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1985Apjs ... 59. .447H the Astrophysical Journal Supplement Series, 59:447-498, 1985 December © 1985. the American Astronomical

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1985Apjs ... 59. .447H the Astrophysical Journal Supplement Series, 59:447-498, 1985 December © 1985. the American Astronomical The Astrophysical Journal Supplement Series, 59:447-498, 1985 December .447H © 1985. The American Astronomical Society. All rights reserved. Printed in U.S.A. 59. ... LONG-SLIT SPECTROSCOPY OF GAS IN THE CORES OF X-RAY LUMINOUS CLUSTERS 1985ApJS Esther M. Hu1 Laboratory for High Energy Astrophysics, NASA/Goddard Space Flight Center; Department of Physics and Astronomy, University of Maryland; and Space Telescope Science Institute Lennox L. Cowie1,2 Center for Theoretical Physics, Center for Space Research, and Physics Department, Massachusetts Institute of Technology; Space Telescope Science Institute; and Physics and Astronomy Department, Johns Hopkins University AND Zhong Wang1 Astronomy Department, Boston University; and Space Telescope Science Institute Received 1984 December 10; accepted 1985 June 4 ABSTRACT We report the results of long-slit spectroscopy obtained for the core regions of 14 clusters of galaxies. Seven of these clusters have optical emission in their cores, and seven do not (down to our limiting detectable surface brightness of between 2x10“17 ergs cm-2 s-1 arcsec-2 and 6xl0-17 ergs cm-2 s_1 arcsec-2 in Ha + [N n] X6584). The presence of the optical emission is shown to depend on whether the central density of the hot X-ray 3 l/1 3 1 1 emitting gas exceeds a critical value of roughly 8 X10" h cm" [h = H0/(100 km s" Mpc" )], correspond- ing to a cooling time of 7X109 A"1/2 years. This result is naturally understood in terms of the “radiative regulation” model, where the hot gas cools and accretes to form the optical systems. We suggest that the length of the cooling time is a weak additional argument for a low value of H0 and point out that the critical density will be a strong function of the z of the clusters observed and of the epoch of cluster formation, so that future observations of clusters at redshifts z > 0.2 will be of great interest. We argue that the emission is shock-excited by shocks in the range 70-90 km s"1 and that abundances in the gas of O, N, and S are near-solar. Except for A1795, which possesses a very extended faint filament system, most of the emission in the clusters is confined within 10 h~l kpc or less of the nucleus of the dominant central galaxy. Study of the morphology and kinematics of these systems shows thick shearing disks, probably rotating in the galaxy potential, which show no preferential alignment with the major or minor axis of the galaxy. In the context of the cooling-flow model, upper bounds on the cluster core specific angular momentum per unit mass of <1000 /z"1 km s"1 kpc are obtained. The magnitude of the projected shear in the optically emitting disks lies 1 1 between 70 and 180 h km s" kpc" and, if interpreted as rotation, is consistent with M/Lv ^ 20 h in the central 3 h~l kpc of these galaxies. We note that the optical emission systems are very similar in morphological and kinematical appearance to those in isolated elliptical galaxies. An additional result is that there is no evidence of any star formation in the central galaxies, either in the spectra or in the color gradients of the central galaxy continuum. Upper limits on the dust content of the cluster gas are discussed. Redshifts and positions are given for the 59 objects (52 galaxies and seven stars) which fell within the slits and which provide an unbiased survey of the cluster core regions to a limiting visual surface 2 brightness fiv = 23 mag arcsec" . Finally we discuss current and future observations in the UV and IR. Subject headings: galaxies: clustering — galaxies: intergalactic medium — galaxies: nuclei — galaxies: redshifts — spectrophotometry I. INTRODUCTION the existence of nuclear components and, more rarely, ex- It is now known that spatially extended, optically emitting tended filaments of emission, and gave luminosities in a few of gas systems are present in and around the central galaxies of a the brighter lines (Ha, [N ii], and [S n]). The search for such number of rich X-ray luminous clusters (Heckman 1981; systems was motivated by the theoretical predictions of the Cowie et al 1983, hereafter CHJY). The differential narrow- “radiative regulation” models of the X-ray emitting gas put band imaging studies described in these papers demonstrated forward by Cowie and Binney (1977) and Fabian and Nulsen (1977). In these models, gas in the center of some of the most X-ray luminous clusters cools in a Hubble time and accretes 1 Visiting Astronomer at Kitt Peak National Observatory, which is to the central regions of the cluster, settling finally onto the operated by the Association of Universities for Research in Astronomy, central galaxy. The salient soft X-ray properties which char- Inc., under contract with the National Science Foundation. 