2 Mm GISMO Observations of the Galactic Center. II. a Nonthermal

Draft version September 19, 2019 A Typeset using L TEX twocolumn style in AASTeX63 2 mm GISMO Observations of the Galactic Center. II. A Nonthermal Filament in the Radio Arc and Compact Sources∗ Johannes Staguhn,1,2 Richard G. Arendt,1,3 Eli Dwek,1 Mark R. Morris,4 Farhad Yusef-Zadeh,5 Dominic J. Benford,6 Attila Kovacs´ ,7 and Junellie Gonzalez-Quiles8,9 1Code 665, NASA/GSFC, 8800 Greenbelt Road, Greenbelt, MD 20771, USA 2Johns Hopkins University 3CRESST2/UMBC 4Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, CA 90095, USA 5CIERA and the Department of Physics & Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA 6Astrophysics Division, NASA Headquarters, 300 E St. SW, Washington, DC 20546, USA 7Smithsonian Astrophysical Observatory Submillimeter Array (SMA), MS-78, 60 Garden St, Cambridge, MA 02138, USA 8Code 667, NASA/GSFC, 8800 Greenbelt Road, Greenbelt, MD 20771, USA 9CRESST2/SURA ABSTRACT We have used the Goddard IRAM 2-Millimeter Observer (GISMO) with the 30 m IRAM telescope to carry out a 2 mm survey of the Galaxy’s central molecular zone (CMZ). These observations detect thermal emission from cold ISM dust, thermal free-free emission from ionized gas, and nonthermal synchrotron emission from relatively flat-spectrum sources. Archival data sets spanning 3.6 µm to 90 cm are used to distinguish different emission mechanisms. After the thermal emission of dust is modeled and subtracted, the remaining 2 mm emission is dominated by free-free emission, with the exception of the brightest nonthermal filament (NTF) that runs though the middle of the bundle of filaments known as the Radio Arc. This is the shortest wavelength at which any NTF has been detected. The GISMO observations clearly trace this NTF over a length of 0.2◦, with a mean 2 mm ∼ spectral index which is steeper than at longer wavelengths. The 2 mm to 6 cm (or 20 cm) spectral index steepens from α 0.2 to 0.7 as a function distance from the Sickle H II region, suggesting ≈ − − that this region is directly related to the NTF. A number of unresolved (at 21′′) 2 mm sources are found nearby. One appears to be thermal dust emission from a molecular cloud that is associated with an enigmatic radio point source whose connection to the Radio Arc is still debated. The morphology and colors at shorter IR wavelengths indicate other 2 mm unresolved sources are likely to be compact H II regions. 1. INTRODUCTION the linear filaments (Yusef-Zadeh et al. 1984, 1997). A Radio continuum observations have identified sev- number of models have considered the origin of NTFs eral systems of nonthermal (radio) filaments (NTFs, associated with molecular clouds or with mass-losing sometimes known as NRFs) as well as isolated indi- stars (e.g. Rosner & Bodo 1996; Shore & LaRosa 1999; arXiv:1909.08026v1 [astro-ph.GA] 17 Sep 2019 vidual filaments within the inner two degrees of the Bicknell & Li 2001). However, there is no consensus Galactic center, with the magnetic field aligned along on the origin of the Galactic center filaments. Among the large number of known NTFs, a bundle of filaments near the Radio Arc at l 0.2◦ is unique in its ∼ [email protected], [email protected] physical interaction with the molecular cloud G0.13-0.13 [email protected], [email protected] (Tsuboi et al. 1997). In addition, these filaments are surrounded by ionized gas, and have an unusually flat [email protected], [email protected] spectrum for a nonthermal polarized source. Here we in- [email protected], [email protected] vestigate the energy spectrum of the emission from the ∗ Based on observations carried out with the IRAM 30 m Tele- radio arc filaments by measuring its spectral index at scope. IRAM is supported by INSU/CNRS (France), MPG (Ger- a wavelength of 2 mm, thus providing additional con- many) and IGN (Spain). 2 Staguhn et al. Figure 1. GISMO 2 mm observations (green) of the Galactic Center, superimposed with 850 µm SCUBA2 observations (blue) and 19.5 cm VLA observations (red). This image highlights different emission mechanisms from different sources. Most generally, red represents synchrotron emission sources, yellow corresponds to free-free emission sources, and cyan features are thermal emission from dust in molecular clouds. High latitude regions of the 2 mm image are masked where the Herschel 500 − ◦ ◦ ′′ µm brightness is < 500 MJy sr 1, affecting some areas at b> 0.1 and b< −0.2 . All images are convolved to match the 21 resolution of the GISMO data. straint on possible mixture of thermal and