Structure in the Neutral Hydrogen Disk of the Spiral Galaxy, IC

Home , IC 342, NGC 2403

STRUCTURE IN THE NEUTRAL HYDROGEN DISK OF THE SPIRAL GALAXY IC 342 LUCIAN P. CROSTHWAITE AND JEAN L. TURNER Department of Physics and Astronomy, UCLA, Los Angeles, CA 90095; lucian=astro.ucla.edu, turner=astro.ucla.edu AND PAUL T. P. HO Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; pho=cfa.harvard.edu Received 1999 September 30; accepted 2000 January 7

ABSTRACT We present 38Aresolution Very Large Array 21 cm continuum and H I line emission observations of the spiral galaxy IC 342, at an adopted distance of 2 Mpc. Kinematic evidence exists for a m \ 2 spiral density wave in the inner disk with a corotation radius located at 4 kpc and a possible four-arm pattern in the outer disk. On smaller scales, outside of the central depression in H I column density, H I is organized into a complex pattern of relatively short (D2È5 kpc), interconnected, spiral arm segments. Numerous ““ holes ÏÏ are distributed throughout the H I disk. By considering the e†ects of shear, structures that are not self-gravitating, such as holes and voids, cannot be long-term phenomena. The timescale, combined with the total energy required to evacuate holes, leads us to reject wind and supernovae origins for the large-scale pattern of H I holes in IC 342. Gravitational instabilities in the disk form on a timescale that is short compared with the rotation period of the disk. The pattern of H I spiral arm segments exists on a scale that is consistent with their being material arms that result from gravitational instabilities. The H I cavities are a natural remnant of the process. Key words: galaxies: individual (IC 342) È galaxies: ISM È galaxies: spiral È galaxies: structure È ISM: H I

