Astron. Astrophys. 325, 923–932 (1997) ASTRONOMYAND

ASTROPHYSICS

The extended CO J=1–0 emission in NGC 253S. Houghton1, J.B. Whiteoak2, B. Koribalski2, R. Booth3, T. Wiklind3, and R. Wielebinski41 Department of Astrophysics & Optics, School of Physics, University of New South Wales, Sydney NSW 2052, Australia2 Australia Telescope National Facility, PO Box 76, Epping NSW 2121, Australia3 Onsala Space Observatory, S-43992, Onsala, Sweden4 Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, D-53121 Bonn, Germany

Received 10 January 1997 / Accepted 11 March 1997

Abstract. The distribution of 12CO(1–0) has been mappedover the spiral galaxy NGC 253 using the Swedish-ESO Sub-millimetre Telescope (SEST). Emission was found to extend22.4 arcmin along the major axis and to be concentrated towardsthe nucleus with an LSR systemic velocity of 235 km s−1. TheCO intensity and velocity distribution suggests three subsys-tems: a dense, highly-rotating nuclear cloud, a disk (with a lineof nodes at a position angle of 63) extending 150 arcsec eachside of the nucleus containing a bar or spiral arms that are lo-cated at an azimuth of 21, and an outer disk with a positionangle of 52. The total gas mass derived for the full extent is2.4 × 109 M, about 2% of the calculated dynamic mass of10.5× 1010 M.

Key words: galaxies: individual: NGC 253 – galaxies: kine-matics and dynamics – radio lines: ISM

1. Introduction

NGC 253 is an edge-on barred spiral galaxy and the brightestmember of the Sculptor group. De Vaucouleurs et al. (1991)have classified it as type SAB(s)c. A distance of 3.4 Mpc hasbeen commonly used (Sandage & Tammann 1975); however wewill adopt the value of 2.5 Mpc from more recent studies (e.g.Davidge & Pritchet 1990, Davidge et al. 1991), in which case1 arcsec corresponds to 12 pc. At a blue magnitude level of 25,Pence (1980) found that the galaxy has an inclination of 78.5and is extended over an area of 27.7 arcmin × 7 arcmin withits major axis at a position angle of 51.

NGC 253 contains arguably the best example of a starburstnuclear region (< 1 kpc in size). It is partially obscured by over-lying dust lanes at optical wavelengths (Prada et al. 1996). How-ever, at infrared and radio wavelengths bright sources within aradius of 10 arcsec are present (e.g. Fomalont 1968; Becklinet al. 1973; Scoville et al. 1985; Antonucci & Ulvestad 1988;

Send offprint requests to: sjh@wodin.phys.unsw.edu.au

Telesco et al. 1993; Rice 1993; Sams et al. 1994). The centralregion also gives rise to prominent optical, infrared and radiospectral-line emission (e.g. Seaquist & Bell 1977; Ulrich 1978;Pence 1981; Munoz-Tunon et al. 1993; Forbes et al. 1993) anddense molecular clouds associated with the nucleus yield someof the strongest microwave spectral lines of extragalactic carbonmonoxide and other molecules (e.g. Mauersberger & Henkel1993).

Extended emission from the disk of the galaxy has beenobserved at X-ray, infrared and radio wavelengths (Cameron1971; Reynolds & Harnett 1983; Fabbiano & Trinchieri 1984;Hummel et al. 1984; Carilli et al. 1992; Pietsch & Trumper1993; Rice 1993). In the inner regions, a bar has been detectedat several different wavelengths extending 120 arcsec each sideof the nucleus at a position angle of 68 (Pence 1981; Scovilleet al. 1985; Forbes & DePoy 1992). In addition to this bar,observations of CO, CS, H I and optical emission lines suggesta second, smaller bar, edge-on ring or possibly a nuclear spiralwithin 30 arcsec of the nucleus (Canzian et al. 1988; Wall et al.1991; Mauersberger et al. 1996; Peng et al. 1996; Koribalski etal. 1995; Sams et al. 1994; Arnaboldi et al. 1995). Gas outflowfrom the nucleus into the halo has been suggested by Combes etal. (1977), Fabbiano & Trinchieri (1984), Turner (1985), Carilliet al. (1992), Dickey et al. (1992), Schulz & Wegner (1992).

On a larger scale, detailed studies of the distribution andkinematics of H I emission associated with NGC 253 (Puche etal. 1991; Koribalski et al. 1995) show that the gas distributionis asymmetric in the outer regions, and that this may be a con-sequence of a warped disk. Modelling involving H I and opticalresults (Puche et al. 1991; Puche & Carignan 1991) suggeststhat dark matter is required to account for rotation velocitiesthat remain high at large radii. However, interpretation of theH I results towards the nuclear region is complicated by the pres-ence of both H I emission and H I absorption against the radionucleus (Koribalski et al. 1995).

