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A Complete Atlas of Hi Absorption Toward Hii Regions The Astrophysical Journal Supplement Series, 211:29 (7pp), 2014 April doi:10.1088/0067-0049/211/2/29 C 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A. A COMPLETE ATLAS OF H i ABSORPTION TOWARD H ii REGIONS IN THE SOUTHERN GALACTIC PLANE SURVEY (SGPS I) C. Brown1,2, J. M. Dickey1,J.R.Dawson1,2, and N. M. McClure-Griffiths2 1 School of Physical Sciences, Private Bag 37, University of Tasmania, Hobart 7001, Australia 2 CSIRO Astronomy and Space Science, ATNF, P.O. Box 76, Epping, NSW 1710, Australia Received 2013 November 28; accepted 2014 February 5; published 2014 April 2 ABSTRACT We present a complete catalog of H i emission and absorption spectrum pairs, toward H ii regions, detectable within the boundaries of the Southern Galactic Plane Survey (SGPS I), a total of 252 regions. The catalog is presented in graphical, numerical, and summary formats. We demonstrate an application of this new data set through an investigation of the locus of the Near 3 kpc Arm. Key word: H ii regions Online-only material: color figures, figure set, machine-readable table, supplemental data 1. INTRODUCTION 15 , combined Parkes and ATCA continuum-subtracted H i cubes (2.2), and combined Parkes and ATCA cubes contain- Combining data from the Australia Telescope Compact Array ing continuum emission (∼1.6). For this work we use the (ATCA) and the Parkes single dish telescopes, the Southern continuum-included combined Parkes-ATCA data. These cubes Galactic Plane Survey (SGPS; McClure-Griffiths et al. 2005) were specifically produced for H i absorption studies, as they provides H i line and 1.4 GHz radio continuum data for the fourth provide accurately calibrated data at the highest angular resolu- Galactic quadrant, with the best combination of resolution and tion of the SGPS. All SGPS continuum and line cube data are sensitivity currently available for that line in this area. Using available online via the Australia Telescope National Facility these data, we have measured 21 cm absorption spectra toward (ATNF) H i Surveys archive.3 a sample of 252 H ii regions. SGPS data has been used extensively in a number of fields, This study produces two distinct data products: a consolidated including H i self-absorption (Gibson et al. 2005;Kavarsetal. census of H ii regions with known radio recombination line 2005), the cold neutral ISM in the outer Galaxy (Strasser et al. (RRL) velocities, taken from the literature, and the H i emission 2007), Galactic structure (McClure-Griffiths et al. 2004)aswell and absorption spectrum pairs toward them—within the bounds as investigations of particular individual sources (Kothes & ◦ ◦ of the SGPS (255 <l<353 ). These two resources will enable Dougherty 2007). further study into the structure and dynamics of the neutral interstellar medium (ISM) in the fourth quadrant (on which 2.2. H ii Regions massive stars and their formation have a significant impact). The catalogs will serve as a data set for numerous studies including First we compiled a list of H ii regions from RRL catalogs for investigations of the spiral structure of the Galaxy (e.g., Strasser the longitude and latitude range of the SGPS. This compilation et al. 2007) and kinematic distance works, both for target regions of H ii regions was then over-plotted on the SGPS radio (e.g., Urquhart et al. 2012) and for the intervening H i clouds continuum maps using the KARMA software suite (Gooch (e.g., Roman-Duval et al. 2009). 1995). Each map was then visually inspected to confirm H ii The sample selection from existing H ii region catalogs is region detection. Target H ii regions were included in this catalog described in Section 2, while the method of spectrum extraction (Section 4)ifasingle, distinct emission source was visible in appears in Section 3. The catalog itself appears in Section 4, the SGPS continuum map. before a discussion of the global properties of H i absorption in Several cataloged H ii regions were coincident with several Section 5 and an illustration of its use: an examination of the radio continuum sources and some targets had coordinates distribution of H i absorption in longitude–velocity (lv) space, coincident with no continuum source. It is not surprising that the ∼ including the locus of the Near 3 kpc Arm. SGPS with angular resolution of 2 detects multiple sources within the larger beam of the early H ii region discovery works, 2. DATA for instance the Parkes beam of Caswell & Haynes (1987)is∼4 . Furthermore, there are often clusters of continuum emission 2.1. Southern