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1991Apj. . .379. .177L the Astrophysical Journal, 379:177-215 .177L .379. The Astrophysical Journal, 379:177-215,1991 September 20 . © 1991. The American Astronomical Society. All rights reserved. Printed in U.S.A. 1991ApJ. MOLECULAR GAS IN ELLIPTICAL GALAXIES Joanna F. Lees and G. R. Knapp Peyton Hall, Princeton University Observatory, Princeton, NJ 08544 Michael P. Rupen Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 AND T. G. Phillips Downs Laboratory of Physics, 320-47, California Institute of Technology, Pasadena, CA 91125 Received 1990 October 19; accepted 1991 April 3 ABSTRACT Observations of 24 early-type galaxies, mostly far-infrared (FIR)-bright ellipticals, in the 12CO(2-l) emission line resulted in 10 detections including eight out of the 20 ellipticals. Eight of these galaxies have not pre- viously been detected. A search of the literature reveals a total of 17 elliptical galaxies, including our eight detections, with observed molecular gas, as well as 27 upper limits. Thus, our study almost doubles the number of elliptical galaxies detected in CO emission. 7 8 The detected ellipticals have typical molecular gas masses of about 10 -10 M0, similar to their H i masses, and an approximately power-law distribution of MnJLB (in contrast to the Gaussian distribution for later type galaxies), extending to much lower values than for the spirals. The lenticular galaxies have CO properties intermediate between the elliptical and spiral systems. The 27 early-type galaxies which are detected in both molecular and atomic gas have the same mean ratio of molecular to atomic gas mass as later type spirals (albeit with a larger dispersion), similar ratios of Ha luminosity and FIR luminosity to molecular gas mass, and identical FIR colors. Thus, these (mostly FIR-bright) ellipticals seem to have global interstellar medium properties similar to those in late-type galaxies, but on a much smaller scale. A comparison of the properties of those ellipticals which have been detected in CO with the undetected galaxies indicates that the molecular gas may occur preferentially in the bluer, lower luminosity dwarf ellip- ticals, which have more than twice the detection rate of the brighter galaxies. Both the detected and unde- tected ellipticals are at the same mean distance; however, it may be more difficult to detect gas in bright galaxies because of their larger line widths. Further study of the bright ellipticals is needed to verify this con- clusion. The detected ellipticals also tend to be much brighter in the far-infrared, implying the presence of cold dust and/or star formation associated with the molecular gas. Our high detection rate, and the power-law distribution of MH2/LB, implies that most FIR-bright ellipticals probably contain a molecular interstellar medium. For those ellipticals in which molecular gas is detected, the lack of correlation between the molecular gas mass and the blue luminosity argues for an external origin for this gas. Subject headings: galaxies: interstellar matter — interstellar: abundances — interstellar: molecules Forman, Jones, & Tucker 1985; Trinchieri & Fabbiano 1985). 1. INTRODUCTION Clearly, it is important to know whether global properties rela- Studies of the interstellar medium of spiral galaxies have ting the interstellar medium and star formation are similar in given us much information on the global processes of star the very different environments of spiral and elliptical galaxies. formation in these galaxies and on the surprisingly uniform Knowledge of these properties could give vital clues to galaxy values from one galaxy to the next of such parameters as rela- formation and the role of the ISM in galaxy evolution, as well tive gas mass, ratio of atomic to molecular gas mass, and ratio as to the more detailed physics of the interstellar medium. of IR or Ha emission to gas mass. On the other hand, initial Since in spiral galaxies the cold gas mass is very closely observations quickly indicated that elliptical galaxies do not related to the global FIR emission, which is usually interpreted contain a substantial cold interstellar medium (e.g., Faber & as thermal emission from dust grains in the gas, we decided to Gallager 1976; Johnson & Gottsman 1979). However, in the observe a number of FIR-bright elliptical galaxies, selected past decade more sensitive measurements have shown that from a compilation of co-added IRAS fluxes of early-type gal- many ellipticals do have some cold gas, either atomic or molec- axies (Knapp et al. 1989), in the 12CO(2-l) line. Knapp et al. ular. About 15% of the ellipticals and 25% of SO’s are detected (1989) list IRAS FIR flux densities for approximately 1150 7 9 in H i emission, with typical atomic gas masses of 10 -10 M0 early-type galaxies, finding that about 45% of the ellipticals are (Knapp 1987, 1990). Close to 60% of ellipticals are also detected at both 60 and 100 /un. We choose to observe in CO detected in optical emission lines (Phillips et al. 1986; Bettoni emission those elliptical galaxies with Si00fim > 1.0 Jy, about 4 5 & Buson 1987) with ionized gas masses of only 10 -10 M0. 