1. INTRODUCTION galaxies. As such, H I is a less reliable tracer of total gas in The gas disks of spiral galaxies have complicated mor- the inner regions of a galaxy, particularly in the inner 1 to phologies with structure on many scales. The grand-design, 2 kpc. two-arm spiral patterns seen both in optical light and in H I IC 342, a nearly face-on, normal Scd galaxy of a size and 21 cm line emission are generally attributed to the presence luminosity similar to the Milky Way, is an ideal candidate of a spiral density wave (SDW). Distinct density wave pat- for studies of disk structure. Its relative proximity allows us terns can be seen in the H I disks of galaxies such as M51 to examine both large- and small-scale features in the disk. (Rots et al. 1990). In contrast, a Ñocculent H I disk pattern The galaxy has a large optical diameter (D20@) on the sky may indicate that a process such as stochastic, self- and an even larger diameter H I disk (D80@; Rogstad, propagating star formation driven by winds from previous Stostak, & Rots 1973). The optical morphology of IC 342 generations of star formation, is taking place. In Ñocculent has several components: a bright nuclear region, an oval galaxies, H I lacks the distinct arm pattern and takes on the central lens, and two inner disk arms that rapidly break up appearance of thick ““ ring ÏÏ (given the commonly seen into a multiarm pattern at increasing distance from the central depression in H I column density) as seen in NGC nucleus (Ables 1971; Elmegreen, Elmegreen, & Montenegro 4414 (Thornley & Mundy 1997). Intermediate between 1992). The large-scale H I distribution is asymmetric (Rots grand-design and Ñocculent galaxies, a pattern of multiple, 1979) and warped at radii beyond 16@ (Newton 1980a). Rots well-organized, spiral arm segments could also be the result (1979) suggests that an interaction with the nearby irregular of strong self-gravity in a disk with high gas content, as galaxy A0355 is the cause of the distorted velocity Ðelds of shown in the simulations of NGC 1365 by Lindblad, Lind- both galaxies. NewtonÏs (1980a, 1980b) H I study indicates blad, & Athanassoula (1996). A spectrum of intermediate the presence of overall spiral structure that breaks up into and combined arm patterns is seen, leading to the classi- multiple, clumpy, spiral arm segments. The inner gas disk Ðcation scheme of Elmegreen & Elmegreen (1987). On (D15@) is known to contain a substantial amount of molecu- smaller scales than the grand design spirals, phenomena lar gas, strongly peaked at the nucleus (Morris & Lo 1978; such as holes and bubbles appear. Heiles (1979, 1984) has Young & Scoville 1982; Sage & Solomon 1991; Crosth- convincingly shown that in the Milky Way, the H I distribu- waite et al. 2000). tion contains shells, bubbles, and holes that may be The distance to IC 342 is fundamental to a discussion of attributable to supernovae (SNe) and winds from the late masses and kinematic properties. McCall (1989) used new evolutionary stages of massive stars. estimates for Galactic extinction and asymptotic B magni- H I is the best-known gas tracer. In gas-rich spiral gal- tude and an extinction-corrected, face-on diameter to arrive axies, the H I disk can extend to radii several times that of at a distance of 1.8 Mpc. Karachentsev & Tikhonov (1993) the optical disk (Bosma 1981). In the Milky Way, H I mass used photometry of the brightest red and blue supergiants dominates the total gas mass beyond the solar circle, and to derive a distance of 2.1 Mpc. Krismer, Tully, & Gioia the atomic and molecular phases reach nearly equal mass (1995) have assigned a distance of 3.6 ^ 0.5 Mpc to the IC proportions at 4 kpc (Kulkarni & Heiles 1987). Similar 342/Ma†ei group, but the group is so close that the size of distributions of atomic gas are seen in other late-type the group itself is a signiÐcant fraction of its distance. We 1720 H I STRUCTURES IN IC 342 1721 will adopt a 2.0 Mpc value, or a linear scale of 0.58 kpc pattern (with one severely underdeveloped arm) can be seen, arcmin~1. confused somewhat by the unresolved source west of the This is the Ðrst of two papers in which we examine the nucleus. large-scale gas disk of IC 342 at higher resolution than The bright, unresolved source northeast of the nucleus previously available. In this paper, we present images of the (a \ 3h47m29s, d \ 68¡08@24A [J2000]) has a 21 cm contin- H I emission made with the Very Large Array (VLA).1 In a uum Ñux of S(21 cm) \ 46 mJy. In a high-resolution 6 cm subsequent paper, we will examine the molecular disk as map, from the data of Turner & Ho (1983), it has the dis- traced by CO and its relation to H I in IC 342 (Crosthwaite tinct double-lobed appearance of a radio galaxy. The same et al. 2000). source is identiÐed in Hummel & Gra veÏs (1990) continuum study as ““ discrete source 4.ÏÏ The steep spectral index 2. OBSERVATIONS (a \[0.87) they found is consistent with this identiÐcation. The unresolved source seen west and slightly south of the The 21 cm continuum and H I line observations were nucleus (a \ 3h46m22s, d \ 68¡05@06A [J2000]) has a 21 cm made in 1985 June and 1984 August using C and D array continuum Ñux of S(21 cm) \ 10 mJy. This source was conÐgurations, respectively. In each array, a full 8 hr track noted in the radio continuum studies of Baker et al. (1977) was taken for uv coverage. The phase center for the obser- and Hummel & Gra ve (1990), the latter assigning a rather vations in B1950.0 coordinates was a \ 3h41m57s.3 and Ñat spectral index of [0.2 to this source identiÐed as d\67¡56@29A (a \ 3h46m48s.4 and d \ 68¡05@48A [J2000]). ““ discrete source 2.ÏÏ The total power for this object is L (21 The channel width for the line observations was 48.83 kHz cm) \ 4.8 ] 1018 WHz~1, or twice that of the supernova (10.3 km s~1), with 63 channels giving a total of 639 km remnant (SNR) Cas A [L (21 cm) \ 2.2 ] 1018 WHz~1], s~1 coverage. Absolute Ñux calibration was achieved by presenting the possibility that this is an IC 342 disk SNR. bootstrapping from 3C 48, which has a Ñux of S(1419 Hummel & Gra ve (1990), however, suggest that this is a MHz) \ 16.0 Jy, accurate to within 2%. The absolute posi- background core-jet source. tional uncertainty is determined by the positional uncer- The unresolved source seen to the northwest, near the tainty of the phase calibrator 0212]735 (B1950.0), known edge of the primary beam at a \ 3h45m23s, d \ 68¡17@00A to less than 0A.05. (J2000) has a 21 cm continuum Ñux of S(21 cm) \ 26 mJy. The C and D array uv data were independently calibrated Hummel & Gra ve (1990) derived a spectral index, and then combined. A zero-spacing Ñux was extrapolated a \[0.8, for this source consistent with an identiÐcation of from the shortest uv spacings for each channel and applied this as a background galaxy. It is less likely that this source in the CLEANing process. The zero-spacing Ñux estimates is within IC 342 since it would have a rather large total- were typically one-third of the total Ñuxes from RotsÏs power output, D6 times that of Cas A. (1979) single-dish observations of IC 342 covering a region 3 times larger than our primary-beam area. The continuum 3.2. TheHIChannel Maps plus line emission cube was CLEANed with a natural Because both IC 342 and its phase calibrator, 0212]735, weight (no taper) for improved sensitivity to the extended are located at low Galactic latitudes (b \ 10¡.6 and b \ H I emission present in this galaxy. The FWHM beam size 12¡.0, respectively), contributions from Galactic absorption for the combined, naturally weighted C ] D array data is and undersampled Galactic emission are possible (Weaver 38A ] 37A, with position angle (P.A.) of 17¡. The largest & Williams 1974). Both the bandpass spectrum for scale structure that can be imaged is limited by the lack of 0212]735 and the spectrum at the location of the contin- short spacings to D15@, signiÐcantly less than the FWHM uum peak in IC 342 were examined, and both show Galac- of the primary beam, which is 32@. The channel cube was tic absorption. In order to produce unbiased integrated corrected for primary-beam attenuation out to the 20% intensity and velocity moment maps, and to prevent the power point and blanked beyond. The rms noise in the unrealistic negative-emission ““ hole ÏÏ that occurs when a channel maps is 1.2 mJy beam~1. The observations can be continuum image, una†ected by Galactic absorption, is converted from units of janskys per beam to a temperature subtracted from the a†ected channels, we correct for this scale using T (K) \ 434S (Jy beam~1). b H I absorption assuming a uniform foreground screen model, RESULTS following Hurt, Turner, & Ho (1996). The correction is 3. small, typically less than 10%, and a†ects only channels in 3.1. T he 21 cm Continuum Map the [54 to 8 km s~1 range. After applying the correction to The 21 cm continuum map shown in Figure 1 is an the CLEANed channels and performing continuum sub- average of 25 line-free channels. The rms noise level, mea- traction, 24 of the channels contained line emission. sured in an emission-free region 10@ from the phase center, is We present gray scale and contour images of the 24 chan- 0.5 mJy beam~1. The 21 cm continuum emission, consisting nels with line emission in Figure 2. The ““ butterÑy ÏÏ pattern largely of synchrotron emission, displays clear spiral struc- characteristic of an inclined rotating disk is formed from the I I ture. A bar-like structure running from the southeast to the lumpy patchwork of H emission. H emission extending northwest through the nucleus is apparent at position angle into the nucleus can be seen in the 11 to 70 km s~1 channels. of 135¡ and with a length of D3 kpc (5@). Hummel & Gra ve The [54 to 8 km s~1 channels show a pattern of low-level (1990) noted the apparent one-arm spiral structure of the di†use emission outside of the normal butterÑy pattern as continuum. In our continuum map, a two-arm spiral compared with the corresponding redshifted channels. Because this di†use emission is at anomalous velocities for its line-of-sight position in the rotating inclined disk of IC ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ I 1 VLA is operated by the National Radio Astronomy Observatory, 342 and is at the velocities expected for Galactic H ,we which is a facility of the National Science Foundation, operated under assume it is Galactic H I emission. The positive-negative cooperative agreement by Associated Universities, Inc. nature of the anomalous emission pattern and the negative background 1722 map to bowl than indicative displayed the convolved Hanning entire order beam removed channel F The The produce IG emission is . 4 surrounding t 0.5 1.ÈMap map o integrated correction observed p a maps produce t was mJy the smoothed [ sources of 3.3. t the o 2, DECLINATION (J2000) 67 55 68 20 is undersampled greater undersampled [ beam twice used of to integrated 4.7 Th 1( 00 05 10 15 not 21 average dotted the the intensity ~1 ] e cm was as the associated the H . i than 10 n moment The 03 49 butterÑy a continuum I contours source 20 v beam Integrated mask applied. elocity three intensity H 1.2 cm map 21cm Continuum di†use with Galactic I ), ~2 size, p to maps, bright column in 1, pattern. emission (1.3 IC , is across 2, isolate the based The 3, emission. presented Intensity 342. and map. continuum mJy 4, the a†ected emission. 5, density, in the three Additional channel 7, the channel on beam IC 10, CROSTHWAITE, emission 48 Map 342. emission 15, a in channels, peaks ~1 channels total and Figure N Gray A maps ) cube H was 30 primary- to I clipping , greater Ñux scale ( in solid the in RIGHT ASCENSION(J2000) were 3. used and was the are the northeast, ranges In contours of 47 TURNER, from ). 1.2 cm for cm 100 from cm typical Newton M IC are vations this S The northwest, H _ Considerable 0 I ~2 ~2 ~2 ] 342. \ apparent the peak t map smaller. o found 2.4 , . . 10 0.030 2.2 There The with by arm numerous Ñux 21 ] (1980a), The is ] & and Rots cm Jy 10 M in arm-interarm regions 10 by H column Typical in is southwest beam the peak 20 H ~2 46 O 3 I also (1979) the \ Rogstad deviations . cm map Jy and ~1 ““ 2.1 ~2 observed km numerous . holes is immediately considerable Contours 4.6 densities hole 0.14 ] of (all down s ~1 ] the 10 Jy ÏÏ contrast et corrected column 9 . 10 from seen beam nucleus The to M al. in c 9 structures olumn o _ units M column 45 ~1 n (1973), , in the total surrounding _ column compared . are the varies densities The for the of from mean density tentatively millijanskys 1 H order distance). densities p disk. 2.6 seen from single-dish (rms) I density mass column ] are with of Values in noise is 10 the 2 identiÐed per the 2.2 1.6 1.6 a 9 imaged t 3.0 o variation Vol. level factor holes M b ] ] ] density 6 disk eam obser- ] , range _ i with 10 10 10 n 119 a 10 s are the by 20 21 21 a of in of 9 a t No. 4, 2000 H I STRUCTURES IN IC 342 1723