Carbon monoxide, regarded as a tracer of molecular hydro-gen gas, gives rise to spectral-line emission which is not onlygenerally detectable in nearby spiral galaxies but is often foundto be concentrated towards the central regions of spirals. Hence

924 S. Houghton et al.: The extended CO J=1–0 emission in NGC 253

studies of this molecule are particularly useful for NGC 253. Amulti-line analysis by Israel et al. (1992) suggested a moleculartorus or spiral about the nucleus, while high resolution CO datafrom Canzian et al. (1988) and Mauersberger et al. (1996) sup-ported a nuclear structure approximately 35 arcsec × 10 arcsecthat could be a molecular spiral or ring. Observations by Scovilleet al. (1985) showed the CO radial distribution with a secondarymaximum which corresponded with an inner disk in the near-infrared (radius ∼ 200′′). However, these results were limitedto the central regions and along the major axis. To better definethe dynamics of the large bar through direct comparison withthe H I results of Koribalski et al. (1995) and possibly detect nu-clear outflow, we have extended the CO coverage by mappingthe J=1–0 12CO transition (rest frequency 115.271 GHz) overNGC 253 using the Swedish-ESO Submillimetre Telescope.This telescope has been described by Booth et al. (1989).

2. Observations and reductions

The 115-GHz CO observations were made during December1993 and October 1994. The beamwidth of the 15-m SEST an-tenna at this frequency is 43 arcsec. The antenna was equippedwith a Schottky diode mixer receiver yielding system tempera-tures which varied between 335 and 1050 K with weather con-ditions and observing elevation; the average value was 630 K.

Spectra were obtained in 1993 with the low-resolution spec-trometer (LRS) providing a bandwidth of 1018 MHz covering1440 channels; the channel separation of 0.707 MHz corre-sponds to a radial velocity of 1.84 km s−1. In 1994, the LRS wassplit into two bands, each providing spectra with bandwidthsof 513 MHz over 720 channels, equivalent to a channel sepa-ration of 0.713 MHz and velocity of 1.86 km s−1. All spectrawere centred on an LSR velocity of 229 km s−1 (Mauersbergeret al. 1996), which is close to the galaxy’s systemic velocity(Pence 1981). Beam-switching with a chopper wheel rotatingat 6 Hz was used to obtain each CO spectrum. A dual beam-switch technique was used, providing two integrations whichwere averaged to obtain a flat spectral baseline – for the first thebeam was switched between the source and a position offset by11′37′′ in azimuth; for the second the telescope was moved sothat the source was in the other beam position and the offset inthe opposite direction.

The radio nucleus of NGC 253 is located at RA(J2000)00h47m33.s18, Dec(J2000)−2517′17.′′2, (precessed B1950 co-ordinates from Turner & Ho 1985; Ulvestad & Antonucci1991). Our spectra were obtained using a square grid of posi-tions centred on the position α(J2000) 00h47m33.s39, δ(J2000)−2517′14.′′1 (as in Mauersberger et al. 1996). The grid ex-tended parallel and perpendicular to the major axis of the galaxywith a position angle of 52. Inadvertently, our second datasetwas observed at a small offset (∼4 arcsec) from the first, re-sulting in slight changes of CO profile shapes. Compared tothe beamwidth of the telescope this offset is negligible forthe low resolution study shown here. To facilitate the possibleapplication of super-resolution techniques, within the central192 arcsec × 96 arcsec the grid interval was 16 arcsec and the

scan integration time 60 seconds. This interval was increased to32 arcsec (integration time 120 seconds) for a larger region of576 arcsec × 256 arcsec. In addition, the major axis was sam-pled out to 864 arcsec each side of the nucleus to obtain a fullview of the rotation of the galaxy. In total, 355 positions wereobserved; in most cases the observations were repeated. Theobservation at each position was preceded by a conventionalcalibration measurement using an ambient temperature load.This provided an antenna temperature scale (T∗A) corrected foratmospheric absorption. All measured intensities were then ad-justed for a main beam efficiency of 0.70, resulting in a scale ofmain-beam temperature Tmb.

Periodic observations of the strong 86-GHz SiO maserR Aqr provided antenna pointing corrections; average pointinguncertainties were ±5 arcsec. CO profiles within the central32 arcsec2 of NGC 253 were observed periodically and usedas additional pointing checks on the basis of their strength anddistinctive shape. As a result of these observations, some spec-tra were discarded and a few were corrected in position by the16 arcsec step-size.

The data reduction was carried out with the CLASS1 soft-ware package. Channels with interference were removed fromthe spectra and linear baseline corrections applied. Because thevelocity resolution of the two sets of observations were differ-ent, both data-sets were resampled to provide a common velocityresolution (resample task in CLASS). Spectra at the same posi-tions were averaged with a weighting according to noise. Twospectral-line cubes of data were created from averaging bothdatasets and ignoring the 4 arcsec pointing offset; one from thespectra within the 576 arcsec × 256 arcsec region, the otherfrom spectra within a region of 64 arcsec × 1728 arcsec alongthe major axis. These cubes were transferred into the AIPS2

software package for subsequent analysis. To compare the re-sults with the recent H I observations of Koribalski et al. (1995)the data cube from the inner region was averaged to a similarvelocity resolution (6.6 km s−1). Moment maps were made byfitting single Gaussians to each point and using (i) the area of theGaussian and (ii) the central velocity of the Gaussian. The sec-ond cube was summed over the direction parallel to the minoraxis at each major axis offset, resulting in a position-velocityimage.

