The 3-D structure of the Coma–A 1367 supercluster: Optical spectroscopy of 102 galaxies

Astronomy and Astrophysics Supplement Series, Jul 2018

Optical spectroscopy of 117 CGCG galaxies, 102 of which are projected in the direction of the Coma–A 1367 supercluster, is reported. These new measurements, added to those found in the literature, bring to 1068 the number of CGCG galaxies in this region with available redshift, out of a total of 1085 objects with . We use these data to infer the 3-D structure of the Coma supercluster.

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The 3-D structure of the Coma–A 1367 supercluster: Optical spectroscopy of 102 galaxies

Astron. Astrophys. Suppl. Ser. The 3-D structure of the Coma{A 1367 supercluster: Optical spectroscopy of 102 galaxies? G. Gavazzi 2 3 L. Carrasco 0 1 2 R. Galli 2 3 0 Observatorio Astronomico Nacional/UNAM , Ensenada B.C. , Mexico 1 Instituto Nacional de Astrofisica , Optica y Electronica, Apartado Postal 51, C.P. 72000 Puebla, Pue. , Mexico 2 Send o print requests to: 3 Universita degli Studi di Milano , Via Celoria 16, 20133 Milano , Italy Optical spectroscopy of 117 CGCG galaxies, decades, as new redshift measurements became available. 102 of which are projected in the direction of the Coma{ The pioneering work of Gregory & Thompson (1978) unA 1367 supercluster, is reported. These new measure- veiled the main structure of the supercluster within the ments, added to those found in the literature, bring to region 11h30m < < 13h30m; 18 < < 32 . By that 1068 the number of CGCG galaxies in this region with time only 238 of the 1130 galaxies brighter than 15.7 available redshift, out of a total of 1085 objects with catalogued in the CGCG (Zwicky et al. 1961-68) in this mp 15:7. We use these data to infer the 3-D structure region, had known redshift values. Ten years later the of the Coma supercluster. analysis of Gavazzi (1987) counted more than twice as many velocity measurements (695) obtained from optical - The Coma{A 1367 supercluster is part of the Great Wall, the major large-scale structure in the Northern Hemisphere (Zabludo et al. 1993) . This supercluster provides us with a unique test-bed for extragalactic investigations since it contains thousands relatively bright galaxies (mp < 15:7) at high galactic latitude, thus little a ected by extinction from our Galaxy, distributed in a variety of environments spanning from rich clusters (Coma + A 1367) to relatively low-density regions. Furthermore, due to its narrow distribution in the redshift space relative to its average recessional velocity (7000 km s−1), the distance spread of its members is minimal. These properties make the Coma supercluster ideal for studying the environmental dependence of the properties of galaxies (e.g. their luminosity function) spanning a local density regime of over an order of magnitude. This structure has been studied in increasing details in the last 2. The sample Galaxies in the present study are selected from the CGCG Catalogue (Zwicky et al. 1961-68) in the region 11h30m < < 13h30m; 18 < < 32 containing the Coma{A 1367 supercluster. There are 1130 galaxies listed in this region. Of these 1085 have mp 15:7. Another 45 belong to multiple systems which were split in their individual components, each of them fainter than the catalogue limiting magnitude 15.7. Out of the 1130 we observed 102 galaxies Tel Spectrog. Loiano Cananea BFOSC LFOSC dispersion A/mm with no z values available in the literature. Only 17/1085 galaxies with mp 15:7 in the Coma region and 45/1130 fainter than this limit remain with unknown redshift, thus the sample is 98% complete at 15.7. Spectra of 3 galaxies belonging to the cluster A 262 and 12 to the Hercules supercluster were also taken as llers. 