THE ASTROPHYSICAL JOURNAL, 511 :730È738, 1999 February 11999. The American Astronomical Society. All rights reserved. Printed in U.S.A.(

MOLECULAR GAS IN 3C 293 : THE FIRST DETECTION OF CO EMISSION AND ABSORPTION IN ANFANAROFF-RILEY TYPE II RADIO GALAXY

A. S. EVANS

Division of Physics, Math, and Astronomy MS 105-24, California Institute of Technology, Pasadena, CA 91125 ; ase=astro.caltech.edu

D. B. SANDERS AND J. A. SURACE1Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822 ; sanders=ifa.hawaii.edu, jason=ipac.caltech.edu

AND

J. M. MAZZARELLA

IPAC, MS 100-22, California Institute of Technology, Jet Propulsion Laboratory, Pasadena, CA 91125 ; mazz=ipac.caltech.eduReceived 1998 June 30 ; accepted 1998 September 1

ABSTRACTThe Ðrst detection of CO emission in a Fanaro† and Riley type II (i.e., edge-brightened radio lobe

morphology) radio galaxy is presented. Multiwavelength (0.36È2.17 km) imaging of 3C 293 shows it tobe a disk galaxy with an optical jet or tidal tail extending toward what appears to be a companiongalaxy 28 kpc away via a low surface brightness envelope. The molecular gas appears to be distributedin an asymmetric disk rotating around an unresolved continuum source, which is presumably emissionfrom the active galactic nucleus (AGN). A narrow km s~1) absorption feature is also(*vabsD 60observed in the CO spectrum and is coincident with the continuum source. Assuming the standard COconversion factor, the molecular gas mass is calculated to be 1.5] 1010 several times the(H2) M

_,

molecular gas mass of the Milky Way. The high concentration of molecular gas within the central 3 kpcof 3C 293, combined with the multiwavelength morphological peculiarities, supports the idea that theradio activity has been triggered by a gas-rich galaxy-galaxy interaction or merger event.Subject headings : galaxies : active È galaxies : individual (3C 293) È galaxies : ISM È

infrared : galaxies È ISM: molecules È radio lines : galaxies

1. INTRODUCTION

Substantial evidence exists that galaxy interactions ormergers are the trigger for the nuclear activity in radiogalaxies. Optical imaging surveys by Heckman et al. (1986)and Smith & Heckman (1989a, 1989b) have shown that asigniÐcant fraction of low-redshift powerful (P408 MHz Z 3] 1025 W Hz~1) radio galaxies possess morphologicalpeculiarities (e.g., tails, fans, bridges, and dust lanes) com-monly associated with the collisions of galaxies. A signiÐ-cant fraction of low-redshift radio galaxies is also known tobe luminous far-infrared sources (Golombek, Miley, &Neugebauer 1988). In many cases, the shape of the spectralenergy distributions (SEDs) at mid- and far-infrared wave-lengths is consistent with thermal emission from dustheated by young massive stars and/or the active galacticnucleus (AGN).

Improvements in millimeter receiver technology and theavailability of moderate size (i.e., total collecting areagreater than 500 square meters) millimeter arrays has madeit possible to detect and spatially map star-forming molecu-lar gas in radio galaxies. Molecular gas, which forms on thesurface of dust grains, is of particular interest in active gal-axies because it is also a likely source of fuel for the centralengine during the initial phases of the merger. To date,several low-redshift radio galaxies have been unam-biguously detected with single-dish telescopes, revealingsubstantial quantities (1 ] 109 to 5 ] 1010 of molecu-M

_)

lar gas (Phillips et al. 1987 ; Mirabel, Sanders, & Kazes1989 ; Mazzarella et al. 1993 ; Evans 1998 ; Evans et al.1999a). To date, however, detections of CO emission have

1 Present address : IPAC, MS 100-22, California Institute of Tech-nology, Jet Propulsion Laboratory, Pasadena, CA 91125.

been limited to radio compact and Fanaro† and Riley I(edge-darkened radio lobe morphology ; Fanaro† & Riley1974) radio galaxies, the latter of which tend to have rela-tively weak radio power.

In this paper, the Ðrst detection of CO emission andabsorption in a Fanaro† and Riley II (edge-brightenedradio lobe morphology) radio galaxy, 3C 293 (P408 MHzDW Hz~1), is presented. The galaxy possesses4.5] 1025optical morphological peculiarities and extended radio andCO emission, which makes it an ideal source for studyingthe relationship between emission in radio galaxies at multi-ple wavelengths.

