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THE BLACK HOLE MASS and EXTREME ORBITAL STRUCTURE in NGC 1399 Karl Gebhardt,1 Tod R

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THE BLACK HOLE MASS and EXTREME ORBITAL STRUCTURE in NGC 1399 Karl Gebhardt,1 Tod R The Astrophysical Journal, 671:1321Y1328, 2007 December 20 A # 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE BLACK HOLE MASS AND EXTREME ORBITAL STRUCTURE IN NGC 1399 Karl Gebhardt,1 Tod R. Lauer,2 Jason Pinkney,3 Ralf Bender,4 Douglas Richstone,5 Monique Aller,5 Gary Bower,6 Alan Dressler,7 S. M. Faber,8 Alexei V. Filippenko,9 Richard Green,2 Luis C. Ho,7 John Kormendy,1 Christos Siopis,5 and Scott Tremaine10 Received 2007 April 11; accepted 2007 August 16 ABSTRACT The largest galaxies, and in particular central galaxies in clusters, offer unique insight into understanding the mech- anism for the growth of nuclear black holes. We present Hubble Space Telescope kinematics for NGC 1399, the 8 central galaxy in Fornax. We find the best-fit model contains a black hole of (5:1 Æ 0:7) ; 10 M (at a distance of 21.1 Mpc), a factor of over 2 below the correlation of black hole mass and velocity dispersion. We also find a dramatic signature for central tangential anisotropy. The velocity profiles on adjacent sides 0.500 away from the nucleus show strong bimodality, and the central spectrum shows a large drop in the dispersion. Both of these observations point to an orbital distribution that is tangentially biased. The best-fit orbital model suggests a ratio of the tangential to radial internal velocity dispersions of 3. This ratio is the largest seen in any galaxy to date and will provide an important measure for the mode by which the central black hole has grown. Subject headings:g galaxies: general — galaxies: nuclei — galaxies: statistics Online material: color figures 1. INTRODUCTION formation on the upper end of black hole correlations, central galaxies in clusters offer unique insight. These galaxies are sub- It is clear that the mass of the central black hole is related to its ject to significant accretion and mergers, and it is important to host galaxy in a fundamental way. Dressler (1989), Kormendy understand whether the black hole grows as the galaxy grows. (1993), Kormendy & Richstone (1995), and Magorrian et al. Houghton et al. (2006) study NGC 1399 using adaptive optics (1998) were the first to highlight a correlation between the black (AO) observations on the Very Large Telescope (VLT); they hole mass and the bulge light. Subsequently, many other correla- findablackholemassof(1:2 0:6) ; 109 M (for a distance tions have been found, with the tightest being that between black Æ of 19.9 Mpc). We find a black hole mass of (5:1 0:7) ; 108 M hole mass and velocity dispersion (Gebhardt et al. 2000; Ferrarese Æ (for a distance of 21.1 Mpc, as used in Lauer et al. [2005]). While & Merritt 2000). Numerous theoretical models have been pro- over a factor of 2 different, we are consistent within 1 (the posed to explain these correlations, and the most compelling to date are those that work through active galactic nucleus (AGN) Houghton et al. result is only a 2 significance for a black hole detection). Furthermore, we find consistent results in terms of feedback mechanisms (Silk & Rees 1998; Fabian 1999; Springel the central orbital structure. We use a distance of 21.1 Mpc to et al. 2005; Robertson et al. 2006). To push further requires more NGC 1399 from Tonry et al. (2001), but scaled to H 70 as in secure observations. The problem is that the uncertainties in the 0 ¼ Lauer et al. (2005). black hole mass estimates are still large (around 30%Y50%) and, more importantly, the extremes of the correlations are not well explored. We have been targeting the largest galaxies in order to 2. DATA study their central black hole mass. There is only a handful of 2.1. HST Observations objects studied with velocity dispersions above 300 km sÀ1. In this paper we study the giant elliptical NGC 1399, the dom- The surface brightness profile comes from Wide Field Plan- inant galaxy in the Fornax Cluster. In addition to providing in- etary Camera 2 (WFPC2) observations for Hubble Space Tele- scope (HST ) programs GO-5990 and GO-8214 (PI: Grillmair). NGC 1399 was observed for 4000 s in F606W and for 5200 s in 1 Department of Astronomy, University of Texas, Austin, TX 78712; gebhardt@ astro.as.utexas.edu, [email protected]. F450W, with the galaxy centered on the PC. The surface bright- 2 National Optical Astronomy Observatory, P.O. Box 26732, Tucson, AZ ness, ellipticity, and color gradient profiles are shown in Figure 1. 