Hidden or missing outflows in highly obscured galaxy nuclei?
Artikel i vetenskaplig tidskrift, 2019

Understanding the nuclear growth and feedback processes in galaxies requires investigating their often obscured central regions. One way to do this is to use (sub)millimeter line emission from vibrationally excited HCN (HCN-vib), which is thought to trace warm and highly enshrouded galaxy nuclei. It has been suggested that the most intense HCN-vib emission from a galaxy is connected to a phase of nuclear growth that occurs before the nuclear feedback processes have been fully developed. Aims. We aim to investigate if there is a connection between the presence of strong HCN-vib emission and the development of feedback in (ultra)luminous infrared galaxies ((U)LIRGs). Methods. We collected literature and archival data to compare the luminosities of rotational lines of HCN-vib, normalized to the total infrared luminosity, to the median velocities of 119 μm OH absorption lines, potentially indicating outflows, in a total of 17 (U)LIRGs. Results. The most HCN-vib luminous systems all lack signatures of significant molecular outflows in the far-infrared OH absorption lines. However, at least some of the systems with bright HCN-vib emission have fast and collimated outflows that can be seen in spectral lines at longer wavelengths, including in millimeter emission lines of CO and HCN (in its vibrational ground state) and in radio absorption lines of OH. Conclusions. We conclude that the galaxy nuclei with the highest L HCN-vib /L IR do not drive wide-angle outflows that are detectable using the median velocities of far-infrared OH absorption lines. This is possibly because of an orientation effect in which sources oriented in such a way that their outflows are not along our line of sight also radiate a smaller proportion of their infrared luminosity in our direction. It could also be that massive wide-angle outflows destroy the deeply embedded regions responsible for bright HCN-vib emission, so that the two phenomena cannot coexist. This would strengthen the idea that vibrationally excited HCN traces a heavily obscured stage of evolution before nuclear feedback mechanisms are fully developed.

Galaxies: ISM

Galaxies: evolution

ISM: jets and outflows

ISM: molecules

Galaxies: nuclei


Niklas Falstad

Chalmers, Rymd-, geo- och miljövetenskap, Astronomi och plasmafysik

F. Hallqvist

Chalmers, Rymd-, geo- och miljövetenskap

Susanne Aalto

Chalmers, Rymd-, geo- och miljövetenskap, Astronomi och plasmafysik

Sabine König

Chalmers, Rymd-, geo- och miljövetenskap, Astronomi och plasmafysik

Sebastien Muller

Chalmers, Rymd-, geo- och miljövetenskap, Onsala rymdobservatorium

Rebeca Aladro


F. Combes

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

A. S. Evans

University of Virginia

National Radio Astronomy Observatory

G. A. Fuller

University of Manchester

J. S. Gallagher III

University of Wisconsin Madison

S. G. Burillo

Observatorio Astronómico Nacional (OAN)

E. Gonzalez-Alfonso

Universidad de Alcala

T. R. Greve

Danmarks Tekniske Universitet (DTU)

University College London (UCL)

C. Henkel


King Abdulaziz University

M. Imanishi

National Astronomical Observatory of Japan

T. Izumi

National Astronomical Observatory of Japan

J. G. Mangum

National Radio Astronomy Observatory

S. Martin

European Southern Observatory Santiago

Atacama Large Millimeter-submillimeter Array (ALMA)

G. Privon

University of Florida

K. Sakamoto

Academia Sinica

S. Veilleux

University of Maryland

P. van der Werf

Universiteit Leiden

Astronomy and Astrophysics

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

Vol. 623 A29


Astronomi, astrofysik och kosmologi

Atom- och molekylfysik och optik




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