LOOPS, DRIPS, AND WALLS IN THE GALACTIC ... - IOPscience

The Astrophysical Journal, 594:833–843, 2003 September 10 # 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A.

LOOPS, DRIPS, AND WALLS IN THE GALACTIC CHIMNEY GSH 277+00+36 N. M. McClure-Griffiths1 Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping, NSW 1710, Australia; naomi.mcclure-griffi[email protected]

John M. Dickey Department of Astronomy, University of Minnesota, 116 Church Street, SE, Minneapolis, MN 55455; [email protected]

B. M. Gaensler Harvard-Smithsonian Center for Astrophysics, Mail Stop 6, 60 Garden Street, Cambridge, MA 02138; [email protected]

and A. J. Green School of Physics, Sydney University, A28, Sydney, NSW 2006, Australia; [email protected] Received 2003 February 24; accepted 2003 May 26

ABSTRACT We present new high-resolution H i images of the Galactic chimney GSH 277+00+36. The chimney is at a distance of
6.5 kpc, is more than 600 pc in diameter, and extends at least 1 kpc above and below the Galactic midplane. Using the Australia Telescope Compact Array and the Parkes Radiotelescope as part of the Southern Galactic Plane Survey, we have imaged the H i associated with this chimney, with a spatial resolution of
6 pc. These are among the highest spatial resolution images of an H i chimney. We find very narrow welldefined shell walls, a remarkably empty interior, and complex small-scale structures. The shell walls show a very steep reduction in emission at the interior edge and a more gradual decline toward the exterior. We suggest that this structure is characteristic of compression and may be used to distinguish stellar by-product shells from shell-like structures resulting from random turbulent motions. The shell and chimney walls also exhibit a great deal of small-scale structure, which we discuss in the context of hydrodynamic instabilities. We find that these structures are primarily cold gas with narrow line widths in the range 1.5–2.5 km s1. Subject headings: Galaxy: kinematics and dynamics — Galaxy: structure — ISM: bubbles — ISM: structure — radio lines: ISM

the number of known shells can only just be explained by the star formation history of the Galaxy (McClure-Griffiths et al. 2002). However, we assume that not all the Galactic shells have been detected. Several alternative formation theories have been proposed to explain the number of large H i shells without requiring extremely high star formation rates. These include theories for creation by gamma-ray bursts (e.g., Loeb & Perna 1998), soft gamma-ray repeaters (C. Heiles 2002, private communication), and the impact of high-velocity clouds with the Galactic disk (e.g., TenorioTagle et al. 1987). Alternatively, Wada & Norman (1999) suggest that a thermally and gravitationally unstable disk can generate large shell-like structures in which dense clumps and filaments of neutral gas surround a hot gas interior. The two-dimensional simulations of an LMC-type galaxy carried out by Wada, Spaans, & Kim (2000) are even able to reproduce the porous filamentary morphology of the LMC with large-scale cavities whose velocity structure mimics that of expanding supershells. Unfortunately, there are few observations of H i shells that allow us to study their detailed physics, in particular their interaction with the surrounding ISM and the process of chimney breakout. Most observational studies of H i shells are limited by images with low spatial resolution. Though large H i shells are clearly detectable in lowresolution images, they typically appear as cavities with solid featureless walls (McClure-Griffiths et al. 2002).

1. INTRODUCTION

The dynamic nature of the interstellar medium (ISM) can be seen over a wide range of spatial scales. Some of the most dramatic examples are neutral hydrogen (H i) shells, supershells, and chimneys. These objects are detected as voids in the H i, with walls of swept-up material. They range in size from tens of parsecs to kiloparsecs and are often the largest discrete objects observed in galaxies. It is believed that most shells were formed from supernovae, stellar winds, and the combined effects of both. The largest of the H i shells have formation energies of 1052–1053 ergs, requiring 10–100 massive stars if they are the product of stellar winds and supernovae. These can grow enough to exceed the scale height of the H i disk and expand unimpeded into the Galactic halo, creating a ‘‘ chimney.’’ There are very few known chimneys in the Milky Way, with some prominent examples being the W4 chimney (Normandeau, Taylor, & Dewdney 1996), the Stockert chimney (Mu¨ller, Wennmacher, & Reif 1989), and the Scutum supershell (Callaway et al. 2000). It has been noted that the number of large shells observed in external galaxies is irreconcilable with star formation rates (e.g., Rhode et al. 1999; Crosthwaite, Turner, & Ho 2000). The situation in the Milky Way is not much better;

