCARS objects were taken. The SDSS magnitudes were calculated using several different models. 7 The differences between values obtained for the different models can reach a magnitude or more. The smallest scatter between models is observed for objects that are close to point sources, and the largest scatter for extended 7

8 OCARS CATALOG 1003 Table 2. Statistics of the OCARS catalog (ICRF2 sources) Number of sources (30.8%) (57.7%) (11.5%) With known types 2489 (72.9%) AGNs 2251 (90.4%) Quasars 1701 (75.6%) BL Lacs 276 (12.3%) Seyferts 180 ( 8.0%) Radio galaxies 238 (25.8%) With redshifts (z) 2401 (70.3%) (31.3%) (58.0%) (10.7%) Unreliable 258 (10.7%) With photometry 3049 (89.3%) (30.6%) (57.5%) (11.9%) With z and photometry 2388 (69.9%) (31.2%) (58.1%) (10.7%) objects, usually galaxies. In accordance with the recommendations of the authors of the SDSS, results obtained for the composite model cmodel are used in the OCARS catalog. Note that bright stars are present in the sky nearby several of the extragalactic radio sources, which could be confused with the OCARS objects in optical observations. Some such coincidences were first noted by O. Titov (private communication). All known cases are described in comments for the corresponding OCARS objects. 4. CONCLUSION Knowledge of the physical characteristics of as large a number of radio sources as possible can play an important role in addressing many astrophysical questions. The OCARS catalog supplements astrometric catalogs of radio sources with optical data about object types, redshifts, and available photometry. The bulk of this information for the OCARS catalog is taken from the NED and SIMBAD large astronomical databases, as well as from the literature. An appreciable fraction of the catalog data was also obtained through specially organized optical observations of astrometric radio sources. One of the main principles of work on the OCARS catalog is the need to update its contents as soon as possible after the appearance of new observational data. On average, this updating is carried out every several weeks. It is important that high-accuracy positional observations of radio sources with known redshifts using VLBI can be used to construct a threedimensional celestial reference frame as an expansion to the modern ICRF. The second version of the OCARS catalog includes a separate file of photometric data in 13 bands in the visible and near infrared. Reliable data on the optical brightnesses of astrometric radio sources are not only required for the overall completeness of the ICRF catalog, they can also be used to address a variety of important tasks, such as the selection of optically bright objects for radio optical frame ties, cross-identification of objects in different catalogs, and separation of nearby objects in the same fields as the radio sources. This last task is related to the fact that several bright optical objects are fairly often observed in the vicinity of a radio source, and reliable identification of a particular one of these sources with the radio source can prove a difficult problem. It stands to reason that the OCARS catalog does not aspire to replace large master catalogs of extragalactic objets, such as LQAC [9] or Milliquas [10], which are essential for many studies. For example, one of the most often used sources of data for astronomical studies is the LQAC-3 catalog, which contains information about quasars. The OCARS catalog has a much narrower sphere of application than such comprehensive databases. However, OCARS also has important advantages, the most important being its use of a broad range of information sources, more active control of content by hand, and continuous support and updating. For example, the OCARS catalog contains 1119 astrometric radio sources with photometric information, and 467 sources with redshifts, that are missing from the LQAC-3. As a result, the OCARS catalog is the most complete and accurate catalog of optical data for the astrometric radio sources it contains. The OCARS catalog is accessible online. 8 Information about updates can be provided to interested users by

9 1004 MALKIN Table 3. Statistics for photometric data and color indices for the OCARS catalog Band (F ) u U B g V r R i I z J H K N Color index (F R): All Quasars BL Lacs Seyferts Galaxies It is proposed to further develop the OCARS catalog in the following directions: inclusion of new radio sources as they are observed in astrometric VLBI programs; inclusion of new optical data as they appear in the literature or through targeted observations; at the suggestion of V. Makarov (private communication), we are also considering expanding the photometric data to encompass the bands in the WISE catalog; expansion of the table of cross-identifications of OCARS objects with objects in the WISE, 2MASS, FGL, and other catalogs. We should note here difficulties in cross-identifying OCARS objects with objects in very large catalogs, due to the frequent presence in the latter of two or more objects that are coincident with the OCARS source within the positional uncertainties, leading to an increased probability of false identifications. In conclusion, we note that, in spite of all measures taken to verify the data it contains, the OCARS catalog is not free of errors, due first and foremost to possible incompleteness and inaccuracy of data taken from the literature and other astronomical catalogs and databases that serve as sources of information for the OCARS catalog. Possible errors in crossidentification of objects in the radio and optical are another source of uncertainty. Therefore, comments from users play a very important role in correcting such errors and helping to further develop the catalog. ACKNOWLEDGMENTS I would like to thank David Gordon (NASA Goddard Space Flight Center), Bryan Gaensler and Elizabeth Mahony (University of Sydney), Tapio Pursimo (Nordic Optical Telescope), Michael Shaw (Stanford University), and Oleg Titov (Geoscience Australia), and Vivienne Wild (Institute of Astrophysics, Paris) for making their data available for this study, including some unpublished data. I would also like to thank Dimitrii Blinov (St. Petersburg State University), Anne-Marie Gontier (Paris Observatory), Yurii Kovalev (Astro Space Center, Lebedev Physical Institute), Valerii Makarov (U.S. Naval Observatory), Marion Schmitz (NED team, IPAC, Caltech/JPL), Jean Souchay (Paris Observatory), and Oleg Titov (Geoscience Australia) for their help and useful discussions. This research has made extensive use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, the SIMBAD database, operated at CDS, Strasbourg, France [34], the CfA Arizona Space Telescope LEns Survey 9 (CASTLES), and data from the SDSS. This work was partly supported by the Russian Government Program of Competitive Growth of Kazan Federal University. REFERENCES 1. Z. Malkin and O. Titov, in Measuring the Future, Ed. by A. Finkelstein and D. Behrend (2008), p A. L. Fey, D. Gordon, C. S. Jacobs, C. Ma, R. A. Gaume, E. F. Arias, G. Bianco, D. A. Boboltz, S. Böckmann, S. L. Bolotin, P. Charlot, A. Collioud, G. Engelhardt, J. M. Gipson, A.-M. Gontier, et al., Astron. J. 150, 58 (2015). 3. O. Titov and Z. Malkin, Astron. Astrophys. 506, 1477 (2009). 4. S. Alam, F. D. Albareti, C. Allende Prieto, F. Anders,S.F.Anderson,T.Anderton,B.H.Andrews, E. Armengaud, E. Aubourg, S. Bailey, S. Basu, J. E. Bautista, R. L. Beaton, T. C. Beers, C. F. Bender, et al., Astrophys. J. Suppl. 219, id. 12 (2015). 5. E. K. Mahony, E. M. Sadler, S. M. Croom, R. D. Ekers, K. W. Bannister, R. Chhetri, P. J. Hancock, H. M. Johnston, M. Massardi, and T. Murphy, Mon. Not. R. Astron. Soc. 417, 2651 (2011). 9

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