A sample of galaxy clusters to study the fundamental plane: Redshift measurements

Astronomy and Astrophysics Supplement Series, Jul 2018

We present the results of spectroscopic observations of galaxies in a sample of nearby clusters. We measured galaxy redshifts and estimated cluster velocity dispersions. These observations were carried out at the Observatoire de Haute Provence as part of a program aiming at a more detailed study of the Cluster Fundamental Plane.

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A sample of galaxy clusters to study the fundamental plane: Redshift measurements

Astron. Astrophys. Suppl. Ser. A sample of galaxy clusters to study the fundamental plane: Redshift measurements? S. Maurogordato 0 D. Proust 0 A. Cappi 0 E. Slezak 0 J.M. Martin 0 Observatoire de Paris-Meudon 0 place J. Janssen 0 Meudon 0 France e-mail: 0 Osservatorio Astronomico di Bologna 0 via Zamboni 0 Bologna 0 Italy e-mail: cappi@astbo 0 .bo.astro.it 0 Observatoire de Nice 0 Le Mont-Gros 0 Nice Cedex 0 France e-mail: 0 ARPEGES 0 Observatoire de Paris-Meudon 0 place J. Janssen 0 Meudon 0 France e-mail: 0 0 Send o print requests to: S. Maurogordato We present the results of spectroscopic obser- and luminosity (West et al. 1989). However these relavations of galaxies in a sample of nearby clusters. We tions have a high dispersion. Schae er, et al. (1993, heremeasured galaxy redshifts and estimated cluster velocity after SMCB) analysed a sample of 19 clusters for which dispersions. These observations were carried out at the luminosities, radii (West et al. 1989) and velocity disObservatoire de Haute Provence as part of a program aim- persions (Struble & Rood 1991) were available. In ading at a more detailed study of the Cluster Fundamental dition to the radius{luminosity relation found by West Plane. et al. (1989), R / L0:5 0:1, SMCB have also de ned a luminosity{velocity dispersion relation, L / 2:0 0:4 - Targets were selected in order to cover a large spread of masses (richness class 1 to 4) and redshifts (0.04 to 0.25): they constitute a sample of 16 Abell/ACO clusters. Su ciently nearby clusters, i.e. A195, A1035, A1045, A1126, A1413, A1468, A1781, A1831, A2034, A2245, A2312, A2457 have been observed at the Observatoire de Haute-Provence, and the results are presented in this paper. The farthest ones, A222, A223, A520, A521, have been observed using the capabilities for multi{object spectroscopy at CFHT and ESO and will be analysed in a separate paper. The sample was a priori composed of relatively regular clusters, in order to derive \unambiguous" velocity dispersions. Nevertheless, in a few cases the velocity distribution (cf. Sect. 4) has shown evidence of subclustering, projection e ects or bimodality, which obviously a ect the estimate of the velocity dispersion. For this reason the three clusters A1035, A1781, A1831 with a complex velocity distribution will not be included in future in our analysis. In addition to the spectroscopic data, we have also obtained photometric measurements based on automatic scans of POSSII plates, which will be presented in a forthcoming paper. 3. Observations and data reduction A catalogue of galaxies up to magnitude mR = 18:5 has been built applying the algorithms described in Slezak et al. (1988) to automatic scans of POSSII plates, thanks to the MAMA facilities; this catalogue included the coordinates of galaxies within a radius of one degree from each cluster centre. Selected galaxies have been observed at the 193 cm telescope of the Observatoire de HauteProvence, using the Carelec spectrograph in long slit mode (Lemaitre et al. 1990) . The choice of the grating represented a compromise between good resolution and reasonable exposure time, and was also constrained by the wavelength range suited for line identi cation at low{redshift ( 3600 − 7300 A). The dispersion was 260 A/mm, corresponding to 7 A/pixel with the CCD TK512 (512 512 pixels of 27 m), and with the slit aperture of 100 the spectral resolution was 8 A. The following results are based on four observing runs (7 nights each one) in April and September 1993, and in April and May 1994. Unfortunately more than half of the nights were lost for bad weather conditions. The exposure time was 45 minutes; two exposures were taken for faint objects. Wavelength calibrations were done using an He lamp and an Ar lamp before each exposure. Standard stars were observed each night (HD 102494, HD 171232, HD 112299, HD 140913). We obtained the spectra of about 8/10 objects per night. Usually we obtained one spectrum per exposure, but in some cases, for instance when sampling the core of rich clusters, we rotated the spectrograph in order to get simultaneously the spectra of a few objects. At the end, we have collected 93 spectra with a su