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Globular Cluster Systems and Their Host Galaxies: Comparison of Spatial Distributions and Colors Haverford College Haverford Scholarship Faculty Publications Astronomy 2014 Globular Cluster Systems and Their Host Galaxies: Comparison of Spatial Distributions and Colors Jonathan Hargis Haverford College, [email protected] Follow this and additional works at: https://scholarship.haverford.edu/astronomy_facpubs Repository Citation Hargis, J. R.; Rhode, K. L. "Globular Cluster Systems and Their Host Galaxies: Comparison of Spatial Distributions and Colors" Astrophysical Journal 796 (1): Article 62. 2014. This Journal Article is brought to you for free and open access by the Astronomy at Haverford Scholarship. It has been accepted for inclusion in Faculty Publications by an authorized administrator of Haverford Scholarship. For more information, please contact [email protected]. The Astrophysical Journal, 796:62 (26pp), 2014 November 20 doi:10.1088/0004-637X/796/1/62 C 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A. GLOBULAR CLUSTER SYSTEMS AND THEIR HOST GALAXIES: COMPARISON OF SPATIAL DISTRIBUTIONS AND COLORS Jonathan R. Hargis1,2 and Katherine L. Rhode2 1 Department of Astronomy, Haverford College, 370 Lancaster Avenue, Haverford, PA 19041, USA; [email protected] 2 Department of Astronomy, Indiana University, 727 East 3rd Street, Swain West 319, Bloomington, IN 47405, USA Received 2014 January 5; accepted 2014 September 22; published 2014 November 5 ABSTRACT We present a study of the spatial and color distributions of four early-type galaxies and their globular cluster (GC) systems observed as part of our ongoing wide-field imaging survey. We use BVR KPNO 4 m+MOSAIC imaging data to characterize the galaxies’ GC populations, perform surface photometry of the galaxies, and compare the projected two-dimensional shape of the host galaxy light to that of the GC population. The GC systems of the ellipticals NGC 4406 and NGC 5813 both show an elliptical distribution consistent with that of the host galaxy light. Our analysis suggests a similar result for the giant elliptical NGC 4472, but a smaller GC candidate sample precludes a definite conclusion. For the S0 galaxy NGC 4594, the GCs have a circular projected distribution, in contrast to the host galaxy light, which is flattened in the inner regions. For NGC 4406 and NGC 5813, we also examine the projected shapes of the metal-poor and metal-rich GC subpopulations and find that both subpopulations have elliptical shapes that are consistent with those of the host galaxy light. Lastly, we use integrated colors and color profiles to compare the stellar populations of the galaxies to their GC systems. For each galaxy, we explore the possibility of color gradients in the individual metal-rich and metal-poor GC subpopulations. We find statistically significant color gradients in both GC subpopulations of NGC 4594 over the inner ∼5 effective radii (∼20 kpc). We compare our results to scenarios for the formation and evolution of giant galaxies and their GC systems. Key words: galaxies: elliptical and lenticular, cD – galaxies: formation – galaxies: individual (NGC 4472, NGC 4594, NGC 5813, NGC 4406) – galaxies: photometry – galaxies: star clusters: general Online-only material: color figures 1. INTRODUCTION The goal of the survey is to provide robust constraints on galaxy formation scenarios by quantifying the funda- Globular cluster (GC) populations are powerful probes of mental ensemble properties of galaxy GC populations (e.g., the assembly histories of galaxies (e.g., Ashman & Zepf 1998; total number and specific frequency of GCs; fractions of Brodie & Strader 2006). GCs are one of the few stellar popu- metal-rich and metal-poor clusters). Accurate measurements of lations in galaxies that survive for a Hubble time (Milky Way these properties allow for key tests of the phenomenological GCs have ages of ∼11–13 Gyr; Dotter et al. 2010) and there- GC/galaxy formation models of the previous two decades— fore they contain an observational record of the chemical and galaxy mergers (Ashman & Zepf 1992), in situ multiphase dis- dynamical conditions present in their host galaxy at the earliest sipational collapse (Forbes et al. 1997), and collapse plus ac- stages of the galaxy’s formation. In the context of hierarchi- cretion (Coteˆ et al. 1998)—and provide constraints on the latest cal galaxy formation theory, GC populations have been used to hierarchical galaxy formation simulations that have investigated probe a number of important physical processes. For example, GC system properties in a ΛCDM context (e.g., Moore et al. observations of the spatial distribution, stellar populations, and 2006; Prieto & Gnedin 2008; Muratov & Gnedin 2010;Griffen kinematics/proper motions of Milky Way GCs have been used et al. 2010). GC populations are also being used to study the to study the role of accretion and in situ formation in build- formation and evolution of early-type galaxies in the context of ing the Galaxy (e.g., Keller et al. 2012; Mackey & Gilmore a “two-phase” (or “inside-out”) assembly scenario (see Brodie 2004; Forbes & Bridges 2010; Dinescu et al. 1999; Searle & et al. 2014 and references therein), where giant galaxies ex- Zinn 1978). perience an early, rapid, dissipational, in situ phase of growth Studies of GC populations outside the Local Group have followed by an ongoing dissipationless, accretion phase (Naab extended this type of work to galaxies with a wider range of et al. 2009; van Dokkum et al. 2010;Oseretal.2010). masses, morphologies, and environments (Brodie & Strader In this paper we study the spatial and color distributions of 2006; Ferrarese et al. 2012; Brodie et al. 2014). We have a subset of four giant E/S0 galaxies in our survey. We perform been carrying out a wide-field optical imaging survey of surface photometry of the host galaxies and compare the radial giant spiral, elliptical, and lenticular galaxies at distances from and azimuthal structure of the galaxy light to the GC system ∼10–30 Mpc in order to characterize the global properties of spatial distribution. We also compare the stellar populations giant galaxy GC systems. The survey design is presented in of the host galaxies to their GC systems using radial color Rhode & Zepf (2001) and previous results are given in Rhode profiles (e.g., B − R versus projected radius). Because both the & Zepf (2001, 2003, 2004; hereafter RZ01, RZ03, RZ04), GC systems and host galaxies are studied using the identical Rhode et al. (2005, 2007, 2010), Hargis et al. (2011; hereafter imaging data (see Section 2), we are able to minimize systematic H11), Hargis & Rhode (2012; hereafter HR12) and Young errors in our comparisons. Our primary goal is to quantify the et al. (2012). shape of the projected two-dimensional (2D) distribution of the 1 The Astrophysical Journal, 796:62 (26pp), 2014 November 20 Hargis & Rhode GC systems and compare the results to the shape of the host radial extent makes general conclusions about the degree of galaxy light distribution. Whenever possible in the analysis, we agreement or disagreement between subpopulations less secure. consider both the total GC population as well as the metal-rich Our imaging has a radial coverage of ∼25 from the galaxy and metal-poor GC subpopulations. center (∼120 kpc at Virgo distances), allowing us to study the The majority of the work on the spatial distributions of galaxy azimuthal distribution out to several effective radii. GC populations has focused on the azimuthally averaged radial Additional motivation for azimuthal distribution studies distribution of clusters. Consequently, the connection between comes from the connection between the projected 2D shapes of the properties of GC system azimuthal distributions and their GC systems, GC system kinematics, and the signature of galaxy host galaxies remains poorly understood (Ashman & Zepf 1998; formation processes. The spatial distribution of point-particles Brodie & Strader 2006). The largest systematic study of the in a large gravitational potential (i.e., GCs in a galaxy+dark mat- azimuthal distribution of GC systems was by Wang et al. (2013), ter potential) should be closely tied to the particles’ kinematic who performed a statistical study of the 2D shape of the 94 early- properties (e.g., rotational flattening, pressure support; Binney type galaxies in the ACS Virgo Cluster Survey (ACSVCS; Cotˆ e´ &Tremaine2008), and therefore the azimuthal distribution of et al. 2004). Wang et al. (2013) found that in modestly flattened a galaxy’s GC system should be linked to the dynamics of the galaxies (ellipticity >0.2) both the metal-rich and metal- system. The differing spatial and kinematic properties of the poor GC subpopulations tend to be aligned to the host galaxy. metal-rich and metal-poor subpopulations of Milky Way GCs Detailed results on the GC system azimuthal distributions of provide a local example of this connection. The Milky Way 23 ACSVCS galaxies were recently presented by Park & Lee metal-rich GC subpopulation is not only more centrally con- (2013). They confirmed the position angle alignment of GCs centrated (radially) than the metal-poor GC subpopulation, it is with the host galaxies discussed by Wang et al. (2013) and also also spatially flattened and shows a modest rotation compared found that the ellipticity of the metal-rich GC subpopulations to the spherical distribution and low-rotation of the metal-poor shows a better correlation with the ellipticity of the bulk starlight GCs (Zinn 1985). In addition, because the dynamical timescales of the host galaxies than the metal-poor GC subpopulations. of GCs in galaxy halos (in particular) are long, the kinematics— However, in both the Wang et al. (2013) and Park & Lee and hence the spatial distribution—may retain the imprint of the (2013) studies, the field of view of the ACS+HST configuration formation processes that created the GC subsystems (e.g., dissi- limits the analysis to only the central ∼2.4 of the galaxies pation in the metal-rich population; accretion in the metal-poor (∼12 kpc at the distance of Virgo).
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