nonthermal expected at 2 mm as previously reported by Reich et al. emission. (2000). The 2 mm GISMO survey of the Galactic Center pro- The 2 mm wavelength is a transition region in the vides a unique view of thermal emission from cold dust spectrum of the general Galactic ISM and specific Galac- in molecular clouds, and free-free emission from ionized tic sources. The Rayleigh-Jeans tail of thermal emission gas in H II regions (Arendt et al. 2019, Paper I). It also from cold dust continues to fade from far-IR wavelengths shows emission from the single brightest NTF, which into the 2 mm band, whereas optically thin free-free runs through the middle of a bundle of parallel NTFs (thermal) radio sources are fading as wavelengths fall known as the Radio Arc. While the large-scale Radio from the radio regime to the mm and sub-mm regime. Arc at low angular resolution has been observed to have Optically thin non-thermal sources typically fall much an inverted radio spectrum (S να with α> 0) at fre- more quickly as a function of decreasing wavelength. ν ∝ quencies as high as 43 GHz (7 mm) (Reich et al. 1988), Figure 1 provides an overview of the 2 mm emission the smaller-scale structure has been reported to have a (green) in comparison with the 850 µm thermal emission steep (α 1) spectrum between 32 and 43 GHz (or 7 from dust (blue; Parsons et al. 2018) and 19.5 cm radio ≈− to 9 mm) (Sofue et al. 1999). The bright NTF had been emission (red). Most molecular clouds are identified by detected in interferometric observations at wavelengths cyan colors with dust emission extending from 850 µm as short as 3 mm Pound & Yusef-Zadeh (2018) and po- to 2 mm. Regions of radio emission with relatively flat larized emission has been detected from the entire Radio spectral indices (free-free emission and flat non-thermal Arc at low resolution at 2 and 3 mm (Culverhouse et al. sources) exhibit yellow colors in this image. 2011). Most NTFs across the GC have relatively steep In this paper, we employ a wide wavelength range of radio spectra (e.g. Law et al. 2008), and any emission archival data sets to focus on the analysis of the emission at 2 mm or shorter wavelengths would be expected to from the brightest of the NTFs. We investigate if the be very faint and confused. The NTFs had not been de- 2 mm observation can provide further insight into the tected at shorter wavelengths (e.g. 850 µm and 1.1 mm), origin and nature of the NTFs, and their relation to which made emission from these features somewhat un- other possibly nearby features. In particular, there is a radio point source of an uncertain nature that lies 2 mm Emission from the Radio Arc NTF 3 atop the NTF, at least in projection. We find 2 mm the Hi-GAL survey (Molinari et al. 2016); and emission at the location of this source and compare the Spitzer IRAC and MIPS observations at 3.6 - SED of this 2 mm point source with other nearby point- 24 µm (Stolovy et al. 2006; Yusef-Zadeh et al. like 2 mm sources in order to understand the nature of 2009). VLA observations at 6 and 20 cm are this source. from Yusef-Zadeh et al. (1984), Yusef-Zadeh & Morris (1987a,b), and Yusef-Zadeh et al. (2009). VLA observations at 90 cm are from LaRosa et al. (2000). 2. DATA Figures 2 and 3 illustrate the emission from the re- The GISMO instrument (Staguhn et al. 2006, 2008), gion of the Radio Arc at wavelengths from 70 µm to paired with the 30 m IRAM telescope (Baars et al. 2 mm. As wavelength increases, emission from warm 1987), was used to map a 2◦ 0.6◦ region at the ∼ × dust becomes surpassed by emission from colder dust, Galactic Center at a wavelength of 2 mm. The beam which in turn begins to be exceeded by free-free emis- size of GISMO observations at the 30 m telescope is sion at 2 mm. Figure 4 further illustrates emission at ′′ 16.6 FWHM. The observations were generally carried radio wavelengths from 6 to 90 cm. The primary beam out under stable atmospheric conditions and with a correction (Napier & Rots 1982) applied to the 6 cm im- zenith opacity of τ2mm < 0.11. Focus in z-direction was age appears valid over the range 0.2◦ <b< 0◦, but − regularly monitored (several times a day) and point- may overcorrect in the corners of the image. The 6 cm ing was frequently checked (about once per observing image is largely dominated by free-free emission. This hour) using the nearby bright quasars J2037+511 and gradually becomes dominated by non-thermal emission J1637+574. Fluxes were calibrated to < 10% accu- as wavelengths increase to 90 cm.

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