Tracing gas accretion in the Galactic center using isotopic ratios^⋆

¹ Instituto de Radioastronomía Milimétrica (IRAM), Av. Divina Pastora 7, Local 20, 18012 Granada, Spain
email: riquelme@iram.es
² Centro de Astrobiología (CSIC/INTA), Ctra. de Torrejón a Ajalvir km 4, 28850, Torrejón de Ardoz, Madrid, Spain
³ Joint ALMA Observatory, Av. Alonso de Córdova 3107, Vitacura, Santiago de Chile, Chile
⁴ European Southern Observarory, Av. Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago, Chile
⁵ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile

Received: 17 May 2010
Accepted: 3 August 2010

Abstract

Aims. We study the ¹²C/¹³C isotopic ratio in the disk of the central molecular zone and in the halo to trace gas accretion toward the Galactic center region in the Milky Way.

Methods. Using the IRAM 30m telescope, we observed the J = 1−0 rotational transition of HCO⁺, HCN, HNC, and their ¹³C isotopic substitutions in order to measure the ¹²C/¹³C isotopic ratio. We observed 9 positions selected throughout the Galactic center region, including clouds at high latitude, locations where the X1 and X2 orbits associated with the barred potential are expected to intersect, and typical Galactic center molecular clouds.

Results. We find a systematically higher ¹²C/¹³C isotopic ratio (>40) toward the halo and the X1 orbits than for the Galactic center molecular clouds (20–25). Our results point to molecular gas that has undergone a different degree of nuclear processing than observed in the gas towards the inner Galactic center region.

Conclusions. The high isotopic ratios are consistent with the accretion of the gas from the halo and from the outskirts of the Galactic disk.