14% of the ellipticals included in the survey of Knapp et al., for Many ellipticals have been detected in X-ray emission from a total of 76 elliptical galaxies. Four of these galaxies do not 108-1010 Mq of hot gas (Nulsen, Stewart, & Fabian 1984; have redshifts, and five more have nearby spiral companions 177 © American Astronomical Society • Provided by the NASA Astrophysics Data System .177L 178 LEES, KNAPP, RUPEN, & PHILLIPS that may be contributing some of the IR flux. We are left with son et al. (1989), respectively. In all, 20 ellipticals (as well as .379. 67 FIR-bright ellipticals. (Currently, 15 of these ellipticals have four other early-type galaxies which are most likely not been detected in CO emission [by us or by other observers], ellipticals) were observed. Eight were definitely detected, and and 11 have sensitive upper limits, leaving 41 galaxies yet to be several more were probably detected. The detection rate was observed. Twenty-one galaxies of the 67 have detected H i approximately 40% on both runs. Each galaxy was observed at 1991ApJ. emission.) A number of other surveys of FIR-bright early-type its optical center in position-switched mode with a throw of galaxies have also been undertaken (Sage & Wrobel 1989; ± 10' in azimuth. Since these galaxies are typically a couple of Thronson et al. 1989; Wiklind & Henkel 1989c; Gordon arcminutes in size, we would have missed molecular gas dis- 1990b). tributed in a ringlike or clumpy configuration in the outer In this paper, we report on 12CO(2-l) observations of 19 of parts of these galaxies. Positions were taken from Dressel & these ellipticals, as well as five other early-type galaxies, includ- Condon (1976) where available; otherwise, from de Vaucou- ing one FIR-faint elliptical, IC 2006. The observations are leurs, de Vaucouleurs, & Corwin (1976, hereafter RC2), presented in § 2. In § 3 we discuss the results on these 24 indi- Sandage & Tammann (1987, hereafter RSA), or Lauberts vidual systems and compare the molecular gas properties to (1982, hereafter ESO). Velocities were taken from these cata- those of the atomic and ionized gas, stellar kinematics, dust, logs (the references are cited by Palumbo, Tanzella-Nitti, & etc. We then combine our observations with previously Vettolani 1983), or from more recent studies in the literature. published results of CO observations of E, SO, and SOa galaxies The noise levels were somewhat worse for the 1990 May to compare the distribution of molecular gas mass in these observations than the 1989 November data because of system- systems, as well as the distributions of MH2/MHI, MH2/LFIR, atic channel noise, due to problems with the AOS spectrometer and infrared colors, in § 4.1. We also compare lumi- (although when we checked the 1989 November data more nosities, colors, IR properties, and H i masses of the CO- carefully, we also saw some evidence for the same problem, but detected ellipticals with the undetected ellipticals in § 4.2. at a lower level than in the 1990 May run). As a result, when we Finally, conclusions are given in § 5. smooth over five channels, the rms noise decreases by some- The distance for each galaxy was estimated from the radial what less than we would expect, although over large scales the -1 -1 velocity assuming H0 = 100 h km s Mpc , with no correc- noise does seem to be uncorrelated. This effect should be kept tions for nonuniform expansion, except for those galaxies in in mind when interpreting our quoted upper limits and errors, obvious groups where the best estimate for the group distance which are just the formal values. We were able to bypass this was assumed (for example, from Turner & Gott 1976 or Davies difficulty to some extent by shifting the central observing fre- et al. 1987); the distance to the Virgo Cluster was taken to be quency by small amounts every few scans, then shifting back 13.5 Mpc (for h = 1). NGC 404 is assumed to be at a distance again before summing the spectra. Unfortunately, this also of 1.5 Mpc. Including corrections for Virgocentric infall might meant a substantial loss of observing efficiency for the 1990 change the distances to galaxies projected close to Virgo, but May run. for our sample these corrections would be less than 10% or The individual 10 minute observations were rejected if they 20% and so can be safely neglected. showed baseline curvature greater than their peak-to-peak noise; a few very noisy scans (rms noise 2-16 times the average) were also excised. In the course of the observations, we found 2.
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