The Astrophysical Journal, 679:420–427, 2008 May 20 A # 2008. The American Astronomical Society. All rights reserved. Printed in U.S.A. NGC 1600: CLUSTER OR FIELD ELLIPTICAL? Rodney M. Smith,1 Vicent J. Mart´ıI´nez,2,3 Alberto Ferna´ndez-Soto,3 Fernando J. Ballesteros,2 and Amelia Ortiz-Gil2 Received 2007 November 5; accepted 2008 February 9 ABSTRACT A study of the galaxy distribution in the field of the elliptical galaxy NGC 1600 has been undertaken. Although this galaxy is often classified as a member of a loose group, all the neighboring galaxies are much fainter and could be taken as satellites of NGC 1600. The number density profile of galaxies in the field of this galaxy shows a decline with radius, with evidence of a background at approximately 1.3 Mpc. The density and number density profile are consistent with that found for other isolated early-type galaxies. NGC 1600 appears as an extended source in X-rays, and the center of the X-ray emission seems not to coincide with the center of the galaxy. The velocity distribution of neighboring galaxies has been measured from optical spectroscopic observations and shows that the mean radial velocity is approximately 85 km sÀ1 less than that of NGC 1600, indicating that the center of mass could lie outside the galaxy. The velocity dispersion of the À1 14 ‘‘group’’ is estimated at 429 Æ 57 km s . The inferred mass of the system is therefore of the order of 10 M ,avaluethat corresponds to a large group. NGC 1600 therefore shares some similarities, but is not identical to, the ‘‘fossil clusters’’ detected in X-ray surveys. Implications of this result for studies of isolated early-type galaxies are briefly discussed. Subject headinggs: galaxies: elliptical and lenticular, cD Online material: color figures 1. INTRODUCTION 73 km sÀ1 MpcÀ1) was included in the sample of SMG04 due to the large magnitude difference between it and other nearby gal- The importance of isolated early-type galaxies in understand- axies, even though it is often quoted as being at the center of a ing galaxy and cluster evolution has led to a surge in the avail- loose group containing a number of NGC-cataloged galaxies, to- able number of catalogs of such objects (e.g., Colbert et al. 2001; Smith et al. 2004; Reda et al. 2004; Denicolo et al. 2005), with gether with several other fainter galaxies. Denicolo et al. (2005) classify it as a field elliptical rather than isolated. The brightest few galaxies in common between them. The variation arises from galaxy within a projected distance of 1 Mpc is NGC 1594, a 13.9 the different selection criteria used in the catalog construction. : A common requirement is that there is no galaxy with a similar B-magnitude (K ¼ 10 2) galaxy at least 800 kpc away. The two closest NGC galaxies, NGC 1601 and NGC 1603 at separations redshift within a certain distance, typically of the order of 1 Mpc. of 1:6 0 and 2:6 0 (30 and 48 kpc), respectively, are both almost However, the incompleteness of current galaxy catalogs, and in 3 mag fainter than NGC 1600. Thus, although there are several particular the lack of complete redshift information, usually re- neighboring galaxies in the field of NGC 1600, it is by far the quires a visual inspection of the field as final confirmation that dominant galaxy in the group. There is a poor group of galaxies the galaxies are isolated. The catalog of Smith et al. (2004, here- (Zwicky cluster 0430.8 0424B; Zwicky et al. 1961–1968) at a after SMG04) only used redshift information for the central gal- À distance of 53 0 to the north of NGC 1600, with a mean velocity axy and discounted any galaxies which had a bright galaxy nearby of 5030 km sÀ1 (Baiesi-Pillastrini et al. 1984), a velocity similar in projection. It thus contains the strictest isolation criteria and all to that of NGC 1600 (4715 km sÀ1). The brightest galaxy in this galaxies in the sample are the most dominant member in the field by at least 2.2 mag within 500 kpc and 0.7 mag within 1 Mpc. group still satisfies the isolation criteria of SMG04, i.e., it is more than 2.2 mag fainter than NGC 1600. Again, visual inspection confirmed that these galaxies were in- The galaxy has been the subject of several previous studies. In deed isolated. The SMG04 catalog will miss several galaxies in the other samples, as no redshift information is applied. However, a photometric study, Matthias & Gerhard (1999) found the gal- axy to have boxy isophotes, while kinematic studies (Bender interpretation of the local faint galaxy environment is simplified et al. 1994; Faber et al. 1989) found that there was little rotation by the lack of other bright galaxies nearby which may have their in the stellar component, with a maximum of 30 km sÀ1, while the own dwarf population. 