85726; [email protected], [email protected]. Due to NGC 1399 being nearly round, the position angle is very 3 Department of Physics and Astronomy, Ohio Northern University, Ada, uncertain and we do not include discussion of it. The reductions OH 45810; [email protected]. 4 Universita¨ts-Sternwarte, Scheinerstrasse 1, 81679 Munich, Germany; are discussed by Lauer et al. (2005). From its surface brightness [email protected]. profile, NGC 1399 is classified as a core galaxy with a break 5 Department of Astronomy, University of Michigan, Dennison Building, radius 3.200 and ¼ 0:12 (approximately the central projected Ann Arbor, MI 48109; [email protected], [email protected]. 6 density slope), from a Nuker law fit (Lauer et al. 2005). For the Computer Sciences Corporation, Space Telescope Science Institute, 3700 surface brightness beyond the HST image, we use ground-based San Martin Drive, Baltimore, MD 21218; [email protected]. 7 Observatories of the Carnegie Institution of Washington, 813 Santa Barbara imaging from Saglia et al. (2000). We match the ground-based Street, Pasadena, CA 91101; [email protected], [email protected]. R-band data to the HST surface brightness in the overlap region. 8 UCO/Lick Observatories, University of California, Santa Cruz, CA 95064; Figure 1 shows a variation in the ellipticity inside of 0.300. [email protected]. However, since the surface brightness is not steep in the central 9 Department of Astronomy, University of California, Berkeley, CA 94720- 3411; [email protected]. regions and the isophotes are nearly round, there are large uncer- 10 Princeton University Observatory, Peyton Hall, Princeton, NJ 08544; tainties in the ellipticities. Thus, the variation could be due to noise, [email protected]. and a constant ellipticity model provides nearly identical residuals. 1321 1322 GEBHARDT ET AL. Vol. 671 We have checked whether the isophotal centers change as a function of radius. This check is important for the discussion in x 4. We find that the center from isophotes at 1000 compared to that derived from the isophotes in the central regions is consistent to within 0.2 pixels, or better than 0.0100. Thus, there appears to be no deviation in the galaxy center. In addition, we find no evi- dence in the residual map (Lauer et al. 2005) for any second component. Houghton et al. (2006) see an elongation in the cen- tral 0.500, suggesting a possible eccentric disk. We find no such structure in our images; furthermore, the HST images have been subsampled and deconvolved, giving a FWHM around 0.0500, better than the 0.07800 as reported for the AO K-band image of Houghton et al. (2006). Still, it is difficult to reconcile the differ- ences; we attribute them to either different structure in K band versus the R band (however, this is unlikely) or an AO artifact. Higher signal-to-noise ratio (S/N) and repeat AO observations will likely help determine the cause. The central surface brightness of NGC 1399 is V ¼ 16:0 mag arcsecÀ2, making it one of the faintest targets we have observed with the HST Space Telescope Imaging Spectrograph (STIS) (Pinkney et al. 2003). For our previous observations, however, we have used a high-resolution grating centered on the Ca ii trip- let region at 8500 8. This region is the best to use since it is not greatly influenced by stellar template mismatch and continuum estimation, as are the bluer regions (Barth et al. 2002). However, the exposure times become prohibitive for targets fainter than V ¼ 16:0 mag, since it typically requires exposure times of lon- ger than 17 hr to obtain adequate signal. Complicating the kine- matic estimate is that these large galaxies tend to have large black hole masses, and therefore large central dispersions. The disper- sion of the central STIS pixel for NGC 4649 is over 600 km sÀ1; given the relatively small equivalent widths of the Ca ii triplet lines, the large dispersion makes the lines almost disappear into the con- tinuum. Our strategy for NGC 1399 is to use a lower resolution grating over the Ca ii H and K region, where the lines remain clear even when the dispersion is that high (Dressler 1984). We obtained 6.67 hr of observations on STIS (Woodgate et al. 1998) using the G430L grating with 5200 ; 0:200 slit align at 117 (along the major axis; see Fig. 3 of Lauer et al. 2005). The wave- length range is 2880Y5690 8, with 2.746 8 pixelÀ1.Webinnedon chip by two, providing 0.1 arcsec pixelÀ1 in the spatial direction. This setup gave us S/N 20 pixelÀ1 in the central regions, and the same S/N at a radius of 1.600 by binning over 10 pixels (1.000). With the low spectral resolution and the wide slit, we have Fig. 1.—HST photometry of NGC 1399. The top panel is the F606W (close to pay special attention to the change in the instrumental resolu- to R band) surface brightness profile.
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