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However, recent hydrodynamic simulations of H i shells and chimneys (e.g., Avillez & Breitschwerdt 2003a, 2003b) show complicated structures with resolved features on parsec and subparsec scales. These simulations reveal hydrodynamic instabilities that have a significant impact on H i shell evolution, including chimney production and eventual shell destruction (e.g., Mac Low, McCray, & Norman 1989; Breitschwerdt, Freyberg, & Egger 2000; de Avillez & Berry 2001). As a large H i shell evolves the dense walls begin to develop Rayleigh-Taylor instabilities, which can lead to fragmentation and cloud production along the shell (Mac Low & McCray 1988; de Avillez 2000). In addition, as shells reach sizes comparable to the H i disk scale height they become Rayleigh-Taylor unstable along their polar caps, which can break the shell and create a chimney. To test the numerical models of H i shell evolution, highresolution images of large H i shells are required. With improved observations we hope to study the development of instabilities along the shell walls that lead to chimney formation and destroy the structural integrity of shell walls. These studies hold the promise of observing small-scale phenomena previously seen only in simulations. In this paper we present new Australia Telescope Compact Array (ATCA) H i observations of the Galactic chimney GSH 277+00+36 (McClure-Griffiths et al. 2000). The ATCA data have been combined with Parkes data from the Southern Galactic Plane Survey (SGPS; McClureGriffiths et al. 2001) to provide sensitivity to angular scales from 30 to 10 , corresponding to 5 to 1100 pc at the distance of the chimney. Using these data we explore the small-scale structure of the chimney, particularly noting the narrow shell walls and the emptiness of the shell interior. In x 2 we present the observational and analysis details. We present images of the chimney in x 3 and discuss the structure of the chimney in x 4. 1.1. GSH 277+00+36: Basic Properties The Galactic chimney GSH 277+00+36 was discovered in the low-resolution Parkes component of the SGPS (McClure-Griffiths et al. 2000). The chimney and a similar, apparently associated structure, GSH 280+00+59, appear as H i voids centered in the Galactic plane with multchannel chimneys extending far above and below the plane. These chimneys are located at the edge of the SagittariusCarina spiral arm at a kinematic distance of 6:5  0:9 kpc. The main portion of the GSH 277+00+36 shell is centered on l ¼ 277 , b ¼ 0 , v ¼ þ36 km s1 and extends over approximately 6 of longitude. The void is apparent over a wide range of local standard of rest (LSR) velocities, from v
15 to v
55 km s1. The wide velocity width suggests that the shell is still expanding with a velocity vexp of 20 km s1. The chimney’s vertical channels do not appear bound at the extremes, and they span more than 10 both above and below the Galactic midplane. Assuming a distance of 6.5 kpc, the main portion of the shell has a diameter of 610  90 pc, and the chimney extensions reach more than 1 kpc above the midplane. The shell is quite massive, with a swept-up mass of Msw ¼ ð2:7 5:6Þ  106 M. McClure-Griffiths et al. (2000) estimate that the equivalent energy initially required at the center of the shell to account for the shell’s current size and expansion is
2:4  1053 ergs. This implies that if the shell were produced by stellar winds and supernovae, several hundred massive stars must have existed within the

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shell. However, the exact formation mechanism for the shell is unclear. McClure-Griffiths et al. (2000) discuss and rule out several possibilities, including the impact of a highvelocity cloud with the Galactic disk. Because of the shell’s distance, no detection of any remaining cluster of O and B stars was possible. In addition, McClure-Griffiths et al. (2000) detect no X-ray, infrared, or radio continuum emission associated with the shell. Without direct evidence for the formation of the shell, it is difficult to place limits on the age of the shell. However, those authors estimated an upper limit to the age of
20 Myr on the basis of the observation that the shell has not experienced significant shearing due to differential rotation in the Galaxy. 2. OBSERVATIONS AND ANALYSIS