4 central velocity dispersion is typical of larger elliptical galaxies NGC 1600, an 11.9 B-magnitude (K ¼ 8:1) E3–E4 galaxy (321 km sÀ1). There is evidence of past and possible ongoing star at a distance of approximately 64 Mpc (NED5 and using H ¼ 0 formation, with the presence of H regions (Trinchieri & di Serego Alighieri 1991) and dust (Ferrari et al. 1999). Thus, it is highly 1 Department of Physics and Astronomy, Cardiff University, Cardiff CF24 likely that NGC 1600 is a merger remnant (Matthias & Gerhard 3AA, UK; [email protected]. 2 1999), although, with broadband colors typical of elliptical gal- Observatori Astrono`mic, Universitat de Vale`ncia, Apartat de Correus 22085, axies (NED), this merger is likely to have occurred a long time E-46071 Vale`ncia, Spain; [email protected], [email protected], [email protected]. ago. Estimates of the age of this galaxy hence range from 4.6 up 3 Departament d’Astronomia i Astrofı´sica, Universitat de Vale`ncia, Apartat to 8.8 Gyr (Trager et al. 2000; Terlevich & Forbes 2002). These de Correus 22085, E-46071 Vale`ncia, Spain; [email protected]. results are in agreement with the study of a sample of iso- 4 All near-infrared magnitudes in this paper have been obtained from the lated early-type galaxies by Reda et al. (2004), who found that 2MASS catalogs. 5 The NASA/IPAC Extragalactic Database (NED) is operated by the Jet Pro- several galaxies had boxy isophotes, evidence of dust and past pulsion Laboratory, California Institute of Technology, under contract with the merger activity but had remained relatively dormant for several National Aeronautics and Space Administration. gigayears. 420 NGC 1600 421 NGC 1600 is also a weak X-ray source (Sivakoff et al. 2004), 2. OBSERVATIONS AND DATA REDUCTION showing extended emission out to at least 100 00 (corresponding to a physical radius of 30 kpc), with a possible central component To obtain an estimate of the radial number density profile associated with NGC 1600 and an outer emission region associated of galaxies around NGC 1600 we have used the APM scans with the group. The extended outer emission is centered to the of United Kingdom Schmidt Telescope (UKST) sky survey northeast of the central galaxy, suggesting that the center of the plates of a circular area of radius 1:5 (corresponding to a scale potential is slightly offset from NGC 1600. The presence of ex- of 1.65 Mpc at the distance of NGC 1600) surrounding this gal- tended material around the galaxy is also suggested from the tailed axy. Only those objects brighter than 20th magnitude in B and X-ray structure around the nearby galaxy NGC 1603, indicating classified as galaxies in both B and R were selected. NGC 1600 an effect of ram pressure stripping. The large magnitude difference lies within 1 of the edge of plate 764 in the survey, and thus the between NGC 1600 and the other galaxies in its neighborhood, adjoining plate, 765, must be matched in both position and together with the extended X-ray emission, would suggest that magnitude. Using the TOPCAT package in the Starlink suite of NGC 1600 may be a fossil group. astronomical data reduction packages, we have used the match- Whereas the radius of about 200 00, or 60 kpc, total soft X-ray ing objects detected by the APM in the overlap region of plate 41 À1 emission of 2 ; 10 ergs s and absolute magnitude of MR ¼ 764 and 765 to determine the accuracy of matching both in mag- À22:5 indicate that this galaxy does not satisfy the criteria to be nitude and position. The average magnitude difference between a fossil group, as stated by Jones et al. (2003), it is worthwhile galaxies that lie in the overlap region and are detected in both remarking that in recent work by Santos et al. (2007) some cases plates is less than 0.05 mag, and thus no magnitude difference is of fossil groups are identified which resemble some of the NGC assumed. In addition, the number of matched objects varies by 1600 group properties. Moreover, detailed studies by Mendes de less than 5% when we vary the angular distance between 1 00 and Oliveira et al. (2006) and Cypriano et al. (2006) have measured 10 00 for those objects that are classified as matched. This indicates properties of fossil groups that make them more similar to clus- that the positional accuracy of the scans is less than 1 00. However, ters than was initially expected.