Extended Stellar Kinematics of Elliptical Galaxies in the Fornax Cluster?,??
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ASTRONOMY & ASTROPHYSICS DECEMBER II 1998, PAGE 325 SUPPLEMENT SERIES Astron. Astrophys. Suppl. Ser. 133, 325–336 (1998) Extended stellar kinematics of elliptical galaxies in the Fornax cluster?,?? A.W. Graham1, M.M. Colless1, G. Busarello2, S. Zaggia2, and G. Longo2 1 Mount Stromlo and Siding Spring Observatories, Australian National University, PMB Weston Creek PO, ACT 2611, Australia 2 Osservatorio Astronomico di Capodimonte, via Moiariello 16, I-80131 Napoli, Italy Received February 4; accepted June 17, 1998 Abstract. We present extended stellar kinematics for 1. Introduction a sample of elliptical galaxies in the Fornax cluster. Out of the 13 galaxies presented here, five (FCC 119, The scaling relations between some global properties of FCC 136, NGC 1373, NGC 1428, FCC 335) have no pre- ellipticals (Es), known as the “Fundamental Plane” (FP; viously published kinematical data. Major-axis velocity Dressler et al. 1987; Djorgovski & Davis 1987), is clearly a dispersion profiles (VDPs) and rotation curves (RCs) are manifestation of a great “regularity” in the properties of given for 12 of the galaxies. A major feature of this data Es. Although a proof of its “universality” is still lacking, is the spatial extension: for 8 galaxies the data extends the FP has been widely applied to various problems: from the study of peculiar motions (Lynden-Bell et al. 1988; beyond 1 Re, and for 5 it extends beyond 2 Re. Compared to the previously available data, this corresponds to an Jørgensen et al. 1996), to the study of the evolution of increase in spatial coverage by a factor from 1 to 5. The M/L ratios (van Dokkum & Franx 1996; Kelson et al. present sample represents 86% of the ellipticals in Fornax 1997), and measurements of q0 (Bender et al. 1998). The “regularity” of the E’s properties, shown by the brighter than BT = 15 mag. Five of the ellipticals in the sample turn out to be ro- FP, must be connected to their gross structural charac- tationally-supported systems, having positive rotation pa- teristics and to their formation and evolutionary mecha- V ∗ nisms (Djorgovski et al. 1995 and references therein). To rameter log( σ ) . One of these five, and another 3 galaxies from the remaining sample, display evidence for bar-like understand this connection is however a very hard task, kinematics. especially in view of the problems posed by the “thin- The data indicate that the true number of “dynami- ness” of the FP itself (Ciotti et al. 1996). Apart from cally hot” stellar systems, is much lower than previously the obvious similarities, it is a matter of fact that Es do thought: of the Es in the present sample only 1/4 are con- not constitute a homogeneous family of galaxies: Es not firmed as “pressure-supported” systems. only have substantially different photometric and kine- The data reveal a host of individual peculiarities, like: matical properties, but they also host a variety of sub- wiggles, strong gradients, and asymmetries in the rotation components, like inner disks, counter-rotating cores, outer curve and/or in the velocity dispersion profile, thus show- rings, dust lanes, etc. (Illingworth 1983; Kormendy & ing that the presence of kinematically distinct components Djorgovski 1989; Caon et al. 1993; Michard & Marchal and/or triaxiality is a common characteristic of this class 1994; Capaccioli & Longo 1994; Scorza & Bender 1996; of object. Graham et al. 1996; Graham & Colless 1997; Busarello et al. 1997; Prugniel & Simien 1997; Carollo et al. 1997, Key words: galaxies: kinematics and dynamics — and references therein). galaxies: elliptical and lenticular, cD — galaxies: The possibility that many Es have disks that escape clusters: individual: Fornax — galaxies: structure — photometric detection (Simien & de Vaucouleurs 1986; galaxies: fundamental parameters Capaccioli 1987; Rix & White 1990; Saglia et al. 1997), and the dichotomy between the two families of boxy and disky Es lead to the possibility of a new classification Send offprint requests to: A.W. Graham, [email protected] ? Based on observations collected at Siding Spring scheme, in which galaxies formerly belonging to the same Observatory. Hubble type are separated according to isophotal shape, ?? Table 3 is presented in electronic form only, and is and in which a continuous sequence exists between Es available from the CDS, Strasbourg via anonymous ftp and S0s (Kormendy & Bender 1996). Since the above sce- to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u- nario is manifestly at odds with the “regularity” of the FP, strasbg.fr/Abstract.html the question arises to what extent the “global” quantities 326 A.W. Graham et al.: Elliptical galaxies in the Fornax cluster entering into the construction of the FP are representative A 1200 l mm−1 grating was used, with a dispersion of of the real status of the galaxies. 