The H i data presented here combine a high-resolution (30 ) mosaic made with the Australia Telescope Compact Array (ATCA) and low-resolution (160 ) single-dish data from the multibeam system on the Parkes Radio Telescope.2 The combined data have a resolution of 30 and are sensitive to angular scales up to 10 . All the Parkes results and the ATCA data covering the region 270  l  280 , 1=5  b  þ1=5 were obtained for the Southern Galactic Plane Survey (SGPS; McClure-Griffiths et al. 2001). We briefly describe here the observations and analysis, focusing in particular on the new ATCA observations. More detailed explanations of the procedures used for the SGPS can be found in McClure-Griffiths (2001) and a forthcoming paper (N. M. McClure-Griffiths et al. 2003, in preparation). The results presented in this paper cover the region between Galactic longitudes 272 and 284 and Galactic latitudes 7 and +5=5. ATCA observations for the region excluding jbj  1 were conducted in three sessions between 2001 October and 2002 March. We used the ATCA in two new compact array configurations, EW 352 and EW 367, for a maximum baseline length of 367 m. For uniform coverage, a total of 841 pointing centers are used: 373 in the region 272  l  284 , 1  b  5=5 and 468 in the region 272  l  284 , 7  b  1 . The pointings are arranged on a hexagonal grid with a separation of 230 . Each pointing was observed with eight 60 s snapshots at widely spread hour angles. We used a correlator configuration that simultaneously records 1024 channels across a 4 MHz bandwidth, centered on 1420.0 MHz, and 32 channels across a 128 MHz bandwidth, centered on 1384 MHz. The former intermediate frequency was used for the H i spectral line work presented here, while the broadband mode will be used to produce continuum images in all four Stokes parameters. PKS B1934638 was observed once per day for bandpass and absolute flux density calibration. The sources PKS B103947 and PKS B084354 were observed approximately every hour for phase and amplitude calibration. PKS B103947 was used for the positive-latitude field and PKS B084354 was used for the negative-latitude field. Data for the region between b ¼ 1 and b ¼ þ1 were taken from the SGPS. These data have 210 pointings in a hexagonal pattern with a separation of 190 . Each pointing 2 The ATCA and the Parkes Radio Telescope are part of the Australia Telescope, which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO.

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GSH 277+00+36

was observed 40 times in 30 s snapshots at evenly spaced hour angles. PKS 0823500 was used for phase and amplitude calibration. All other aspects of the observations were as described above. Calibration and imaging was performed in the MIRIAD data reduction package3 by using standard techniques. Prior to imaging, continuum subtraction was performed in the u-v domain by using a linear fit to the emission in off-line channels. The individual pointings were linearly combined and imaged using a standard grid and fast Fourier transform technique scheme with superuniform weighting. The mosaicked images were deconvolved using a maximumentropy deconvolution algorithm implemented in MIRIAD’s MOSMEM (Sault, Staveley-Smith, & Brouw 1996). The Parkes data were obtained from the SGPS by using the inner seven beams of the multibeam system, a 13 beam 21 cm receiver package at the prime focus on the Parkes telescope (Staveley-Smith et al. 1996). The observations were made by mapping ‘‘ on the fly,’’ scanning through 3 in Galactic latitude, while recording data in 5 s samples. The complete data set covers the region jbj  10=5. The data were recorded in frequency-switching mode to allow for robust bandpass calibration. The frequency center was switched between 1419.0 and 1422.125 MHz every 5 s, with an instantaneous bandwidth of 8 MHz across 2048 channels. The narrow-line IAU standard calibration regions, S6 and S9, were observed daily for bandpass and absolute brightness temperature calibration. Brightness temperature calibration was performed for each beam and each polarization separately. The data were shifted to the LSR frame by applying a Doppler correction in the form of a phase shift in the Fourier domain. Finally, the calibrated data were imaged using Gridzilla (Barnes et al. 2001), employing a weighted-median technique with a Gaussian-smoothing kernel of FWHM 180 , a cutoff radius of 100 , and a cell size of 40 . The single-dish images were regridded to the same grid as the deconvolved mosaics and multiplied by a calibration scaling factor to account for differences in brightness temperature calibration for the two telescopes. Finally, the deconvolved interferometer and single-dish images were combined in the Fourier domain (Stanimirovic´ 2002). The final H i cube has a synthesized beam of 3