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«Spectroscopy of Globular Clusters out to Large Radius in the Sombrero Galaxy Terry J. Bridges Department of Physics, Queen’s University, Kingston, ...»

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Spectroscopy of Globular Clusters out to Large Radius in the

Sombrero Galaxy

Terry J. Bridges

Department of Physics, Queen’s University, Kingston, ON K7L 3N6, Canada;


Katherine L. Rhode1

Astronomy Department, Wesleyan University, Middletown, CT 06459;



Department of Astronomy, Yale University, New Haven, CT 06520

Stephen E. Zepf

Department of Physics & Astronomy, Michigan State University, East Lansing, MI 48824;

zepf@pa.msu.edu Ken C. Freeman Research School of Astronomy & Astrophysics, Australian National University, Mount Stromlo Observatory, Weston Creek, ACT 2611, Australia; kcf@mso.anu.edu.au


We present new velocities for 62 globular clusters in M104 (NGC 4594, the Sombrero Galaxy), 56 from 2dF on the AAT and 6 from Hydra on WIYN. Com- bined with previous data, we have a total sample of 108 M104 globular cluster velocities, along with BVR photometry for each of these. We use this wide-field dataset to study the globular cluster kinematics and dark matter content of M104 out to 20′ radius (∼ 60 kpc). We find no rotation in the globular cluster system.

The edge-on nature of M104 makes it unlikely that there is strong rotation which is face-on and hence unobserved; thus, the absence of rotation over our large radial range appears to be an intrinsic feature of the globular cluster system in M104. We discuss ways to explain this low rotation, including the possibility that angular momentum has been transferred to even larger radii through galaxy 1 NSF Astronomy & Astrophysics Postdoctoral Fellow –2– mergers. The cluster velocity dispersion is ∼ 230 km/s within several arcmin of the galaxy center, and drops to ∼ 150 km/s at ∼ 10′ radius (∼ 30 kpc). We derive the mass profile of M104 using our velocity dispersion profile, together with the Jeans equation under the assumptions of spherical symmetry and isotropy, and find excellent agreement with the mass inferred from the stellar and gas rotation curve within 3′ radius. The M/LV increases from ∼ 4 near the galaxy center to ∼ 16 at 7′ radius (∼ 20 kpc, or 4 Re ), thus giving strong support for the presence of a dark matter halo in M104. More globular cluster velocities at larger radii are needed to further study the low rotation in the globular cluster system, and to see if the dark matter halo in M104 extends beyond a radius of 30 kpc.

Subject headings: galaxies: star clusters — galaxies: formation — galaxies: dynamics

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The prevailing view of galaxy formation is that galaxies assemble hierarchically from smaller structures that are composed of both dark and baryonic matter. These structures collide and merge to create larger structures, with the eventual result being a bound galaxy in which the luminous, baryonic matter exists within a much more massive halo of dark matter. Testing this paradigm is crucial to our developing a complete, self-consistent picture of cosmology and galaxy formation.

Although one can in theory measure the masses and mass profiles of galaxies using a variety of methods — e.g., observations of integrated starlight, HI in late-type galaxies, and X-ray-emitting gas in luminous ellipticals — in practice this can be difficult, particularly for early-type galaxies. The challenges for early-type galaxies are that they normally lack significant amounts of extended HI gas, many do not have luminous, extended hot gaseous halos for X-ray studies, and their integrated starlight can only be measured to a few effective radii (e.g. Kronawitter et al. 2000). Globular clusters (GCs) are luminous, compact objects that are distributed more or less spherically around giant galaxies, number in the hundreds to thousands, and are readily detected in wide-field imaging out to 10−15 Re (e.g. Rhode & Zepf 2001, 2004), and for these reasons make uniquely valuable tracers of galaxy structure.

Furthermore, they may be less kinematically biased than other types of dynamical tracers (e.g., planetary nebulae; see Dekel et al. 2005). Since most GCs are old and are likely markers of the major star formation episodes that a galaxy has undergone (e.g., Ashman & Zepf 1998, Brodie & Strader 2006), they also provide an observable record of the formation and assembly history of galaxies. More specifically, many galaxies have been found to have –3– two populations of GCs: a blue, metal-poor population and a red, more metal-rich one, that appear to have formed in different episodes (e.g. Gebhardt & Kissler-Patig 1999; Kundu & Whitmore 2001). Galaxy formation models that predict how these populations arose in the context of a galaxy’s formation often predict that the red and blue populations will have different kinematics. Measuring GC velocities therefore provides a test of the proposed formation scenarios.