The paper is divided into Ðve sections. Section 2 is adiscussion of the optical, near-infrared, and millimeterobservations of 3C 293. The data reduction methods andmolecular gas mass calculations are summarized in ° 3.Section 4 contains an estimate of the dynamical mass of 3C293 based on the CO data, a discussion of the 2.7 mmcontinuum emission and CO absorption, and a detailedcomparison of the morphologies of the galaxy at optical,near-infrared, millimeter, and radio wavelengths. Section 5summarizes the results.

Throughout this paper, we adopt km s~1H0 \ 75Mpc~1 and Thus, for a source at a redshift ofq0\ 0.0.0.045, 815 pc subtends 1A in the sky plane.

2. OBSERVATIONS

The interpretation of the CO data presented in this paperhas beneÐted from observations of the galaxy at otherwavelengths. Below, the ground-based U@-, B-, I-, andK@-band imaging observations, as well as the millimeter

730

MOLECULAR GAS IN 3C 293 731

observations, are summarized.2 Additional radio data andHubble Space Telescope (HST ) data have also beenobtained courtesy of J. P. Leahy and the HST Archive.

2.1. Ground-based ImagingGround-based imaging observations of 3C 293 were

made at the University of Hawaii (UH) 2.2 m telescope. TheU@-, B-, and I-band images were obtained on 1998 March 25using the Orbit Semiconductor 2048 ] 2048 CCD camera.The original scale is pixel~1 (Wainscoat 1996), but the0A.09CCD was read out with 2] 2 pixel binning. Four ditheredexposures were taken, with integration times of 480, 360,and 360 s each for U@, B, and I, respectively. Near-infrared,K@ observations of 3C 293 were also obtained on 1996 April24 using the UH Quick Infrared Camera (QUIRC; Hodappet al. 1996), which consists of a 1024] 1024 pixel HgCdTeAstronomical Wide Area Infrared Imaging (HAWAII)array. The near-infrared observations were done at f/10,providing a Ðeld of view of 3@] 3@. Five dithered exposureswere taken, each with an integration time of 180 s.

2.2. CO SpectroscopyThe initial millimeter observations of 3C 293 were made

with the NRAO3 12 m telescope on 1996 January 24. Thetelescope was conÐgured with two 256] 2 MHz channelÐlterbanks and dual polarization SIS spectral line receiverstuned to the frequency 110.35 GHz, corresponding to aredshift of 0.045 (Sandage 1966 ; Burbidge 1967) for theCO(1] 0) emission line. Observations were obtained usinga nutating subreÑector with a chop rate of D1.25 Hz. Sixminute scans were taken, and a calibration was done everyother scan. Pointing was done on 3C 273 prior to the obser-vations, and the pointing was checked at the end of theobservations using Mars. The total duration of the obser-vations was 8.8 hr.

Follow-up aperture synthesis maps of CO(1 ] 0) and 2.7mm continuum emission in 3C 293 were made with theOwens Valley Radio Observatory (OVRO) MillimeterArray during three observing periods from 1997 Septemberto November. The array consists of six 10.4 m telescopes,and the longest observed baseline was 242 m. Each tele-scope was conÐgured with 120] 4 MHz digital correlators.Observations done in the low-resolution conÐguration(1997 September and October) provided a (FWHM)D4A.0synthesized beam with natural weighting, and observationsin the high-resolution conÐguration (1997 November) pro-vided a beam of (FWHM). During the observations,D2A.5the nearby quasar HB 89 1308]326 (1.27 Jy at 110 GHz;B1950.0 coordinates was13h08m07s.56, ]32¡36@40A.23)observed every 25 minutes to monitor phase and gain varia-tions, and 3C 273 was observed to determine the passbandstructure. Finally, observations of Uranus were made forabsolute Ñux calibration.

3. DATA REDUCTION AND RESULTS

3.1. Imaging DataThe U@-, B-, and I-band data reduction was performed

using IRAF. The data reduction consisted of Ñat-Ðeldingindividual images, scaling each image to its median value tocorrect for o†sets in individual images, then shifting and

2 The UH U@- and K@-band Ðlters have central wavelengths of 3410 A�and 21,700 respectively.A� ,

3 The NRAO is a facility of the National Science Foundation operatedunder cooperative agreement by Associated Universities, Inc.

median combining the images. The Ðnal images were thenboxcar smoothed by 4 ] 4 pixels.

Figure 1 shows two three-color images of 3C 293. InFigure 1a, 3C 293 is presented in a linear stretch to show thehighest surface brightness features. Figure 1b shows thegalaxy with a logarithm stretch, which reveals additional,extended low surface brightness structure.