3. Observations and data reduction Low dispersion spectra of 117 galaxies were obtained in several observing runs since 1995 using the imaging spectrographs BFOSC and LFOSC attached to the Cassini 1.5 m telescope (Loi) at Loiano (Italy) and the 2.1 m telescope (Can) of the Guillermo Haro Observatory at Cananea (Mexico), respectively. Table 1 lists the characteristics of the two spectrographs in the adopted con gurations: The observations at Loiano were performed using a 2.0 or 2.5 arcsec slit, depending on the seeing conditions, oriented E-W. The exposure time ranged between 20 and 90 min according to the brightness of the target object. Observations at Cananea were carried out with a 3.1 arcsec xed slit, oriented N-S. Every galaxy spectrum was preceded and followed by an exposure of a HeAr lamp (Loiano) or XeNe lamp (Cananea) to secure the wavelength calibration. Data reduction was performed in the IRAF-PROS environment. After bias subtraction, when 3 or more frames of the same target were obtained, these were combined (after spatial alignment) using a median lter to help cosmic rays removal. Otherwise the cosmic rays were removed under visual inspection. The wavelength calibration was checked on known sky lines. These were found within 1 A from their nominal value, providing an estimate of the systematic uncertainty on the derived velocities of 50 km s−1. After subtraction of the sky background, one-dimensional spectra were extracted from the frames. These spectra were analyzed with either of two methods: 1) individual line measurement: all spectra taken at Cananea and those obtained at Loiano prior to 1996 were inspected and emission/absorption lines were identi ed. Emission lines include H , N[II] and S[II]. Absorption lines include the Mg[I], CaFe and Na. The galaxy redshift was obtained from these individual measurements. If more than one line was identi ed, the galaxy redshift was derived as the weighted mean of the individual measurements, with weights proportional to the line intensities. 2) Cross correlation technique: spectra obtained at Loiano after 1996 were analyzed using the crosscorrelation technique of Tonry & Davis (1979) . This method is based on a \comparison" between the spectrum of a galaxy whose redshift is to be determined, and a ducial spectral template of a galaxy (or star) of appropriate spectral type to contain the wanted absorption/emission lines. The basic assumption behind this method is that the spectrum of a galaxy is well approximated by the spectrum of its stars, modi ed by the e ects of the stellar motions inside the galaxy and by the systemic redshift. For this purpose high signal-tonoise spectra were taken of two template galaxies: M105 (absorption lines) and NGC 1330 (emission lines), whose redshifts are 866 and 1039 km s−1 respectively. The observed redshifts (Vobs) were rst transformed to Heliocentric (Vhel), then corrected for the motion of our galaxy relative to the Cosmic Microwave Background (VCMB) according to Kogut et al. (1993) . 4. Results The velocity measurements obtained in this work are listed in Table 2 as follow: Column 1: the CGCG designation (Zwicky et al. 1961-68) . Columns 2, 3: (B1950) celestial coordinates, measured with few arcsec uncertainty. Column 4: the photographic magnitude (Zwicky et al. 1961-68) . Columns 5, 6: the derived heliocentric velocity with uncertainty. The latter quantity is obtained by adding in quadrature systematic and statistical errors. Column 7: an asterisk marks uncertain redshifts. These are either low signal spectra or spectra with only one absorption line that could be misidenti ed. Column 8: type of lines (A = absorption; E = emission; EA = both). Column 9: method used for the redshift measurement (CC = cross correlation; IL = individual line measurement). Column 10: observing run. Figure 1 gives a representation in celestial coordinates of 1085 galaxies (panel a) and a wedge diagram (VCMB vs. R.A.) is given in panel b. Small symbols mark galaxies Qual. Lines Method run (8) Figure 2 gives a histogram of the galaxy velocity distribution (panel a). The present measurements are given in a separate histogram (panel b). Since, for obvious reasons, we measured galaxies fainter than average, the fraction of background objects (panel c) is higher than in the general taken from the literature, lled circles mark the measure- sample. However, even at the present limiting magnitude, ments obtained in this work. nearly 50% of the new redshifts are found in the range 6000 − 8500 km s−1, typical of the Coma supercluster. 