3. Results

In Fig. 1 the positions at which the spectra were observed havebeen overlaid on an ESO Schmidt plate of NGC 253. Fig. 2shows the integrated CO spectrum obtained for the positions onthe 32′′ grid. (Figs. 2, 3 and 4 depict spectra taken from the 1993observations only). The profile shows evidence of the double-horn shape characteristic of edge-on spiral galaxies and showsemission covering a wide velocity range of 25–450 km s−1. A

1 Continuum and Line Analysis of Single-dish Software from theObservatoire de Grenoble and the Institute de Radio-AstronomieMillimetrique2 Astronomical Image Processing System of the US National RadioAstronomy Observatory

S. Houghton et al.: The extended CO J=1–0 emission in NGC 253 925

Fig. 1. Boxes marking the 355 positions observed using the SEST are superimposed on an ESO Schmidt plate. The step size is 16 arcsec nearthe nucleus and 32 arcsec elsewhere.

Fig. 2. The integrated CO(1–0) spectrum for the 273 spectra on thegrid of positions spaced by 32 arcsec.

first estimate of the LSR systemic velocity is given by the centreof the velocity range: 237 km s−1 (244 km s−1 heliocentric).The two peaks at the outer velocities of 47 and 422 km s−1,contributed by CO in the outer disk, appear similar; howeverthe CO emission between the peaks is not symmetrical aboutthe systemic velocity. The profile differs from its H I counterpart(Koribalski et al. 1995) because of the presence of H I absorptionat the central velocities. Nevertheless, the velocities of the two

Fig. 3. A composite of CO(1–0) profiles within the central 96 arcsecof NGC 253. The step size between spectra is 16 arcsec. The scale foreach spectrum is −0.1–2.1 K in Tmb and 0–500 km s−1in velocity.

outer CO peaks are similar to those (58 and 423 km s−1) of theH I features.

Fig. 3 shows spectra from the central 7 × 7 sampled po-sitions. The rms noise of each spectrum varies from 0.02 K to0.1 K depending on the number of spectra taken at each position.Several features are noticeable. The value of Tmb at the centre

926 S. Houghton et al.: The extended CO J=1–0 emission in NGC 253

Fig. 4. a The thick line shows the CO(1–0) spectrum towards the centralposition of NGC 253; the other profiles are offset ±16 arcsec alongthe major axis. b The central spectrum and the fitted profile, includingthe four Gaussian components of the fitted profile.

of the galaxy reaches 1.7 K but the peak temperature (2.1 K)occurs in the spectrum at a position offset of −16 arcsec alongthe major axis and at a velocity of 300 km s−1. This asymme-try in emission is consistent with recent studies finding strongnear-infrared emission to the south-west of the nucleus (Pinaet al. 1992; Sams et al. 1994). Most profiles appear to consistof several features. The central CO spectrum shows significantstructure and consists of at least two components. The relativestrengths of these components vary across the nucleus. This vari-ation is seen more clearly in Fig. 4a, which shows the centralprofile and profiles offset±16 arcsec along the major axis. Thecentral profile can be fitted well by four Gaussian componentsas shown in Fig. 4b, with fitted parameters listed in Table 1.Although the two outermost features are relatively weak, theyincrease in intensity in spectra at offsets further along the ma-jor axis. The features are present in spectra observed along theouter spiral arms.

CO emission was detected as far out as 672 arcsec (8 kpc)along the major axis each side of the nucleus of the galaxy. How-ever, the total CO extent of 22.4 arcmin along the major axis isless than the H I extent of 26 arcmin observed by Koribalski etal. (1995). The strongest CO emission in the outer regions ofthe galaxy coincide with H I maxima off the major axis whichdefine spiral arms in the outer part of the galaxy.

The distribution of CO emission for successive radial veloci-ties is shown in Figs. 5a & b, with each image covering a velocityrange of 18 km s−1. The images show a prominent central regionat almost all velocities, with a peak temperature of 2.1 K near

Table 1. Parameters for Gaussian fits of the central spectrum ofNGC 253 as shown in Fig. 4b.

Line Main-Beam Velocity Width AreaIntensity (K) (km s−1) (km s−1) (K km s−1)

1 0.19 72 46 9.72 1.14 157 98 120.03 1.53 278 150 243.84 0.10 402 33 3.7

290 km s−1. The uncorrected half-intensity diameter at this ve-locity is 55 arcsec; (the value corrected for antenna beamwidthis 34 arcsec). The position of peak temperature moves 95 arcsecbetween velocities 38 km s−1 and 416 km s−1, suggesting amolecular cloud rotating around the centre of the galaxy. Theforked distributions at intermediate velocities, and the extendedemission along the disk (e.g. 38 and 416 km s−1 for NGC 253),are consistent with the typical velocity fields of inclined spiralgalaxies. However, asymmetric emission in the forked distribu-tions occurring to the east at low velocities (e.g. 110 km s−1)and to the west at high velocities (e.g. 326 km s−1) could beevidence of a bar-like structure. In maps at velocities near thesystemic velocity (218 and 236 km s−1) there is evidence of aspiral shape in the outer contours.