The K@-band data reduction was done in a similarmanner to the U@-, B-, and I-band data reduction, exceptthat after the median level of each image was subtracted, theindividual frames were averaged together without spatialshifting using a min/max averaging routine. This procedureremoves the contribution of astronomical sources in theindividual frames to produce an object-free ““ sky ÏÏ image.This sky image was then subtracted from the individualframes before they were shifted and averaged. The resultantwide-Ðeld K@-band image is discussed in ° 4.5.

3.2. NRAO 12 m T elescope DataThe NRAO 12 m telescope data were reduced using the

IRAM data reduction package CLASS. The individualscans were checked for baseline instabilities, then averagedtogether. The emission line was observed to span the veloc-ity range [500 to 350 km s~1 (where 0 km s~1 correspondsto the systemic velocity of 3C 293) ; thus a linear baselinewas subtracted excluding this velocity range. The spectrumwas then smoothed to 43 km s~1.

Figure 2 shows the CO(1 ] 0) spectrum. The emissionline is moderately broadÈthe velocity width at half themaximum intensity, of D400 km s~1. For com-*vFWHM,parison, the mean value for infrared luminous galaxies is250 km s~1 (Sanders, Scoville, & Soifer 1991). An absorp-tion feature is also observed at the systemic velocity of 3C293 (D13,500 km s~1), which corresponds to the velocity ofthe H I absorption feature associated with the galaxy (Baan& Haschick 1981).

The CO emission line has a intensity, ofICO\ Tmb*v,2.0^ 0.4 K km s~1, where is the main-beam brightnessTmbtemperature. Assuming a Kelvin-to-Jansky conversionfactor of 25.2 Jy K~1 (P. Jewell 1995, privatecommunication), the emission-line Ñux, is 51^ 11SCO*v,Jy km s~1.

3.3. OV RO DataThe OVRO data were reduced and calibrated using the

standard Owens Valley array program MMA (Scoville et al.1992). The data were then exported to the mappingprogram DIFMAP (Shepherd, Pearson, & Taylor 1995).

The resultant continuum and integrated intensity mapsare shown in Figure 3. The CO emission in 3C 293 isextended over a region 7A (5.7 kpc) in diameter. Fourspectra have also been extracted, showing clearly thenarrow absorption feature km s~1) seen in the(*vabsD 60single-dish spectrum. In contrast to the CO emission, thecontinuum emission is unresolved and appears to belocated at the center of the molecular gas morphology. Boththe synthesized maps of the CO absorption (not shown) andthe 2.7 mm continuum are spatially coincident, which indi-cates the presence of CO-emitting gas along the line of sightto the continuum source.

The total Ñux measured within a 7A diameter aperture is53 ^ 10 Jy km s~1, consistent with the Ñux obtained fromthe 12 m in a 74A beam.

732 EVANS ET AL. Vol. 511

FIG. 1.ÈU@-, B-, and I-band images of 3C 293. The U-, B-, and I-band images are displayed as blue, green, and red, respectively. The images are shownwith (a) linear and (b) logarithmic stretches. North is up, and east is to the left.

3.4. Molecular Gas MassFor a universe, the luminosity distance for aq0\ 0.0

source at a given redshift, z, is

DL\ cH0~1z(1] 0.5z)(Mpc) . (1)

Given the measured Ñux, the CO luminosity of aSCO*v,source at redshift z is

LCO@ \A c22klobs2

BSCO*vD

L2(1] z)~3 , (2)

No. 2, 1999 MOLECULAR GAS IN 3C 293 733

FIG. 2.ÈNRAO 12 m CO(1] 0) spectrum of 3C 293. The spectrum isplotted in units of main-beam brightness temperature. The spectrum has a1 p rms of 0.52 mK per velocity channel, where the resolution of a singlechannel is D43 km s~1.

where c is the speed of light, k is the Boltzmann constant,and is the observed frequency. For a luminosity dis-lobstance expressed in units of Mpc, can be written asL CO@

L CO@ \ 2.4] 103A SCO *vJy km s~1

BA DL2

Mpc2B(1] z)~1

(K km s~1 pc2) . (3)

For 3C 293, Jy km s~1 and Mpc;SCO *v\ 51 DL\ 180

thus K km s~1 pc2. To calculate the massL CO@ \ 3.8] 109of molecular gas in 3C 293, a reasonable assumption ismade that the CO emission is optically thick and therma-lized and that it originates in gravitationally bound molecu-lar clouds. Thus, the ratio of the mass and the COH2luminosity is given by a \ M(H2)/L CO@ P [n(H2)]1@2/Tb