68 30 153 km/s 142 km/s

67 50

40 68 30 132 km/s 122 km/s

DECLINATION (J2000) 67 50

40 68 30 111 km/s 101 km/s

67 50

40 03 50 48 46 44 03 50 48 46 44 RIGHT ASCENSION (J2000)

IG I F . 2.ÈH channel maps of IC 342. Each channel is labeled with the channel velocity (V_). Twenty-four channels contain emission. Gray scale ranges from 0 to 0.07 Jy beam~1. The 1 p (rms) noise level in the maps is 0.72 mJy beam~1. Contours are at [7.2, [3.6 (dotted contours), 3.6, 7.2, 18, and 36 mJy beam~1. The beam size is 38A ] 37A. Primary-beam correction has not been applied.

To more accurately determine a mean H I column density account, IC 342 has an average column density N 7 H I D for IC 342, we need to estimate the amount of missing Ñux ] 1020 cm~2, similar to estimates of the Milky Way that could not be fully recovered by our estimate of the average H I column density, N \ 6.2 ] 1020 cm~2 H I zero-spacing Ñux. We do this by comparing our map with (Dickey & Lockman 1990). If we assume a Milky Way value RotsÏs (1979) single-dish observations. When convolved to for the H I gas scale height of 120 pc (Burton 1976), the the same 10.8@ beam size, comparison of our column density mean H I density is 0.8 cm~3. The addition of the under- contours with those of Rots shows that on average we sampled H I Ñux (h[15@), at a level of D0.3 Jy beam~1 km resolve out N \ 2.3 ] 1020 cm~2, or roughly 30% of the s~1 Ñux spread over the entire galaxy, is below the Ðrst H I Ñux in the mapped region. Taking the missing Ñux into contour in Figure 3 and would not a†ect the clumpy 1724 CROSTHWAITE, TURNER, & HO Vol. 119

68 30 91 km/s 80 km/s

67 50

40 68 30 70 km/s 60 km/s

DECLINATION (J2000) 67 50

40 68 30 49 km/s 39 km/s

67 50

40 03 50 48 46 44 03 50 48 46 44 RIGHT ASCENSION (J2000)

FIG. 2.ÈContinued

appearance of the H I. Correcting for missing Ñux, inclina- disk. The spiral arms can be recognized in the isovelocity tion, and the He fraction, we obtain a mean gas surface map as distinct kinks in the velocity contours along the density & \ 6.5 M pc~2. spiral arms. There is no indication of warping in the inner 9 gas _ kpc (16@) radius of the H I disk, since the line of nodes is I 3.4. A Kinematic Analysis of the H Disk: T he ““ Inner ÏÏ and perpendicular to the isovelocity symmetry axis. Beyond 10 ““ Outer ÏÏ Disks kpc (17@) we begin to see a clockwise shift in the line of The intensity-weighted radial velocity and velocity dis- nodes, indicative of a warped disk. In general, the largest persion maps of IC 342 are presented in Figure 4. Fluxes values in the velocity dispersion map coincide with large greater than 1.5 p were used to produce both moment maps. N . The velocity dispersion peaks near the nucleus at 35 H I D The velocity map with contours has the overall ““ spider ÏÏ km s~1, even though the H I column density drops, imply- pattern characteristic of a di†erentially rotating inclined ing that some gas component other than H I (H ) is contrib- 2 No. 4, 2000 H I STRUCTURES IN IC 342 1725

68 30 29 km/s 19 km/s

67 50

40 68 30 8 km/s -2 km/s

DECLINATION (J2000) 67 50

40 68 30 -12 km/s -23 km/s

67 50

40 03 50 48 46 44 03 50 48 46 44 RIGHT ASCENSION (J2000)

FIG. 2.ÈContinued

uting to the total gas surface density (Sage & Solomon 31¡ inclination), 170 ^ 35 km s~1, is attained at a radius of 1991; Crosthwaite et al. 2000). 4.8 kpc (500A ^ 15A). The total dynamical mass derived A Brandt model rotation curve was Ðtted to the radial from the Brandt model is M \ 1.1 ^ 0.3 ] 1011 M , tot _ velocity map. Derived values for the systemic velocity, same as found by Newton (1980a). We note that in our dynamical center, inclination, and position angle are listed rotation curve the rotation velocity decreases from D4.8 in Table 1. The rotation curve is plotted in Figure 5, includ- kpc (500A)toD12.6 kpc (1300A). The rise in rotation veloci- ing NewtonÏs (1980a) rotation curve. Considering that ties between D12.6 and D14.5 kpc may be because of inter- NewtonÏs rotation curve was generated from velocities action with a companion (Rots 1979) and may be related to along the major axis only, it is still consistent with ours the warp seen in the disk beyond 17@. when one allows for di†erences in inclination and The morphology of the velocity residuals, di†erences resolution. The maximum rotation velocity (corrected for between the rotation model and the observed velocities, can 1726 CROSTHWAITE, TURNER, & HO Vol. 119

68 30 -33 km/s -43 km/s

67 50

40 68 30 -54 km/s -64 km/s

DECLINATION (J2000) 67 50

40 68 30 -74 km/s -85 km/s

67 50

40 03 50 48 46 44 03 50 48 46 44 RIGHT ASCENSION (J2000)