The distribution of the CO emission integrated over veloc-ity is shown in Fig. 6a. For levels above 5% of the peak value,the CO occupies a region of dimensions 8 arcmin × 3 arcmin.The distribution shows a well-defined nuclear cloud centredat α(J2000) 00h47m33.s1, δ(J2000) −2527′14′′, with a half-intensity diameter of 63 arcsec (46 arcsec when corrected forthe antenna beamwidth). A bar-like region extending approx-imately 130 arcsec (1.6 kpc) each side of the nucleus is at anangle of 11 east of the major axis (i.e. at a position angle of63).

Fig. 6b shows the velocity field associated with the CO emis-sion shown in Fig. 6a, obtained from the moments of the spec-trum at each position. Although the velocity field is consistentwith a general regular galactic rotation in the outer parts of thegalaxy, in the central ±100 arcsec the isovelocity lines suggesta warp towards a lower position angle. However, the velocityfield shown in Fig. 6b represents only the bulk motions and isinsensitive to fine structure. The latter is better represented inposition-velocity (pv) distributions.

The pv distribution of the CO is shown as contours in Fig. 7overlaid with the optical results (dots) from Arnaboldi et al.(1995) and H I results (greyscale) from Koribalski et al. (1995).The CO distribution is similar to that derived by Scoville et al.(1985) within the major axis range they observed (±315 arcsecfrom the nucleus). The distribution shows a very steep veloc-ity gradient in the nuclear region, covering a velocity range(±200 km s−1) which nearly exceeds that due to the rotation ofthe outer disk of the galaxy; the intensity peak is offset to highervelocities. Within an offset of±50 arcsec, the velocity gradientof the ridge of maximum intensity is large and relatively constantwith change of position, a phenomenon that is consistent with

S. Houghton et al.: The extended CO J=1–0 emission in NGC 253 927

Fig. 5. Channel maps of the CO(1–0)data with a velocity resolution of18 km s−1. The central velocity of thechannel used is written in each frame,with axes of RA and Dec offsetsfrom the positionα(J2000) 00h47m33.s4,δ(J2000)−2517′14′′. The contour lev-els are 3, 5, 10, 20, 30, 50, 70 and 90%of the peak emission. The beamsize isshown in the bottom left corner of thefirst frame.

solid-body rotation. Beyond an offset of ±150 arcsec, the in-tensity ridge shows ‘flat’ rotation. An estimate of the systemicvelocity more accurate than found from the central spectrumcan be calculated from the symmetry of the outer parts of thepv distribution: 235 km s−1.

At offsets greater than ±100 arcsec from the nucleus, thepv distribution shows many similarities to the H I distributionof NGC 253 from Koribalski et al. (1995). However, at smalleroffsets the H I pv distribution is confused by the presence ofabsorption; therefore the CO distribution better defines the gen-eral kinematics. In fact, the CO and H I results differ markedlynear the nucleus – whereas the CO velocities extend from 40to 430 km s−1, with an emission peak near 300 km s−1, theH I absorption (which can originate only in front of the nu-cleus) extends from 45 to 349 km s−1, with peak absorptionat 190 km s−1. At the same time, H I emission is also presentat higher velocities and presumably originates from behind thenucleus.

4. Discussion

4.1. The central region

The (0,0) CO spectrum towards our central position (Fig. 4b) hasa similar shape to the corresponding spectrum from Mauers-berger et al. (1996) obtained with a higher angular resolu-tion of 23 arcsec. The integrated area of the (0,0) spectrum(Fig. 2) is found to be 264 K km s−1 for T∗A or 377 K km s−1

for Tmb. These values compare favourably with 272 K km s−1

(T∗A) from Scoville et al. (1985) with a beamwidth of 50 arcsecand 360 K km s−1 (Tmb) from Mauersberger et al. (1996) scaledto a similar resolution. Other parameters also compare well.

As mentioned earlier, four components can be identifiedin the (0,0) spectrum. The two weaker components (at 73 and401 km s−1) can be traced, over the full range of radius along themajor axis, to the outer spiral arms of the galaxy. This impliesthat these spiral features extend to within 43 arcsec (516 pc) ofthe centre of the galaxy. The pv distribution of Fig. 7 suggests

928 S. Houghton et al.: The extended CO J=1–0 emission in NGC 253

Fig. 5. Continued

that the central CO is associated with a rapidly rotating cloudcomplex enveloping a massive nucleus, which is supported bythe two kinematically distinct CO maxima at radii of ∼ 10′′

found by Israel et al. (1992) and Mauersberger et al. (1996).Arnaboldi et al. (1995) found two inner Lindblad resonancesnear the nucleus and suggested that these may be causing anuclear spiral within a radius of 25 arcsec.