4.1. The 3-D structure of the Coma{A 1367 supercluster The aim of this work is to try an objective determination of the \aggregation" state of galaxies in the CGCG (1) Coma{A 1367 supercluster, i.e. to establish the member- enhancement centered at 7200 km s−1, stretching through ship of galaxies to structures of a given size and complexity the entire R.A. window. Conspicuous features are: the (e.g. clusters, groups, multiplets etc.) or their degree of \ ngers of god" of the two clusters Coma and A 1367 \isolation". This is not a trivial exercise since this de - which span the interval 4000 < V < 10000 km s−1 and the nition depends on two arbitrary quantities: the scale on large \void" in front of the supercluster. The remaining which the local density is computed and the local density objects, belonging to the bridge between the two clusters itself. Consider for example a pair of galaxies with a have a narrower velocity distribution, which lies within separation of say 100 kpc. This doublet could be relatively the interval of roughly 6000 < V < 8500 km s−1. isolated in space, or embedded in a group of galaxies, or To reconstruct the 3-D structure of the supercluster in a much larger cluster containing hundreds of galaxies. we proceed as follows: These three cases, in spite of their similar local densities (i.e. computed on 100 kpc radius), are subject to ex- 1) Clusters (Agg = 1 to 4): we identify the members tremely di erent environmental conditions: the rst of the two rich clusters using a preliminary conservative (isolated pair) is dominated by the gravitational (tidal) positional criterion: in the velocity range 4000 < V < force induced by its companion, whereas in the second 10000 km s−1 there are 72 galaxies within 1 deg radius example the prevailing conditions are dictated by the (Agg = 1) and 177 within 2 deg (Agg = 2) of the Coma group potential and in the third by the large-scale cluster cluster and 59 galaxies within 0.5 (Agg = 3.0) and 81 potential or might be severely influenced by the di use within 1 deg radius of A 1367 (Agg = 4.0). We subtract intracluster gas (e.g. ram pressure). These prevailing these objects from the sample. environmental conditions are thus \described" by a density parameter which must be computed within a 2) \Homunculus" (Agg = 16): we identify on a purely radius similar to the typical scale of the aggregate. positional basis the members of the \legs of the homuncuOnce the densities are computed, these can be converted lus" (see de Lapparent et al. 1986) (Agg = 16) and we into (mutually exclusive) \aggregation" classes using subtract these objects from the sample. appropriate density thresholds. 4.2. The 3-D algorithm The structure of the Coma{A 1367 supercluster stands out clearly in Figs. 1 and 2 as the pronounced density 3) Foreground (Agg = 8 to 18): we subtract from the remaining sample all galaxies with V < 6000 km s−1, which are considered as foreground objects. These are empirically assigned to individual groups and structures on the basis of their 3-D coordinates (Agg = 8 to 18 as de ned in Gavazzi 1987) (see Table 3 below for details). Figure 3 gives a representation in celestial coordinates of the foreground galaxies (including Agg = 16) panel a) and a wedge diagram (VCMB vs. R.A.) is given in panel b. 4) Background (Agg = 19.0): we subtract from the remaining sample all galaxies with V > 8600 km s−1, which are considered as background objects. 5) Groups (Agg = 5.): on the remaining sample (6000 < V < 8600 km s−1) we run an algorithm that counts around each galaxy the number of galaxies found within 0.9 Mpc projected radius, satisfying the additional requirement that their velocity is within 600 km s−1 from the mean velocity of the aggregate under study (850 km s−1 for group 5.3). If there are at least 8 such galaxies, these are assigned to the \group" class (Agg = 5). 5 groups are found by the algorithm (N3937, N4065, IC 762, N4213 and IC 3165) (see Table 3 for details). Figure 4 gives a representation in celestial coordinates of the groups members (panel a) and a wedge diagram is given in panel b. Di erent symbols are used for the various groups. 6) Multiplets (Agg = 6.2 to 6.5): on the remaining sample (6000 < V < 8600 km s−1) we run an algorithm that counts around each galaxy the number of galaxies found within 0.3 Mpc projected radius satisfying the additional requirement that their velocity is within 600 km s−1 from the mean velocity of the aggregate under study. If there is at least 1 such galaxy, these are assigned to the \multiplet" class (Agg = 6). (6.2 = doublets, 6.3 = triplets... 6.5 = quintuplets). 7) Contact multiplets (Agg = 6.1): we repeat 6) with a more stringent requirement that the companion galaxy lies within 50 kpc. these are contact doublets and triplets. Figure 5 gives a representation in celestial coordinates of the members to multiplets (panel a) and a wedge diagram is given in panel b. 8) Isolated (Agg = 7.0): on the restricted velocity range 6000 < V < 8000 km s−1 we count galaxies which do not have a companion within 0.3 Mpc projected radius. We call these isolated (Agg = 7.0). 