Submillimeter imaging of the Galactic Center starburst Sgr B2. Warm molecular, atomic, and ionized gas far from massive star-forming cores
Abstract
Context. Star-forming galaxies emit bright molecular and atomic lines in the submillimeter and far-infrared (FIR) domains. However, it is not always clear which gas heating mechanisms dominate and which feedback processes drive their excitation.
Aims: The Sgr B2 complex is an excellent template to spatially resolve the main OB-type star-forming cores from the extended cloud environment and to study the properties of the warm molecular gas in conditions likely prevailing in distant extragalactic nuclei.
Methods: We present 168 arcmin2 spectral images of Sgr B2 taken with Herschel/SPIRE-FTS in the complete ~450−1545 GHz band. We detect ubiquitous emission from mid-J CO (up to J = 12−11), H2O 21,1−20,2, [C I] 492, 809 GHz, and [N II] 205 μm lines. We also present velocity-resolved maps of the SiO (2−1), N2H+, HCN, and HCO+ (1−0) emission obtained with the IRAM 30 m telescope.
Results: The cloud environment (~1000 pc2 around the main cores) dominates the emitted FIR (~80%), H2O 752 GHz (~60%) mid-J CO (~91%), [C I] (~93%), and [N II] 205 μm (~95%) luminosity. The region shows very extended [N II] 205 μm emission (spatially correlated with the 24 and 70 μm dust emission) that traces an extended component of diffuse ionized gas of low ionization parameter (U ≃ 10−3) and low LFIR / MH2 ≃ 4−11 L⊙M⊙−1 ratios (scaling as ∝Tdust6). The observed FIR luminosities imply a flux of nonionizing photons equivalent to G0 ≈ 103. All these diagnostics suggest that the complex is clumpy and this allows UV photons from young massive stars to escape from their natal molecular cores. The extended [C I] emission arises from a pervasive component of neutral gas with nH ≃ 103 cm−3. The high ionization rates in the region, produced by enhanced cosmic-ray (CR) fluxes, drive the gas heating in this component to Tk ≃ 40−60 K. The mid-J CO emission arises from a similarly extended but more pressurized gas component (Pth / k ≃ 107 K cm−3): spatially unresolved clumps, thin sheets, or filaments of UV-illuminated compressed gas (nH ≃ 106 cm−3). Specific regions of enhanced SiO emission and high CO-to-FIR intensity ratios (ICO / IFIR ≳ 10−3) show mid-J CO emission compatible with C-type shock models. A major difference compared to more quiescent star-forming clouds in the disk of our Galaxy is the extended nature of the SiO and N2H+ emission in Sgr B2. This can be explained by the presence of cloud-scale shocks, induced by cloud-cloud collisions and stellar feedback, and the much higher CR ionization rate (>10−15 s−1) leading to overabundant H3+ and N2H+.
Conclusions: Sgr B2 hosts a more extreme environment than star-forming regions in the disk of the Galaxy. As a usual template for extragalactic comparisons, Sgr B2 shows more similarities to nearby ultra luminous infrared galaxies such as Arp 220, including a "deficit" in the [C I] / FIR and [N II] / FIR intensity ratios, than to pure starburst galaxies such as M 82. However, it is the extended cloud environment, rather than the cores, that serves as a useful template when telescopes do not resolve such extended regions in galaxies.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- May 2021
- DOI:
- 10.1051/0004-6361/202040221
- arXiv:
- arXiv:2103.17177
- Bibcode:
- 2021A&A...649A..32S
- Keywords:
-
- ISM: clouds;
- photon-dominated region;
- infrared: ISM;
- Galaxy: center;
- ISM: molecules;
- cosmic rays;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Accepted for publication in A&