The apparent solid-body rotation in our low resolution COresults, in which the radial velocity changes by 372 km s−1 over60 arcsec (a radius of 360 pc), would be equivalent to a centraldynamical mass of 2.9 × 109 M. This total mass is uncertainas it will scale with the angular size of the cloud complex whichis not well defined in our results. Our value is consistent withthe dynamical mass of 2.6× 109 M derived by Mauersbergeret al. (1996) for the inner 480 pc.

4.2. The Bar

In Fig. 6a the inner contours are not aligned with those of themore extended disk, but appear to define a separate system of

extent ±130 arcsec at a position angle of 63. They are also‘bending’ back into the main disk at the outer extremities. Thisfeature is comparable with an infrared bar-like feature found byScoville et al. (1985) at 2.2 µm. The infrared contour map showsthe bar with a position angle of 68 and a size of 240 arcsec. TheCO feature can also be seen to be slightly bending back towardsthe major axis at its extremities. Near-infrared photometric dataof Forbes & DePoy (1992) also show a bar of size∼300 arcsecat a position angle of ∼70, with a “spiral structure emanatingfrom the ends”. There does not appear to be any evidence tosupport the existence of nuclear outflow to the north as foundin the OH results of Turner (1985).

The velocity field distortion already pointed out in Fig. 6bappears to be related to the feature discussed above. However,the line of nodes for the isovelocity contours is not aligned withthe extension of the CO emission. The two results could bereconciled if there is an inner disk in a plane with a major axisdefined by the isovelocity contours (position angle ∼40), butcontaining bars or spiral arms extending out from the nucleus

S. Houghton et al.: The extended CO J=1–0 emission in NGC 253 929

200 100 0 -100 -200

100

-100

0

100

-100

0

Offset along Major Axis (arcsec)

Offs

et a

long

Min

or A

xis

(arc

sec)

Fig. 6. Moment maps of the CO(1–0) emission towards NGC 253 withaxes of RA and Dec offsets from the position α(J2000) 00h47m33.s4,δ(J2000) −2517′14′′. (a) The integrated CO intensity map; contourlevels are 2, 3, 5, 7, 9, 11, 13, 15, 20, 25, 30, 50, 70 and 90% of thepeak. (b) The mean velocity field; contour levels are 90, 120, 150, 180,210, 240, 270, 300, 330, 360 and 390 km s−1.

at an azimuth in the plane that is appropriate to the extensionin Fig. 6a. If we assume that the inclination of this plane is thesame as the outer region (78.5), then the azimuth of the barwould be 21.

4.3. Comparison of CO and H I

The channel maps of CO emission (Fig. 5) show a resemblanceto H I line maps published by Puche et al. (1991) in both theextended structure along the disk and the forked distribution.However, the H I shows symmetrical emission intensity in the‘forks’ and no evident distortion from regular rotation near thenucleus. The CO results are more biased towards the molecularbar.

The H I gas (Fig. 1a of Koribalski et al. 1995) appears to beconcentrated within a box-shape in the disk. This would indicatethe size of the inner disk where the molecular bar is located, asedge-on bars reveal box or peanut shapes because of verticalresonances (Combes 1994). A comparison of CO and H I pvdistributions within 100 arcsec of the nucleus indicates that thehigher velocity gas is primarily behind the nuclear continuumemission, whereas the lower velocity gas (absorbing) is in front.The two major line profile components in the CO may representtwo sides of a nuclear ‘bar’ or ‘mini-spiral’.

We find many similarities between the neutral emission (Ko-ribalski et al. 1995) and the molecular emission: the pv distri-butions are similar at radii outside the nuclear region and COspectra coincide with H I maxima near the spiral arms in theouter regions of the galaxy. This would not be expected if themolecular gas and the neutral gas generally avoided each other,

400600 200 0 -200 -400 -600

200

100

0

-200

-100

Offset along Major Axis (arcsec)

Vel

ocity

Offs

et (

km/s

)Fig. 7. Overlay of position-velocity distributions: CO (contours) and H I

(greyscale) from Koribalski et al. (1995) and optical (dots) from Arn-aboldi et al. (1995). Axes show offsets fromα(J2000) 00h47m33.s4,vLSR

235 km s−1. The greyscale range is −0.05–0.5 Jy/beam and contourlevels are 5, 10, 20, 40, 60, 80, 90, 95% of the peak.

as suggested by Sofue et al. (1995) and Honma et al. (1995).Also, the H I emission within ∼200 arcsec (Fig. 7) is alignedwith the two weaker components of the central CO spectrum(see Sect. 3).

Since gravitational torques will depopulate the co-rotationregion in a spiral galaxy (e.g. Garcıa-Burillo & Guelin 1995,Combes 1996), the lack of both molecular and atomic gas couldindicate the position of this region. Arnaboldi et al. (1995) usedthe length of the observed bar to deduce a co-rotation radius of325 arcsec, taking into account the projection of the bar into theline of sight. Our data roughly agree with this prediction; theCO emission weakens between radii of 350–400 arcsec (e.g.Fig. 7). A better defined position of 370′′ is provided by the H I

gas, especially toward the SW where the CO emission is veryweak.

