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«NGC 4314. III. In owing Molecular Gas Feeding a Nuclear Ring of Star Formation1 G. Fritz Benedict McDonald Observatory, University of Texas, Austin, ...»

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NGC 4314. III. In

owing Molecular Gas Feeding

a Nuclear Ring of Star Formation1

G. Fritz Benedict

McDonald Observatory, University of Texas, Austin, TX 78712

Beverly J. Smith

IPAC, California Institute of Technology, Pasadena, CA 91125

and

Je rey D. P. Kenney

Astronomy Dept., P.O. Box 208101, Yale University, New Haven, CT 06520

Received ; accepted

1 Based on observations collected at the Owens Valley Radio Observatory

{2{

ABSTRACT

NGC 4314 is an early-type barred galaxy containing a nuclear ring of recent star formation. We present CO(1-0) interferometer data of the bar and circumnuclear region with 2:300  2:200 spatial resolution and 13 km s 1 velocity resolution acquired at the Owens Valley Radio Observatory. These data reveal a clumpy circumnuclear ring of molecular gas. We also nd a peak of CO inside the ring within 200 of the optical center that is not associated with massive star formation. We construct a rotation curve from these CO kinematic data and the mass model of Combes et al. (1992). Using this rotation curve, we have identied the location of orbital resonances in the galaxy. Assuming that the bar ends at corotation, the circumnuclear ring of star formation lies between two Inner Lindblad Resonances, while the nuclear stellar bar ends near the IILR.

Deviations from circular motion are detected just beyond the CO and H ring, where the dust lanes along the leading edge of the bar intersect the nuclear ring.

These non-circular motions along the minor axis correspond to radially inward streaming motions at speeds of 20 90 km s 1 and clearly show in owing gas feeding an ILR ring. There are bright H ii regions near the ends of this in ow region, perhaps indicating triggering of star formation by the in ow.

{3{

1. Introduction Inner Lindblad resonances (ILRs) are thought to strongly in uence gas motions and star formation in the central regions of galaxies. Gas surface densities often peak near the ILR (Combes et al. 1992; Kenney et al. 1992), lending support to the idea that the inward ow of gas along galaxian bars slows down and piles up between the OILR and the IILR (Combes 1988; Shlosman et al. 1989). Studies of the CO (1-0) distribution within the central regions of barred spirals (Kenney et al. 1992; Kenney 1995; Regan, Vogel, & Teuben 1995; Rand 1995; Sakamoto et al. 1995) show a variety of molecular gas morphologies near the ILR, including rings, spiral arms, and \twin peaks" with two maxima where the dust lanes along the primary bar cross the ILR. In the case of the starburst galaxy NGC 3504, the CO peaks near the nucleus, probably inside the ILR (Kenney et al. 1993). The dynamical and evolutionary di erences responsible for this variety are not yet understood, but are critical for understanding starbursts and the formation and evolution of compact circumnuclear stellar disks (Kormendy 1993) and stellar bulges (Pfenniger & Norman 1990).

At present, only a handful of barred spirals have been mapped in CO at sucient resolution to study the e ects of ILRs on the gas distribution and kinematics. The barred spiral galaxy NGC 4314 is a particularly good galaxy to study because it exhibits evidence for many features associated with resonances and it is nearly face-on (i  23). It has a large-scale stellar bar of diameter 13000 (6.3 kpc) and a prominent circumnuclear ring of star formation of diameter 1000 (500 pc) that is visible in H (Pogge 1989), radio continuum (Garcia-Barreto et al. 1991; hereafter G-B), and optical color maps (Benedict et al. 1992;

hereafter P1). CO (1-0) mapping of NGC 4314 (Combes et al. 1992) at 500 resolution revealed the presence of a molecular ring slightly smaller than that of the ring of star formation. Outside of these rings, a blue elliptical feature of diameter 20-2500 (1 kpc) is seen in optical color maps, which may correspond to a ring of relatively young but non-ionizing {4{ stars (P1). This feature, which can be seen on the unsharp masked optical image shown in Fig. 1 (from P1), is elongated perpendicular to the primary bar, suggesting that it extends outward to the IILR. Combes et al. (1992) suggest that in this galaxy star formation is propagating inwards. This is in contrast with the evolutionary scenario proposed by Kenney et al. (1993) for strong starbursts, in which star formation devours gas most rapidly in the center, ultimately forming a ring of gas near the ILR.

Inside the H ii region ring, a nuclear stellar bar of diameter 800 (400 pc) is seen in Hubble Space Telescope (HST) I band data (Benedict et al. 1993; hereafter P2). This nuclear bar is aligned parallel with the large scale bar, which suggests that the nuclear bar has the same pattern speed as the primary bar, and lies within the IILR (Binney & Tremaine 1987).