FIG. 2.ÈContinued

be used to locate SDW resonances (Canzian 1993). Because cyclic frequencies, i, derived numerically from the rotation of streaming motions along the spiral arms, for a spiral curve, along with the assignment of R , were used to assign CR pattern with m arms, there will be m ] 1 pairs of the pattern speed, at 40 km s~1 kpc~1 (0.40 ^ 0.06 km s~1 approaching-receding (a-r) residual arms outside the coro- arcsec~1), and locate the Outer Lindblad Resonance (OLR) tation radius (CR) and m [ 1 pairs inside corotation. The radius, R \ 6.9 ^ 0.9 kpc ([email protected] ^ [email protected]) (see Fig. 5). No OLR corotation radius, R \ 4.2 ^ 0.7 kpc ([email protected] ^ 1.2@ ) was Inner Lindblad Resonance (ILR) is indicated from the CR assigned by determining where the inner bipolar pattern angular velocity plot, but the 38A beam provides insufficient changes into a multiple a-r pattern (Fig. 6). This is not a resolution for a detailed analysis of the nuclear region. The sharply deÐned region and the assignment of the a-r pairs Turner & Hurt (1992) study of the inner kinematics suggests outside of corotation is subject to interpretation (three have that, if an ILR exists at all, it lies within the inner 50 pc of IC been tentatively assigned). Angular velocities, ), and epi- 342. R inner km Corresponding disk shall inside bations for appearing evaluation This resonance No. mined with [ F As We Our the s IG ~1 R and 4, di†ers . the designate primary we portion . divide CR and 3.ÈH location Resolution from 2000 across as locations an shall DECLINATION (J2000) R near outside ““ of considerably I CR beam. column ““ integrated 67 50 68 30 outer our optical outer of spiral \ the see of regions R 40 00 10 20 of the discussion 15 CR the 03 51 densities disk.ÏÏ of expected found the in ÏÏ @ . arms. suggested overall intensity disk Strictly tracers, corotation principal 4 ° map . with 4, from DISCUSSION by are based Intensity Integrated HI since is of map H amplitude D obtained R Elmegreen speaking, R the the 38 \ resonances b I CR of on y A 50 the are disk . location R \ Newton IC H Features CR morphological using markedly properties 6.0 342. I of disk as @ both of et Gray IC N ““ and extended of H agrees inner velocity (1980b) 49 al. into I 342 (cm the H scale lie (1992) R di†erent, ~2 o an OLR I corotation within that disk f fairly ) ranges STRUCTURES over \ the ““ from changes \ pertur- inner 7.9 RIGHT ASCENSION(J2000) deter- ÏÏ has 48 12 angular disk well ] from and the we an @ .3 10 a ÏÏ . 20 0 S t o scales H 3 I (Jy 47 J atomic strongly greatest. ular stood, vanelli less. common disappears, that 2 diameter M. y beam beam kpc greater Neutral The IN P. It gas in Rupen ~1 ~1 IC is (3 appearance & the state the 4.1. component than @ km km peaked here ) feature of Haynes 46 342 o atomic outer hydrogen s s and f ~1 ~1 80 in Morphology 1999, 15 that I ). . C @ @ the The All are (Rogstad at at of 342 disk. 1988). we of private outer o contours hydrogen lower many the not and f 45 the expect H but gas resolved Much nucleus moment I For concurrent regions of spiral in resolution, communication). et is are changes the the the reasons not al. at is of because maps 44 Inner (Morris 0.4, galaxies inner interaction 1973 to depleted the entirely 0.8, a presented from star arm-interarm Disk not mainly it ; of 1.2, disk M can missing & (Bosma formation completely 1.6, . , 43 absent. a within R L A is have with appear o and . predominantly \ molecular di†erent Holdaway 1978 short R been 2.4 1981 the CR the This Jy contrast ; to feature- baselines. corrected Young molec- under- beam ; be inner from 1727 Gio- one, is its ~1 & a However, of 100 d contours 1728 4 distribution treated aged & 2000). contours matics A \ F The Scoville the east IG N N M V Position M R v Distance Inclination v l Scale Decl. Hubble R.A. 68¡5 k , _ LSR . max surface ( m b H H d e f c b a V H dyn d 4.ÈH missing a dynamical The Newton I I max Total McCall Obtained Kinematic Dynamical o I d ...... at in from d f (with (observed) ...... e (J2000) and f from @ ...... (J2000) 47 ) 7, ...... DECLINATION (J2000) d a our without type ...... 67 50 68 30 1982 c 10, Angle A separate I two dynamical [ ...... of , radial missing densities d 21 40 00 10 20 1989 03 51 1980b, 60 and b a observed ...... b H center from ...... CO which Property ...... parameters ; d to cm center phases ...... 14 Sage ; I center velocity V ...... Karachentsev [ a Ñux, radial the Ñux adjusted and km continuum b 10 paper. mass P 50 etter based in ROPERTIES NASA/IPAC o s & km estimate) i ~1 f s this its H have (Table and IC based from . Solomon TABLE the s All 49 t determination on ~1 I coincident relation o 342 velocity disk is a , of Ðt Ðt and & on equivalent 2 H ...... OF not the 1) of Mpc 48 of Tikhonov at peak Brandt 1 IC Brandt is Extragalactic solid I determined Brandt moment a dispersion a 1991 hole distance. o†set to 342 radius secure 47 at contours 4 1 4 2 3 2 2 3 7 1 3 138¡ 3 68¡6 0.58 S H model . . . 9 1 . . 1 7 7 h rotation c ] with 7 1 8 0 1 rotation 46 1993. d ¡ ¡ ¡ 0 ¡ ; .7 .1 the ] ] ^ maps I k M ^ ^ at . of ^ azimuthally Crosthwaite 10 @ m 2 b kpc ^ ^ 46 p 5 in CROSTHWAITE, , 49 of p 10 10 0.3 y maps A c 1¡ 6¡ 20 35 result. 10¡ Ðt. the c ^ Database. centroid 0¡ 0¡ Value a (500 s .3 20 9 IC 1 from from . . 3 arcmin ] presented .6 cm 2 2 \ the km model. 5 M 1 ^ model. kpc cm A 45 A distribution km km of 10 A ~2 _ 342 3 0 ^ north s ~2 0 11 s IC .3 h ~1 Both the s s 46 t ~1 15 nucleus. (5 ~1 ~1 o M 342. will 44 A m @ 130 of ). have ) _ aver- kine- et 48 The and ( the the a km s .8 be al. RIGHT ASCENSION(J2000) ) been 43 (a) Radial , s ~1 primary-beam TURNER, .( velocities b 67 50 68 30 d far-IR symmetry. some Hodge nent the ( m positive predominantly map This (counterclockwise approaching-receding structure, weakness also enhanced The single of spiral aligned terns, the P.A. present ing analysis (1992) regions) major of tracers H ) contours V 40 00 10 20 \ 03 51 \ In Beyond elocity I northern the leading maps explanation 68¡5 hole 100 D 2 large-scale lens, contains Figure V degree with arm (Fig. structure, axis in 100 suggest dispersion mode star [ & with located dispersion in indicate to k @ gray corrected. of ) 50 48 & except ( 15¡). of m symmetry with a Kennicutt spiral the or predominantly k The the and edge the A the superposed \ formation the m( , and scale H inner the of 6) respectively). 7, SDW, ““ 49 3 O central same that a The our southeast, fat h contours bar optical disk winding southern negative spiral we the of 46 fat along structure 21 substantial and map for starting inside bar rotation). ÏÏ arm. m the 48 H has cm optical a compare 49 H contours, 100 pattern or images the the in of (1983 weak I bar structure s .1 that northern I gray arm tracers maps image integrated no the ““ ), modes, map IC The , 47 for pattern preponderance k inner boxy of residuals at the d noted an m structures signature ; scale the bar. \ While northern 342. m contours D leads star points of speeds, pair the show trace simultaneous indication corotation map location 46 an \ of 68¡5 180¡ ÏÏ arm IC (thermal, ranging northern The the inner along b formation bar, 3 IC ends is An optical y ), intensity pattern 342 the @ the the some mode 45 52 contains e leads at with to distinctly m overlap and in 21 arlier 342 but examination A 10 30 arm. from \ H inner two of with H provides of the far-IR cm and km 44 ] of I degree contains 2 pitch 21 o to I nonthermal, within H is image velocity the inner investigators. has surface f 0 s inner Elmegreen continuum and expected 18 occurring a passage maps. ~1 the northwest t simultaneously a a II star cm at o arm. a bar (b) 43 \ bifold intervals, 2 regions optical bipolar kpc angle bar the 5 illusion c of m the continuum an 3 R entral k spiral ( arm. formation contours potential. m h density. \ a All Vol. CR residuals two-arm 46 In location of (5 intrigu- s by promi- of , ~1 3 ““ D m @ single for along all show dotted ] 48 barÏs bar from et their their , with map H pat- arm The and two 10¡. of with 119 bar 10¡ All s .4 al. 3 of II a a ), ÏÏ @ , , No. 4, 2000 H I STRUCTURES IN IC 342 1729