M_(K km s~1 pc2)~1, where and are the density ofn(H2) T

bH2and brightness temperature for the CO(1 ] 0) transition

(Scoville & Sanders 1987 ; Solomon, Downes, & Radford1992). Multitransition CO surveys of molecular clouds inthe Milky Way (e.g., Sanders et al. 1993) and in nearly

starburst galaxies (e.g., et al. 1993) have shown thatGu� stenhotter clouds tend to be denser such that the density andtemperature dependencies tend to cancel each other. Thevariation in the value of a is approximately a factor of 2 fora wide range of kinetic temperatures, gas densities, and COabundance (e.g., a \ 2È5 [K km s~1 pc2]~1 ; Radford,M

_Solomon, & Downes 1991). We adopt a value of 4 M_(K km s~1 pc2)~1 for a, which is similar to the value deter-

mined for the bulk of the molecular gas in the disk of theMilky Way (Strong et al. 1988 ; Scoville & Sanders 1987).4Thus, the molecular gas mass of 3C 293 is 1.5 ] 1010 M

_(D5 times the molecular gas mass of the Milky Way) ;however, note that could be as low as 7.3 ] 109M(H2) M

_(D2.5 times the Milky Way molecular gas mass). In addi-tion, the concentration of molecular gas within the inner 3kpc of 3C 293 is D530 pc~2. Such a molecular gasM

_concentration is 4È400 times greater than that of nearbyearly-type spiral galaxies (e.g., Young & Scoville 1991) buton the low end of concentrations determined for aH2sample of merging galaxies studied by Sco-L IR [ 1011 L

_ville et al. (1991) and Bryant (1996).

4. DISCUSSION

As mentioned in ° 1, several other radio galaxies havebeen detected in CO by single-dish telescopes. Millimeterinterferometry of these radio galaxies is currently under-way ; for the present moment, the global properties of 3C293 will be brieÑy compared with these radio galaxiesbefore discussing 3C 293 in detail.

Table 1 summarizes the infrared and CO properties of 3C293 relative to other CO-luminous radio galaxies. Thegalaxy 3C 293 is one of the most molecular gas rich of theradio galaxies detected to date but has a relatively lowinfrared luminosity and low ratio. The ratioL IR/L CO@is commonly referred to as the star formationL IR/L CO@““ efficiency ÏÏ ; if the infrared luminosity is dominated by rep-rocessed light from young, massive stars, then the ratio indi-cates how efficiently molecular gas is converted into stars.In the case of 3C 293, a ratio of 8.9 indicates that the galaxyis producing stars at the rate of quiescent spiral galaxies(e.g., Sanders & Mirabel 1996). All of the more infraredluminous radio galaxies have very high ratios, butL IR/L CO@caution should be taken when interpreting their infraredluminosities ; in the most luminous infrared galaxies, there isstrong evidence that the infrared luminosity is heavily con-

4 The equivalent cm~2 (K km s~1)~1.XCO\ n(H2)/ICO\ 2.5] 1020

TABLE 1

PROPERTIES OF CO-LUMINOUS RADIO GALAXIES

Parameter 3C 293 Other CO Luminous Radio Galaxiesa

DL

(Mpc) . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 30È500L IR (8È1000 km) (]109 L

_) . . . . . . 33 9È1700

zCO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.0446 0.0018È0.12SCO*v (Jy km s~1) . . . . . . . . . . . . . . . . 51^ 11 30È200*vFWHM (km s~1) . . . . . . . . . . . . . . . . . . 400 250È400L CO@ (]108 K km s~1 pc2) . . . . . . . 3.7 2.5È120M(H2) (]109 M

_) . . . . . . . . . . . . . . . . 15 1È50

L ir/LCO@ (L_/K km s~1 pc2) . . . . . . 8.9 16È220

a Data of other CO-luminous radio galaxies have been compiled from the followingsources : Phillips et al. 1987 ; Mirabel et al. 1989 ; Mazzarella et al. 1993 ; Evans et al. 1999a.