Galactic Plane Survey, SGPS sources surrounding a cataloged region’s coordinates, such that The SGPS covers 325 deg2 of the Galactic plane over the no attempt can be made to distinguish the background emission fourth and first Galactic quadrants (SGPS I and SGPS II, (see Section 3). There are several H ii regions that are included in respectively). For this work, SGPS refers to the fourth quadrant more than one H ii region RRL velocity catalog (Section 2.2.1) component of the survey only (i.e., the SGPS I). The SGPS I and therefore become duplicate sources when the catalogs are provides both the H i line data, as well as the continuum combined. Furthermore, there were H ii regions that did not maps (Haverkorn et al. 2006) used for source detection and appear as continuum sources in the SGPS, or had continuum identification. temperatures Tcont < 5 K, these were not included in this catalog. The SGPS provides three distinct data products: Parkes continuum-subtracted cubes, with an angular resolution of 3 http://www.atnf.csiro.au/research/HI/common 1 The Astrophysical Journal Supplement Series, 211:29 (7pp), 2014 April Brown et al. Table 1 H ii Region Catalog: Sample Longitude Latitude SIMBAD ID Qual Tcont VRRL Ref 254.676 +0.299 NGC 2579 A 134 64 1 267.947 −1.066 GAL 267.95−01.07 A 969 1 1 268.0 −1.1 GAL 268.0−01.1 B 31 1.8 4 305.551 −0.005 GAL 305.55−00.01 A 117 −45 1 305.787 +0.140 GAL 305.79+00.14 E 11 −43 1 319.157 −0.423 GAL 319.16−00.42 A 49 −22 1 − − − 337.665 0.048 GAL 337.67 00.05 C 29 55 1 Figure 1. Brightness temperature of sample Ton (peak brightness, shown in red) 337.949 −0.476 GAL 337.95−00.48 A 380 −40 1 and average off source (black) spectra for G269.133−1.137. The gray envelope 338.014 −0.121 GAL 338.01−00.12 B 46 −54 1 around the average off source spectrum represents a 3σ variation between the 338.131 −0.173 GAL 338.13−00.17 A 61 −53 1 three off source positions. Components of Equations (1)and(2) are labeled. 353.035 +0.748 [L89b] 353.035+00.748 C 44 −9.1 3 (A color version of this figure is available in the online journal.) Notes. Columns are as follows: (1) Galactic longitude, (2) Galactic latitude, With a 95% detection rate the GBTHRDS selected its target (3) SIMBAD identification, (4) quality factor, (5) Tcont (K), (6) RRL velocity, (7) reference for H ii region coordinates and velocity. sample from spatially coincident mid-infrared and radio con- References. (1) Caswell & Haynes 1987; (2) Bania et al. 2010;(3)Lockman tinuum emission. The Spitzer MIPSGAL survey (Carey et al. 1989; (4) Wilson et al. 1970. 2009) provided 24 μm data, while 21 cm continuum emission (This table is available in its entirety in a machine-readable form in the online was sourced from the SGPS, VGPS (Stil et al. 2006) and NVSS journal. A portion is shown here for guidance regarding its form and content.) (Condon et al. 1998). Lockman (1989). The canonical northern hemisphere catalog of Lockman (1989) extends into the fourth quadrant, and The total number of individual, distinct H ii regions, visible in therefore into the SGPS coverage area. This large survey the SGPS continuum is 252. Throughout this work, the name (in provides RRL velocity detections for 462 H ii regions. the form “G longitude ± latitude”) reported for each H ii region Wilson et al. (1970). Wilson et al. (1970) detected the is taken from the catalog from which the region is provided (see H109α RRL toward 130 H ii regions visible in the southern Section 2.2.1); hence the inconsistencies in decimal precision. ◦ ◦ sky (261 <l<50 ) using the Parkes Radio Telescope in 2.2.1. H ii Region Catalogs 1968. We use the catalog of Caswell & Haynes (1987) as the basis 3. EMISSION/ABSORPTION METHOD OF H i of our target list, supplementing that catalog with further H ii ABSORPTION SPECTRUM EXTRACTION regions sourced from other works. Our final source list is a compilation of the H ii region RRL velocity catalogs of Caswell The emission/absorption (E/A) method observes H i spectra & Haynes (1987), the Green Bank Telescope H ii Region coincident with, and adjacent to, discrete continuum sources. In Discovery Survey (GBTHRDS; Bania et al. 2010), Lockman order to derive absorption, e−τ , the brightness temperature as a (1989) and Wilson et al. (1970) (see Table 1). We describe each function of velocity, V,bothon(Ton) and off the source (i.e., the of these works below. emission spectrum, Toff) are compared. The simplest radiative There are several other fourth quadrant studies which include transfer situation gives H ii region candidates (Reifenstein et al. 1970; Walsh et al.
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