THE ASTRONOMICAL JOURNAL, 116:2793È2804, 1998 December ( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE AGES OF DISTURBED FIELD ELLIPTICAL GALAXIES. II. CENTRAL PROPERTIES DAVID R. SILVA1 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany; dsilva=eso.org AND GREGORY D. BOTHUN1 Department of Physics, University of Oregon, 120 Willamette Hall, Eugene, OR 97403; nuts=moo.uoregon.edu Received 1998 June 3; revised 1998 August 13 ABSTRACT The formation of elliptical galaxies via the merger of gas-rich disks has received considerable attention in recent years, with many studies strongly supporting the merger hypothesis. When investigated in detail, the dynamics of a major merger that produces a high phase-space density of material (e.g., the center of an elliptical galaxy) invariably produces the ubiquitous signature of a centrally concentrated burst of star formation. We have searched for this central burst of star formation in a sample of Ðeld elliptical galaxies that exhibit morphological Ðne structure thought to be indicative of merging. Out of this sample of 32 galaxies, we Ðnd only two galaxies, NGC 3610 and NGC 5322, with sufficiently red central near-IR colors to be consistent with the asymptotic giant branch light reÑective of the central burst of star formation a few gigayears ago. Using NGC 3610 and 5322 as case studies, we discuss pos- sible astrophysical links between global and central properties and their implied constraints on elliptical galaxy formation and evolution. In particular, we conclude that the available evidence argues against mergers of disk galaxies within the last 3È4 Gyr as being the primary formation mechanism for morpho- logically disturbed Ðeld elliptical galaxies. Key words: galaxies: elliptical and lenticular, cD È galaxies: evolution È galaxies: formation È galaxies: stellar content È galaxies: structure 1. INTRODUCTION In this scenario, the strong Hb and blue UBV colors are produced by the stellar population created in this merger- Many current-epoch elliptical galaxies in low-density driven star formation event. environments (so-called Ðeld ellipticals) exhibit morpho- However, the near-IR colors of these same galaxies logical Ðne structure and/or the presence of kinematically present a di†erent picture. As discussed inSilva & Bothun unsettled features (e.g., counterrotating cores and central (1998,hereafter Paper I), their global near-IR colors place dust lanes). As such features can only persist for 1È3 Gyr theSS92 galaxies in the same locus of points in the J[H before being destroyed via phase mixing, it has been widely versus H[K plane as elliptical galaxies with no signs of suggested that they were produced by a recent merger event recent merger activity. This strongly constrains the global (seeBender et al. 1989; Schweizer & Seitzer 1992; fractional amount of intermediate-age (1È3 Gyr) stellar Illingworth& Franx 1989; van Dokkum & Franx 1995; mass to be no more than 10%È15%, and in most cases, the Quinn 1984;Hernquist & Quinn 1988, 1989; Barnes 1992). data are consistent with all the stellar mass being old But is such kinematic/dynamical evidence alone sufficient (T [ 10 Gyr) and metal-rich ([Fe/H] º [0.3). We argued to establish that most Ðeld ellipticals were indeed formed inPaper I that any recent merger activity was therefore not through mergers? As it would seem unlikely that dynamical accompanied by a signiÐcant episode of distributed star features could be produced without some kind of augmen- formation within the region deÐned byR \ 1.5Re and that tation of the stellar population, there should be an addi- the bluer UBV colors observed in this region for some of tional signature of the merger event contained in the these galaxies are due to metallicity e†ects driven by the integrated light of these galaxies. accretion of lower metallicity stars or by relative abundance Two possible stellar population signatures have been di†erences. noted bySchweizer et al. (1990) and Schweizer & Seitzer ThePaper I result, however, does not rule out a centrally (1992,hereafter SS92). They found that elliptical galaxies concentrated younger stellar population. The existence of with morphological Ðne structure tend to have stronger such a population in recent merger products is a ubiquitous central Hb absorption and bluer integrated UBV colors prediction of models that include gasdynamics. In such within their e†ective radii than elliptical galaxies with little models, the majority of the gas quickly Ñows to the bottom or no morphological Ðne structure.SS92 argued that both of the potential well and reaches high enough densities to observations could be