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THE ROTATING NUCLEAR STAR CLUSTER in NGC 4244 Anil C

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THE ROTATING NUCLEAR STAR CLUSTER in NGC 4244 Anil C The Astrophysical Journal, 687:997Y1003, 2008 November 10 A # 2008. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE ROTATING NUCLEAR STAR CLUSTER IN NGC 4244 Anil C. Seth,1, 2 Robert D. Blum,3 Nate Bastian,4 Nelson Caldwell,1 and Victor P. Debattista5 Received 2008 June 16; accepted 2008 July 15 ABSTRACT We present observations of the nuclear star cluster in the nearby edge-on spiral galaxy NGC 4244 using the Gemini Near-Infrared Integral Field Spectrograph (NIFS) with laser guide star adaptive optics. From a previous study of edge-on galaxies, this nuclear star cluster was found to be one of a sample of clusters that appear flattened along the plane of their host galaxies disks. Such clusters show evidence for multiple morphological components, with younger/ bluer disk components and older/redder spheroidal components. Our new observations of NGC 4244 show clear rota- tion of 30 km sÀ1 within the central 10 pc (0:500) of the cluster. The central velocity dispersion is found to be 28 Æ 2kmsÀ1. The multiple stellar populations inferred from the optical colors and spectra are seen as variations in the CO line strength in the NIFS spectra. The rotation is clearly detected even in the older, more spheroidal stellar component. We discuss evidence for similar structures and kinematics in the nuclear star clusters of other galaxies including M33 and the Milky Way. Our observations support two possible formation mechanisms: (1) episodic accretion of gas from the disk directly onto the nuclear star cluster; or (2) episodic accretion of young star clusters formed in the central part of the galaxy due to dynamical friction. Subject headinggs: galaxies: formation — galaxies: individual (NGC 4244) — galaxies: kinematics and dynamics — galaxies: nuclei — galaxies: spiral — galaxies: star clusters Online material: color figures 1. INTRODUCTION In Seth et al. (2006, hereafter Paper I), we found from a study of edge-on galaxies that in some NSCs these multiple stellar pop- Prominent nuclear star clusters (NSCs) are found in 75% ulations appear to be associated with different morphological com- of late-type spiral galaxies (Bo¨ker et al. 2002) and early-type ponents. These clusters are flattened along the major-axis of the galaxies (Coˆte´ et al. 2006). These NSCs are intrinsically very galaxy and have color maps and morphologies that suggest they luminous, with typical M À12, and sizes similar to globular I consist of young disks and older more spheroidal components. clusters (r A 5 pc; Bo¨ker et al. 2004). Recently, multiple stud- e We present here follow-up observations of the NSC in NGC ies (Ferrarese et al. 2006; Wehner & Harris 2006; Rossa et al. : 2006) have shown that NSC masses are coupled to the mass of 4244, the nearest galaxy in the Paper I sample (D ¼ 4 3Mpc, 100 21 pc; Seth et al. 2005). In Paper I we presented optical spec- their host galaxy, following a similar M- relation as massive ¼ tra of this cluster that indicate the presence of multiple stellar pop- black holes (MBHs) and extending this relation to lower mass ulations. Although the detailed star formation history is not well galaxies. This result suggests that the formation of both types of constrained, populations of 50Y100 Myr combined with older objects are linked to the formation of their host galaxies. Some populations are required to provide a good fit to the spectra. Red- galaxies host both NSCs and MBHs, but the relationship be- shifted emission identified with an H ii region offset by 19 pc along tween these two types of objects is unknown (Seth et al. 2008). the major axis of the cluster gives a lower limit to the mass of Unlike black holes, the assembly history of NSCs can, in prin- 2:5 ; 106 M . This mass measurement is consistent with the ciple, be inferred from studies of their stellar populations and ki- masses derived from the stellar population fitting of the spectrum nematics. Thus, NSCs provide important tools for studying the history of galaxy centers and their connection to the galaxies as a as well. In 2 we describe the observations and data reduction. Our whole. x results on the kinematics, morphology, and stellar populations In spiral galaxies, integrated optical spectra of NSCs show that they nearly always have multiple stellar populations (Walcher of the cluster are presented in x 3. We discuss the results in the context of other galaxies and formation models in 4. et al. 2006; Rossa et al. 2006). More specifically, NSCs typically x have a young stellar population (<100 Myr) but are dominated in 2. OBSERVATIONS AND DATA