New Astronomy Reviews 46 (2002) 155–158www.elsevier.com/ locate /newar

HST NICMOS and WFPC2 observations of molecular hydrogenand dust in the central galaxies in cluster cooling flows

*Megan Donahue , Jennifer MackSTScI, 3700 San Martin Drive, Baltimore, MD 21218, USA

Abstract

We present preliminary results of HST imaging observations of three central galaxies in X-ray luminous clusters ofgalaxies with putative major cooling flows in their cores: NGC 1275 in the Perseus cluster, Abell 2597, and PKS 0745-19.Narrow-band NICMOS imaging at 2 microns reveals extended, warm (T | 2000–3000 K) molecular hydrogen structures inthe cores of Abell 2597 and PKS 0745-19 that appear to be co-spatial with the ionized hydrogen revealed in Ha 1[N II]images obtained with WFPC2. The H /Ha emission line ratio is unexpectedly high in Abell 2597 and PKS 0745-191: too2

21high to be explained by shocks with v . 50 km s or by power-law photo-ionization. Photo-ionization by the surroundingX-ray gas is unlikely in Abell 2597. Fluorescent heating by hot stars is plausible in both Abell 2597 and PKS 0745-191. Noextended H emission was discovered in NGC 1275. The H /H ratio allowed by our detection limits are consistent with2 2 a

shocks or nuclear photo-ionization in NGC 1275. A paper by Donahue et al. is in preparation. 2002 Elsevier ScienceB.V. All rights reserved.

Keywords: Cooling flows; Radio continuum: Galaxies; X-rays: Galaxies: Clusters

1. Introduction luminous, extended emission line nebulae (e.g. Hec-kman et al., 1989), which appear to be ionized and at

The high electron density of the hot intracluster least partially heated by hot, young stars (Voit andmedium (ICM) at the centers of many if not most Donahue, 1997; Cardiel et al., 1995, 1998; Hansen etclusters of galaxies implies that the central ICM may al., 1995). The star formation rates inferred fromcool over a Hubble time. The short cooling time of such observations are similar to those in somethis gas prompted suggestions that the ICM in these starburst galaxies (e.g. McNamara, 1997), but fall atclusters cools and flows towards the center at a rate least an order of magnitude short of the inferred

21of |100 M yr , where it subsequently condenses cooling flow rates. The optical emission-line(

(see Fabian, 1994 and references therein for a luminosities from the ‘‘cooling-flow’’ nebulae arereview.) The central galaxies of clusters suspected to strongly correlated (99.94% confidence) with theharbor such ‘‘cooling flows’’ frequently display inferred mass flow rate, albeit with a dispersion

spanning up to two orders of magnitude (Heckman etal., 1989). The emission-line nebulae are common,appearing in |40% of X-ray selected clusters ofgalaxies (Donahue et al., 1992). The emission-line*Corresponding author.nebulae in clusters seem to avoid clusters withoutE-mail addresses: donahue@stsci.edu (M. Donahue),

mack@stsci.edu (J. Mack). central cooling times shorter than about a Hubble

1387-6473/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved.PI I : S1387-6473( 01 )00171-3

156 M. Donahue, J. Mack / New Astronomy Reviews 46 (2002) 155 –158

time. Yet the optical emission line gas itself seems to 2. Observations: HST NICMOS and WFPC2be dusty (Donahue and Voit, 1993; Sparks et al.,1993), and thus is not likely to be a direct condensate Three cluster targets, Perseus /NGC 1275, Abellfrom the ICM. 2597, and PKS 0745-191, were chosen for imaging

All central cluster galaxies with large inferred with the Hubble Space Telescope (HST) NICMOSmass deposition rates and powerful optical emission- camera in order to study the the relationship betweenline systems yet observed with sufficient sensitivity the warm molecular hydrogen, the optical line-emit-emit strong molecular hydrogen emission in the 1-0 ting gas, the dust, and the radio source in the centralS(1) line (Elston and Maloney, 1992, 1994). More few arcseconds of X-ray luminous clusters. All threerecent IR spectroscopic observations of radio galax- targets are embedded in cooling flows with signifi-

21~ies associated with strong cooling flows reveal that cant cooling rates (M . 100–300 M yr ) (e.g.(

these galaxies emit H emission while other radio Edge et al., 1992). The nuclei of all three targets2

galaxies do not (Falcke et al., 1998; Jaffe and were known a priori to be bright, radiating infraredBremer, 1997). The infrared spectra show H vi- H lines whose observed wavelengths fall within the2 2

brational line ratios characteristic of |1000–2000 K band-passes of narrow-band NICMOS filters (Elstonmolecular gas. Apparently, the presence of warm and Maloney, 1994; Falcke et al., 1998). Starmolecular hydrogen may be as prevalent as optical formation rates inferred from the blue optical excess