cient signal-to-noise ratio to allow a good redshift determination. Data reduction was carried out with IRAF, using the MULTIRED package. MULTIRED is an integrated multi{slit spectra reduction task (Le Fevre et al. 1995) which allows to process raw 2D spectra CCD frames (with one or several spectra) into bias/flat/sky corrected 2D spectra and wavelength calibrated 1D spectra. Radial velocities have been determined using the cross{correlation technique (Tonry & Davis 1979) implemented in the RVSAO package (developed at the Smithsonian Astrophysical Observatory). Finding charts for each cluster are displayed in Fig. 1. They cover 150 150 square regions extracted from the Space Telescope Science Institute Digital Sky Survey1 around the position of the cluster in the Abell/ACO catalogue (300 300 for A1413, A1781 and A2457). An identifying number is displayed to the right of each target. Velocity measurements are listed in Table 1. The columns are as follows: Column (1): Abell cluster number (Abell et al. 1989) ; Column (2): identi cation number of each target galaxy in the cluster as shown in the nding charts; Columns (3) and (4): right ascension and declination (J2000.0) of the target galaxy; Column (5): best estimate of the radial velocity resulting from the cross{correlation technique; Column (6): estimated error; Column (7): other published measurement of the radial velocity. The subscript e indicates that the measurement refers to the emission line velocity. 4. Velocity analysis of the clusters The number of measured redshifts per cluster is often quite small (from 5 to 20 galaxies). It is therefore di cult to have reliable estimates of the velocity dispersion. Using the NED database2 we searched for other galaxies with measured redshifts within a radius of one degree from the center of each target cluster. This allowed us to add one more velocity to A1035 (Batuski et al. 1991) , three to A1413 (Stocke et al. 1991; Allen et al. 1992) , one to A1045 (Allen et al. 1992), sixteen to A1831 (NED 1992 and Owen et al. 1995) , two to A2034 (Crawford et al. 1995) , four to A195 (Giovanelli & Haynes 1993) , four to A2245 (from Rhee & Katgert (1988) and NED 1992), six to A 2457 1 Based on photographic data of the National Geographic Society { Palomar Observatory Sky Survey (NGS-POSS) obtained using the Oschin Telescope Palomar Mountain. The NGS-POSS was funded by a grant from the National Geographic Society to the California Institute of Technology. The plates were processed into the present compressed digital form with their permission. The Digitized Sky Survey was produced at the Space Telescope Science Institute under US Government grant NAG W-2166. Copyright c 1994, Association of Universities for Research in Astronomy, Inc. All rights reserved. 2 The NASA/IPAC Extragalactic Database (NED) is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. HEL. VEL. V ( km s−1) 12464 12369 12318 ERROR Other measurements V ( km s−1) V ( km s−1) 37 34 52 HEL. VEL. V ( km s−1) ERROR Other measurements V ( km s−1) V ( km s−1) * emission lines [OII], H , [OIII], H , [SI]. (Quintana & Ramirez 1995; Hewett et al. 1995; Watson median vM and the bi{weight vBI as location estimators. et al. 1991) . Velocity histograms are shown in Fig. 2. In the small{intermediate case (10 < Nv < 20), we used the median vM and the bi{weight vBI as estimators of the We have applied to our cluster data set the updated location, and the canonical standard deviation SSD, the ROSTAT package (version 1.2) developed by Beers et al. bi{weight SBI, the gapper Sgap. The mean estimator of (1990). We chose the most appropriate estimators accord- the location has also been listed, although it is known ing to Beers et al. (1990) , taking into account the number to be a poor estimator in the case of a non{Gaussian of available velocities Nv. The results are listed in Table distribution. Table 2 lists in Col. (1), the Abell cluster 2. In the \tiny case", when Nv 5, we have used the number, in Col. (2) and (3) the right ascension and declination of the cluster center (Abell, et al. 1989) precessed at J2000.0, in Cols. 4, 5 and 6, bi-weight, median and mean estimates of the redshift of the cluster from our analysis, in Col. 7 the previous measurement of the cluster redshift from literature (referenced in Col. 10) and the number of galaxies used for this estimate, in Cols. 11, 12 and 13, the standard deviation, bi-weight, and gap estimates of the velocity dispersion within the cluster, and in Col. 13 the number of cluster members used for the previous determinations. The velocity analysis is indeed limited by the small number of galaxies with available redshift by cluster. The various estimates of velocity dispersions are listed in Table 2 so that the degree of self-consistency can