The Astronomical Journal, 149:170 (21pp), 2015 May doi:10.1088/0004-6256/149/5/170 © 2015. The American Astronomical Society. All rights reserved. THE STRUCTURE OF NUCLEAR STAR CLUSTERS IN NEARBY LATE-TYPE SPIRAL GALAXIES FROM HUBBLE SPACE TELESCOPE WIDE FIELD CAMERA 3 IMAGING* Daniel J. Carson1, Aaron J. Barth1, Anil C. Seth2, Mark den Brok2, Michele Cappellari3, Jenny E. Greene4, Luis C. Ho5,6, and Nadine Neumayer7 1 Department of Physics and Astronomy, University of California Irvine, 4129 Frederick Reines Hall, Irvine, CA 92697, USA 2 Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA 3 Sub-Department of Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK 4 Department of Astrophysical Sciences, Princeton University, Peyton Hall—Ivy Lane, Princeton, NJ 08544, USA 5 Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China 6 Department of Astronomy, School of Physics, Peking University, Beijing 100871, China 7 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany Received 2014 November 1; accepted 2015 January 20; published 2015 April 28 ABSTRACT We obtained Hubble Space Telescope/Wide Field Camera 3 imaging of a sample of ten of the nearest and brightest nuclear clusters (NCs) residing in late-type spiral galaxies, in seven bands that span the near-UV to the near-IR. Structural properties of the clusters were measured by fitting two-dimensional surface brightness profiles to the images using GALFIT. The clusters exhibit a wide range of structural properties, with F814W absolute magnitudes that range from −11.2 to −15.1 mag and F814W effective radii that range from 1.4 to 8.3 pc. For 6 of the 10 clusters in our sample, we find changes in the effective radius with wavelength, suggesting radially varying stellar populations. In four of the objects, the effective radius increases with wavelength, indicating the presence of a younger population that is more concentrated than the bulk of the stars in the cluster. However, we find a general decrease in effective radius with wavelength in two of the objects in our sample, which may indicate extended, circumnuclear star formation. We also find a general trend of increasing roundness of the clusters at longer wavelengths, as well as a correlation between the axis ratios of the NCs and their host galaxies. These observations indicate that blue disks aligned with the host galaxy plane are a common feature of NCs in late-type galaxies, but are difficult to detect in galaxies that are close to face-on. In color–color diagrams spanning the near-UV through the near-IR, most of the clusters lie far from single-burst evolutionary tracks, showing evidence for multi-age populations. Most of the clusters have integrated colors consistent with a mix of an old population (>1 Gyr) and a young population (∼100–300 Myr). The wide wavelength coverage of our data provides a sensitivity to populations with a mix of ages that would not be possible to achieve with imaging in optical bands only. The surface brightness profiles presented in this work will be used for future stellar population modeling and dynamical studies of the clusters. Key words: galaxies: nuclei – galaxies: spiral – galaxies: star clusters: general Supporting material: extended figures, machine-readable and VO table 1. INTRODUCTION et al. 2013). In a recent HST archival study, Georgiev & Böker (2014) analyzed the photometric and structural properties of a Hubble Space Telescope (HST) imaging surveys have shown sample of 228 NCs in spiral galaxies, and confirmed that most that most galaxies contain bright, compact stellar systems NCs are similar in size to GCs, but found that the largest and known as nuclear clusters (NCs) in their photometric and ( brightest NCs in spiral galaxies occupy the regime between dynamical centers Carollo et al. 1997; Matthews et al. 1999; ultra-compact dwarfs (UCDs) and the nuclei of early-type Böker et al. 2002). Similar in many respects to high-mass – ( ) galaxies in the size luminosity plane, which is consistent with globular clusters GCs , NCs typically have velocity disper- the idea that some UCDs and GCs are the remnant nuclei of – −1 ( ) sions of 15 35 km s , effective radii or half-light radii of a tidally disrupted spiral galaxies (Böker 2008; Mieske 6– 7 ( few parsecs, and dynamical masses of 10 10 M Böker et al. 2013; Seth et al. 2014). They also found that the ) et al. 1999, 2002; Walcher et al. 2005 . Unlike GCs, NCs in effective radius of NCs tends to be smaller when measured in spiral galaxies have complex star formation histories, under- bluer filters, suggesting that younger stellar populations tend to going repeated bursts of star formation, with spectra often be more concentrated than the rest of the stars in NCs. We note consistent with a continuous star formation history (Rossa et al. that the Milky Way’s NC also has a population of young stars 2006; Seth et al. 