The Green Bank Ammonia Survey (GAS): First Results of NH3 mapping the Gould Belt

Abstract: We present an overview of the first data release (DR1) and first-look science from the Green Bank Ammonia Survey (GAS). GAS is a Large Program at the Green Bank Telescope to map all Gould Belt star-forming regions with $A_V \gtrsim 7$ mag visible from the northern hemisphere in emission from NH$_3$ and other key molecular tracers. This first release includes the data for four regions in Gould Belt clouds: B18 in Taurus, NGC 1333 in Perseus, L1688 in Ophiuchus, and Orion A North in Orion. We compare the NH$_3$ emission to dust continuum emission from Herschel, and find that the two tracers correspond closely. NH$_3$ is present in over 60\% of lines-of-sight with $A_V \gtrsim 7$ mag in three of the four DR1 regions, in agreement with expectations from previous observations. The sole exception is B18, where NH$_3$ is detected toward ~ 40\% of lines-of-sight with $A_V \gtrsim 7$ mag. Moreover, we find that the NH$_3$ emission is generally extended beyond the typical 0.1 pc length scales of dense cores. We produce maps of the gas kinematics, temperature, and NH$_3$ column densities through forward modeling of the hyperfine structure of the NH$_3$ (1,1) and (2,2) lines. We show that the NH$_3$ velocity dispersion, ${\sigma}_v$, and gas kinetic temperature, $T_K$, vary systematically between the regions included in this release, with an increase in both the mean value and spread of ${\sigma}_v$ and $T_K$ with increasing star formation activity. The data presented in this paper are publicly available.

Overview of the Green Bank Ammonia Survey (GAS) - First Results

The Green Bank Ammonia Survey (GAS) represents an extensive campaign focused on mapping ammonia (NH$_3$) emissions in prominent star-forming regions within the Gould Belt as observable from the northern hemisphere. The study leverages the Green Bank Telescope (GBT) to target star-forming regions with visual extinctions greater than approximately 7 magnitudes, which have been shown to coincide with dense molecular gas conducive to star formation.

Methodology and Data Release

In this initial phase, the survey encompassed four Gould Belt clouds: B18 in Taurus, NGC 1333 in Perseus, L1688 in Ophiuchus, and Orion A North. Each of these regions presents unique features, ranging from the filamentary configurations in Taurus to the high-mass star-forming complexes in Orion. These maps were meticulously constructed using the GBT's K-Band Focal Plane Array, capturing NH$_3$ alongside carbon-bearing molecules like HC$_5$N and C$_2$S in certain configurations.

Key Findings

NH$_3$ Emission Characteristics

The GAS data revealed that NH$_3$ emissions efficiently trace regions of higher H$_2$ column density, aligning closely with thermal dust emissions observed through instruments like Herschel. Importantly, across most surveyed regions, NH$_3$ was detected in over 60% of lines-of-sight where visual extinction exceeded the 7 mag threshold, emphasizing its role as a reliable dense gas tracer.

Variability Across Regions

Investigations into NH$_3$ hyperfine structure allowed for the determination of gas kinematics, temperature, and NH$_3$ column densities. Notably, velocity dispersion and kinetic temperature varied systematically with increasing star-forming activity among the regions surveyed. These results provide insight into the role of environmental conditions in shaping the physical properties of dense gas.

Star Formation Activity Correlations

The study identified correlations between star formation activity level and gas properties. Regions with higher star formation activity displayed increased mean values and broader distributions of velocity dispersion and kinetic temperature. This aligns with theoretical expectations that more dynamic environments influence molecular turbulence and thermal characteristics.

Filaments and Other Molecular Tracers

The GAS data underscored NH$_3$ emissions' spatial alignment with filamentary structures previously discerned in dust continuum studies. Additionally, carbon-chain emissions, though less ubiquitous, offered insights into chemical stratification within star-forming cores.

Implications and Future Directions

The findings from GAS enhance our understanding of different star formation environments by detailing the interplay of physical conditions in nearby molecular clouds. As further data and analyses become available, GAS will undoubtedly enrich theoretical frameworks governing gas dynamics in star-forming regions. Observations of NH$_3$ and complementary tracers continue to be vital for constraining models of molecular cloud evolution and star formation initiation.

Future work will explore the complexity of gas structures, leveraging these initial results to refine simulations and enhance empirical models. The expanding dataset from GAS not only provides a detailed inventory of the gas-phase conditions in star-forming clouds but also offers a robust platform for upcoming research into the mechanisms governing star formation.