734 EVANS ET AL. Vol. 511

FIG. 3.ÈOVRO map of the CO(1] 0) emission and 3 mm continuum emission in 3C 293. The continuum emission corresponds to a position of(B1950.0). The 1 p rms is 0.00436 and 0.00335 Jy beam~1 for the high and low]high resolution data, respectively. The13h50m03s.20, ]31¡41@32A.8

high-resolution data are plotted as 60%, 70%, 80%, 90%, and 99% contours, where 60% corresponds to 3 p rms and 100% corresponds to a peak Ñux of0.0218 Jy beam~1. The low]high resolution data are plotted as 50%, 60%, 70%, 80%, 90%, and 99% contours, where 50% corresponds to 3 p rms and100% corresponds to a peak Ñux of 0.0201 Jy beam~1. Spectra extracted at several positions in the CO emission show clear evidence of rotation, as well asabsorption associated with the central continuum source. The spectra have a 1 p rms of 0.021 Jy per velocity channel, where the resolution of a single channelis D10 km s~1. A low-resolution CO map of 3C 293 can be found in Evans (1998).

taminated by reprocessed AGN light (e.g., Evans et al.1998a). We will brieÑy return to this discussion in ° 4.6.

4.1. Optical MorphologyHeckman et al. (1986) described 3C 293 as a primary

galaxy connected to a southwestern companion galaxy by abridge of emission that fans into a westward extending tidaltail beyond the companion. Although the images in Figure1 do not have a wide enough Ðeld of view to show the tidaltail associated with the companion galaxy, Figure 1b clearlyshows the low surface brightness emission that appears tobe connecting the two galaxies. Indeed, the halo appears tocompletely envelope the primary galaxy.

The high surface brightness morphology of the primarygalaxy is clearly that of a disk galaxy (Fig. 1a). The bulgecomponent has redder colors than the disk component andcontains a prominent, warped dust lane (see also vanBreugel et al. 1984). The disk component also appears to bewarped, but its apparent morphology may simply be anartifact of dust obscuration. It is not well understood whatcauses galaxy warps, but in the case of 3C 293, the warpmay result from gravitational interactions with the com-panion galaxy, gas infall, and/or a misalignment betweenthe disk and halo (e.g., Binney 1992).

4.2. CO EmissionThe CO distribution of 3C 293 appears to be asymmetric

No. 2, 1999 MOLECULAR GAS IN 3C 293 735

(Fig. 3) and may be subject to the same warping observed inthe dust lanes and possibly the large-scale disk. As is clearfrom the four extracted spectra and CO morphology, themolecular gas is distributed in a disk rotating around theunresolved continuum source. Taking a disk radius of 3A.5(2.8 kpc) and a velocity width at half the maximum intensityof km s~1, the dynamical mass is*vFWHMD 400

MdynBr*vFWHM2bG sin2 i

\ 1.0] 1011 [(sin2 i)~1 M_] , (4)

where G is the gravitational constant, i is the disk inclina-tion, and b is approximately unity (e.g., Bryant & Scoville1996). Thus the molecular gas mass is D10% or less of theestimated dynamical mass. The remainder of the masswithin this radius consists of stars and the central engine.

A molecular gas warp has been inferred in the radiogalaxy Centaurus A by Quillen et al. (1992). From models oftheir single-dish telescope maps, Quillen et al. (1992) havespeculated that the gas is in a triaxial potential. If the molec-ular gas in 3C 293 is also in such a potential, equation (4)may overestimate the true dynamical mass (i.e., if the gas isin elliptical orbits instead of circular). Thus the moleculargas in the inner 2.8 kpc may constitute more than 10% ofthe total estimated gas mass.

4.3. T he 2.7 mm Continuum EmissionWhat is the source of the 2.7 mm continuum emission?

The continuum emission, which has a Ñux density of 0.19 Jy,Ðts the extrapolated power-law relation of thefl P l~ahigher frequency radio Ñux densities,5 indicating that the2.7 mm continuum emission is nonthermal.

As a double check, assume that the 2.7 mm emission isdue to thermal emission from dust, as is the case for Arp220. Arp 220, which is at a distance of 77 Mpc, has a 2.7 mmÑux density of 0.035 Jy (Scoville, Yun, & Bryant 1997).Thus, the 2.7 mm luminosity of 3C 293 is 30 times higherthan that of Arp 220. If the assumption is made, as for Arp220, that the emission at km is emanating fromj0[ 200optically thin (qD 1) dust radiating at a temperature ofD40 K, the dust mass is given by

Mdust BSobsDL

2(1] z)i0B(l0, T )

, (5)

where is the observed Ñux density, is the rest-Sobs i0frequency mass absorption coefficient with a value of 0.085g~1 cm2 (i.e., 10 g~1 cm2 at 250 km scaled to 2.7 mm;Hildebrand 1983), and T ) is the rest-frequency value ofB(l0,the Planck function. The derived dust mass of 3C 293 isthus 2.0 ] 1010 Further, if the dust in 3C 293 is radi-M

_.

ating at a temperature of 15 K, the derived dust mass is afactor of 3 higher. Given that the standard gas-to-dust ratiois 100È200 and that the 2.7 mm continuum emission is notspatially extended like the CO emission, the 2.7 mm contin-uum emission is unlikely to be thermal emission emanatingfrom dust associated with the molecular gas. Thus, the con-tinuum emission most likely emanates from processesdirectly associated with a supermassive nuclear black holeor circumnuclear accretion disk.

