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Internal Kinematics and Stellar Populations of Compact Elliptical Galaxies Discovered in the Sloan Digital Sky Survey

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Internal Kinematics and Stellar Populations of Compact Elliptical Galaxies Discovered in the Sloan Digital Sky Survey OBSERVING REQUEST University of Arizona Observatories Year: 2006 Term: May - Aug Proposal Number: Internal Kinematics and Stellar Populations of Compact Elliptical Galaxies Discovered in the Sloan Digital Sky Survey PI: David Burstein (ASU) CoI(s): Ronald Marzke (SFSU), Luiz da Costa, Paulo Pellegrini (Observatorio Nacional, Brazil) Proposal Type: short-term Abstract of Scientific Justification We request one night with the MMT Blue Channel in the cross-dispersed echellette mode to obtain spectra of eleven compact elliptical galaxies discovered in the Sloan Digital Sky Survey. The high spectral resolution offered by this mode is required to obtain precise velocity dispersions of these low-mass dwarfs, which resemble the rare compact elliptical galaxy M32 at the bright end and the brightest ultracompact dwarf galaxies at the faint end. In addition, the broad wavelength coverage provides stellar absorption line strengths from NH at 3360A˚ to the Na doublet at 5890A˚ as well as nuclear emission lines from OII to SII . Together, these observations will allow the first systematic study of the formation history of compact elliptical galaxies and of the relationship between compact ellipticals and other spheroidal stellar systems. Summary of observing runs requested for this project Run Telescope Instrument No. Nights Moon Cage Optimal months Accept. months 1 MMT Blue/Echellette 1 gray f9 May May-June 2 3 4 Scheduling constraints and non-usable dates (up to two lines). May 24-30 conflicts with KPNO 2.1m allocation for the redshift-survey component of the program proposed here. Is sharing nights possible and/or advisable? If scheduled after 6/3/2006, two first halves would be appropriate Approval for Instrument Use from PI? For MMT requests only: Percentage of time to be assigned to UAO and CfA 1.0, 0.0 Page 2 Observing List (attach list if longer than 10 objects) Object RA DEC Magniude SDSSJ110404.4+451619 11:04:04.39 +45:16:19.1 rAB = 16.35 NGC 3979A 11:56:00.56 –02:43:12.3 rAB = 17.38 NGC 4073A 12:04:17.32 +10:15:02.8 rAB = 16.69 NGC 4073B 12:04:36.73 +01:53:33.5 rAB = 17.04 SDSSJ121456.31+473106.1 12:14:56.31 +47:31:06.1 rAB = 16.58 NGC 4621A 12:42:11.04 +11:38:41.2 rAB = 16.08 NGC 4621B 12:41:55.33 +11:40:03.7 rAB = 17.21 SDSSJ144049.71+032803.2 14:40:49.71 +03:28:03.2 rAB = 17.83 SDSSJ163214.81+480825.2 16:32:14.81 +48:08:25.2 rAB = 16.92 NGC 5846B 15:06:34.27 +01:33:31.6 rAB = 15.01 CONTACT INFORMATION: Observer(s) Institution: David Burstein Email(s): [email protected] Phone Num- ber(s) : 480-965-4336 GRADUATE STUDENTS: Whose second year project is this? Whose thesis is this? For each graduate student named on the front page, provide the following information: Student’s Name Advisor’s Name Advisor’s Signature Page 3 Scientific Justification The observed structure of elliptical galaxies motivated early models of galaxy formation and remains one of the definitive tests of modern theory. Relationships among luminosity, surface brightness, effective radius, velocity dispersion and metallicity have played a particularly important role in the the development of the cold dark matter model (Faber, Blumenthal & Primack 1984, Dekel & Silk 1986). These scaling relations divide elliptical galaxies into two families with markedly different structural properties (Kormendy 1985, Binggeli, Sandage & Tarenghi 1984, Bender, Burstein & Faber 1992). Among giant ellipticals and spiral bulges, the faintest are the most compact, while the vast majority of dwarf ellipticals (dEs) follow the opposite trend and are, at their faintest, among the most diffuse galaxies known. The rare and extremely compact dwarf ellipticals M32, NGC 4486B, and NGC 5846A are puzzling exceptions to this rule and appear more aligned with the giant-elliptical and bulge scaling relations than with those of typical dEs (Wirth & Gallagher 1984). Because the faint bulges of late-type spirals are often obscured by bright, dusty disks, the less encumbered compact ellipticals offer unique constraints on the formation of low-mass spheroids. Although the structural similarity between compact and giant ellipticals hints at a common origin, the proximity of the few known compact ellipticals to massive companions suggests that tidal interactions play a role (King 1962), either during their formation (Burkert 1994) or over the course of their evolution (Faber 1973a). In one view (Nieto 1990, Bekki et al. 2001), M32 is interpreted as the remnant bulge of a spiral galaxy stripped of its disk by M31. With so few compact ellipticals available, the detailed measurements of their structure (Kent 1987, Choi et al. 2001, Graham 2002), stellar populations (Faber et al. 1973b, Davidge 1991, Bender et al. 1993, Worthey 2004, Rose et al. 2005 ) and kinematics (Bender et al. 1992) are difficult to place into the broader context of elliptical