4.4. Mass estimates

If we assume the total integrated CO emission and the densityof H2 are directly related, the mass of molecular hydrogen insolar masses, M(H2), is given by

M (H2) = 3.7× 10−19 X D2 〈ICO〉 (1)

where X is the constant of proportionality (X = N(H2)/〈ICO〉), Dis the distance to the galaxy in Mpc and 〈ICO〉 =

∫ ∫TmbdvdΘ

(K km s−1 arcsec2) is the main-beam brightness temperature in-tegrated over the velocity range and solid angle (Θ) covered bythe CO emission.

930 S. Houghton et al.: The extended CO J=1–0 emission in NGC 253

Assuming the generally adopted value of X = 3 × 1020

(Solomon et al. 1987), our central spectrum yields a mass of H2

of 4.5×108 M. This is consistent with both the molecular mass(4.2 × 108 M) derived by Krugel et al. (1990) using the dustcontinuum emission of the nucleus and the value (4.1×108 M,converted to our values of X and D) from Scoville et al. (1985)for the central 50 arcsec, but does not compare well with thevalue of 2.2 × 108 M calculated by Peng et al. (1996) for CSwithin a radius of 20 arcsec. We also note that if the gas in thenucleus is optically thin and the conversion factor is reducedto X = 0.3 × 1020 (Mauersberger et al. 1996), then our centralmolecular mass would be in exact agreement with CO(2–1)measurements from Mauersberger et al. (1996): 0.45×108 M.

Our results for the 576×256 arcsec2 area (Fig. 6) give a totalH2 mass of 1.6× 109 M. Scoville et al. (1985) gave an upperlimit of 4×109 M for a radius of 270 arcsec (converted value).A mass estimate for the full extent of the CO can be made if weassume that we only observe half of the emission in the outerregions (see Fig. 1) of the galaxy. This results in an H2 mass of1.8× 109 M and a total gas mass of 2.4× 109 M, assumingthat the mean atomic weight in the interstellar medium per Hatom is 1.36 (Allen 1991).

In the general rotation of an edge-on spiral galaxy, motionis commonly described by a relation given by Brandt (1960)resulting in a dynamical mass M (solar masses) of (e.g. Rogstadet al. 1967)

M = (1.5)3/n 7.45× 104 V2max Rmax D (2)

where rotational velocity V (km s−1) reaches a maximum value(Vmax) at the turnover point Rmax (in arcmin) in galactocentricradius and the galaxy distance D is in Mpc. For a spiral galaxywith inclination i, the observed radial velocity is V sin i. Theindex n, with a commonly accepted value of 3, is a measure ofthe extent of the flatness of the rotation curve.

For NGC 253, the dynamics are complicated by the exis-tence of several kinematic systems. These affect the velocitiesobserved along the major axis (which are generally used to de-rive the parameters used in Eq. 2). However, the pv distributioncan be used to separate the velocity contributions. The mas-sive rotating nucleus is responsible for the high central veloc-ity gradient. Further out, to a radius of 200 arcsec, the innerbar/spiral arms dictate the major motions (i.e. the emission atthe extreme velocities). At larger radii, the maximum velocitiesare associated with the rotation of the outer disk – its velocitywould be zero towards the nucleus, and probably follows thelower velocity envelope at radii out to 300 arcsec. These outerregions of the galaxy will provide the greatest contribution tothe total mass. Although a well-defined velocity turnover is notpresent in Fig. 7, a maximum velocity of 190 km s−1 (the aver-age of both sides) may be present near R = 600 arcsec. If thesevalues and an inclination of 78.5 are adopted then the totalmass is 10.5 × 1010 M. This compares well with the value of9.8×1010 M (nuclear, bulge and disk components) calculatedby Sofue (1996).

Fig. 8. The angular velocity (Ω) as a function of radius. The solidline represents the fit to the data. The open circles are the CO resultsfrom this paper, the filled circles are optical results from Arnaboldi etal. (1995) and the stars are H I results from Koribalski et al. (1995).The Ω ± κ/2 and Ω ± κ/4 curves are the dashed and dotted linesrespectively. The extent of the nuclear starburst region, the molecularbar, the outer molecular ring and the likely location of the corotationradius are indicated. The corresponding bar pattern speeds calculatedhere and in Arnaboldi et al. (1995) are shown as the dash–dot lines.

4.5. Dynamical analysis

Fig. 7 can be used to deduce the approximate locations of variousresonances within NGC 253. However, a rotation curve of analmost edge-on, barred galaxy will be affected by the resonancesas well as the unknown projection angle of the bar. The locationof the resonances are therefore only approximate.

