High Resolution Study of the ‘Carina Flare’ Supershell - an Hi-H2 Galactic Chimney J. R. Dawson, N. M. McClure-Griffiths, Y. Fukui

1. Introduction Supershells are some of the largest and most energetic structures in the ISM and play a vital role in its evolution, disrupting and reshaping the gas on scales of hundreds of parsecs. Most supershells are thought to be formed by stellar feedback from OB clusters. The stellar winds and supernovae inject vast amounts of energy (∼1051 to 1053 ergs) into the ISM, creating a giant cavity of hot, tenuous gas surrounded by a cool shell of swept-up material. Many expand sufficiently that they exceed the disk scale height, blowing their tops and venting their hot interior gas out into the Halo. These are sometimes called Galactic ‘chimneys’. Supershells have long been thought to trigger the formation of molecular clouds and stars. But the mechanisms of such feedback and its relative importance in the Galactic ecosystem are still poorly understood. The supershell may trigger star formation in existing molecular material, compressing it until it becomes unstable (e.g. Boss et al. 1995). Alternatively, molecular clouds may condense out of the cool shell walls, and these clouds may go on to become the parents of a new stellar generation. This second mode is closely linked with the formation of instabilities in the Hi shell and its eventual break up. Proposed mechanisms include gravitational instability and fragmentation of the shell into bound, co-moving clouds (e.g. McCray & Kafatos, 1987; Elmegreen et al. 2002); and the formation of sheets of cool gas and cold molecular ‘drips’ through fluid instabilities (Avillez, 2000, hereafter A00; McClure-Griffiths et al. in preparation, hereafter McG1). However, although the basic validity of supershell-triggered star formation has been demonstrated by stellar population studies (e.g. Dopita et al. 1985; Oey et al. 2005), there have been virtually no attempts to investigate the earlier stages of the process from the point of view of the evolution of the ISM. High resolution studies of Hi supershells are still rare, and few authors have studied their structures in detail. Furthermore, a lack of wellresolved Hi-H2 supershell candidates means that detailed investigations of triggered molecular gas formation have not yet been attempted. This project is a detailed study of the ‘Carina Flare’, a rare example of an Hi-H2 chimney. We propose high resolution observations of the atomic component of the supershell, to investigate the mechanisms by which supershell feedback affects the ISM. We intend to use the combination of the Hi and H2 morphology to study instabilities in the shell, seeking to discriminate between the various models of fragmentation and cloud formation. In addition, high resolution observations will enable us to study how the physical properties of the gas change with location in the supershell, allowing us to investigate its effect on the evolution of the cold, warm and ionized phases. This work also has the potential to contribute to the study of triggered star formation, by linking supershell instability mechanisms to the creation of star-forming molecular clouds. These observations will be combined with existing high sensitivity data from the Parkes Southern Galactic Plane Survey (SGPS) (McClure-Griffiths et al. 2005, hereafter McG05).