galaxy formation. At distances beyond the Virgo cluster, compact ellipticals are difficult to distinguish from stars in typical seeing conditions, and their measured abundance is thus a weak lower limit (Disney 1976). Extrapolation of the surface-brightness distribution of early-type galaxies from the Sloan Digital Sky Survey (SDSS, Gunn et al. 2006) suggests that the population of undiscovered compact ellipticals could be substantial (Shen et al. 2003). Indeed, the recent discovery of two M32 counterparts in HST/ACS imaging of Abell 1689 (Mieske et al. 2005) and the discovery of much fainter, ultra- compact dwarfs in the Fornax cluster (Phillips et al. 2001) suggest that compact stellar systems are, at the very least, under-represented in traditional galaxy surveys. Recently, Hasegan et al. (2005) and Jones et al. (2006) discovered faint, ultracompact dwarfs in the Virgo cluster, but the field abundance of compact and ultracompact dwarfs remains completely unknown. In order to compile the first statistically meaningful sample of compact elliptical galaxies, we are conducting a redshift survey of compact candidates drawn from the SDSS/DR4 (Adelman- McCarthy et al., 2006). We have selected bright sources with r-band Petrosian half-light radii 00 r50 < 1.6 and ugriz colors consistent with low-redshift galaxies (Fig 1). Most of our candidates are rejected as possible stellar contaminants during spectroscopic targeting of the primary SDSS galaxy samples (Strauss et al. 2002, Eisenstein et al. 2001), but we have identified a subsample of nine galaxies with SDSS redshifts that are confirmed compact ellipticals. Remarkably, one of these lies in the halo of the field elliptical NGC 5846, which already hosts the prototypical compact elliptical NGC 5846A. Two of the other confirmed candidates have no sizable companions and thus provide unique constraints on the tidal origin of compact elliptical galaxies. Among the sources rejected by the SDSS targeting algorithm, we have identified two high-probability candidates in the halo of the Virgo cluster elliptical NGC 4621. One of these candidates appears on an archival HST/ACS image, and its compact surface brightness profile places it between the brighter compact elliptical prototypes and the Fornax ultracompact dwarfs observed with HST/STIS by de Propris et Page 4 al. (2005). Already, our sample nearly doubles the number of previously known compact ellipticals, and another 124 candidates are targeted for spectroscopy at the KPNO 2.1m in May 2006. Here, we request one night with the MMT Blue Channel in its echellette mode to obtain high- resolution spectra of eleven compact elliptical galaxies with absolute magnitudes ranging from Mr = −13.9 to Mr = −18.3. Nine of these galaxies are spectroscopically confirmed in the SDSS, and the other two are the newly-discovered companions to NGC 4621. Our goals are to measure precise velocity dispersions, absortion-line strengths, and (in some cases) emission-line ratios of these galaxies in order to compare their kinematics, stellar populations and nuclear activity to those of other spheroidal stellar systems. We have submitted a complementary proposal to obtain surface brightness profiles and color gradients of these same galaxies using the ACS High Resolution Camera on HST. We outline our strategy below. Kinematics: Correlations among luminosity, surface brightness and velocity dispersion constrain the dark-matter content of hot stellar systems (Burstein et al. 1997). At the high-mass end, giant 0.25 ellipticals and bulges follow the Faber-Jackson relation, σ0 ∝ L (Faber & Jackson 1976). At the low-mass end, globular clusters appear to contain little dark matter and follow a steeper luminosity- 0.5 velocity dispersion relation: σ0 ∝ L (McLaughlin & van der Marel 2005). Little is known about the mass range between these extremes. The internal kinematics of seven faint, ultracompact dwarfs (UCDs) in the Virgo cluster align more with massive globular clusters (Hasegan et al. 2005), while a handful of brighter UCDs in Fornax appear to follow an extrapolation of the Faber- Jackson relation (Drinkwater et al. 2003). Although brighter UCDs have been identified as possible remnants of stripped, nucleated dwarf ellipticals, their kinematics may indicate a closer kinship with compact elliptical galaxies like M32. Precise velocity dispersions of our sample of compact ellipticals will begin to fill the essentially unexplored region of the fundamental plane between M32 and the ultracompact dwarfs and will thus constrain the link between globular clusters and galaxies. Figure 2 shows simulated spectra of our targets, which require both high resolution and high SNR for precise velocity dispersion measurements (see Experimental Design). Line Strengths: Faber (1973a) notes that the compact ellipticals M32, NGC 4486B and NGC 5846A have heavy-element abundances characteristic of more luminous galaxies and suggests that compact ellipticals are the stripped cores of brighter ellipiticals. On the other hand, Nieto (1990) and Bekki et al. (2001) suggest that M32 was once the bulge of a spiral galaxy.
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