9) Strictly isolated (Agg = 7.1) we repeat 8) with the more stringent requirement that the galaxy under study is isolated within 0.5 Mpc projected radius. Figure 6 gives a representation in celestial coordinates of the isolated objects (panel a) and a wedge diagram is given in panel b. 10) Extended clusters: nally we re-inject in the sample the cluster objects (see step 1) and try an alternative, less restrictive de nition of cluster membership, which includes more peripheral objects. We count around each previously de ned as belonging to a group, multiplet or galaxy, in the interval 4000 < V < 10000 km s−1, the even be considered isolated. number of galaxies found within 2.0 Mpc projected ra- The criteria used above are summarized in Table 3 as dius. Within 2 < R < 8:5 deg of the center of Coma and follows: For any Agg class (Col. 1) a decription is given 1 < R < 3:5 deg of the center of A 367 we allow for a in Col. 2. velocity di erence of 1500 km s−1 from the mean velocity Column 3: the minimum number of objects used to of the cluster. In both aggregates we consider a galaxy to compute the threshold density of each aggregate. belong to the \extended cluster" if there are >20 galaxies Column 4: the scale (R in Mpc; in degrees) used to satisfying the above criterion. Figure 7 gives a representa- compute the threshold density of each aggregate. tion in celestial coordinates of the cluster members (panel Columns 5, 6: the \window" in celestial coordinates a) and a wedge diagram is given in panel b. Di erent sym- where each aggregate was found. A single coordinate bols are used for the core, outskirt and extended cluster gives the center of the aggregate (with radius R or ). If members. a pair of coordinates is given this indicates the interval Figure 8 gives a representation in celestial coordinates where the aggregate was found. of the supercluster members (panel a) and a wedge dia- Column 7: the allowed velocity di erence from the gram is given in panel b. average velocity of the aggregate. All the above aggregation classes (1-19) are mutu- Columns 8, 9: the velocity interval where the aggregate ally exclusive, except the de nition of extended clus- was found. ter. A galaxy belonging to the extended cluster can be Column 10: the average velocity of the aggregate. Column 11: the number of objects belonging to each aggregate. 5. Conclusions and summary We obtained 117 new redshift measurements of GCGC galaxies, 102 of which in the direction of the Coma{ A 1367 supercluster. With the new data, the total number of available redshifts in this direction of the sky is 1068 (mp 15:7). The supercluster is centered at 7200 km s−1 and it extends from 6000 to 8500 km s−1. It contains two rich clusters (Coma + A 1367) whose \ ngers of God" stretch from 4000 to 10000 km s−1. By computing of the local galaxy density around each object, we separate galaxies belonging to ve groups, from several multiplets and relatively isolated objects within the supercluster. This classi cation will be used to study the environmental dependence of properties of galaxies spanning over an order of magnitude in local density. Acknowledgements. We wish to thank the TACS of the Loiano and Cananea telescopes for the generous amounts of time allocated to this project. G.G. wishes to thank the students of his course for their contribution during the observations and the data reduction. L.C. has had support from CONACYT (Mexico) research grant No. 211290-5-1430PE. de Lapparent V. , Geller M. , Huchra J. , 1986 , ApJ 302 , L1 Gavazzi G. , ApJ 320 , 96 Gavazzi G. , Randone I. , Branchini E. , 1995 , ApJ 438 , 590 Gavazzi G. , Boselli A. , 1996 , Astroph. Lett. Comm . 35 , 1 Giovanelli R. , Haynes M. , 1985 , ApJ 292 , 404 Gregory S. , Thompson L. , 1978 , ApJ 222 , 784 Huchra J. , Geller M. , de Lapparent L., Corwin H. , 1990 , ApJS 72 , 433 Kogut A. , et al., 1993 , ApJ 419 , 1 Nilson P. , 1973 , Uppsala Obs . 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G. Gavazzi, L. Carrasco, R. Galli. The 3-D structure of the Coma–A 1367 supercluster: Optical spectroscopy of 102 galaxies, Astronomy and Astrophysics Supplement Series, 227-235, DOI: 10.1051/aas:1999209