4.4. T he Nature of the CO AbsorptionH I absorption features in several low-redshift radio gal-

axies have been extensively studied (e.g., Baan & Haschick

5 The radio densities were obtain from data compiled by the NASAExtragalactic Database (NED).

1981 ; Mirabel 1989 ; Conway & Blanco 1995). Such studieshave attempted to determine the location and mass of H I

within the host galaxies. In 3C 293, the depth of the COabsorption is shallower in the single-dish spectrum than inthe OVRO spectrum, which indicates that beam dilution ismasking the true absorption depth. Given this, the opticaldepth, of 3C 293 isqCO, qZ ln [Icont/(Icont [ *Iabs)]Z 0.69,where is the continuum emission Ñux density andIcont *Iabsis the CO absorption depth.

Although the velocity dispersions of individual absorbingclouds are probably typical of giant molecular clouds (i.e.,

km s~1), the dispersion of the absorption feature[10km s~1) may be due to the motion of the clouds(*vabsD 60

around the nucleus of the galaxy. Assuming this is the case,the absorption occurs in clouds at a distance of rcloud\

pc from the continuumM(:r)G/*vabs2 D 120(M(:r)/108 M_)

source, where is the stellar/gaseous/black hole massM(:r)interior to the absorbing clouds and G is the gravitationalconstant. This is comparable to the metric size of the dusttorus surrounding the nucleus of the nearby FR II radiogalaxy 3C 270 (NGC 4261), detected using the HST (Ja†e etal. 1996).

Is the velocity dispersion of the absorbing clouds reallydue to circumnuclear clouds in circular orbits ? Similarstellar dispersion velocities have been observed in severalgalaxies believed to contain quiescent black holes (i.e., gal-axies that may have once been radio galaxies or quasars) ; inthe dwarf spheroidal M32, which is believed to possess a2 ] 106 black hole, the velocity dispersion is 60 km s~1M

_and increases to D90 km s~1 within the central 1A (4 pc ;e.g., Kormendy & Richstone 1995). The elliptical galaxyNGC 3377, believed to harbor a 2] 108 black hole,M

_has a velocity dispersion of 90 km s~1 that increases toD158 km s~1 within the central 1A (48 pc ; Kormendy et al.1998). However, if the dynamical mass within the inner 2.8kpc of 3C 293 is on the order of 1 ] 1011 3C 293 as aM

_,

whole may have a mass comparable to a massive ellipticalgalaxy Within 100 pc of the nucleus, the(Z3 ] 1011 M

_).

dispersion may actually be D150È250 km s~1, similar tothose observed for NGC 3115 and NGC 4594 (The Som-brero Galaxy ; e.g., Kormendy & Richstone 1995). The COabsorption may then be occurring in clouds in noncircularmotion well outside of the circumnuclear region, as isbelieved to be the case for some of the clouds responsible forthe molecular absorption observed in Centaurus A(Wiklind & Combes 1997).

4.5. Comparison between the CO Distribution of RadioMorphology

In Figure 4, the large-scale radio emission of 3C 293(Leahy, Pooley, & Riley 1986) is superposed on the wide-Ðeld K@-band image. The large jets appear to be orientednearly perpendicular to the major axis of the primarygalaxy. Based on the major axis of the CO distribution (Fig.3) and the large-scale structure, one might naively assumethat the radio jets have simply escaped along the path ofleast resistance, the path perpendicular to the molecular gasand stellar distribution. Such a scenario would be consistentwith the idea that the central engine is fed by a moleculartorus/accretion disk and that the radio jets escape along theaxis perpendicular to the accretion disk (e.g., Blandford1984).

