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Identification of Active Galaxies Behind the Coma Cluster of Galaxies C Ledoux, D

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Identification of Active Galaxies Behind the Coma Cluster of Galaxies C Ledoux, D Identification of active galaxies behind the Coma cluster of galaxies C Ledoux, D. Valls-Gabaud, H. Reboul, D. Engels, P. Petitjean, O. Moreau To cite this version: C Ledoux, D. Valls-Gabaud, H. Reboul, D. Engels, P. Petitjean, et al.. Identification of active galaxies behind the Coma cluster of galaxies. Astronomy and Astrophysics Supplement Series, EDP Sciences, 1999, 138, pp.109-117. 10.1051/aas:1999266. hal-02160086 HAL Id: hal-02160086 https://hal.archives-ouvertes.fr/hal-02160086 Submitted on 19 Jun 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. ASTRONOMY & ASTROPHYSICS JULY 1999, PAGE 109 SUPPLEMENT SERIES Astron. Astrophys. Suppl. Ser. 138, 109–117 (1999) Identification of active galaxies behind the Coma cluster of galaxies? C. Ledoux1, D. Valls–Gabaud1,2,H.Reboul3,D.Engels4, P. Petitjean5,6, and O. Moreau7,8 1 UMR 7550 CNRS, Observatoire Astronomique de Strasbourg, 11 rue de l’Universit´e, F–67000 Strasbourg, France 2 Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK 3 GRAAL, Universit´e Montpellier II, F–34095 Montpellier Cedex 5, France 4 Hamburger Sternwarte, Gojenbergsweg 112, D–21029 Hamburg, Germany 5 Institut d’Astrophysique de Paris - CNRS, 98bis Boulevard Arago, F–75014 Paris, France 6 DAEC, Observatoire de Paris-Meudon, F–92195 Meudon Principal Cedex, France 7 Laboratoire d’Astronomie de l’Universit´e Lille I, Impasse de l’Observatoire, F–59000 Lille, France 8 Centre d’Analyse des Images, Observatoire de Paris, 61 avenue de l’Observatoire, F–75014 Paris, France Received January 12; accepted May 21, 1999 Abstract. We describe the results of an efficient survey When the foreground is a cluster of galaxies, the quasars to identify bright B ∼< 18 active galactic nuclei (AGN) behind the cluster can be used to map not only the warm within ≈ 4◦ (4.7 h−1 Mpc projected radius) of the cen- gas content of the intra-cluster medium (via absorption tre of the Coma cluster of galaxies. The candidates have lines), but also its dark matter distribution (via micro- been carefully selected either as unresolved UV-excess ob- lensing). jects with no detected proper motion, or from identifi- The discovery of a diffuse extreme UV-excess in cations on objective-prism plates including a few ROSAT nearbyclustersbytheExtreme Ultraviolet Explorer sources. A global success rate larger than 30% is achieved. (EUVE) satellite (Lieu et al. 1996a,b) was initially Low-resolution spectra are presented for the extragalactic interpreted as evidence for a warm gas component at objects identified in the course of the survey: the sample about 500 000 K in the intra-cluster medium. This contains 16 emission-line objects, a BL Lac candidate and would constitute a major mass component and raises an elliptical galaxy. The AGNs of this list are unique tar- enormous problems (Fabian 1996). A non-thermal ex- gets to study the association of the Lyman-α forest with planation, based on inverse Compton scattering, seems large-scale structures and galactic haloes at low redshift, more plausible and consistent with the hard X-ray to search for warm gas within the Coma cluster and nearby tail observed by BeppoSAX (Ensslin & Biermann 1998; filaments, and for optical monitoring to detect the micro- Sarazin & Lieu 1998). However the distorted shape of lensing variability produced by the baryonic dark matter the EUV isophotes may be more consistent with a shock in the cluster. heated medium, so the situation remains very confused. Although attempts have been made to detect the warm Key words: quasars: general — quasars: absorption gas through UV emission lines (Dixon et al. 1996), lines — galaxies: Seyfert — galaxies: starburst — absorption lines are far more sensitive to the presence of techniques: spectroscopic — surveys this component. Hence, Hubble Space Telescope (HST) spectroscopy of background QSOs should reveal absorp- tion lines at the redshift of the cluster if the thermal interpretation is correct. 