(a) (b) 200 1.5

150 OLR 1

100 Newton's 21cm data

C+D array 21cm data 0.5 Brandt Model 50

0 0 0 500 1000 1500 0 500 1000 1500 Radius (arcsec) Radius (arcsec)

FIG. 5.ÈH I rotation curve and circular frequency models for IC 342. (a) Rotation velocity from the H I line emission annuli are plotted as circles. A Brandt model Ðt to the rotation data is shown with a solid curve. The triangles are the average of velocities along the major axis to northeast and southwest from NewtonÏs (1980a) rotation curve assuming an inclination of 25¡. (b) Circular, epicyclic, and resonance frequencies were calculated from the Brandt model Ðt to the rotation curve. The location of an outer Lindblad resonance and the spiral density wave pattern speed are shown with hatched area assuming corotation is located at R D 435A ^ 75A. density wave patterns increases the concentration of gas in the optical lens and the two inner arms. H I features tend to the arm beyond that possible by either mode alone, which strongly correlate with optical structure outside of the two then increases the rate of formation of massive stars, which inner arm region. However, the inner disk can only be com- in turn leads to the higher thermal dust emission, higher pletely understood when the molecular gas is included. synchrotron emission, and an increase in the density of H II 4.2. Morphology of the Outer Disk, R [ R regions seen along the arm. However, this requires the exis- CR tence of the m \ 3 SDW, which we cannot conÐrm from our Outside R , the appearance of the H I disk of IC 342 CR H I observations. depends on the scales being considered. On the scale of a Just inside of R , the two inner arms bifurcate into few kiloparsecs, a feathery pattern is seen, reminiscent of the CR multiple arms and the strong two-arm pattern disintegrates. Ñocculent pattern seen in optical images of other galaxies. The termination of the inner two spiral arm pattern and There are numerous short, parallel, interconnected spiral bifurcation of the spiral arms are both optical signatures of arm segments, and the disk appears to be riddled with holes the corotation radius (Elmegreen 1991; Elmegreen & and clumps scaled on a variety of sizes. On a slightly larger Elmegreen 1995). Beyond the two-arm spiral structure, the scale, the two optical inner arms bifurcate into a many- 21 cm continuum emission traces the beginning of H I spiral armed pattern that extends beyond R , and continue to CR structure. In general, the 21 cm continuum emission lies wind intact through large azimuthal angles, but at a much slightly inside (0.3 kpc or 30A) corresponding strong H I larger pitch angle (D25¡) than the inner two arms. The arms. The H II regions tend to lie along arms seen in the leading edges of the bifurcating outer arms are outlined by optical as well as in neutral hydrogen. This displacement of H I emission. As these optical arms fade, they appear to nonthermal continuum and H I arms can be understood if continue in H I, as can be seen in the H IÈDigital Sky Survey the H I emission is the downstream by-product of star for- (DSS) overlay (Fig. 7). The outer stellar/H I spiral arms mation. A density wave causes compression and induces appear to be continuous, deÐned by their stellar content star formation as traced by the continuum. Further down- near R and H I outside of R . The majority of H I regions CR CR stream, the predominantly molecular gas is dissociated by lie on the H I arms. H I in these arms is no longer predomi- UV from star formation, forming H I arms, which will even- nantly downstream but rather coincident with other spiral tually recombine into molecular form (Tilanus et al. 1988; arm tracers, indicating that dissociation of H plays a less 2 Tilanus & Allen 1989, 1993). In this case, one expects the dominant role in the appearance of the H I disk than it does inner spiral structure to be better deÐned by molecular gas inside of R . CR tracers such as CO emission (Crosthwaite et al. 2000). Once Outside of 6 kpc (10@), the D2È5 kpcÈlong H I arm seg- the optical arms begin to bifurcate, the H I arm structure ments appear to be loosely organized in a four-arm pattern. can be seen as a continuation of the optical arm pattern. The southeastern arm is twice as broad (4 kpc) and twice as ] To summarize, within the inner disk, IC 342 undergoes a massive (2.2 108 M_) as the other three large-scale arms. transition from a bar morphology to a two-arm spiral. Although we have some evidence for an m \ 2 mode SDW These arms ultimately bifurcate in the outer disk, beyond with an OLR at D7 kpc (12@), it is difficult to explain the corotation. All of the star formation tracers are primarily presence of this four-arm pattern between 6 and 15 kpc. A contained within R . H I emission is patchy in the region of weak m \ 4 SDW with a completely di†erent pattern speed CR we be absent M51, the 1730 & grand-design IC shorter and disk While resolution on to with outline in Mpc) size arms gray F It Mundy that interpreted take 342 the IG two thick (Fig. set is . would beyond while o scale the 6.ÈRotation IC instructive (Rots r of appearance o arm are the dash inner NGC f pattern 7 342 H resonance (gray 1997). b but lumpy, comprised

