Tracing Shocks and Photodissociation in the Galactic Center Region1

Martín

2010

A&A

Aims. We study the chemistry in the harsh environments of galactic nuclei using the nearest one, the Galactic center (GC). Methods. We have obtained maps of the molecular emission within the central five arcminutes (12 pc) of the GC in selected molecular tracers: SiO(2-1), HNCO(5 0,5 -4 0,4 ), and the J = 1 → 0 transition of H 13 CO + , HN 13 C, and C 18 O at an angular resolution of 30 (1.2 pc). The mapped region includes the circumnuclear disk (CND) and the two surrounding giant molecular clouds (GMCs) of the Sgr A complex, known as the 20 and 50 km s −1 molecular clouds. Additionally, we simultaneously observed the J = 2 → 1 and J = 3 → 2 transitions of SiO toward selected positions to estimate the physical conditions of the molecular gas using the large velocity gradient approximation. Results. The SiO(2-1) emission shows all the molecular features identified in previous studies, covering the same velocity range as the H 13 CO + (1-0) emission, which also presents a similar distribution. In contrast, HNCO(5-4) emission appears in a narrow velocity range mostly concentrated in the 20 and 50 km s −1 GMCs. A similar trend follows the HN 13 C(1-0) emission. The HNCO column densities and fractional abundances present the highest contrast, with difference factors of ≥60 and 28, respectively. Their highest values are found toward the cores of the GMCs, whereas the lowest ones are measured at the CND. SiO abundances do not follow this trend, with high values found toward the CND, as well as the GMCs. By comparing our abundances with those of prototypical Galactic sources we conclude that HNCO, similar to SiO, is ejected from grain mantles into gas-phase by nondissociative C-shocks. This results in the high abundances measured toward the CND and the GMCs. However, the strong UV radiation from the Central cluster utterly photodissociates HNCO as we get closer to the center, whereas SiO seems to be more resistant against UV-photons or it is produced more efficiently by the strong shocks in the CND. This UV field could be also responsible for the trend found in the HN 13 C abundance. Conclusions. We discuss the possible connections between the molecular gas at the CND and the GMCs using the HNCO/SiO, SiO/CS, and HNCO/CS intensity ratios as probes of distance to the Central cluster. In particular, the HNCO/SiO intensity ratio is proved to be an excellent tool for evaluating the distance to the center of the different gas components.

Section: Notessupporting

confidence: 87%

Section: Integrated Intensity Maps and Fractional Abundancesmentioning

confidence: 98%

Mapping photodissociation and shocks in the vicinity of Sagittarius A^∗

Amo-Baladrón

Martín

2010

A&A

“…The comparison with other molecular clouds dominated both by UV radiation and shocks, in the Galactic Center and in external galaxies, shows that M 82 is the galaxy with the highest CH 3 CCH abundance observed up to now. SgrA* (-30,-30) and G+0.18-0.04, used as templates of molecular clouds dominated by photodissociating radiation in the Galactic Center of the Milky Way (Martín et al 2008a), have relative abundances larger than a few 10 −9 . On the other hand, the relative abundances we find in NGC 253 and Maffei 2 are 5.2 × 10 −9 and 2.0 × 10 −9 respectively.…”

Section: Evolved Starburst: M 82mentioning

confidence: 99%

“…Martín-Pintado et al 1997;Hüttemeister et al 1998), those pervaded by UV dissociating radiation from the massive stars that create large photo dissociation regions (PDRs) (e.g. Martín et al 2008a), and also those which study regions affected by X-rays (XDRs) (e.g. Martín-Pintado et al 2000;Amo-Baladrón et al 2009), allows to use the chemical composition to infer the physical processes dominating the heating of ⋆ This work is based on observations with the IRAM 30-m telescope.…”

Section: Introductionmentioning

confidence: 99%

CS, HC₃N, and CH₃CCH multi-line analyses toward starburst galaxies

Aladro

Martín

et al. 2010

A&A

Self Cite

Aims. We aim to study the properties of the dense molecular gas towards the inner few 100 pc of four nearby starburst galaxies dominated both by photo dissociation regions (M 82) and large-scale shocks (NGC 253, IC 342 and Maffei 2), and to relate the chemical and physical properties of the molecular clouds with the evolutionary stage of the nuclear starbursts. Methods. We have carried out multi-transitional observations and analyses of three dense gas molecular tracers, CS, HC 3 N (cyanoacetylene) and CH 3 CCH (methyl acetylene), using Boltzmann diagrams in order to determine the rotational temperatures and column densities of the dense gas, and using a Large Velocity Gradients model to calculate the H 2 density structure in the molecular clouds.Results. The CS and HC 3 N data indicate the presence of density gradients in the molecular clouds. These two molecules show similar excitation conditions, suggesting that they arise from the same gas components. In M 82, CH 3 CCH has the highest fractional abundance determined in a extragalactic source (1.1 × 10 −8 ). Conclusions. The density and the chemical gradients we have found in all galaxies can be explained in the framework of the starburst evolution, which affects the chemistry and the structure of molecular clouds around the galactic nuclei. The young shock-dominated starburst galaxies, like presumably Maffei 2, show a cloud structure with a rather uniform density and chemical composition which suggests low star formation activity. Molecular clouds in galaxies with starburst in an intermediate stage of evolution, such as NGC 253 and IC 342, show clouds with a large density contrast (two orders of magnitude) between the denser regions (cores) and the less dense regions (halos) of the molecular clouds and relatively constant chemical abundance. Finally, the galaxy with the most evolved starburst, M 82, has clouds with a rather uniform density structure, large envelopes of atomic/molecular gas subjected to UV photodissociating radiation from young star clusters, and very different chemical abundances of HC 3 N and CH 3 CCH.

“…To identify which molecular radial velocities are associated with the Fe o line emission, we have used the criterion of the best morphological coincidence between the CS and the Fe o line emissions. We have used the CS emission because it is one of the best tracers of high density gas, it is moderately affected by shocks [6], and it seems to survive UV radiation fields [7]. Therefore, we expect that CS emission will trace the large column densities of neutral material required to produce the Fe o line emission [4].…”

Section: Observations and Resultsmentioning

confidence: 99%

Correlation between SiO and X-ray emission in the galactic center

Amo-Baladrón

Morris

et al. 2008

J. Phys.: Conf. Ser.

Abstract. We present emission maps (spatial resolution ∼40") of the Sgr A molecular cloud complex at the Galactic Center (GC) in the J=2→1 SiO line, observed with the IRAM-30m telescope at Pico Veleta. We have compared our SiO(2-1) data cube with CS(1-0) maps, and we have found a correlation between the SiO/CS intensity ratio and the equivalent width of the X-ray Fe line at 6.4 keV. We discuss the SiO abundance enhancement in the two most plausible scenarios for the origin of the Fe line, which is, at the moment, under debate: fluorescence in an X-ray Reflection Nebula (XRN), or impact by Low-Energy Cosmic Rays (LECRs) followed by electronic relaxation. Both could explain the enhancement in the SiO/CS intensity ratio with the intensity of the Fe line, but both scenarios present difficulties: the first one requires a population of very small grains to produce the enhancement in the SiO/CS intensity ratio, and the second needs higher column densities than the ones derived from observations in the region.