John M. Dickey

GLIMPSE. I. An SIRTF Legacy Project to Map the Inner Galaxy

ABSTRACT The Galactic Legacy Infrared Mid‐Plane Survey Extraordinaire (GLIMPSE), a Space Infrared Telescope Facility (SIRTF) Legacy Science Program, will be a fully sampled, confusion‐limited infrared survey of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

FIRST GLIMPSE RESULTS ON THE STELLAR STRUCTURE OF THE GALAXY

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Rotation Measures of Extragalactic Sources Behind the Southern Galactic Plane: New Insights into the Large-scale Magnetic Field of the Inner Milky Way

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Radio Polarization from the Inner Galaxy at Arcminute Resolution

The GLIMPSE (Galactic Legacy Mid-Plane Survey Extraordinaire) Point Source Catalog of ~30 million mid-infrared sources toward the inner Galaxy, 10° ≤ |l| ≤ 65° and |b| ≤ 1°, was used to determine the distribution of stars in Galactic longitude, l, latitude, b, and apparent magnitude, m. The counts versus longitude can be approximated by the modified Bessel function N = N0(l/l0)K1(l/l0), where l0 is insensitive to limiting magnitude, band choice, and side of Galactic center: l0 = 17°-30° with a best-fit value in the 4.5 μm band of l0 = 24° ± 4°. Modeling the source distribution as an exponential disk yields a radial scale length of H* = 3.9 ± 0.6 kpc. There is a pronounced north-south asymmetry in source counts for |l| 30°, with ~25% more stars in the north. For l = 10°-30°, there is a strong enhancement of stars of m = 11.5-13.5 mag. A linear bar passing through the Galactic center with half-length Rbar = 4.4 ± 0.5 kpc, tilted by = 44° ± 10° to the Sun-Galactic center line, provides the simplest interpretation of these data. We examine the possibility that enhanced source counts at l = 26°-28°, 315-34°, and 306°-309° are related to Galactic spiral structure. Total source counts are depressed in regions where the counts of red objects (mK-m[8.0] > 3) peak. In these areas, the counts are reduced by extinction due to molecular gas, high diffuse backgrounds associated with star formation, or both.

The Magnetic Field of the Large Magellanic Cloud Revealed Through Faraday Rotation

We present new Faraday rotation measures (RMs) for 148 extragalactic radio sources behind the southern Galactic plane (253° ≤ l ≤ 356°, |b| ≤ 1.5°), and use these data in combination with published data to probe the large-scale structure of the Milky Ways magnetic field. We show that the magnitudes of these RMs oscillate with longitude in a manner that correlates with the locations of the Galactic spiral arms. The observed pattern in RMs requires the presence of at least one large-scale magnetic reversal in the fourth Galactic quadrant, located between the Sagittarius-Carina and Scutum-Crux spiral arms. To quantitatively compare our measurements to other recent studies, we consider all available extragalactic and pulsar RMs in the region we have surveyed, and jointly fit these data to simple models in which the large-scale field follows the spiral arms. In the best-fitting model, the magnetic field in the fourth Galactic quadrant is directed clockwise in the Sagittarius-Carina spiral arm (as viewed from the north Galactic pole), but is oriented counterclockwise in the Scutum-Crux arm. This contrasts with recent analyses of pulsar RMs alone, in which the fourth-quadrant field was presumed to be directed counterclockwise in the Sagittarius-Carina arm. Also in contrast to recent pulsar RM studies, our joint modeling of pulsar and extragalactic RMs demonstrates that large numbers of large-scale magnetic field reversals are not required to account for observations.

Resolving the Kinematic Distance Ambiguity toward Galactic H II Regions

The Southern Galactic Plane Survey (SGPS) is an H I and 1.4 GHz continuum survey of the fourth quadrant of the Galaxy at arcminute resolution. We present here results on linearly polarized continuum emission from an initial 28 deg2 test region for the SGPS, consisting of 190 mosaicked pointings of the Australia Telescope Compact Array and covering the range 325.5d<l<332.5d, -0.5d<b<3.5d. Complicated extended structure is seen in linear polarization throughout the test region, almost all of which has no correlation with total intensity. We interpret the brightest regions of polarized emission as representing intrinsic structure in extended polarization, most likely originating in the Crux spiral arm at a distance of 3.5 kpc; fainter polarized structure is imposed by Faraday rotation in foreground material. Two large areas in the field are devoid of polarization. We argue that these voids are produced by foreground H II regions in which the magnetic field is disordered on scales of ~0.1-0.2 pc. We also identify a depolarized halo around the H II region RCW 94, which we suggest results from the interaction of the H II region with a surrounding molecular cloud.