), DECLINATION (J2000) inner 67 50 68 30 segments. curves. change I et spiral the the in can optical 4414 as scale of al. 40 00 10 20 at the Alternatively, 03 51 ring. velocity IC approaching-receding a to termination be The 1990). a appear ranges tight pattern of (18 Ñocculent consider locations if 342 greater classiÐed proposed arms, For The a o IC Mpc). residuals f thick, Residuals Velocity H The from winding multiple, to 342 comparison, appearance of I the be integrated distance, how [ four corotation When H may of galaxy as lumpy the 50 10 and was discrete long, I the in to ““ of the large-scale H kinematic be parallel, grand arms. 25 galaxies convolved viewed the four-arm I continuous, NGC present appearance km ring region ring similar of intensity the structures. four Approaching s 49 design the ~1 CROSTHWAITE, features with in two interconnected, 4414 where ). with such outer spiral ring Velocity in IC pattern to t H o this ÏÏ four map 342 the (Thornley a a of in the two-arm, is based M51 I s arms. 3 residual arms arms IC three 48 region. similar the M51 @ residuals might RIGHT ASCENSION(J2000) spiral beam takes to same 342. H (10 o a be of in - If is n Velocity r arms I arm in the TURNER, 47 ( pair dark is novae ments. smaller H ingly present, in the seen This of the residuals range pattern similar We First The D a gray I 4.3. location H spurs, galaxy 25 because Ðlamentary higher of . ) are I The We overall , are [ km from disk, changes scale Fine 46 is the 1a to & still indicated not consider ““ 0.4 s as n the angular ~1 holes,ÏÏ that of H d1k ““ H which IC structure distant holes left t O to impression the Ðt the o kpc I Structure 342 a ms of and found 1 single by with primary 45 ILR ~1 a kpc ÏÏ ~1 three is and Brandt resolution. the Flocculent is Ñocculent as are extremely and are so a seen the in range thick for - M51, spaces r possibilities indicated pair nearby shells the in R model is necessity an driver CR in solid the that is 44 of Milky unless m D bounded the Arms between H t Disk size Ðlamentary o \ created curves, ; in 7 a the for i I t H kpc 4 white appearance SDW, would to scales Way imaged of rotation mode, for ? I Ðne-scale by receding arms, are IC explain 43 in this by the a (Heiles pair order 342 implied. although for not a curve multiple and at spiral Ðne the pattern of arms : the to correspond- much be Shells structure can ovals. are Ñocculent. structure. numerous more 1979). H apparent Vol. ( light arm displayed only clearly I super of or speed clearly holes gray 119 H seg the be in - I ) No. 4, 2000 H I STRUCTURES IN IC 342 1731

68 30

68 25 HI/DSS HI in 3' beam

20 20

10 10

00 00

67 55

67 50 50 (b) 45 (a) 40 03 51 50 49 48 47 46 45 44 43 03 50 49 48 47 46 45 44 43

DECLINATION (J2000) 68 15 HI/100µ m/H α 68 15 HI/21cm

10 10

05 05

00 00

67 55 67 55 03 48 30 00 47 30 00 46 30 00 45 30 00 03 48 30 00 47 30 00 46 30 00 45 30 00 RIGHT ASCENSION (J2000)

FIG. 7.ÈIC 342 emission morphology comparisons. (a)HIintegrated intensity in gray scale range from 0 to 3.2 Jy beam~1 km s~1, with uncalibrated contours of the DSS image. The white ovals bound the probable location of corotation. (b)HIintegrated intensity map convolved to a 3@ beam size in order to display large-scale features. Gray scale ranges from 0 to 35 Jy beam~1 km s~1.(c)HIintegrated intensity with IRAS 100 km HIRES contours and the location of H I regions. Contours are at 30, 45, 60, 90, 225, and 650 MJy sr~1. The IRAS 100 km HIRES map was produced by 200 iterations of the maximum correlation method. The e†ective beam varies over the HIRES map with an average 88A ] 70A FWHM and position angle of 62¡. Squares mark the locations of H II regions as traced by Ha emission (Hodge & Kennicutt 1983). (d)HIintegrated intensity with 21 cm continuum contours. Contours are at 0.001, 0.003, 0.005, 0.01, and 0.05 Jy beam~1. studies of Holmberg II (Puche et al. 1992), M31 (Brinks & shell in velocity space. Another indicator for SN shell candi- Bajaja 1986), M33 (Deul & den Hartog 1990), and NGC dates is a ring of H II regions surrounding a hole. Such a 2403 (Thilker, Braun, & Walterbos 1998) Ðnd similar pattern would be expected from a second generation of star results. The most reliable way to identify such holes is formation occurring in the compressed gas of the shell. through their kinematic signatures. SpeciÐcally we looked Our best candidates are the three ““ shells ÏÏ shown in for evidence of midplane, kinematic, ““ hollow ÏÏ shell struc- Figure 8, located just to the west of the nuclear region. The tures (““ type 3 ÏÏ structures deÐned and used by Brinks & holes have radii D0.9 kpc (1.5@). They are surrounded by Bajaja 1986 and Deul & den Hartog 1990). We examined H II regions, as seen in Ha, occurring at the shell boundary, spectra taken from the channel maps at ““ hole ÏÏ locations and the holes occur at only one velocity channel. In order to for multiply peaked proÐles. We examined the maps at hole estimate the energy required to produce these holes via locations for a pattern of emission at the location of a SNR expansion, we use the expression derived from SN ““ hole ÏÏ in a channel, followed by one or more channels with expansion models (Chevalier 1974; Heiles 1979): no or lower emission, followed again by a channel with emission. That is, we looked for ““ caps ÏÏ of an expanding E \ 5.3 ] 1043n1.12R3.12V 1.4 , 0 sh 1732 CROSTHWAITE, TURNER, & HO Vol. 119

-12 km/s -23 km/s 68 05

00 -33 km/s -43 km/s 68 05

DECLINATION (J2000) 04

00 03 46 30 15 00 45 45 15 00 45 45 RIGHT ASCENSION (J2000)

FIG. 8.ÈH I ““ shell ÏÏ candidates in IC 342 channel maps. Four channels of emission in the vicinity of a H I ““ hole ÏÏ are shown. Small squares are locations of H II regions as traced by Ha emission (Hodge & Kennicutt 1983). The ““ shell caps ÏÏ are seen in the extreme velocity channels.

