Sagittarius B2

Introduction

Sagittarius B2 (abbreviated as Sgr B2) is one of the most massive molecular clouds in the Galaxy. It is located at a distance of 8.34±0.16 kpc (Reid et al. 2014) and has a projected distance of 107 pc from Sgr A*, the compact radio source associated with the supermassive black hole located at the Galactic Center.

Hüttemeister et al. (1993) distinguish three different parts in Sgr B2:

a low density envelope
a moderate density region, that is extended around
several small clumps, which are the most compact and densest molecular regions.

At least three small clumps are sites of active star formation (Gordon et al. 1993). They are historically named according to their relative location in an equatorial coordinate system: Sgr B2(North), Sgr B2(Main) and SgrB2(South). These sources contain a plethora of (ultra-compact) Hii regions, X-ray sources, dense cores, embedded protostars, and molecular masers.

Sketch of the Sagittarius B2 region, adapted from Hüttemeister et al. (1995).

Continuum radiative transfer modeling

Data: We model the thermal dust and free-free continuum emission using our Pandora framework. For this approach it is crucial to use multi-wavelength, multi-scale data to properly constrain the structure of Sgr B2. Towards the hot cores Sgr B2(N) and Sgr B2(M), the Herschel/HIFI spectral surveys (HEXOS project, PI: Ted Bergin) provide the continuum information from the sub-mm to the far-infrared regime. High-resolution interferometric maps towards both hot cores obtained with the Submillimeter Array (SMA) and the Very Large Array (VLA) provide the necessary spatial resolution on small scales. To cover the large-scale structure, we use dust continuum maps obtained within the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) and the Herschel infrared Galactic Plane Survey (HiGAL).

Large-scale modeling:

Sgr B2, large scale continuum maps. From left to right: ATLASGAL 870 um, Hi-GAL 500 um, 350 um, 250 um, and 70 um. First row: Data. Bottom row: Simulation.

Small-scale modeling:

After managing to reconstruct the large-scale structure, we take a detailed look at the two hot cores Sgr B2(N) and Sgr B2(M). In this step we make use of the high resolution maps obtained with the VLA and the SMA. These maps allow to include the Hii regions (free-free emission) and the small-scale dust cores.

Sagittarius B2(N):

Interferometric continuum map from the VLA of Sgr B2(N). Left: Data. Right: Simulation.

Interferometric continuum maps from the SMA of Sgr B2(N). Left: Data. Right: Simulation.

Sagittarius B2(M):

Interferometric continuum map from the VLA of Sgr B2(M). Left: Data. Right: Simulation.

Interferometric continuum maps from the SMA of Sgr B2(M). Left: Data. Right: Simulation.

Stellar contribution: Stars provide the heating. In our models, they are assumed to be point sources.

We account for observed early-type high-mass stars by including stars embedded in the known Hii regions (Mehringer et al. 1993; Gaume et al. 1995; De Pree et al. 1998).
We account later spectral types are included (stars which produce Hii regions non-detectable with current observations). These stars are randomly drawn from Kroupas initial mass function (IMF).

Details on this approach are included in our paper, submitted to A&A. The final stellar column density is plotted below.

Center map: Stellar column density map of Sgr B2. The projected distribution of the stars along the x- and y-axis are shown in the right and top panel, respectively.

Spectral energy distribution: In addition to the standard SED, where the continuum maps at each wavelength are convolved with the same beam (Figure to the left), it is now possible to convolve each continuum map with the telescope size, resulting in a wavelength-dependence of the beam size. This approach allows us to fit the SEDs obtained from the Herschel/HIFI observations (Figure to the right).

Spectral energy distribution plots using a fixed beamsize 30 arcsec. The orange solid line is the RADMC-3D dust and free-free best fit, the orange dotted line represents the free-free contribution, the dashed-dotted line represents the contribution from dust emission. The dark blue markers represent the observational results from Goldsmith+1990, Goldsmith+1992. The data point at 70 um was taken from the HiGAL PACS map. The dashed blue line represent the best fit from Etxaluze+2013 convolved to a beamsize of 30 arcsec. The surface, where the dust optical depth equals one is plotted in olive, the corresponding axis is shown on the right. The z-axis points towards the observer, Sgr B2(M) is located at z = 0 and Sgr B2(N) is located at z = 7e5 au.

Spectral energy distribution of the HIFI data convolved to the frequency dependent beam of the 3.5 m Herschel telescope (in grey). The data are unsmoothed, what looks like noise are actually individual spectral lines. The best fit towards Sgr B2(M) and N are represented by the solid orange line. The fit includes dust and free-free emission. The olive line gives the location along the line of sight, where the optical depth is equal to 1, i.e. the dust becomes optically thick.

More analysis is included in our paper, which is published in A&A (ADS).