0.555 A˚ pixel−1 over a range of 1000 A˚ centered on In this respect, an important question is whether the 5200 A.˚ The CCD used was a SITe chip (1752×532) with 00 central velocity dispersion σ0 is fully representative of the 15 µm pixels. The spatial scale on the chip was 0.91 dynamical status of a galaxy. There is much evidence that pixel−1. We used a spectrograph slit of 200 on the sky and this is not the case: due to differences in velocity disper- of length greater than the spatial extent of the CCD. The sion profiles and anisotropy, masses derived from σ0 can best collimator focus gave a FWHM for the arc lines of be wrong by a factor of ten (Tonry 1983; Richstone & 2.7pixelsor1.50A,˚ giving a resolution of 86 km s−1 at Tremaine 1986; Mathews 1988); in presence of central, 5200 A.˚ During the observations the seeing varied from kinematically distinct sub-components, the value of σ0 is 1.5 to 2.5 arcsec. likely to be more connected to the sub-component than The observed galaxies are listed in Table 1, along with to the galaxy as a whole; Es are dynamically non-homo- their photometric parameters (taken from Caon et al. logous systems: the slope of the velocity dispersion profile 1994, we also refer to this paper for a detailed photomet- is correlated to σ0 in such a way that there is a systematic ric description). We obtained our estimates of the galaxy departure of the kinetic energy from a simple scaling of major-axis PAs based on careful inspection of the position σ0 (Graham & Colless 1997; Busarello et al. 1997); finally, angle (PA) profiles (Caon et al. 1994). Only our estimate the rotation, although not dominant in Es, plays a non- for the major-axis PA of NGC 1419 differed from the value negligible role (Busarello et al. 1992; Prugniel & Simien adopted in Caon et al. (1994), where we have adopted 50◦ 1994, 1996; Busarello et al. 1997; D’Onofrio et al. 1997). rather than 65◦. The number of spectra for each galaxy To fully understand the complex kinematics of Es, and and the total exposure times used are also listed in the to analyze the connection between the individual prop- table. The spectra were exposed in 30 − 45 min blocks. erties of Es and the scaling laws, it is essential to have A Ne-Ar lamp was observed before and after each expo- access to kinematical data as accurate, extended and ho- sure for wavelength calibration and four template stars mogeneous as possible. One of the objectives of the present (of spectral types from G8III to K3III) were observed at work is to obtain homogeneous sets of data for magnitude- the beginning, middle, and end of each night. The usual limited samples of elliptical galaxies belonging to different dome and sky flats were taken, as were bias frames and clusters. measurements of the dark current. In the present paper we present new stellar kinematical data for 13 galaxies in the Fornax cluster. Section 2 describes the observations and the data re- 2.2. Data reduction duction. In Sect. 3, we give a short description of the ma- jor features of each of the kinematical profiles. Section 4 The data reduction included, besides the usual CCD provides a comparison of our data with previous observa- cosmetics, noise removal along the spatial direction by tions, and in Sect. 5 we briefly summarize our findings. adaptive filtering techniques (Richter et al. 1992). After The data themselves are presented in Appendix A and in wavelength calibration and sky subtraction, the spectra Table 3 (available in electronic form from the CDS). were normalized to the continuum, obtained by fitting with a 6th order polynomial. The reliability of the sky sub- traction was iteratively tested by comparing the bright- 2. Observations and reduction ness profiles of the sky-subtracted spectra to the profiles 2.1. Observations from Caon et al. (1994). The slit function, as derived from twilight spectra, varied by at most 0.5%, so that no further Ferguson (1989) lists 340 likely Fornax cluster members correction was needed. The low-frequency residual varia- within an area of ∼40 sq deg centered on the cluster. Of tions were reduced by filtering in Fourier space. Finally, these, 14 are classified as elliptical galaxies brighter than the spectra were processed by the Fourier Correlation BT =15.0 mag. We were able to obtain kinematical data Quotient (FCQ) technique (Bender 1990). All four tem- for 12 of these galaxies making our investigation 86% com- plate stars were used, and the relative results were com- plete at that limiting magnitude. In addition, we have ob- pared to test their consistency: as expected, no relevant tained kinematical data for FCC 119, (classified as a S0 difference was detected (see Bender 1990).