explained if these elliptical galaxies trigger star formation. The preexisting stars are more had been formed by disk-disk mergers roughly 3È5 Gyr uniformly distributed throughout the potential well ago, which resulted in a distributed burst of star formation. (Negroponte& White 1983; Barnes & Hernquist 1991; Mihos, Bothun, & Richstone 1992;Mihos & Hernquist 1996; Barnes& Hernquist 1996). In the case of ultralumi- ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ nous IRAS galaxies, large collections of central gas and 1 Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, operated by the Association of Uni- associated bursts of star formation are directly observed versities for Research in Astronomy, Inc., under cooperative agreement (Mihos& Bothun 1998; Downes & Solomon 1998). We with the National Science Foundation. therefore would expect similar, but scaled down, levels of 2793 2794 SILVA & BOTHUN Vol. 116 TABLE 1 OBSERVATIONAL SAMPLE Class [MB AB VR p Hot NGC & (RSA) (mag) (mag) (km s~1) log DN (km s~1) Population? Core (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) Elliptical galaxies: 547....... ... E1 ... 0.12 5524 0.59 171 . 596....... 4.60 E0 21.25 0.12 1817 0.93 151 G ,Hb P 636....... 1.48 E1 20.76 0.10 1805 0.83 156 GT,Hb ... 821....... ... E6 ... 0.16 1716 0.86 1992 . 1453 ...... 1.48 E2 22.26 0.24 3906 0.78 290 . 1700 ...... 3.70 E3 22.50 0.12 3881 0.90 233 G ,Hb P 2974 ...... 0.00 E4 20.88 0.11 1924 0.95 2222 . 3156 ...... 1.70 E5: 18.82 0.04 1296 0.68 112 G ,Hb ... 3193 ...... 0.00 E2 20.13 0.08 1378 0.93 2050 . 3377 ...... 1.48 E6 19.66 0.06 689 1.06 131 G ,Hb P 3379 ...... 0.00 E0 20.44 0.05 922 1.24 2010 . C 3605 ...... 2.70 E5 18.48 0.00 686 0.57 120 Hb P 3608 ...... 0.00 E1 19.87 0.00 1197 0.89 204 . C 3610 ...... 7.60 E5 21.37 0.00 1765 1.02 159 G ,Hb ... 3640 ...... 6.85 E2 21.01 0.10 1302 1.03 176 G1,Hb ... 4125 ...... 6.00 E6 22.28 0.04 1340 1.11 229 GT,Hb ... 4168 ...... 3.00 E2 19.63 0.04 2307 0.74 1821 . C 4660 ...... 0.00 E5 19.59 0.00 1115 0.91 198 . 4697 ...... 0.00 E6 21.76 0.04 1210 1.22 165 G P 4915 ...... 5.48 E0 21.11 0.04 3152 0.76 209 G ,H0 b ... 5322 ...... 2.00 E4 22.14 0.00 1804 1.06 224 G1,Hb ... 5831 ...... 3.60 E4 20.28 0.14 1683 0.77 1662 Hb ... S0 galaxies: 315....... ... EL ... 0.26 4956 0.82 352 . 584....... 2.78 S0 (3, 5) 21.76 0.12 1875 1.06 217 Hb ... 1023 ...... ... SB01 (5) 21.17 0.25 661 . P 1052 ...... 1.78 E3/S01 20.89 0.06 1475 1.00 206 . 1172 ...... ... S0 (0, 3) 19.48 0.10 1669 0.58 121 . P 2549 ...... 0.00 S01 (7) 19.80 0.12 1082 . 2685 ...... 2.48 S01,2(7) pec 19.55 0.15 881 . 2768 ...... 0.00 S03 (6) 21.55 0.02 1363 1.03 198 . 3489 ...... 0.00 S01,2/Sa 19.13 0.02 659 . 3607 ...... 0.00 S03(3) 20.71 0.00 951 1.10 248 . Ongoing merger: 3 3921 ...... 8.84 Merger . 0.00 5926 . NOTES.ÈCol. (1): NGC number. Col. (2): Morphological Ðne-structure parameter fromSS92. Col. (3): Morphological classiÐcation from Revised Shapley-Ames Catalog (RSA;Sandage & Tammann 1981). Col. (4): Absolute B magnitude, from RSA. Col. (5): Foreground Galactic absorption fromBurstein et al. 1987 or Burstein & Heiles 1984. Col. (6): Heliocentric radial velocity, from Burstein et al. 1987. Col. (7): logDN fromBurstein et al. 1987. Col. (8): Adopted central velocity dispersion, fromDavies et al. 1987. Col. (9): ““ Hot ÏÏ population indicators: Hb \ strong central Hb absorption(Schweizer et al.1990 or Gonzalez 1993) and G \ global UBV color excess indicated by SS92. Based on the discussion in SS92, galaxies are classiÐed as follows:G \ possible recent (1È3 Gyr) disk-disk merger product with signiÐcant merger-driven star formation;G \ possible older1 (D5 Gyr) disk-disk merger product with signiÐcant merger-driven star formation; G \ possible sites2 of recent (1È3 Gyr) star formation events induced by gas transfer only;G \ globally blue UBV colorsT but conÑicting evidence about recent merger activity and/or the origin of the blue light. See0SS92 for further details. Col. (10): Core morphology, as deÐned byFaber et al. 1997 based on HST observations. C \ core, P \ power law. central gas collection and star formation in any elliptical et al.1983; Frogel, Mould, & Blanco 1990). As Persson et galaxy that formed from a recent disk-disk merger. al. demonstrated, stellar populations that contain a signiÐ- Given this picture, elliptical galaxies that formed from cant fraction of such intermediate-age AGB stars have dis- disk-disk mergers in the last few gigayears should have tinctive near-IR colors. In particular, their H[K color is central stellar populations that are younger than their outer quite red for their J[H color relative to clusters without populations.