To date, only a small sample of galaxies have had substantial numbers (∼100 or more) of their GC velocities measured. Three of these — M87 (Cohen 2000, Cˆt´ et al. 2001), oe NGC 4472 (Zepf et al. 2000; Cˆt´ et al. 2003), and NGC 1399 (Richtler et al. 2004) — oe are luminous ellipticals located near the centers of galaxy clusters, one (NGC 5128; Peng et al. 2004) is a moderate-luminosity elliptical with a peculiar morphology (possibly due to a recent merger) and two are spirals — our own galaxy (see Harris 1996 for a compilation) and M31 (e.g., Perrett et al. 2002). Studying GC kinematics in galaxies over a wider range of luminosities and environments is necessary before we can begin to draw general conclusions about galaxy formation and how galaxy mass profiles change with overall galaxian properties.

We also need to measure GC velocities over a larger radial range than typically has been done in past studies, which have for the most part concentrated on the central regions of galaxy GC systems. Covering a large radial range is especially important for quantifying the distribution of dark matter in galaxy halos, and for studying GC kinematics at large radius.

The Sombrero galaxy (NGC 4594, M104) is an interesting target for a GC spectroscopic study because it is the closest undisturbed field galaxy with a luminous bulge/spheroid.

M104 has MV = −22.4, typical of giant ellipticals, and is intermediate in its properties between spiral and elliptical galaxies. Table 1 summarizes some of these properties. It is sometimes classified as an Sa spiral (e.g., de Vaucouleurs et al. 1991), but its large bulge-todisk ratio and bulge fraction are more like that of an S0 (Kent 1988), and its optical colors are likewise similar to those of S0s (Roberts & Haynes 1994). M104 has two advantages, however, over giant elliptical galaxies. First, measurement of its disk rotation out to ∼ 3′ gives an independent constraint on the mass profile out to moderate radii (see Section 4.3). Second, since M104 is reasonably isolated, GC kinematics probe only its gravitational potential, and not that of a surrounding galaxy group or cluster.

The Sombrero is relatively nearby (9.8 Mpc; Tonry et al. 2001) and its GC system has been studied with photographic plates (e.g., Harris et al. 1984), CCD detectors (Bridges & Hanes 1992, Rhode & Zepf 2004; hereafter RZ04), and Hubble Space Telescope imaging (Larsen et al. 2001, Spitler et al. 2006). RZ04 imaged the galaxy out to a radius of ∼65 kpc with a mosaic CCD detector and multiple broadband filters, and used these data to derive global properties for the GC system. Selecting GC candidates in multiple filters reduced –4– the contamination from foreground and background objects, although contamination from stars remains significant because of the Sombrero’s location toward the Galactic bulge. RZ04 found that M104 has ∼1900 GCs, a spatial extent of ∼50 kpc, and a specific frequency SN (GC number normalized by the V -band luminosity of the galaxy, as defined by Harris & van den Bergh 1981) of 2.1±0.3. The color distribution of the system is bimodal, with about 60% blue (metal-poor) GCs and 40% red (metal-rich). The blue GC population is slightly more extended than the red population, producing a shallow color gradient in the overall system.

Spectroscopy of GCs in M104 has been published in two previous studies. Bridges et al. (1997; hereafter B97) used the William Herschel Telescope (WHT) to measure radial velocities of 34 GCs out to 5.5′ (∼16 kpc) from the galaxy’s center, with velocity errors of 50−100 km s−1. From these velocities they estimated a mass of 5+1.7 ×1011 M⊙ for M104, −1.5 and found that the M/L increases with radius, in other words that M104 possesses a dark matter halo. The second spectroscopic study was done by Larsen et al. (2002; hereafter L02), who measured spectra of 14 GCs in M104 with the Keck I telescope. The GCs in the L02 study are located within 5′ of the galaxy center, with nearly all (80%) of them in the central 2′. L02 estimated the galaxy’s mass within this central region, and also combined their velocities with those of B97 to determine a projected mass of (5.3±1.0)×1011 M⊙ within 17 kpc.

In this paper, we present the results from spectroscopy of 62 GCs in M104. Fifty-six of the GCs were observed with the 3.9-m Anglo-Australian Telescope (AAT) and 2dF multiber spectrograph. Six more GC spectra were obtained with the 3.5-m WIYN telescope and the Hydra fiber positioner and bench spectrograph2. The target objects were identified in the mosaic CCD survey of RZ04 and are located between 2 and 20′ from the galaxy center.