The structure in the inner few arcseconds, however, com-plicates matters : Figure 4a shows a high-resolution 5 GHz

736 EVANS ET AL. Vol. 511

FIG. 4.ÈRadio (1.5 GHz) image of 3C 293 superposed on the logarithmic near-infrared (2.17 km) image. The near-infrared image has been boxcarsmoothed 4 ] 4 pixels ; the unsmoothed pixel scale is pixel~1. (a) High-resolution 5 GHz MERLIN image and high-resolution CO(1 ] 0) distribution0A.19(see Fig. 3) superposed on the 7000 HST image. The pixel scale of the 7000 image is pixel~1. North is up, and east is to the left.A� A� 0A.046

MERLIN map (Akujor et al. 1996) superposed on both thehigh-resolution CO map and an archival HST 7000 of 3CA�293. The radio map has been registered with the CO map byassuming that the core of the 5 GHz emission is coincidentwith the 2.7 mm continuum emission (the coordinates of the5 GHz core and the 2.7 mm continuum emission di†er byless than which is much less than the resolution of the0A.3,OVRO data). The registration of the MERLIN map withthe HST image was done by Ðrst aligning the radio knotswith features observed in a recently obtained near-infraredimage of 3C 293 taken with the Near-Infrared Camera andMultiobject Spectrometer (NICMOS) aboard HST (Evanset al. 1999b). Such an alignment places the core of the 5GHz emission on what appears to be the nucleus of thegalaxy. Note that the nuclear dust lane observed in Figure 1

breaks up into multiple dust lanes in the HST image, whichmakes the nuclear region of 3C 293 resemble that of Cen-taurus A, which is dissected by a prominent warped dustlane (Sandage 1961).

The axis of the inner radio emission that extends (2.02A.5kpc) from the nucleus is rotated D30¡ relative to the outer1.5 GHz radio emission, which itself spans 200A (160 kpc)from the tip of the bright northwest lobe to the tip of thefainter southeast lobe. Indeed, the twist of the eastern jet isapparent in the large-scale emission, which curves from thecore toward the southern lobe. Such a jet morphology canbe caused by (1) the redirection of the jets from a glancingimpact with a molecular cloud or from a sudden change inthe density gradient (e.g., van Breugel et al. 1984), (2) by amerger event or interaction, causing a warp in the

No. 2, 1999 MOLECULAR GAS IN 3C 293 737

galaxy and thus shifting the radio axis (as in IRASP09104]4109 ; Hines et al. 1999), or (3) by a precessingradio jet. The Ðrst scenario is supported by the fact that theeastern jet appears to twist northward after passing the7000 o†-nuclear knot, then southward after passing theA�centroid of the western CO component. At larger radii, thegalactic density gradient would drive the large-scale radioemission to be perpendicular to the molecular and stellardistribution of the galaxy. However, it is difficult to recon-cile this scenario with the relativistic velocity of the large-scale radio jets, as implied by the apparent Dopplerboosting (dimming) of the approaching northwest (recedingsoutheast) jets ; an impact with molecular clouds wouldundoubtedly cause the inner jet to lose substantial amountsof kinetic energy. The second scenario is supported by themorphologies of the primary and companion galaxies (Fig.1 ; Heckman et al. 1986). Given this, the inner radio jets mayrepresent a more recent outburst triggered by a galaxy-galaxy interaction. Following Akujor et al. (1996), if it isassumed that the jet propagation speed is 0.1c, the radiooutburst resulting in the large-scale structure occurred3 ] 106 yr ago, and the companion outburst was producedonly 3 ] 104 yr ago. The third scenario is related somewhatto the second ; the creation of a radio jet in a dynamicallyunrelaxed environment could result in the observed precess-ion, and thus the high-resolution MERLIN map reÑects thepresent pole axis of the supermassive black hole.

4.6. T he Interaction/Merger Status of 3C 293Much of the interpretation of the dynamic state of 3C 293

is dependent on whether or not the companion galaxy is thetrigger for the strong interaction/merger. There is indeedstrong morphological evidence that the companion isassociated with 3C 293 (° 4.1), but to our knowledge, thereexists no published redshift of the companion. However, thecompanion appears to be a galaxy and not a foregroundstar ; the measured FWHM of the companion in the 2.17km image is as compared with for stars in the Ðeld.2A.1, 1A.2

From the morphologies of 3C 293 and the companion, itmight at Ðrst appear that the companion penetratedthrough the nuclear region of 3C 293 approximately (28kpc/250 km s~1)D 1 ] 108 yr ago, leaving a trail ofoptical/near-infrared debris (e.g., Figs. 1 and 4). Alternative-ly, the bridge of optical/near-infrared emission between 3C293 and the companion may be a nonaxisymmetric tidalfeature viewed nearly edge-on, created as the orbit of thecompanion decays via dynamical friction (see simulationsby Hernquist & Mihos 1995). However, a determination ofthe relative masses of the two galaxies suggests that thecompanion is not massive enough to have caused the mor-phological disturbances observed in 3C 293 (Fig. 4a). Therelative masses of the two galaxies can be estimated fromthe fact that the spectral energy distribution of supergiantstars peaks at D1.6 km, and thus the observed 2.17 kmluminosity is correlated with the mass of the galaxy. Theratio of companion mass to the mass of 3C 293 is thereforeestimated to be D0.03. Note that this ratio does not includelight from the nucleus of 3C 293, which may emanate fromthe active galactic nucleus.