where E is the energy (ergs), n is the average density gies, on the order of 1051 ergs, we need several hundred SNe 0 (cm~3), R is the shell radius (pc), and V is the shell expan- to produce the holes examined here. sh sion velocity (km s~1). The mean energy required to We have four arguments against an SN origin for the H I produce expanding shells of this size is 3.2 ] 1053 ergs, holes: which we regard as a lower limit since we have not adjusted the density for contributions from the molecular gas 1. Most of the holes do not show the kinematic signatures content, which increases the average density by a factor of we would expect for SN-generated holes. While there is D2 at the locations of these holes. For Type II SNe ener- some evidence for an expanding shell at the previously men- No. 4, 2000 H I STRUCTURES IN IC 342 1733 tioned locations, we lack similar convincing evidence at the timescales are too short for wind-driven holes to retain their locations of most of the holes. What we Ðnd is decreased shapes. We consider initially spherical, 10 pcÈradius shells emission at the location of a hole with the peak emission in placed at galactic radii R \ 2, 5, and 8 kpc. In lieu of any the hole at the center channel of the velocity spread. The measurement of an expansion rate for the holes in IC 342, majority of the holes have singly peaked spectra centered at we assume a 12 km s~1 expansion rate, based on the mean the mean radial velocity for that portion of the disk. The calculated for type 3 holes in M33 (Deul & den Hartog peaks of the hole spectra are not displaced in velocity from 1990). The e†ect of di†erential rotation on the appearance surrounding regions. This indicates that we are not simply of the shells as they expand in a face-on disk, with IC 342Ïs looking at a shell that has ““ blown ÏÏ out of one or more rotation curve, can be seen in Figure 9. The time required surfaces in the gas plane. for shells to uniformly expand to the 0.5 and 0.9 kpc radii of 2. Most of the H II regions are not located where they are our two previously mentioned holes is D35 Myr and expected for wind-driven holes. Puche et al. (1992), examin- greater than 70 Myr, respectively. Shells in the R \ 2È5 kpc ing holes in Holmberg II, and Thilker et al. (1998), studying range will be severely sheared in these time frames. Typi- expanding shells in NGC 2403, found that in general the cally, 30 Myr is a sufficient timescale to severely distort an smaller H I holes were Ðlled with Ha emission, while Ha was expanding shell. found at the edges of the larger ones. We do not see this correlation between Ha and H I holes (Fig. 7c). H II regions Undoubtedly the wind and especially the SN-driven seem to simply line up along the optical and H I spiral arm shells would expand at much higher rates early in their and increased column density features, suggesting that star evolution, which reduces the shearing time frame and sub- formation is occurring at the locations of large H I clouds as sequent distortion of the hole (Chevalier 1974; Chevalier & might be expected. Gardner 1974; Brinks & Bajaja 1986; Deul & den Hartog 3. Too many (greater than 300) massive stars (assuming 1990; and references therein). The initial expansion rate is the high end of the Type II SNe energy range, 1051 ergs) are expected to slow substantially after a mass of the ambient required to produce holes in regions of the disk where there medium equivalent to the ejected SN mass is swept up. A is no evidence for the required level of star formation. The simple calculation, assuming 20 M_ ejected and a 800 km thermal (100 km), nonthermal (21 cm continuum), and s~1 expansion rate, shows this occurs within a 10 pc radius ionized gas (Ha) tracers of massive star formation fall to the on the order of 104 yr, an insigniÐcant fraction of size and limits of detection at radii where holes in the H I disk are timescales for our holes. If these are wind-driven shells, the still prominent (Fig. 7). The holes in the outer disk are of current expansion velocities must be lower than the 10 km sizes comparable to those located in regions where massive s~1 channel width of our H I data, and the shell would have star formation tracers exist. SpeciÐc examples include the had to have been expanding over timescales comparable to holes located at (J2000): a \ 3h45m04s, d \ 67¡58@56A; those calculated here. a \ 3h45m42s, d \ 68¡12@51A; a \ 3h47m48s, d \ 68¡18@32A; We considered other forces that might lead to less elon- a \ 3h48m28s, d \ 68¡01@44A; a \ 3h48m48s, d \ 68¡05@34A. gated shapes for the holes. The azimuthal expansion veloci- Many other examples can be readily located from a visual ties may be less than the radial expansion velocities as the examination of the H I disk. Because of the energies distance to discrete driving sources increases faster in the required to create them via SNe, Deul & den Hartog (1990) shearing direction. While this would lead to a more spher- conclude that the larger holes found in a study of M33 ical shape for a hole, at least over the lifetime of the energy could not be created by a single event and are just interarm sources, the age of the holes is several times larger than the regions bordered by spiral arm segments. We are led to the age of the OB associations needed to drive the shells same conclusion in the case of IC 342. (assuming that the self-regulatory nature of massive star 4. The strongest argument that these large holes are not formation prohibits a subsequent generation of OB stars at wind driven is the response to di†erential rotation. Shearing the same location). Tidal acceleration, V 2/R, might lead to

t = 20 Myr t = 40 Myr t = 60 Myr

2 kpc 5 kpc 2 kpc 5 kpc 2 kpc 5 kpc

FIG. 9.ÈExpanding H I holes in a di†erentially rotating disk. This sequence of frames illustrates the appearance of shells expanding at a uniform rate of 12 km s~1 in a rotating disk whose rotation curve deÐned by the Brandt model solution derived for IC 342. The disk is viewed at a face-on inclination (i \ 0¡). The holes are initially spherical shells of radius 10 pc. Each point on the surface of the shell is allowed to expand in its initial direction while being sheared by di†erential rotation. Contours of constant galaxy radius are supplied, and rotation is counterclockwise. Each frame is labeled with the number of years elapsed from the initial 10 pcÈradius hole. 1734 CROSTHWAITE, TURNER, & HO Vol. 119

increase the a†ected area along the gas arms and cannot in 15 itself account for the majority of the short radially directed structure seen in the disk. T hird, the Ñocculent H I structure is the result of instability in the gas disk. In this case the holes are artifacts, not unlike the voids seen in simulations of an initially homogeneous gas that is allowed to gravitationally collapse. We look for 10 evidence that this is actually the case. Toomre (1964) speci- Ðed a critical wavelength, j , which deÐnes the size scale crit for the onset of unstable structure in a di†erentially rotating disk,