2. The Carina Flare: A Molecular Galactic Chimney The ‘Carina Flare’ supershell is a unique example of an Hi Galactic chimney with a significant mass of molecular gas. Located on the near side of the Carina Spiral Arm (D∼2.6±0.4kpc), it was first detected in molecular tracers by Fukui et al. (1999). These authors used the NANTEN 4m telescope to map the molecular gas at 2.60 resolution in the 115GHz 12 CO(J=1-0) line, revealing an extensive cloud complex located unusually far above the Galactic plane. Denser condensations were imaged in the same line of the rarer isotopomer 13 CO, and at least two of these are actively forming intermediate mass stars (Dawson et al. 2006). One particularly impressive structure (∼104 M ) is located at z ∼350pc - five times the molecular gas scale height. We have used low resolution SGPS data to investigate the atomic phase (Dawson et al, in preparation). In the Hi 21cm line, the molecular clouds reveal themselves to be co-moving parts of a moderately sized (∼250×300pc), gently expanding (vexp ∼10km/s) atomic supershell, which shows strong evidence of galactic plane blowout. Linewidths, measured from confusion-free components, reach values as low as ∼5km/s, indicating the presence of significant quantities of cool gas at T<500K. Even at the low resolution of the SGPS dataset (160 ∼12pc), ∼ the Hi shows considerable substructure, with thick, elongated filaments and clumps showing clear morphological relationships with the molecular gas. The total mass of associated molecular matter is estimated at 3×105 M , and includes structures ranging in size from parsec-scale cloudlets (the resolution limit) to a single giant molecular cloud squashed along the supershell’s bottom rim. This is a considerable fraction of the estimated atomic mass of ∼7×105 M . Selected channel maps of both datasets are given in figure 1. In addition, SHASSA Hα images (Gaustad et al. 2001) reveal faint but unmistakable ionized ‘streamers’ running parallel to the direction of blowout (see figure 1, inset). Several delineate the inner rims of Hi structures, arguing strongly for a genuine association. This makes the Carina Flare one of only a few Galactic supershells for which multi-phase information is available. 1

Figure 1: Selected channel maps. Greyscale: velocity integrated Parkes Hi 21cm line emission. Filled contours: NANTEN 12

CO(J=1-0) emission (levels: 1.5+3.5 Kkm/s). Dotted lines enclose regions observed with NANTEN. Maps shown in a, b and c represent part of the approaching limb (-25.0 < vlsr < -22.5km/s) the velocity centroid (-20.0 < vlsr < -17.5km/s), and part of the receding limb (-12.6 < vlsr < -10.1km/s), respectively. Inset image: SHASSA Hα emission of unknown velocity.

3. A Closer Look at Some Substructure Figure 2a shows a highly blue shifted cloud complex located at z∼350pc, which may be plausibly interpreted as a fragment blown out of the top-front of the shell. With a total molecular mass of ∼2×104 M , it is unlikely that the molecular clouds could either have been pre-existing at such high latitudes, or that they could have been moved here from the molecular disk without being severely disrupted. They may therefore be good candidates for clouds formed in-situ from the atomic gas. We note that the size and location of the Hi component are broadly consistent with supershell-produced structures found in the simulations of A00. Figure 2b shows a lone molecular cloud of ∼103 M that forms the head of an atomic ‘tail’. The tail points directly away from the shell center and shows a small but significant velocity gradient along its length, with the head lagging behind the tail in the shell expansion. The molecular cloud was recently the target of observations with the Mopra 22m telescope. Mapped at sub-parsec resolution in 12 CO(J=1-0), it shows an unusual arc-like structure. We consider this morphology and velocity behavior to be highly suggestive of a pre-existing cloud that has been overrun by the shell, and whose atomic component is being swept back and accelerated faster than its denser molecular component. Higher resolution observations are needed to map the small-scale structure of the tail to help confirm this interpretation. Figure 2c shows two Hi filaments (white dotted lines) whose velocities just place them on the approaching wall of the supershell. A highly elongated molecular cloud is located along the inner filament, culminating in a dense star-forming clump at the point where two converge. The inner rim of the structure is bright in Hα and shows a particularly sharp and coherent edge. This, combined with its unusual morphology, makes it one of the clearest examples of the supershell’s influence. Also visible in this image are molecular condensations that appear displaced away from the strongest Hi emission, towards the inside of the shell (dotted circles). We speculate that these may be of a similar nature to the molecular ‘drip’ discovered by McG1 in the inner wall of the supershell GSH 277+00+36. Higher resolution observations are now needed to clarify the relationship between the atomic and molecular gas.