Given that interactions with the companion are anunlikely trigger for the present state of 3C 293, the linearfeature extending from the host galaxy is most likely a tidalremnant from a merger event occurring more than 109 yrago (see simulations by Barnes & Hernquist 1992, 1996).

Similar one-sided tidal features are observed in advancedmergers such as Mrk 273 (e.g., Mazzarella & Boroson 1993).A major di†erence between 3C 293 and advanced mergerssuch as Mrk 273, Arp 220, and the radio galaxy 3C 120 isthat 3C 293 has a relatively low ratio. This,L IR/LCO@however, may simply indicate that the star formationand/or AGN activity in 3C 293 that is typically responsiblefor boosting the infrared luminosity is waning.

The instabilities resulting from the merger haveundoubtedly induced a loss of angular momentum in themolecular disk, causing a fraction of the gas to fall inward,ultimately serving as fuel for the nuclear activity. Even-tually, the stellar distribution will dynamically relax intothat of an elliptical or an early-type spiral galaxy, possiblymaking 3C 293 resemble more classical FR II radio galaxiessuch as Cygnus A.

5. SUMMARY

In this paper, the Ðrst detection of CO emission in an FRII type radio galaxy, the galaxy 3C 293, has been presented.The analysis of the CO data has beneÐted from data takenat a wide range of wavelengths. The following conclusionsare drawn:

1. The optical morphology of 3C 293 shows a primaryand what appears to be a companion galaxy enveloped by alow surface brightness halo, consistent with previous opticalimages of the galaxy. In addition, the primary galaxy has awarped dust lane and a possible warped disk, most likelyresulting from an interaction/merger, gas infall, and/or mis-alignment between the disk and halo of the primary galaxy.

2. The CO emission line is relatively broad (*vFWHMD400 km s~1) and contains a km s~1 absorption*vabsD 60feature at the same velocity as the H I absorption featurepreviously observed. Assuming the standard CO-to-H2mass conversion factor, the inferred molecular gas mass of3C 293 is 1.5] 1010 M

_.

3. The CO emission in 3C 293 is observed to be extendedover a 7A (5.7 kpc) diameter region, and the concentra-H2tion within the inner 3 kpc is D530 pc~2. An unre-M

_solved, 2.7 mm continuum source is also detected, which isspatially coincident with the systemic absorption feature.The morphology and kinematics of the CO emission areconsistent with a disk of molecular gas rotating around thecontinuum source.

4. The Ñux density of the 2.7 mm continuum source (0.19Jy) is consistent with a power-law extrapolation of the radiowave Ñux densities. This, and the fact that the continuumemission is not coincident with the CO emission, indicatesthat the 2.7 mm continuum emission is nonthermal innature and that it most likely emanates from a circumnu-clear accretion disk around a supermassive black hole andnot from dust associated with the molecular gas.

5. The radio jets of 3C 293 appear to twist by about 30¡from the inner 10A to the radio lobes. The cause of the twistis either due to the e†ects of a varying interstellar densitygradient or precession, both of which are connected with astrong interaction or merger event in the recent history of3C 293.

Based on the above conclusions, the observed propertiesof 3C 293 result from a strong interaction or merger event.The resultant instabilities have most likely induced gas inthe inner molecular disk to lose angular momentum and fall

738 EVANS ET AL.

into the center of the gravitational potential well. Such ascenario provides a plausible explanation of how the centralengines of radio galaxies are fueled.

A. S. E. thanks A. Readhead, M. Shepherd, N. Scoville,and T. Pearson for many useful discussions and J. P. Leahyfor providing the 1.5 and 5 GHz radio maps. We thank thesta†s of the NRAO 12 m telescope, OVRO, and the UH 2.2m telescope for their assistance. We also thank the refereefor many useful suggestions. A. S. E. and D. B. S. were

supported in part by NASA grants NAG5-3042 and NAG5-3370, respectively. J. M. M. was supported by the Jet Pro-pulsion Laboratory, California Institute of Technology,under contract with NASA. The NRAO is a facility of theNational Science Foundation operated under cooperativeagreement by Associated Universities, Inc. The OwensValley Millimeter Array is supported by NSF grants AST93-14079 and AST 96-13717. This research has made use ofthe NASA/IPAC Extragalactic Database (NED), which isoperated by the Jet Propulsion Laboratory.

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