SOFIA/FORCAST Galactic Center Legacy Survey: Overview

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 $\mu$m 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 arcmin$^2$ (2180 pc$^2$), 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 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.

• The paper details the high-resolution mid-infrared mapping of the Galactic Center using SOFIA/FORCAST, significantly enhancing our understanding of star formation.
• It outlines a survey of key regions including Sgr A, B, and C, employing strategic observations at 25 and 37 μm and addressing calibration challenges with Spitzer and MSX comparisons.
• The findings reveal intricate dust structures and dynamic processes that provide critical insights for future astrophysical models and star formation rate analyses.

Overview of the SOFIA/FORCAST Galactic Center Legacy Survey

The paper "SOFIA/FORCAST Galactic Center Legacy Survey: Overview" by Hankins et al. provides a comprehensive summary of the first survey from the Stratospheric Observatory for Infrared Astronomy (SOFIA) using the Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST) at mid-infrared wavelengths. The Galactic Center (GC) is known for its unique and complex environment with extreme conditions for star formation, making it a key subject for astrophysical studies. The survey aimed to map mid-infrared bright regions in the GC to enhance our understanding of star formation and the physical processes at play in this peculiar region.

Survey Design and Observations

The survey focused on the brightest mid-infrared sections of the Galactic Center, covering prominent star-forming complexes like Sgr A, B, and C over a total survey area of 403 arcmin$^2$. The observations were conducted at 25 and 37 $\mu$m wavelengths using the FORCAST instrument aboard SOFIA. The choice of wavelengths was intended to resolve the limitations of existing Spitzer data, particularly in regions saturated at 24 $\mu$m. This strategic choice highlights the need for higher spatial resolution imaging of warm dust, which is critical to identifying young stellar objects (YSOs) and to measuring star formation rates accurately.

Data Processing and Challenges

Data were processed using established pipeline procedures, and mosaic maps were created for both 25 and 37 $\mu$m observations. Key challenges included managing the effects of the secondary mirror issue, which caused elongation in images, and aligning older data with current cycle observations given advancements in data reduction techniques. This required careful calibration against existing Spitzer and MSX datasets to ensure accurate astrometric solutions. The survey overcame these obstacles to produce high-resolution maps critical for tracing recent star formation activities.

Scientific Insights

• Sgr A Complex: The FORCAST data enabled detailed imaging of dust structures in the CND and surrounding regions, revealing connections to X-ray lobes and streamer-like features extending from the Sgr A complex. The observations provide insights into outflow processes and interactions of warm dust with ambient environments. The presence of dusty nebulae associated with known emission line stars indicates dynamic star formation activities in Sgr A.
• Arched Filaments and Sickle H II Regions: The survey provided enhanced views of these prominent GC features, showcasing "finger"-like structures in the Sickle region suggestive of interactions with magnetic fields or shocks. The enhanced resolution offers clearer insights into the cloud morphology and underlying processes influencing ionization and heating by neighboring massive clusters such as the Arches and Quintuplet clusters.
• Sgr B and Sgr C Complexes: In Sgr B2, the survey captured several compact and reddened sources, illuminating the active star formation within ultra-compact H II regions. The survey elucidates the ongoing transformation and stellar birth in a GC region known for its significant molecular mass richness. Meanwhile, the Sgr C observations revealed intricate dust structures near the Sgr C H II region and molecular cloud, consistent with ongoing high-mass star formation, providing a continuum-observed pairing with known radio spectral lines.
• Isolated Sources: The survey documented several mid-infrared sources between Sgr A and Sgr C that may represent more isolated modes of star formation. The morphology and infrared colors measured point to features that deviate from typical massive star clusters, suggesting varied evolutionary processes and environmental conditions within the GC.

Implications and Future Directions

The paper highlights the significance of the survey for assessing star formation efficiency in the GC and understanding the interplay between stars and the interstellar medium under extreme conditions. The data serve as a crucial resource for validating models of star and cluster formation, galaxy evolution diagnostics, and galactic-scale bar dynamics. Future directions include detailed analysis of individual regions, production of source catalogs, and integration with Spitzer/MIPS datasets to enrich the understanding of star formation in heavily saturated or unresolved areas. The potential to address the star formation rate discrepancy in the GC could have broad implications for theoretical models across numerous galactic environments.

In conclusion, the SOFIA/FORCAST Galactic Center Legacy Survey represents a substantial contribution to the understanding of the complex and dynamic processes governing star formation within one of the most active and densely packed regions of the Milky Way. By addressing the observational deficits of previous infrared surveys with high-resolution imaging, the findings from this survey set the groundwork for future studies that explore unraveling the unique astrophysical phenomenology of the GC.