The Southern Galactic Plane Survey: the test region

We have measured the Faraday rotation toward a large sample of polarized radio sources behind the Large Magellanic Cloud (LMC) to determine the structure of this galaxys magnetic field. The magnetic field of the LMC consists of a coherent axisymmetric spiral of field strength ∼1 microgauss. Strong fluctuations in the magnetic field are also seen on small (<0.5 parsec) and large (∼100 parsecs) scales. The large bursts of recent star formation and supernova activity in the LMC argue against standard dynamo theory, adding to the growing evidence for rapid field amplification in galaxies.

A stellar wind bubble coincident with the anomalous X-ray pulsar 1E 1048.1-5937: Are magnetars formed from massive progenitors?

Kinematic distance determinations in the inner Galaxy are hampered by the near-far kinematic distance ambiguity. Here we resolve the ambiguity for 49 H II region complexes with known recombination-line velocities in the first Galactic quadrant. We measured the 21 cm H I absorption spectrum toward each source with the Very Large Array in the C array. The maximum velocity of H I absorption was used to discriminate between the near and far kinematic distances. The number ratio of far to near sources, ~3, can be entirely explained as a geometrical effect. The kinematic distances that we derive are compared with previous determinations for the same sources. Although our distance determinations are largely in agreement with previous measurements, there are 22 discrepancies that we discuss. Using our distance determinations, we create a face-on Galactic map of the H II region complexes and compare it with a kinematically derived profile of the distribution of CO-traced molecular hydrogen. The H II region complexes delineate the large-scale features seen in the molecular gas. The 5 kpc molecular ring and the Sagittarius spiral arm are clearly evident, and a few H II region complexes lie in the Perseus arm.

NGC 1058 - Gas motions in an extended, quiescent spiral disk

The Southern Galactic Plane Survey (SGPS) is a project to image the H I line emission and 1.4 GHz continuum in the fourth quadrant of the Milky Way at high resolution using the Australia Telescope Compact Array and the Parkes Radio Telescope. This paper describes the survey details and goals, presents 21 cm continuum data, and discusses H I absorption and emission characteristics of the SGPS test region (3255 ≤ l ≤ 3335; -05 ≤ b ≤ +35). We explore the effects of massive stars on the interstellar medium through a study of H I shells and the H I environments of H II regions and supernova remnants (SNRs). We find an H I shell surrounding the H II region RCW 94, which indicates that the region is embedded in a molecular cloud. We give lower limits for the kinematic distances to SNRs G327.4+0.4 and G330.2+1.0 of 4.3 and 4.9 kpc, respectively. We find evidence of interaction with the surrounding H I for both of these remnants. We also present images of a possible new SNR G328.6-0.0. Additionally, we have discovered two small H I shells with no counterparts in continuum emission.

Milky Way Kinematics. I. Measurements at the Subcentral Point of the Fourth Quadrant

We present 21 cm H I observations from the Southern Galactic Plane Survey of the field around the anomalous X-ray pulsar 1E 1048.1-5937, a source whose X-ray properties imply that it is a highly magnetized neutron star (a magnetar). These data reveal an expanding hydrogen shell, GSH 288.3-0.5-28, centered on 1E 1048.1-5937, with a diameter of 35 × 23 pc (for a distance of 2.7 kpc) and an expansion velocity of ≈7.5 km s-1. We interpret GSH 288.3-0.5-28 as a wind bubble blown by a 30-40 M☉ star, but no such central star can be readily identified. We suggest that GSH 288.3-0.5-28 is the wind bubble blown by the massive progenitor of 1E 1048.1-5937 and consequently propose that magnetars originate from more massive progenitors than do radio pulsars. This may be evidence that the initial spin period of a neutron star is correlated with the mass of its progenitor and implies that the magnetar birthrate is only a small fraction of that for radio pulsars.