2Alfred P. Sloan Foundation Fellow. acterize such gas inflow and accretion are (1) strongly peaked © American Astronomical Society • Provided by the NASA Astrophysics Data System .447H 448 HU, COWIE, AND WANG 59. surface brightness reflecting the high local density of cooling Details of the instrumentation, observing configuration, and ... gas about the nucleus of the central galaxy, as was indeed seen processing and calibration of the data are given in § II, along in X-ray imaging data (Jones and Forman 1984), and (2) the with a discussion of the estimated measurement accuracies presence of a low-temperature component ( ~ 107 K), which and comparison with previous work. In § III we present a was detected in X-ray spectrometric experiments with sensitiv- detailed description of our data. Redshifts are tabulated for all 1985ApJS ity at the soft spectral end (Mushotzky etal. 1981; Cañizares objects in our slit exposures which were sufficiently bright to 1981; Mushotzky 1984). Cowie, Fabian, and Nulsen (1980) be measured (§ IIIa). Emission-line profiles and the corre- suggested that such cooling material would eventually be sponding continuum contributions along each slit were con- detectable as filamentary condensations of optically emitting structed by forming 100 A bandpass regions centered on and gas, which may be tentatively identified with the Heckman near the wavelength of the redshifted Ha for the central and CHJY systems. galaxy (§ Illb). Apart from the presence of narrow emission It remains to be convincingly demonstrated that the opti- features, the spectrum of the central galaxy appears nearly cally emitting gas which is seen is indeed the base of such an identical from cluster to cluster, and line ratios were computed accretion flow rather than being produced by the central from residual spectra formed by subtracting a scaled con- galaxy itself. Even within the context of the cooling-flow tinuum template of the underlying central galaxy (§ IIIc). models, there remain fascinating questions: large mass infall Contour plots showing the kinematics of gas in the extended 2 -1 rates of -100 h~ M0 yr have been inferred from the emission systems are given in § \l\d. Finally, B — V and X-ray data (Mushotzky 1984; Jones and Forman 1984; Fabian V — R color distributions along the slit were constructed along et al 1981 a), and many of these cooling clusters are observed each slit to study the possible effects of star formation on the to have cD galaxies at their cores. The nature of the gas-galaxy central galaxy colors (§ Hie). Annotated descriptions of indi- interaction, the ionization mechanism, and the ultimate fate of vidual clusters are given in the last subsection (§ III/). the gas seen in these cases have yet to be determined. Star A discussion of the consequences of these results is given in formation at the base of such high-pressure flows may favor § IV. (The reader may skip directly to this section if he or she low-mass objects (Jura 1977; Cowie and Binney 1977; is interested only in the final interpretation.) In § IVa we show Cañizares 1981). “Accretion population” models of star for- that the presence of optical emission is tied to the properties mation under such conditions calculated by Sarazin and of the hot gas in the cluster and not to the morphology of the O’Connell (1983) indicate that such a stellar population might central galaxy or cluster, demonstrating that the optical sys- have colors which are marginally distinguishable from those of tems are indeed formed by cooling of hot gas. Cooling flows the parent galaxy. Valentijn (1983) has recently claimed that occur when the gas density exceeds a critical central value there are large systematic radial color gradients [A(2? - F) > found to be roughly 8X10-3 hl/2 cm""3 [h = Hq/(100 0.5 within 50 kpc of the nucleus of the central galaxy] for km s-1 Mpc-1)], which corresponds to a cooling time of 9 1 1/2 l/2 several of these cooling-flow candidates, a result which would 7X10 [Tgas/(1 XlO K)] h~ yrs. We argue that the lead to important constraints on star formation if verified, but length of this time scale weakly favors low values of H0. In which we do not find evidence for in our own measurements. § IVb we discuss the kinematics of the gas flows, pointing out Finally, many of these central galaxies have powerful central that the gas within the cD galaxies appears to form thick radio sources associated with them. The question of whether shearing or rotating disks which do not align with major or the cold accreting material is powering these sources is clearly minor axes. Using the shear of the disk to measure the of great interest. potential of the cDs, we find typical M/Lv ratios of ~ 20 /z Detailed spectroscopy of these systems can answer many of within 3 A-1 kpc.
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