REDUCTION mass by stars older than >1 Gyr. Further support to this picture comes from the observations of several distinct populations in K-band spectra of the NGC 4244 nuclear star cluster were ob- the color-magnitude diagram of the nucleus of the Sgr dwarf tained using the Near-Infrared Integral Field Spectrograph (NIFS; spheroidal (M54; Monaco et al. 2005; Siegel et al. 2007). McGregor et al. 2003) on Gemini North, an image-slicing integral field unit spectrograph. The spectrograph was used in conjunction with the ALTAIR Laser Guide Star system. The cluster itself was 1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street Cambridge, used as a tip/tilt star but is too faint (r ¼ 16:18) to get the highest MA 02138; [email protected]. Strehl performance. The cluster was observed in K band on the 2 Center for Astrophysics Fellow. nights of 2007 February 9 and April 29. In total 5 ; 900 s expo- 3 National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, sures were obtained on-source and 4 ; 900 s exposures off-source AZ 85721. 4 Institute of Astronomy, University of Cambridge. (sky). The air mass of the observations ranged between 1.11 and 5 Centre for Astrophysics, University of Central Lancashire, Preston PR1 2HE, 1.42, and an A0 star, HIP 50459, was observed on both nights at UK; RCUK Fellow. similar air mass to be used as a telluric calibrator. The exposures 997 998 SETH ET AL. Vol. 687 all had similar image quality, with the PSF FWHM varying by <30% between the exposures. We note that H-band spectral ob- servations of the NGC 4244 NSC were obtained as well, but due to a number of observational problems and a significantly lower resolution, these data are not discussed here. The data were reduced using the Gemini version 1.9 IRAF package, using pipelines based on the nifsexamples scripts. To obtain a clean telluric absorption spectra from the HIP 50459 observations, the spectra were processed as described for the sci- ence images below, after which one-dimensional spectra were extracted and the Br absorption line was fit and removed. For the science exposures, an off-source exposure was sub- tracted from each on-source exposure; for the February 9 data only one off-source exposure was available and was thus subtracted from both images. The images were then sliced up and rectified based on Ronchi mask and arc lamp images using nftransform and spatially rebinned into image cubes using nifcube.Inthis process, the original 0:04300 ; 0:100 pixels get rebinned into 0:0500 pixels which are block replicated in the long (slice) direc- tion. Each data cube was then corrected for telluric absorption using the nftelluric program, which allows for interactive correction of the small differences in the wavelength calibration and air mass between the observations of the calibrator and each exposure. The final data cubes from each individual on-source exposure were combined using gemcombine, correcting for spa- tial and spectral offsets. The resulting data cube has a 300 ; 300 field of view with 0:0500 spatial pixels and 2.132 8 spectral pix- els from 2.00Y2.44 m. Based on measurements of the sky lines, the spectral resolution is 4.1 8 (53 km sÀ1) FWHM. The spatial resolution in the combined image was measured using four com- pact sources after subtracting the NSC model (x 3.1) and found to be 0:2300 FWHM. These four compact sources are shown as crosses in Figure 1; three of the four are also detected as pointlike sources in the higher resolution (0:0800 FWHM) HST images. The spatial resolution of our data cube is roughly 2 times lower Fig. 1.—Top: Spectrum within 0:100 of the nuclear star cluster center. The than that achieved when using a brighter tip/tilt star (e.g., for our plotted spectrum is the normalized flux ratio of the galaxy spectrum to the Br - corrected A0 V telluric calibrator spectrum. The CO band heads dominate the telluric calibrators). red part of the spectrum while Mg i (21080, 22830 8), Br (21676 8), Na i 00 A spectrum from the inner 0:1 of the cluster is shown in Fig- (22075 8), and Ca i (22644Y77 8) lines are also seen. Bottom: K-band image ure 1. The plotted spectrum is the normalized flux ratio of the made from collapsing the final data cube along the spectral direction. Contours galaxy spectrum to the Br -corrected A0 V telluric calibrator are at 16.5, 15.5, 14.5, 13.5, and 12.5 mag arcsecÀ2. Crosses show the four com- spectrum. All the spectral analysis presented below is done using pact sources used to determine the spatial resolution of the data. The compass in the top left shows the rotation of the field (P:A: ¼ 147); the x-axis runs roughly flux-ratioed spectra. In order to achieve a reasonable signal-to- along the major axis of the galaxy.
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