1. Introduction In addition, background QSOs can be used as probes of intervening gaseous clouds and of their evolution along Bright quasars (QSOs) can be used as background sources the line of sight. Adjacent lines of sight provided by close to probe the physical properties of foreground objects. pairs of QSOs can also constrain the size of the absorbers, Send offprint requests to:C.Ledoux their spatial distribution and their connection with galax- ([email protected]) ies. In this context, targeting quasars behind low-redshift ? Based on observations carried out at the 1.93 m telescope clusters is a very efficient way to reveal the connec- of the Observatoire de Haute-Provence (CNRS), France. tion of the intergalactic medium on any scale with these 110 C. Ledoux et al.: Identification of active galaxies behind the Coma cluster concentrations of galaxies. Coma (Abell 1656, at a red- down to a limiting magnitude of about B<18. We shift of <zComa > =0.0229) is ideal in this respect describe the selection methods in Sect. 2 and present the since it is one of the best studied areas of the sky as far observations in Sect. 3. In Sects. 4 and 5, we discuss the as redshift coverage and galaxy properties are concerned characteristics of the extragalactic objects identified in (Biviano 1998). this way, and conclude in Sect. 6 on the global efficiency Quasar pairs with projected separations of no more of the selection. than a few arcminutes yield interesting constraints on Throughout this paper, we assume a standard the size, physical structure and kinematics of galac- Einstein-De Sitter cosmological model and a Hubble −1 −1 tic haloes, clusters and filaments (Smette et al. 1995; constant H0 = 100 h km s Mpc . Petitjean et al. 1998; D’Odorico et al. 1998). The ideal project however would be to observe a large number of QSOs in a small solid angle on the sky to probe 2. Target selection all scales at the same time. Since absorption is ex- tremely sensitive to the total amount of warm gas, AGN candidates were selected with the criterion < this would probe even small column densities inside B ∼ 18 from 3 different sources: [1] the OMHR sur- the cluster and also reveal, in the outskirts of the vey of UV-excess objects at the North Galactic pole cluster, the connection with the large-scale filamen- (Moreau & Reboul 1995), [2] the Hamburg Quasar tary network (Cen et al. 1994; Petitjean et al. 1995; Survey (HQS) by Hagen et al. (1995) and [3] the ROSAT Hernquist et al. 1996; Miralda-Escud´e et al. 1996). All-Sky Survey (RASS) using identifications on HQS Besides the mapping of the warm gas in the cluster plates (Bade et al. 1998). and in the neighbouring environment, a third goal of We also selected the object US 370 (Usher 1981), a QSO survey behind clusters is the mapping of their which was classified as a QSO candidate with V =18and baryonic dark matter content. This can be achieved U−V =−0.55 by Berger et al. (1991). The proper motion by monitoring the background QSOs searching for the of this target could not be measured due to the proximity expected micro-lensing signature in their light curves. of a bright star as mentioned by Moreau & Reboul (1995). The micro-lensing optical depth produced by a cluster A slit-less observation was performed by Weedman (1985) is very large, a few 10−3, and for lens masses ranging and we provide here a spectrum of US 370 to confirm its −5 −3 nature and measure its redshift properly. from 10 to 10 M the events will have typical time scales of days (Walker & Ireland 1995; Tadros et al. 1998; Wu & Xue 1998). A program to monitor a few tens of 2.1. OMHR candidates QSOs behind Coma for instance, for 5 months with a daily sampling rate, should reveal dozens of micro-lensing Berger et al. (1991) performed a photometric (U, B, V ) events and produce a 2-D mapping of the baryonic dark and astrometric analysis of 1221 UV-excess unresolved ob- matter within the cluster. jects in a 40.5ut field almost centred on the selected area There are however very few quasars known to date SA 57: behind low-redshift clusters and clearly additional back- 12h52m08.6s <α < 13h21m39.2s ground quasars, and more generally galaxies with an ac- J2000 ◦ 0 00 ◦ 0 00 tive galactic nucleus, are badly needed for all these studies. 26 01 12 <δJ2000 < 32 14 39 . These sources have to be bright enough to be accessible They used the MAMA (Machine Automatiquea ` Mesurer for medium-resolution spectroscopy with the HST and for pour l’Astronomie) machine to scan three plates taken in optical monitoring with small/medium-size ground-based 1962 at the Palomar 48-inch Schmidt. This field was fully telescopes. reduced in U, B and V by Moreau & Reboul (1995) with a Until now optical searches for quasars have only been visual limiting magnitude 20.3. Amongst the ∼ 130 000 de- performed systematically behind Virgo (He et al. 1984; tected objects, the authors extracted a list of 1759 quasar Impey & He 1986), where 29 QSOs were found out of candidates selected mainly as UV-excess unresolved ob- 82 candidates observed at the Palomar 5-meter tele- jects.
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