In this paper, we present new CO (1-0) maps of NGC 4314, obtained at twice the spatial resolution as the Combes et al. (1992) data. The observations are described in Section 2, while the CO distribution and kinematics are discussed in Sections 3 and 4, respectively. In Section 5 we compare NGC 4314 with other galaxies and discuss star formation in this galaxy. Some general properties of NGC 4314 are tabulated in Table 1.

2. Observations

We observed NGC 4314 in the CO (1-0) line using the Owens Valley Radio Observatory (OVRO) millimeter-wave interferometer (Padin et al. 1991) in ve congurations between April and June 1991. Projected baselines ranged in length from 10 to 200 meters. At that time OVRO consisted of a three element array with 10.4m dishes (HPBW = 6500 and SIS receivers. The 32 5 MHz receivers give 13 km s 1 resolution and an instantaneous bandpass of 416 km s 1. The quasar 1219 + 285 was used for phase calibration, Uranus and Neptune for ux calibration.





{5{ After calibration, channel maps were made with both uniform and natural weightings using AIPS. Uniform weighting produces higher resolution maps, with less sensitivity, while natural weighting provides greater sensitivity with less resolution. The latter reduction mode provides higher signal to noise information on the regions in the primary bar of NGC 4314 containing fainter CO intensity.

All maps were made as 256  256 arrays with 0: 5 pixel 1. Natural weighting produced a resolution of 3:1  2: 7 with channel noise 0.022 Jy beam 1. These maps were CLEANed to a 0.045 Jy beam 1 level. Using uniform weighting, we obtained a 2:3  2: 2 beam with channel noise 0.028 Jy beam 1, CLEANed to a 0.07 Jy beam 1 level. For both weightings we have made primary beam corrections. Fig. 2 presents our uniformly weighted channel maps. These maps have absolute positional accuracy of about 0: 5. Table 2 summarizes the interferometer-related parameters.

The total CO ux observed by OVRO is 245 Jy km s 1 (Table 2), while the CO luminosity is 4:7  107 K km s 1 pc2. This luminosity is consistent with the single dish observations of G-B and with the Nobeyama Millimeter Array results of Combes et al.

(1992) (see Table 1). Therefore we have recovered essentially all of the CO ux from this galaxy with our OVRO observations.

–  –  –

Uniformly weighted and naturally weighted total intensity maps were constructed using the AIPS moment routines. Fig. 3 presents the uniformly weighted CO intensity map. The strongest CO emission arises from an incomplete, clumpy ring with an o -center minimum.

There are ve clumps of emission along the ring, separated by gaps. The largest of these {6{ gaps is at p.a. = 239. A secondary gap occurs at p.a. = 330. In addition to the clumps in the ring, we nd strong CO emission within 200 of the optical center.

Two symmetrically located features along p.a. 20 extend out from the ring. These are associated with the dust lanes that lie along the leading edge of the primary stellar bar.

These lanes are detected in CO as they curve around to intersect the nuclear ring. We refer to these features as in ow spurs, since the CO velocity eld shows clear evidence for in ow motions here (x4.2).

Fig. 4 shows the naturally weighted CO intensity distribution, covering about 4 times the area on the sky as Fig. 3. Note the CO clumps extending on either side out from the ring along p.a. 148. These extensions lie along the leading edge of the primary stellar bar (P1).

–  –  –

In Fig. 5a we overlay the uniform-weight CO map contours on the optical unsharp masked image shown in Fig. 1. The dust and CO are relatively coincident for the CO arcs to the SE and NW, while the dust lanes lie outside the CO ring to the NE and SW. Along the primary bar, a dusty area is seen to the SE (see Fig. 1), a region called the `dust bowl' in P1. A comparison of optical and CO intensity from the naturally-weighted data for a larger area of the galaxy (Fig. 5b) shows weak ( 3) CO sources coincident with the dust bowl. Note in both Fig. 5a and 5b the anticoincidence of CO with the outer ellipse of relatively young stars.

In Fig. 6 the Pogge (1989) H + [N II] map is overlaid on a uniform weight CO {7{ intensity contour plot. The Wakamatsu & Nishida (1980) H -bright knots A (p.a.  90) and B (p.a.  178 ) are the second and third strongest H sources in the Pogge map. The H peaks tend to lie outside of the CO ring (see also the proles in Fig. 7 - 9). At the largest CO gap, H is also weak.

The H + [N II] map resembles the 6 cm radio continuum map shown in G-B and our observations conrm the results of Combes et al. (1992) that the ring of star formation has a larger radius than the molecular ring.