In the linear resonance theory, the angular velocity is definedas Ω = V (R)/R where V (R) is the circular rotational velocityat galactocentric radius R. The radial epicyclic frequency, κ, isthen expressed by,

κ2 = 2Ω(

2Ω + RdΩdR

)(3)

In Fig. 8 we plot the angular velocity as a function of radius(no correction is made for the galaxy’s inclination which will bea constant scaling factor). The observed values are representedby open circles (CO), filled circles (optical from Arnaboldi etal. 1995), stars (H I from Koribalski et al. 1995) and the fit bythe solid line. Two different fits have been applied for the COresults: (i) following the ridge of maximum intensity and (ii)approximating a curve without the central rotating mass. Thetwo noticeably discrepant CO results at a radius of 50′′ fromfit (i) are thus due to the smoothing of the true rotation curve.These points would move towards the solid line in Fig. 8 if thefeatures (rotating central mass and outer disk) were followedindependently for the rotation curve. Outer and inner Lindbladresonances (OLR and ILR) occur when the bar pattern speed

S. Houghton et al.: The extended CO J=1–0 emission in NGC 253 931

ΩB = Ω±κ/m form = 2. The location of the ultraharmonic res-onance (4/1 resonance) is found when m = 4. These curves arealso plotted in Fig. 8 (dashed for m = 2 and dotted for m = 4).

As co-rotation occurs when the pattern speed equals the an-gular velocity, our result calculated from the lack of emission(see Sect. 4.3) corresponds to a value of 43 km s−1 kpc−1 forΩB. This compares well with the 48 km s−1 kpc−1 from Arn-aboldi et al. (1995). Although our results are in part based ontheir data, it is likely that the point of co-rotation is in the region3.5–4.5 kpc with a bar pattern speed of 50–40 km s−1 kpc−1.Since Ω is relatively flat at large radii, the value of ΩB is lessuncertain than the radius of co-rotation.

Due to the steep slope of Ω−κ/2 at small radii, the locationof the ILR is relatively insensitive to the value of ΩB. Gas withinthe outer ILR (OILR) and the inner ILR (IILR) is subjectedto a negative gravitational torque (Combes 1996) and shouldaccumulate at the IILR. The positions of the OILR at ∼400 pcand the IILR at ∼100 pc are very close to the nucleus of thegalaxy and are consistent with the CO maxima located at radii of∼10 arcsec (Fig. 7 and Mauersberger et al. 1996). It is possiblethat the molecular gas is concentrated in a nuclear ring closeto the IILR and is being consumed by vigorous star formationactivity. However, the location of the IILR is strongly dependenton the rotation curve at small radii which is not well sampledand is derived using a linear theory in a region with a possiblestrong resonance.

In the plot of the pv distribution of both the CO and H I data(Fig. 7), two concentrations can be seen at±200 arcsec from thenucleus. If these complexes represent a ring, as is often seen inbarred galaxies (Combes 1996), they should be associated withthe 4/1 resonance. This resonance is located at a radius of 2.4–2.6 kpc (Fig. 8) for the pattern speeds calculated above. Theseresults are consistent with the molecular ring being producedby gas flowing from the co-rotation point towards the ILRs andbecoming trapped in symmetric orbits (hence, not subject to thegravitational torque exerted by the bar). We must, however, bearin mind that the location of the 4/1 resonance is more sensitiveto the fit of the observed rotation curve than the location of theOILR.

5. Conclusion

The SEST has been used, with a 43 arcsec beam, to map the dis-tribution of the J=1–0 transition of CO across the southern spiralgalaxy NGC 253. The following conclusions were reached:

(a) CO emission with an LSR systemic velocity of235 km s−1 has been detected (rms noise averaging 0.05 K inT∗A) to an offset of ±672 arcsec from the nucleus, and is highlyconcentrated towards the nucleus. The emission of CO in theouter regions is displaced from the major axis and is associatedwith H I spiral arm features.

(b) The nucleus is embedded in a dense molecular cloudrotating as a solid body with maximum rotational velocity of186 km s−1. The corresponding dynamical mass for a diameterof 60 arcsec is 2.9× 109 M.

(c) The CO intensity and velocity distribution suggests threesubsystems: a dense highly rotating cloud, a disk containing abar or spiral arms that are located with an azimuth 20, and anouter disk.

(d) The H2 mass from within 576 arcsec × 256 arcsec is1.6 × 109 M (X = 3 × 1020). The total gas mass for the fullextent of CO emission is estimated as 2.4× 109 M.

(e) The dynamical mass given by the maximum rotationalvelocity of the outer disk (194 km s−1) at 600 arcsec is 10.5×1010 M, resulting in the gas mass being 2% of the total massof NGC 253.

(f) A co-rotation radius of 4.4 kpc with a linear theory ofresonance results in the location of the nuclear molecular gasbeing consistent with a ring close to the inner ILR. A molecularring seen in the pv distribution at ∼200 arcsec is close to theinferred 4/1 resonance.

Acknowledgements. SEST is operated jointly by ESO and the SwedishNational Facility for Radio Astronomy, Onsala Space Observatory atChalmers University of Technology.

SH and BK acknowledge financial support from the Max-Planck-Forschungspreis (1992) awarded to RW and JBW.

SH thanks Thierry Forveille and Stephane Guilloteau from Institutde Radio-Astronomie Millimetrique, Grenoble and Lewis Knee fromthe Onsala Space Observatory for assistance with the CLASS package.