easily be checked. Determinations for clusters with less that 10 redshift measured are listed between braces. Note on individual clusters Some clusters of our sub-sample show a complex velocity distribution. For instance, it is clear from Fig. 2 that the velocity distribution of A1035 is bimodal, with a group of 9 galaxies clustered around vBI = 20052 km s−1with small dispersion ( 280 km s−1) and a group of 10 galaxies belonging a to a more massive structure around vBI = 23530 km s−1( 880 km s−1). A1781 shows a complex structure that cannot be well sampled with our data. The eld of this cluster was recently examined in detail by Ramella et al. (1995 ; RGHT), as a loose group of galaxies at z 0:04 is found in that region of the sky. This results in a projection of the loose group on the more distant Abell cluster (at z 0:065 from Postman et al. 1992) , of which galaxies 2, 3, 4, and 6 are probably genuine members. In Fig. 3), the chain of four galaxies in the north corresponds to a foreground structure, the redshifts of these galaxies being respectively, from south to north, 11019 km s−1 (RGHT), 13699 km s−1(our galaxy No.1), 11928 km s−1 (RGHT), and 11940 km s−1 (our galaxy No. 7). A1831 shows two peaks in the velocity histogram, at vBI = 18585 km s−1 and vBI = 22907 km s−1, with velocity dispersions of respectively 700 km s−1 and 820 km s−1. As mentioned before, we will not pursue the analysis of the three previous clusters. A1413 shows a main concentration of 9 galaxies at a mean velocity of vBI = 42686 km s−1(including the central cD galaxy) identi ed as the main cluster. The estimate of its velocity dispersion (listed in Table 2) is SSD = 1460 km s−1. This value is quite di erent from the estimate with the bi{weighted method (SBI = 370 km s−1). This can mean that the number of galaxies is still not su cient to use the bi{weighted technique. Previous estimates of the velocity dispersion in A1413 have to be taken with are carrying on further spectroscopical observations with care until more redshift measurements are available. multi{object facilities in order to increase e ciently the Finally, we note the presence of a foreground group of data for each cluster. The information which can be de4 galaxies around v = 30000 km s−1. rived from these data is of course useful for various studies, In A2457, the spectrum of galaxy 7 (Fig. 3) shows from large{scale structure to cluster dynamics. Our main three very broad systems of emission lines. The rst com- aim is to use this database to constrain the parameters plex includes a blend of Hγ and [O III], with a FWHM of de ning the cluster fundamental plane. about 89 A, the second one (with a FWHM of 140 A) Acknowledgements. We would like to thank all the sta of includes H and the [OIII] doublet in the tail of H , the the Observatoire de Haute Provence for their e cient assisthird one (FWHM 115 A) includes a blend of H and tance during the successive observational runs. S.M. would like [N II]. Accurate ts with a gaussian could be done when to thank Veronique Cayatte, Catherine Boisson and Philippe deblending the [OIII] doublet and the H and [N II] com- Prugniel for fruitful discussions. We are grateful to Jean ponents, giving a mean velocity of v = 16962 100 km s−1. Guibert and the MAMA team for their scans of the POSSII The cross{correlation technique for the absorption lines plates. This research has made use of the NASA/IPAC exgives v = 16888 117 km s−1. This object has been in- tragalactic database (NED) which is operated by the Jet cluded for its H and [SII] lines in the catalog of the Propulsion Laboratory, Caltech, under contract with the Palomar Transit Grism Survey (Schneider et al. 1994) , National Aeronautics and Space Administration. which gives a velocity v = 17388 300 km s−1. The difference with our estimate is easily explained by the fact References that in the case of the PTGS H and [N II] could not be deblended, given the low{resolution of that survey; as a consequence, the wavelength of the H line {and the redshift{ is overestimated. The existence of very broad permitted emission lines and narrower forbidden ones suggests that this object is a Seyfert 1 galaxy. 5. Conclusion We have presented the rst results of an observational program devoted to the measure of cluster redshifts and velocity dispersions. At the present stage of this work, estimates of the velocity dispersions have to be considered as preliminary results as a consequence of the small number of measured redshifts per cluster. 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S. Maurogordato, D. Proust, A. Cappi, E. Slezak, J. M. Martin. A sample of galaxy clusters to study the fundamental plane: Redshift measurements, Astronomy and Astrophysics Supplement Series, 411-422, DOI: 10.1051/aas:1997165