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SOFIA/FORCAST Galactic Center Legacy Survey: Overview

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SOFIA/FORCAST Galactic Center Legacy Survey: Overview Draft version April 15, 2020 Typeset using LATEX modern style in AASTeX62 SOFIA/FORCAST Galactic Center Legacy Survey: Overview Matthew J. Hankins,1 Ryan M. Lau,2 James T. Radomski,3 Angela S. Cotera,4 Mark R. Morris,5 Elisabeth A. C. Mills,6 Daniel L. Walker,7, 8 Ashley T. Barnes,9 Janet P. Simpson,4 Terry L. Herter,10 Steven N. Longmore,11 John Bally,12 Mansi M. Kasliwal,1 Nadeen B. Sabha,13 and Macarena Garc´ıa-Marin14 1Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA 2Institute of Space & Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan 3SOFIA-USRA, NASA Ames Research Center, MS 232-12, Moffett Field, CA 94035, USA 4SETI Institute, 189 Bernardo Ave., Mountain View, CA 94043, USA 5Dept. of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USA 6Department of Physics and Astronomy, University of Kansas, 1251 Wescoe Hall Dr., Lawrence, KS 66045, USA 7National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo, 181-8588, Japan 8Joint ALMA Observatory, Alonso de C´ordova 3107, Vitacura, Santiago, Chile 91Argelander-Institut f¨urAstronomie, Universit¨atBonn, Auf dem H¨ugel 71, 53121, Bonn, Germany 10Department of Astronomy, Cornell University, Space Sciences Bldg, Ithaca, NY 14853-6801, USA 11Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK 12Department of Astrophysical and Planetary Sciences, University of Colorado, 389 UCB, Boulder, CO 80309, USA 13Institut fr Astro- und Teilchenphysik, Universitt Innsbruck, Technikerstr. 25, 6020 Innsbruck, Austria 14European Space Agency, 3700 San Martin Drive, Baltimore, MD 21218, USA ABSTRACT The Galactic Center contains some of the most extreme conditions for star formation in our Galaxy as well as many other phenomena that are unique to this region. Given our relative proximity to the Galactic Center, we are able to study details of physical processes to a level that is simply not yet possible for more distant galaxies, yielding an otherwise inaccessible view of the nuclear region of a galaxy. We recently carried out a targeted imaging survey of mid-infrared bright portions of the Galactic Center at 25 and 37 µm using the FORCAST instrument on SOFIA. This survey was one of the inaugural Legacy Programs from SOFIA cycle 7, observing a total area of 403 arcmin2 (2180 pc2), including the Sgr A, B, and C complexes. Here we present an overview of the survey strategy, observations, and data reduction as an accompaniment to the initial public release of the arXiv:2001.05487v2 [astro-ph.GA] 14 Apr 2020 survey data. We discuss interesting regions and features within the data including extended features near the circumnuclear disk, structures in the Arched Filaments and Sickle H II regions, and signs of embedded star formation in Sgr B2 and Sgr C. We also feature a handful of less well studied mid-infrared sources located between Sgr A and Sgr C that could be sites of relatively isolated star formation activity. Last, we discuss plans for subsequent publications and future data releases from the survey. Keywords: Galaxy: center, ISM: H II regions Corresponding author: Matthew J. Hankins [email protected] 2 Hankins et al. 1. INTRODUCTION The origin of these massive field stars is some- The environment in the Galactic Center (GC) what of a mystery. While several sources may is unlike any other part of our Galaxy. The be former cluster members which have been dy- region contains high molecular gas densities namically ejected or removed due to tidal evap- (Guesten & Henkel 1983), high temperatures oration, the entire population of these sources (Morris et al. 1983; Guesten et al. 1985), and cannot be accounted for with these mechanisms large turbulent motions (Bally et al. 1987), all (Habibi et al. 2014). Instead, some fraction inside a deep gravitational potential well (Mor- of the field stars likely originates from a more ris & Serabyn 1996). The conditions in the isolated mode of star formation, and there is GC mirror those found in the nuclei of lumi- evidence of ongoing massive star and stellar nous infrared galaxies and high-redshift sys- cluster formation within parts of the GC (e.g., tems near the peak of cosmic star formation Barnes et al. 2019, and references therein). history (Kruijssen & Longmore 2013), but its There have also been efforts to identifying pos- relative proximity (d = 8:0 ± 0:5 kpc; Reid sible clusters associated with GC field stars 1993) enables us to study the physical pro- which has proven observationally challenging cesses there at a level of detail that is simply (Steinke et al. 2016; Dong et al. 2017), though not