4n2G& j \ gas . 5 crit i2 Over the range of 4È8 kpc, the surface density of H I does not vary substantially, & 8 M pc~2, while the epi- H I D _ cyclic frequency, i, steadily declines (see Fig. 5). As a result, if instabilities in the gas disk are the primary cause of structure, then we expect to see size scales for gravitationally 0 10 15 20 bound gas on the order of j and an increase in the crit radius (arcmin) maximum size of the gas clumping at larger galactic radii. To arrive at an estimate of the clumping scale, unbiased FIG. 10.ÈIC 342 azimuthal clump sizes and j . Plotted here are the crit azimuthal sizes of H I gas condensations vs. radius. A clump is deÐned as a by the confusing shapes, we measure the lengths of clump region in which the contiguous surface density, & , remains greater structures by their azimuthal extent (arclength) above a clump than 140% of the azimuthally averaged surface density, & , at that radius. speciÐed threshold & at a given radius. This ignores the avg H I Individual clumps sizes are indicated by diamonds. The long-dashed dash pitch angle of arm segments and subclumping within an line is j at that radius; the short-dashed line is the same but includes a crit already dense structure. Because the overall pitch angle is gas ampliÐcation factor derived from a two-Ñuid analysis of galactic disks. low, 25¡, the azimuthal measurement yields scales close to the major-axis lengths of subclumps within an arm segment formed by a bead of clumps. When measured over the entire an elongation of hole shapes in radial dimension. But this outer disk, R [ 5 kpc, any trends in the clumping scales @ would only be e†ective for holes in the inner 1.2 kpc (2 ) with radius should become apparent. We corrected the inte- galactic radius, the region of the rising rotation curve. grated intensity image for inclination and missing Ñux and Outside 1.2 kpc, the rotation curve Ñattens, and the di†er- made polar plots of the deprojected gas disk, and then we ence in tidal acceleration across the hole will be negligible. computed the average intensity at a given radius. We then In short, although there are many uncertainties in this counted the number and size of gas clumps along that model, the e†ects of shear cannot be avoided. radius that exceeded 1.4 ] & . While the 1.4 ] & selec- Second the I holes are the result of low or negative avg avg , H ““ ÏÏ tion factor was somewhat arbitrary, it was the largest value shear that creates feathery features projecting perpendicu ““ ÏÏ - that did not severely Ñatten the spectrum of sizes into beam- larly from existing gas arms . Balbus (1988) showed that there sized clumps but was still well above the mean surface are preferred directions for the growth of perturbations in density. spiral arms (along the arms and perpendicular to the arms). Using the epicyclic frequency i(R) and a smoothed curve The relative growth rates for perturbations are modiÐed in for the mean gas surface density, & (R), we compute avg regions of strong compression, favoring growth perpendicu- j (R). This j does not take into account the stellar crit crit lar to the arms. Tightly wound, growing wave crests can be content of a galaxyÏs disk. From a two-Ñuid stability distorted by a changing shear direction. Either case pro- analysis, Jog & Solomon (1984) have shown that there is an motes the formation of ““ spurs ÏÏ and ““ branches ÏÏ and an increase in the e†ective & when the gas is embedded in a gas overall ““ feathery ÏÏ appearance. In the case of IC 342, higher surface density stellar disk. They argue that much of ““ holes ÏÏ are then the low-density regions between arms and the ““ messy distribution ÏÏ of spiral arm features can be spurs, beam-smeared into rounded elliptical and circular attributed to randomly occurring two-Ñuid gravitational shapes. Shear along a spiral arm, A , di†ers from the arm instabilities. In this case, the numerous arm segments are average shear, A (OortÏs constant A)by 0 material arms rather than the coherent large-scale arms & expected from a SDW. They found that the e†ective & A \ A 2 [ arm , gas arm 0A & B was increased by a factor of D3.5 for regions of the disk 0 where & /& ^ 0. 1. We expect this e†ect to be more gas stars where & is the surface density along the arm and & is the pronounced at the inner (4 kpc) end of our radius range, arm 0 average. Assuming that an ““ epoch of negative shear ÏÏ when where the stellar content of disk is still apparent in the DSS the H I arm density distribution was substantially di†erent image (Fig. 7). By 8 kpc, the stellar contribution to the disk did not exist, only a small fraction of gas arms are candi- is less obvious and the H I surface density becomes the dates for spurring because of negative shear. Nearly all of dominant contributor to the total disk surface density. The these lie at large radii, R [ 6 kpc, and are located in patches measured spectrum of clump sizes can be expected to be a along densest parts of arm structure. Including areas of low little closer to a two-Ñuid wavelength, 3.5j (R) at 5 kpc, crit shear, those less than 0.05A , does not substantially and closer to j (R) at the disk extremities. A more accu- 0 crit No. 4, 2000 H I STRUCTURES IN IC 342 1735 rate assessment of j (R) requires measurements of the dissociated molecular gas resulting from star formation in crit stellar component of the disk that are not currently avail- the inner arms. The m \ 2 mode pattern does not propagate able. to the outer disk. Instead, it bifurcates into a multiple-arm Figure 10 displays the results. The size of the individual pattern in the vicinity of R . All of the strong star forma- CR clumps, the average size, and the calculated value of j as tion tracers are contained within R . crit CR well as of 3.5j are shown. We see a trend to azimuthally In the outer disk, the H I gas disk has a Ñocculent appear- crit clump on larger scales at larger galactic radii. The largest ance, with short spiral arm segments, interconnected by clump sizes are bounded by j and 3.5j . The increase of ““ spurs ÏÏ and Ðlled with ““ holes. ÏÏ When the pattern is com- crit crit H I arm segment or clump width with galactic radius is pared with the bifurcating optical arms, the H I segments consistent with the interpretation of the holes in the outer can be seen as continuations of stellar arms out to larger disk of IC 342 being the voids in between self-gravitating radii. Well beyond R , the Ñocculent H I arm pattern CR arm segments. If the voids form on the same timescales as begins to organize itself into an overall four-arm spiral the largest clumps, they form in less than a rotation period, pattern that extends beyond the OLR for the interior m \ 2 typically 10 Myr for IC 342 (Toomre 1964). On these time- mode SDW. scales, the voids would be less likely to show shearing e†ects On a smaller scale, we considered three possible explana- compared to SN/wind-driven holes, which require larger tions for the rich structure seen primarily in the outer disk: timescales to reach the same size. (1) the holes are created by numerous localized supernovae; CONCLUSIONS (2) spurring of gas arms due to negative shear creates 5. ““ feathery ÏÏ arms; and (3) the Ñocculent arms are gas insta- We have presented H I maps of the nearby spiral galaxy bilities and holes are what remains after the formation of IC 342 with better resolution (38A) than previously avail- Ñocculent structures. Rotational shearing would tend to able. We Ðnd a mean column density N \ 4.7 ] 1020 distort the spherical shapes in the timescales shorter than H I cm~2. The mean column density becomes 7.0 ] 1020 cm~2 that required for the holes, produced by an instantaneous when an adjustment for missing Ñux is taken into account. burst of SNe or winds, to reach the observed sizes. There is Analysis of the column density and spatial scale for the not enough surface density enhancement in the H I spiral missing Ñux indicates that the structures seen in the H I disk arm segments to promote negative shear and the sub- of IC 342 are real. Considerable structure is seen in the disk sequent formation of perpendicular spurs. The conclusion in the form of spiral arms and ““ holes ÏÏ with typical column most consistent with the available observational data is that densities 1.6 ] 1021 and 1.6 ] 1020 cm~2, respectively. the majority of the H I spiral structure represents material A kinematic analysis of the H I disk allows us to locate arms formed from instabilities in the gas disk. The timescale the corotation radius at 4 kpc for the inner two-arm SDW. for the formation of instabilities is a fraction of the rotation The properties of the H I disk inside and outside of corota- period, so the shearing of voids produced in the process will tion are sufficiently di†erent to warrant distinguishing be minimized. The clump size and the change in clump size between the ““ inner ÏÏ (R \ R )(and ““ outer ÏÏ disk R\R). with radius are close to the size scales predicted from a CR CR Within the inner disk, H I is depleted relative to the mean simple analysis of the gas stability. The formation of disk H I column density, known to result from a change in material arms from instabilities results in a Ñocculent the gas state from an atomic one to a molecular one. While appearance. The holes seen in the H I disk are voids left no large-scale bar is seen in the H I surface density, the from the formation of material arms. kinematic signatures of a bar are seen in the velocity residuals. The large-scale bar can be seen in the 21 cm This work was supported in part by NSF grant AST continuum and 100 km far-IR and at optical wavelengths as 94-17968 to J. L. T. We made use of the NASA/IPAC and a lens or ““ fat ÏÏ bar. An m \ 2 mode SDW is detected in the IRAS HIRES data reduction facilities as well as the Space velocity residuals, and an inner two-arm spiral is seen in the Telescope Science Institute Digital Sky Survey facilities. previously mentioned tracers. H I downstream of the non- The authors are grateful for the helpful suggestions of the thermal continuum in the inner disk indicates this H I is anonymous referee.

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