Bibcode
García-Burillo, S.; Viti, S.; Combes, F.; Fuente, A.; Usero, A.; Hunt, L. K.; Martín, S.; Krips, M.; Aalto, S.; Aladro, R.; Ramos Almeida, C.; Alonso-Herrero, A.; Casasola, V.; Henkel, C.; Querejeta, M.; Neri, R.; Costagliola, F.; Tacconi, L. J.; van der Werf, P. P.
Bibliographical reference
Astronomy and Astrophysics, Volume 608, id.A56, 13 pp.
Advertised on:
12
2017
Journal
Citations
33
Refereed citations
33
Description
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 ≃ 35″ (2.5 kpc) region of
the disk of NGC 1068 with a spatial resolution 1.̋0 ×
0.̋7 (≃ 50-70 pc). We used maps of the dust continuum emission
obtained at 349 GHz by ALMA to derive the H2 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 ≃ 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 Paα
emission complexes imaged by the Hubble Space Telescope (HST). However,
the brightest C2H emission originates from a r ≃ 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 ≃ 400 pc. We derived the
fractional abundances of C2H (X(C2H)) assuming
local thermodynamic equilibrium (LTE) conditions and a set of excitation
temperatures (Tex) constrained by the previous multiline CO
studies of the galaxy. Our estimates range from X(C2H) ≃
a few 10-8 in the SB ring up to X(C2H) ≃ 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(H2)
≥ 104 cm-3) and moderately UV-irradiated gas
(UV-field ≤ 10 × Draine field, where 1 Draine field ≡
2.74 × 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 105 yr or as late as
107 yr. However, the high fractional abundances estimated for
C2H in the outflow region can only be reached at very early
times (T ≤ 102-3 yr) in models of UV or X-ray irradiated
dense gas (n(H2) ≥ 104-5 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.
Related projects
Nuclear Activity in Galaxies: a 3D Perspective from the Nucleus to the Outskirts
This project consists of two main research lines. First, the study of quasar-driven outflows in luminous and nearby obscured active galactic nuclei (AGN) and the impact that they have on their massive host galaxies (AGN feedback). To do so, we have obtained Gran Telescopio CANARIAS (GTC) infrared and optical observations with the instruments
Cristina
Ramos Almeida