possible in more distant systems (see the there has been recent progress in this area us- recent review by Mills 2017). ing large proper motion studies (?). Observations of the GC often challenge the- Large extinction toward the GC (AV ∼30; oretical models of star formation, which often Fritz et al. 2011) makes it impossible to ob- break down in this complex region. For exam- serve massive stars and protostars at optical ple, while the GC comprises less than 0.01% and ultraviolet wavelengths. Instead, studies of of the total volume of the Galactic disk, its the region have relied on observations at other −1 wavelengths to examine the distribution and star formation rate (SFR) (∼ 0:1 M yr ; Immer et al. 2012; Barnes et al. 2017) is a birth environment of stars. Numerous studies considerable fraction of the total SFR of the of star formation in the GC have been con- −1 ducted with mid-infrared observations between Galaxy (∼ 1:2 M yr ; Lee et al. 2012). How- ever, the global GC SFR is more than an or- ∼3.6 { 24 µm (e.g., Ram´ırezet al. 2008; Yusef- der of magnitude smaller than one would ex- Zadeh et al. 2010; An et al. 2011; Immer et al. pect based on scaling relations with its dense 2012). In particular, warm dust emission at molecular gas content (Lada et al. 2012; Long- ∼ 24 µm is a valuable probe for identifying more et al. 2013). The inefficiency of the GC young stellar objects (YSOs) and estimating in converting dense gas to stars presents a sig- star formation rates (e.g., Calzetti et al. 2007). nificant quandary for the region with much However, the most active regions within the broader implications. For example, SFR mea- inner ∼200 pc of our Galaxy are strongly satu- surements are universally used as a fundamen- rated in the Spitzer/MIPS 24 µm data (Yusef- tal diagnostic tool for understanding the under- Zadeh et al. 2009). Earlier observations with lying physics in galaxies and for understanding the Midcourse Space Experiment (MSX; Egan galaxy evolution. et al. 2003) at 21.3 µm (Band E) provide un- Even with the aforementioned star formation saturated images, but are relatively low spatial deficiency, the stellar inventory of the GC is resolution (∼20" or ∼0.8 pc) when compared relatively rich with numerous types of massive with Spitzer/MIPS at 24 µm (∼6" or ∼0.2 pc). evolved stars, such as Luminous Blue Variables The MSX data suffer from significant confu- and Wolf-Rayet stars (e.g., Figer 2009). The sion in these complex regions, which presents GC is home to three known massive stellar a considerable hurdle in our understanding of clusters (Lu 2018) - the Arches cluster (Cotera these very active portions within the GC. Fur- et al. 1996), the Quintuplet cluster (Okuda thermore, the lack of high-quality mid-infrared et al. 1990; Nagata et al. 1990), and the Central data in these regions represents an essential, cluster (Krabbe et al. 1991, 1995) in addition to missing piece of the rich multi-wavelength pic- a large number of massive field stars which are ture of the GC that has emerged over the last spread throughout the region (e.g., Muno et al. decade. 2006; Mauerhan et al. 2010; Dong et al. 2012). In order to create improved mid-infrared maps of the brightest portions of the inner SOFIA/FORCAST Galactic Center Survey Overview 3 ∼200 pc of our Galaxy, we set out to con- We chose to conduct the survey using the FOR- duct a targeted survey of regions within the CAST 25 µm filter because it is close in wave- GC using SOFIA/FORCAST. This survey was length to the MIPS 24 µm filter and only a selected as one of the inaugural Legacy Pro- slight color-correction ([24]-[25]=0.15 mag for grams in SOFIA cycle 7. Observations were a Vega-like source) is needed to compare pho- obtained at 25 and 37 µm using the Faint Ob- tometry between the two data sets where pos- ject infraRed CAmera for the SOFIA Telescope sible. The FORCAST and MIPS data sets are (FORCAST), enabling us to create a high- complementary since the higher bright source resolution (FWHM∼2.3" or ∼0.07 pc at 25 µm limit of FORCAST allows for observations of and FWHM∼3.4" or ∼0.1 pc at 37 µm) mosaic objects and regions where the MIPS data are of portions within the GC including the Sgr A saturated, while the higher sensitivity of the complex and other prominent star forming re- MIPS probes fainter sources than FORCAST gions such as Sgr B and Sgr C. can detect in reasonable integration times. Our In this paper we present a description of the nominal sensitivity per field was selected to observations along with the survey strategy achieve a 5-σ point source depth of 250 mJy and an initial look at the data set with vari- at 25 µm which is equivalent to a 3-σ extended ous regions of interest highlighted.
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