References

Allen C.W., 1991, Astrophysical Quantities. The Athlone Press, Lon-don

Antonucci R.R.J., Ulvestad J.S., 1988, ApJ 330, L97Arnaboldi M., Capaccioli M., Cappellaro E., Held E.V., Koribalski B.,

1995, AJ 110, 199Becklin E.E., Fomalont E., Neugebauer G., 1973, ApJ 181, L27Booth R.S., Delgado G., Halstrom M. et al., 1989, A&A 216, 315Brandt J.C., 1960, ApJ 131, 293Cameron M.J., 1971, MNRAS 152, 403Canzian B., Mundy L.G., Scoville N.Z., 1988, ApJ 333, 157Carilli C.L., Holdaway M.A., Ho P.T.P., De Pree C.G., 1992, ApJ 399,

L59Combes F., 1994. In: Munoz-Tunon C. (ed.) V Canary Islands Winter

School: The Formation of Galaxies, p. 29Combes F., 1996. In: Buta R., Crocker D.A., Elmegreen B.G. (eds.)

Proc. IAU Coll. 157, Barred Galaxies. ASP Conf. Ser., p. 286Combes F., Gottesman S., Weliachew L., 1977, A&A 59, 181Davidge T.J., Pritchet C.J., 1990, AJ 100, 102Davidge T.J., Le Fevre O., Clark C.C., 1991, ApJ 370, 559de Vaucouleurs G., de Vaucouleurs A., Corwin H.G. jr. et al., 1991,

Third Reference Catalogue of Bright Galaxies. SpringerDickey J.M., Brinks E., Puche D., 1992, ApJ 385, 501Downes D., Solomon P.M., Radford S.J.E., 1993, ApJ 414, L13Fabbiano G., Trinchieri G., 1984, ApJ 286, 491Fomalont E.B., 1968, ApJS 151, 177Forbes D.A., Ward M.J., Rotaciuc V. et al., 1993, ApJ 406, L11Forbes D.A., DePoy D.L., 1992, A&A 259, 97Garcıa-Burillo S., Guelin M., 1995, A&A 299, 657Honma M., Sofue Y., Arimoto N., 1995, A&A 304, 1Hummel E., Smith P., van der Hulst J.M., 1984, A&A 137, 138Israel F.P., White G.J., Baas F., 1995, A&A 302, 343

932 S. Houghton et al.: The extended CO J=1–0 emission in NGC 253

Koribalski B., Whiteoak J.B., Houghton S., 1995, Proc. Astron. Soc.Aust. 12, 20

Krugel E., Chini R., Klein U. et al., 1990, A&A 240, 232Kutner M.L., Ulich B.L., 1981, ApJ 250, 341Mauersberger R., Henkel C., 1993, Reviews in Modern Astronomy 6,

ed. G. Klare, Astronomische Gesellschaft, p. 69Mauersberger R., Henkel C., Wielebinski R., Wiklind T., Reuter H.-P.,

1996, A&A 305, 421Munoz-Tunon C., Vilchez J.M., Castaneda H.O., 1993, A&A 278, 364Pence W.D., 1980, ApJ 239, 54Pence W.D., 1981, ApJ 247, 473Peng R., Zhou S., Whiteoak J.B., Lo K.Y., Sutton E.C., 1996, ApJ 470,

821Pietsch W., Trumper J., 1993, Adv Space Research 13, 171Pina R.K., Jones B., Puetter R.C., 1992, ApJ 401, L75Prada F., Manchado A., Canzian, B. et al., 1996, ApJ 458, 537Puche D., Carignan C., 1988, AJ 95, 1025Puche D., Carignan C., 1991, ApJ 378, 487Puche D., Carignan C., Van Gorkom J.H., 1991, AJ 101, 456

Radford S.J.E., Solomon P.M., Downes D., 1991, ApJ 368, L15Reynolds I., Harnett J.I., 1983, Proc. Astron. Soc. Aust. 5, 235Rogstad D.H., Rougoor G.W., Whiteoak J.B., 1967, ApJ 150, 9Rice W., 1993, AJ 105, 67Sams B.J.III, Genzel R., Eckart A., Tacconi-Garman L., Hofmann R.,

1994, ApJ 430, 33Sandage A., Tammann G.A., 1975, ApJ 196, 313Schulz H., Wegner G., 1992, A&A 266, 167Scoville N.Z., Soifer B.T., Neugebauer G. et al., 1985, ApJ 289, 129Solomon P.M., Rivolo A.R., Barrett J., Yahil A., 1987, ApJ 319, 730Seaquist E.R., Bell M.B., 1977, A&A 60, L1Sofue Y., 1996, ApJ 458, 120Sofue Y., Honma M., Arimoto N., 1995, A&A 296, 33Telesco C.M., Dressel L.L., Wolstencroft R.D., 1993, ApJ 414, 120Turner J.L., 1985, ApJ 299, 312Turner J.L., Ho P.T.P., 1985, ApJ 299, L77Ulrich M.-H., 1978, ApJ 219, 424Ulvestad J.S., Antonucci R.R.J., 1991, AJ 102, 875Wall W.F., Jaffe D.T., Israel F.P., Bash F.N., 1991, ApJ 380, 384

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