4. Project Aims The Carina Flare is a unique laboratory for investigating the effect of supershell feedback on the evolution and structure of the ISM. This project proposes to mosaic the Carina Flare at a resolution of 30 , corresponding to ∼2.5pc at D=2.6kpc, obtaining some of the highest resolution images of any known supershell. Previous high resolution mappings of Hi shells have resolved cool parsec-scale drips, narrow filaments, and highly structured, scalloped walls (McClure-Griffiths et al. 2003). Structures such as these would be easily resolved by our observations and provide insights into the physics of the dominant instability mechanisms in the shell. (The above authors found their structures to be consistent with Rayleigh-Taylor instabilities.) In the case of the Carina Flare, the NANTEN 12 CO data provide extra information on the distribution and properties of cold, dense gas within the supershell, which has not been available to previous researchers. We intend to use the combination of the Hi and H2 morphology to study instabilities in the shell, seeking to discriminate between the various models of fragmentation and cloud formation. Combined with SHASSA Hα observations, our data coverage will span three phases of the ISM at parsec-scale resolutions. We plan to utilize them to study how the physical properties of the gas change with location in the supershell, investigating its effect on the evolution of 2

the cold, warm and ionized phases. The Carina Flare is also notable as one of only a few known Galactic Chimneys. The low resolution SGPS data show faint ‘billowing’ structures above the blowout mouth in central velocity channels, which should be resolved at the proposed resolution and sensitivity. These delicate structures, and the neutral gas distribution around the Hα streamers, should enable us to investigate the physics of the blowout process, providing insights into how supershells supply gas to the Galactic halo.

5. Observing Strategy We request 130 hours to image the Hi emission associated with the Carina flare to an angular resolution of 20 and a brightness temperature sensitivity of 4K per km/s channel. These observations will cover an 81 deg2 region spanning 283 < l < 292◦ , 1.5 < b < 10.5◦ . Coverage of the bottom rim of the shell will be provided by existing high resolution SGPS data (McG05) in the region 0.5 < b < 1.5◦ . We will use the EW352 and EW367 arrays for uniform baseline coverage from 30m. These data will be combined with Hi data of the region obtained from the low resolution SGPS for full sensitivity to angular scales from 3 arcminutes to 10 degrees. The proposed region will be covered with a mosaic of 653 pointings. These pointings will be arranged in a hexagonal pattern with a separation of 230 between adjacent pointings. Although this separation is less than Nyquist, the sensitivity function will only vary by ∼ 2.5% between pointings and the smaller number of pointings will allow us to obtain more snapshots per pointing to improve the u-v coverage. The observing strategy will be to obtain fifteen 40 second integrations on each pointing at a wide range of hour angles. We will use the FULL 4 2048 correlator configuration for 0.4km/s channel widths, which we will smooth to 0.8km/s for combination with the Parkes data, but which we can use at full resolution for comparison of small scale features with the CO emission. Including overheads and time for calibration the total time requested is 130 hours. We ask that 65 hours be allocated in the 07APRS term with the EW367 array and the remaining 65 hours be allocated early in the 07OCTS term with the EW352 array to complete the project. References Boss, A. P. 1995, ApJ, 439, 224 Dawson, J. R., Kawamura, A., Fukui, Y. 2006, in IAU Symp 237, Triggered Star Formation in a Turbulent Medium de Avillez, M. A. 2000, MNRAS, 315, 479 Dopita, M. A., Mathewson, D. S., & Ford, V. L. 1985, ApJ, 297, 599 Elmegreen, B. G., Palous, J., & Ehlerova, S. 2002, MNRAS, 334, 693 Gaustad, J. E., McCullough, P. R., Rosing, W., & Van Buren, D. 2001, PASP, 113, 1326 McClure-Griffiths, N. M., Dickey, J. M., Gaensler, B. M., & Green, A. J. 2003, ApJ, 594, 833 McClure-Griffiths, N. M., Dickey, J. M., Gaensler, B. M., Green, A. J., Haverkorn, M., & Strasser, S. 2005, ApJS, 158, 178 McCray, R., & Kafatos, M. 1987, ApJ, 317, 190 Oey, M. S., Watson, A. M., Kern, K., & Walth, G. L. 2005, AJ, 129, 393

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