22 21emission line nebulae in the central galaxies of in all 3 systems span 20–40 h M yr (McNa-50 (

cluster cooling flows. mara, 1997).Dusty nebular filaments do not only appear in High signal-to-noise observations of the molecular

cluster cooling flows. Such structures have also been hydrogen and continuum were acquired with thedetected in early type galaxies in small groups HST NICMOS2 camera (Thompson et al., 1998)(Goudfrooij and Trinchieri, 1998), and in interacting using narrow and broad-band filters. For NGC 1275,gas-rich galaxies (Kenney et al., 1995; Donahue et the narrow band F216N filter allows the observational., in preparation). The characteristics in common of the H 1-0 S(1) line whose rest wavelength at2

with cooling flow optical emission-line filaments are 2.12 microns. The other two targets were observed inthe size scales (kiloparsecs), the prominent forbidden the 1-0 S(3) line (rest wavelength 1.956 mm) whoseline and recombination emission ([NII], Ha). Some nuclear flux is nearly equal to the 1-0 S(1) flux inof these sources have had recent interactions. Most these systems (Falcke et al., 1998; Elston andof these sources also have hot ISM or are embedded Maloney, 1994). This line redshifts into the F215Nin a cluster or group. In none of these objects is the and F212N NICMOS filter wavelength coverage forsource of the filament energy understood. PKS 0745-191 and A2597 respectively. We obtained

Our project goals were: (1) to image three cluster broad band F160W images for continuum observa-cores /central cluster galaxies (NGC 1275, Abell tions. The bandpass of the F160W filter (|H-band)2597, and PKS 0745-19) with previously known Ha is sufficiently wide and the emission lines suffi-and H line emission with narrow-band filters cen- ciently weak such that the F160W image is of nearly2

tered on a vibrationally excited H emission line and pure continuum.2

the HST NICMOS camera, (2) to determine whether To map the optical-line emission at a similarthe H emission is confined to the central nuclear spatial resolution as our molecular line maps, we2

regions of the galaxies, (3) to map the H emission acquired HST WFPC2 data, both from our own2

and reveal any correlation with the Ha and radio program and from the Hubble Data Archive. Foremission line structures, and (4) to evaluate the NGC 1275, we utilized archival WFPC2 data taken

˚plausibility of various heating mechanisms for the H with the linear ramp filter at 6676 A to centered on2

gas, shocks and photo-ionization, by using measures Ha and [N II]. For A2597 we used archival WFPC2of local energetics such as X-ray emission and local imaging of [OII]3727 emission with F410M andionizing UV radiation, H /Ha line ratios, and H Ha 1[NII] emission seen through the F702W filter2 2

surface brightnesses. (Koekemoer et al., 1999). The stellar continuum

M. Donahue, J. Mack / New Astronomy Reviews 46 (2002) 155 –158 157

images were taken through the F702W filter forNGC 1275 and the F160W filter for A2597. Ha

emission line images of PKS 0745-19 were takenthrough the WFPC2 linear ramp filter, with bluecontinuum imaging through the F439W filter. TheH-band continuum images were used to model thecontinuum isophotes for all three targets, since dustabsorption significantly contaminated the opticalcontinuum images.

3. Results

Off-nuclear molecular hydrogen was detected intwo of the three targets: Abell 2597 and PKS 0745-19. Figs. 1 and 2 show full color superpositions ofthe NICMOS and WFPC2 data (North is up and Eastto the left). The red intensity represents the H-bandstellar continuum (NICMOS). Green coding mapsthe Ha 1[N II] emission line regions, while bluecoding maps the net H emission. White therefore, is2 Fig. 2. Central 7.5’’ by 7.5’’ of PKS 0745-191.where the color scheme saturates in all three maps.The purple in Fig. 1 is H and starlight (Ha is2

strongly absorbed) near the center. In our maps and in side by side comparisons, theH emission and the Ha emission appear to be2

morphologically similar but not exactly identical: theH emission appears to be more lumpy (although this2

effect could arise from the lower signal to noise inthe continuum-subtracted H images.) Prominent2

dust lanes are visible in all continuum images for allthree galaxies, even in the 1.6 micron images (H-band). The 1-0S(3) H /Ha 1[N II] emission ratios2

in the H knots in PKS 0745-191 and Abell 2597 are2

between 5 and 15%, unusually high for photo-ioniza-tion by power-law AGN or for shocks with v . 50

21km s or so. The Ha /Pa emission (scaled fromnuclear spectra) does not appear to be absorbed bymore than a factor of 2 over Case B.

The H emission, just like the Ha emission,2

cannot be from the X-ray cooling gas, because themolecular emission is too luminous by factors of100–1000. As of this poster, we have not completedour analysis of the energetics of shocks, thermalexcitation, X-ray photo-ionization, and UV fluores-cence, but we have some preliminary assessments.Shocks (e.g. Hollenbach and McKee, 1989) and

Fig. 1. Central 11.30 by 11.30 of Abell 2597. power-law photo-ionization are ruled out by the high

158 M. Donahue, J. Mack / New Astronomy Reviews 46 (2002) 155 –158

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