3.2.2. Proles

The proles in Fig. 7 - 10 o er a more detailed and quantitative comparison between the distributions of CO intensity, blue light (B, P1), (H +[NII]) (uncalibrated H +[N II] surface intensity, Pogge 1989), I J and B H (respectively identied as the best tracer of dust and of new stars in P1). Fig. 7, 8, and 9 are primary bar major axis, primary bar minor axis, and E-W cuts across the galaxy, respectively. Fig. 10 shows a cut through the dust bowl, perpendicular to the primary bar. For these plots the uniformly-weighted CO intensity has been converted to H2 (M pc 2) assuming (Bloeman et al. 1986, Kenney et al. 1992) H2 (M pc 2 ) = 470 ICO (Jy km s 1 arcsec 2 ) H is a tracer of the local star formation rate (SFR). These plots also contain the log of the ratio of (H +[N II])/CO. In H ii regions, where the observed H +[N II] emission is dominated by H and the ionization is due to OB stars, this ratio is proportional to the gas depletion timescale, or Star Formation Eciency.

In Fig. 7, the dust lanes at r = 5: 5, seen as reddening in the I J prole, are very prominent in CO. There is also considerable CO emission near the nucleus; Fig. 7, 8, and 9 {8{ show the CO surface density near the nucleus as high as in the ring of star formation. The (H +[NII]) proles corroborate that the H ii regions in the ring lie slightly outside the CO ring. Dips in the B H prole are coincident with the new stars at the H ring, r = 700.

The (H +[N II])/CO ratio shows a relatively higher SFE near the nuclear ring, especially to the NW.

There is also a secondary source in (H +[N II]), peaking  100 S of the center. This is likely due to strong [N II] emission contaminating the H +[N II] image near the center. Keel (1983) identied NGC 4314 as having an \[N II]"-type optical spectrum. Measured values of [N II]/H for the NGC 4314 nucleus range between 1 and 2.5 (Keel 1983; Wakamatsu & Nishida 1980; Stau er 1982; Smith et al. 1987) and Keel (1983) identied NGC 4314 as an \[N II]"-type galaxy. The ratios of H , H, [N II] 6584, [O III] 5007, and [O I] 6300 given in Smith et al. (1987) are consistent with the LINER denition given in Heckman (1980) & Dahari (1985). Therefore the nuclear peaks in H +[N II] and (H +[N II])/CO may not be due to star formation.

Note the coincidence of an increase in I J prole, a decrease in B, and a peak in the H2 prole to the SE. These all argue that CO and dust are associated. How good is this correlation? Is there CO only where there is dust? The peak in the H2 prole to the SE (Fig. 7) is well-described by a gaussian centered at r = 5: 4, with FWHM = 4: 14. From the HST I data presented in P2, we nd the dust lane can also be described by a gaussian centered at r = 5: 5, with FWHM = 1: 60. Along this p.a. the OVRO synthesized beam can be considered to be a gaussian with FWHM = 2: 20. Convolving the OVRO beam with the dust prole produces a distribution with FWHM = 2: 70. Evidently, the CO is not conned to the dust lane, but spills over in a symmetric distribution. We next deconvolve the OVRO beam from the Fig. 7 H2 prole and determine that the actual CO distribution has FWHM = 3: 51 and a peak H2 = 2550 M pc 2 at the dust lane center.

{9{ Along a slice perpendicular to the primary stellar bar (Fig. 8), we again nd strong CO near the nucleus and near the dust lanes associated with the ring. (H +[N II]) peaks outside the CO to the NE, but inside to the SW. The (H +[N II])/CO ratio picks up near r = 400 at a gap in the CO ring. There is an increase in the SFR at r = 100, due to the observed CO deciency, or to [N II] contamination from the LINER phenomenon. To the SW, the redder dust lane at r = 600 is present in CO as well. The CO maximum at r  4005 to the NE is not as pronounced in the color indices as that to the SW, to be expected if the SW side of the galaxy is nearer.

For completeness we provide a set of proles (Fig. 9) passing East-West through knot A, the second most intense H ii region seen in the Pogge (1989) data. The dust to CO correlation is not as striking, primarily because dust signatures are weak in I J and B H.

There is one other location where comparisons with optical data might prove useful; at the \dust bowl" in the primary stellar bar to the SE at r  2300. In Fig. 10 we plot proles passing through the dust bowl, perpendicular to the primary stellar bar. The proles are B, V I, and B V (P1) along with H2 derived from the natural weight map (Fig. 3).

Again, dust and CO are well correlated, with reddening in V I matching increases in H2 all along the prole.



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