The data presented here double the number of known GC velocities for this galaxy and, combined with data from B97 and L02, bring M104 into a sample of only seven galaxies with 100 measured GC velocities. Furthermore, this study increases the radial coverage for M104 by nearly a factor of four compared to the previous studies. This enables us for the first time to probe the kinematics of M104’s outer halo and GC system, and to trace the galaxy’s mass distribution to many effective radii.

In the following section, we describe the observations and the steps used to reduce and analyze the data. In Section 3 we present our sample of new radial velocity measurements for 62 M104 GCs, which in combination with previous data yields a total sample of 108

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GC radial velocities in M104. In Section 4 we present and discuss our results, including an analysis of the kinematics of the GC system, the GC velocity dispersion profile, and the mass profile of M104. Finally, in Section 5, we summarize the main results of this study.

Throughout this paper, we adopt a distance of 9.8 Mpc for M104 (Tonry et al. 2001), and an effective radius Re = 105′′ (Burkhead 1986).

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To create a list of targets for this study, we began with a preliminary list of GC candidates produced from BV R images of M104 taken with the Mosaic Imager on the Kitt Peak National Observatory 4-m Mayall telescope. The final results from the 4m-Mosaic survey of M104’s GC system are published in RZ04. The survey techniques are detailed there; briefly, objects qualify as GC candidates if they appear as point sources in the 36′ x 36′ Mosaic images, are detected in all three filters, and have BV R magnitudes and colors consistent with what one would expect for GCs at the distance of the galaxy (see RZ04 for details of the selection methods). RZ04 identified 1748 unresolved GC candidates in M104; the final set of candidates have V magnitudes between 18.96 and 24.3, B − V colors in the range 0.32−1.24 and V − R colors between 0.23 and 0.78.

2.1.1. AAT/2dF Targets and Observations

A preliminary photometric calibration of the Mosaic data was done in 2001 and a list of ∼1900 GC candidates was produced. (The photometric calibration was later redone using new data, and revised magnitudes and colors were calculated for all the Mosaic sources.

Some of the original GC candidates were rejected based on the revised photometry; the final list of GC candidates includes the 1748 objects mentioned above.) Starting with this list, we selected a subset of 584 objects with 19.0 V 21.5. The Mosaic images were calibrated astrometrically using tasks in the IRAF3 IMCOORDS package and coordinates for stars in the USNO-A2.0 Catalog (Monet et al. 1998). The astrometric solution has an rms of ≤ 0.4′′, and this accuracy has been confirmed by matching Chandra sources with several of the 3 IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.

–6– RZ04 object positions. This input list was then weighted by magnitude and radius, with bright candidates at large radius given the highest weight.

199 of these 584 GC candidates were observed with the 2dF multi-fiber spectrograph on the AAT in April 2002. The 2dF instrument has 400 fibers over a two-degree field of view (FOV), making it well-suited to wide-field GC spectroscopy (Lewis et al. 2002). The 2dF Configure software was used to automatically select these 199 objects; this pointing also included 78 fibers positioned on blank sky, and 4 fiducial fibers for field acquisition and guiding. 2dF has two spectrographs, with each spectrograph receiving 200 fibers. We used 600V gratings in both spectrographs, centered at 5000 ˚, with spectral coverage from A ˚. The dispersion is 2.2 ˚/pixel, and the resolution was 4.5 and 5.5 ˚ for the two 3900−6100 A A A spectrographs (spectrograph #1 has poorer resolution). The 2dF fiber size varies between 2−2.1′′ across the field.

Our observing sequence consisted of a fiber flatfield at the beginning, followed by 1800 sec object exposures; CuAr+CuHe arcs were taken after every two object exposures. For each sequence we also obtained 3×300 sec offset sky exposures, where the telescope is offset a few arcmin from the field; in the end, however, we did not use these for sky subtraction (see Section 2.2.1). On 17 April 2002, we obtained 2×1800 sec object exposures under poor conditions (seeing ranging from 2.4−3′′ ). On 18 April, conditions were better (some haze, seeing starting at 2.0′′, improving to 1.5−1.8′′ through the night), and we obtained 14×1800 sec object exposures. Thus, we obtained a total of 8 hours on-source over the two nights.

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