2006; Walcher et al. 2006). Young (<100 concentrated in the central 0.5 pc (Paumard et al. 2006;Lu Myr) populations are found in most spiral NCs, and there is et al. 2013; Schödel et al. 2014). limited evidence for complex populations in early-type nuclei Two main scenarios have been proposed to explain the as well (Monaco et al. 2009; Seth et al. 2010; Lyubenova formation of NCs: in situ formation from the accretion of low angular momentum gas onto the galactic nucleus (Milosavlje- * Based on observations made with the NASA/ESA Hubble Space Telescope, vić 2004), and mergers of globular clusters (GCs) migrating obtained at the Space Telescope Science Institute, which is operated by the toward the galactic nucleus due to dynamical friction (Lotz Association of Universities for Research in Astronomy, Inc., under NASA ) ( ) contract NAS 5-26555. These observations are associated with program GO- et al. 2001 . Hartmann et al. 2011 compared simulations of 12163. the migratory/merger scenario with integral field unit (IFU) 1 The Astronomical Journal, 149:170 (21pp), 2015 May Carson et al. observations of NGC 4244, and found that in situ formation Studies of NCs have mainly been limited by the angular accounts for at least 50% of the NC’s mass. Antonini resolution of existing telescopes. The next generation of large (2013, 2014) and Arca-Sedda & Capuzzo-Dolcetta (2014) ground-based telescopes, such as the Thirty Meter Telescope, demonstrated that the migratory/merger scenario predicts will be able to resolve the kinematic structure of stars within correlations between the masses of NCs and global properties the sphere of influence of BHs with much lower mass than is of their host galaxies that match observed correlations. The 4 currently possible (down to ~10 M ), and will be crucial for young populations observed in most NCs rule out the sinking future dynamical searches for IMBH within NCs (Do ( of GCs as the sole process for NC formation i.e., the sinking et al. 2014). In a simulation of observations of NCs with the ) of younger star clusters or gas accretion must play some role . European Extremely Large Telescope, Gullieuszik et al. (2014) However, our understanding of the formation of NCs is found that observations of NCs with telescopes that can resolve incomplete and the reason for the ubiquity of NCs still remains individual stars in the cluster will lead to great improvements in a mystery. stellar population and age-dating studies, which are currently NCs and BHs are known to coexist in galaxies of all Hubble fi types across a wide range of masses (Seth et al. 2008a), but limited by the various degeneracies that come with tting some NCs have no central BH down to highly constraining stellar population synthesis models to the integrated cluster limits. The NC in NGC 4935, for example, contains a NC light. The nearest and brightest NCs will be prime targets for (Filippenko & Ho 2003) as well as a Type 1 active galactic the next generation of large ground-based telescopes. nucleus (AGN) powered by accretion onto a central BH We present a set of uniform, high signal-to-noise ratio ( ) (Filippenko & Sargent 1989), with mass estimates ranging S/N , unsaturated images of NCs with wavelength coverage 4 5 from the near-UV to the near-IR for a sample of ten of the from 5 ´ 10 to 3.6´ 10 M (Peterson et al. 2005; Edri et al. 2012), while dynamical studies of the NC in M33, a nearest late-type galaxies hosting NCs. Our data represent a galaxy with a similar mass to NGC 4395, have placed substantial improvement in spatial resolution, wavelength remarkably tight upper limits of 1500–3000 M on the mass coverage, and homogeneity over previous imaging studies of of any central BH (Gebhardt et al. 2001; Merritt et al. 2001). NCs. Although all of the galaxies in our sample have archival These studies indicate that at least some NCs can host a central HST images available from previous studies, the archival data BH, but the overall occupation fraction of BHs in NCs is are very heterogeneous, including images that are saturated in unknown. The masses of both NCs (Ferrarese et al. 2006; the core of the NC and images taken with several different Wehner & Harris 2006) and BHs (Ferrarese & Merritt 2000; cameras. The homogeneity of our data allows for a more Gebhardt et al. 2000; Häring & Rix 2004) are correlated with detailed analysis of the structure and stellar populations of NCs the masses of their host galaxy bulges. Galaxies with mass than would be possible with archival data alone. Detailed 10 above 10 M tend to host BHs, while less massive galaxies structural information of the best resolved NCs across a wide tend to host NCs, with a transition region for galaxies with range of wavelengths may provide clues about formation 8– 10 ( mass 10 10 M where both objects coexist Graham & mechanisms for NCs, which are currently poorly understood.