ALMA imaging of C2H emission in the disk of NGC 1068
Journal article, 2017

Aims. We study the feedback of star formation and nuclear activity on the chemistry of molecular gas in NGC 1068, a nearby (D = 14 Mpc) Seyfert 2 barred galaxy, by analyzing whether the abundances of key molecular species such as ethynyl (C2H), which is a classical tracer of photon dominated regions (PDR), change in the different environments of the disk of the galaxy. Methods. We used the Atacama Large Millimeter Array (ALMA) to map the emission of the hyperfine multiplet of C2H(N = 1-0) and its underlying continuum emission in the central r similar or equal to 35" (2.5 kpc) region of the disk of NGC 1068 with a spatial resolution 1.0 x 0.7 (similar or equal to 50-70 pc). We used maps of the dust continuum emission obtained at 349 GHz by ALMA to derive the H-2 gas column densities and combined these with the C2H map at matched spatial resolution to estimate the fractional abundance of this species. We developed a set of time-dependent chemical models, which include shocks, gas-phase PDRs, and gas-grain chemical models to determine the origin of the C2H gas. Results. A sizeable fraction of the total C2H line emission is detected from the r similar or equal to 1.3 kpc starburst (SB) ring, which is a region that concentrates the bulk of the recent massive star formation in the disk traced by the Pa alpha emission complexes imaged by the Hubble Space Telescope (HST). However, the brightest C2H emission originates from a r similar or equal to 200 pc off -centered circumnuclear disk (CND), where evidence of a molecular outflow has been previously found in other molecular tracers imaged by ALMA. We also detect significant emission that connects the CND with the outer disk in a region that probes the interface between the molecular disk and ionized gas outflow out to r similar or equal to 400 pc. We derived the fractional abundances of C2H (X(C2H)) assuming local thermodynamic equilibrium (LTE) conditions and a set of excitation temperatures (T-ex) constrained by the previous multiline CO studies of the galaxy. Our estimates range from X(C2H) similar or equal to a few 10(-8) in the SB ring up to X(C2H) similar or equal to a few 10(-7) in the outflow region. The PDR models that incorporate gas-grain chemistry are able to account for X(C2H) in the SB ring for moderately dense (n(H-2) >= 10(4) cm(-3)) and moderately UV-irradiated gas (UV-field <= 10 x Draine field, where 1 Draine field 2.74 x 10(-3) erg s(-1) cm(-2)) in a steady-state regime, which depending on the initial and physical conditions of the gas may be achieved by 10(5) yr or as late as 10(7) yr. However, the high fractional abundances estimated for C2H in the outflow region can only be reached at very early times (T < 10(2-3) yr) in models of UV or X-ray irradiated dense gas (n(H-2) >= 10(4-5) cm(-3)). Conclusions. We find that the transient conditions required to fit the high values of X(C2H) in the outflow are likely due to UV or X-ray irradiated non-dissociative shocks associated with the highly turbulent interface between the outflow and molecular gas in NGC 1068. Although the inferred local timescales are short, the erosion of molecular clouds by the active galactic nucleus (AGN) wind and/or the jet likely resupplies the interface working surface continuously, making a nearly steady state persist in the disk of the galaxy.

galaxies: ISM

galaxies: kinematics and dynamics

galaxies: individual: NGC 1068

galaxies: Seyfer

galaxies: nuclei

radio lines: galaxies

Author

S. G. Burillo

Spanish National Observatory (OAN)

S. Viti

University College London (UCL)

F. Combes

Centre national de la recherche scientifique (CNRS)

A. Fuente

Spanish National Observatory (OAN)

A. Usero

Spanish National Observatory (OAN)

L. K. Hunt

Arcetri Astrophysical Observatory

S. Martin

Atacama Large Millimeter-submillimeter Array (ALMA)

European Southern Observatory Santiago

M. Krips

Institut de Radioastronomie Millimétrique (IRAM)

Susanne Aalto

Astronomy and Plasmaphysics

Rebeca Aladro

Astronomy and Plasmaphysics

C. Ramos Almeida

University of La Laguna

Instituto de Astrofísica de Canarias

A. Alonso-Herrero

Centro de Astrobiologia (CAB)

V. Casasola

Arcetri Astrophysical Observatory

C. Henkel

King Abdulaziz University

Max Planck Society

M. Querejeta

Spanish National Observatory (OAN)

European Southern Observatory (ESO)

R. Neri

Institut de Radioastronomie Millimétrique (IRAM)

Francesco Costagliola

Astronomy and Plasmaphysics

L. J. Tacconi

Max Planck Society

P.P. van der Werf

Leiden University

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 608 A56- A56

Subject Categories

Astronomy, Astrophysics and Cosmology

DOI

10.1051/0004-6361/201731862

More information

Latest update

9/7/2018 1