Volume Density Structure of the Central Molecular Zone NGC 253 through ALCHEMI Excitation Analysis
Journal article, 2024
We present a spatially resolved excitation analysis for the central molecular zone (CMZ) of the starburst galaxy NGC 253 using the data from the Atacama Large Millimeter/submillimeter Array Comprehensive High-resolution Extragalactic Molecular Inventory, whereby we explore parameters distinguishing NGC 253 from the quiescent Milky Way’s Galactic center (GC). Non-LTE analyses employing a hierarchical Bayesian framework are applied to Band 3-7 transitions from nine molecular species to delineate the position-position-velocity distributions of column density ( N H 2 ), volume density ( n H 2 ), and temperature (T kin) at 27 pc resolution. Two distinct components are detected: a low-density component with ( n H 2 , T kin ) ∼ ( 10 3.3 cm − 3 , 85 K ) and a high-density component with ( n H 2 , T kin ) ∼ ( 10 4.4 cm − 3 , 110 K ) , separated at n H 2 ∼ 10 3.8 cm − 3 . NGC 253 has ∼10 times the high-density gas mass and ∼3 times the dense-gas mass fraction of the GC. These properties are consistent with their HCN/CO ratio but cannot alone explain the factor of ∼30 difference in their star formation efficiencies (SFEs), contradicting the dense-gas mass to star formation rate scaling law. The n H 2 histogram toward NGC 253 exhibits a shallow declining slope up to n H 2 ∼ 10 6 cm − 3 , while that of the GC steeply drops in n H 2 ≳ 10 4.5 cm − 3 and vanishes at 105 cm−3. Their dense-gas mass fraction ratio becomes consistent with their SFEs when the threshold n H 2 for the dense gas is taken at ∼104.2−4.6 cm−3. The rich abundance of gas above this density range in the NGC 253 CMZ, or its scarcity in the GC, is likely to be the critical difference characterizing the contrasting star formation in the centers of the two galaxies.
Author
Kunihiko Tanaka
Keio University
J. G. Mangum
National Radio Astronomy Observatory
Serena Viti
Leiden University
University College London (UCL)
S. Martin
European Southern Observatory Santiago
Atacama Large Millimeter-submillimeter Array (ALMA)
N. Harada
The Graduate University for Advanced Studies (SOKENDAI)
National Astronomical Observatory of Japan
Academia Sinica
Kazushi Sakamoto
Academia Sinica
Sebastien Muller
Chalmers, Space, Earth and Environment, Onsala Space Observatory
Y. Yoshimura
University of Tokyo
Kouichiro Nakanishi
The Graduate University for Advanced Studies (SOKENDAI)
National Astronomical Observatory of Japan
R. Herrero-Illana
Spanish National Research Council (CSIC)
European Southern Observatory Santiago
K.L. Emig
National Radio Astronomy Observatory
S. Muhle
University of Bonn
Hiroyuki Kaneko
Joetsu University of Education
Niigata University
T. Tosaki
Joetsu University of Education
E. Behrens
University of Virginia
Víctor M. Rivilla
Arcetri Astrophysical Observatory
Centro de Astrobiologia (CAB)
L. Colzi
Centro de Astrobiologia (CAB)
Y. Nishimura
University of Tokyo
National Astronomical Observatory of Japan
Pedro Humire
Max Planck Society
M. Bouvier
Leiden University
K. Y. Huang
Leiden University
Joshua Butterworth
Leiden University
David S. Meier
New Mexico Institute of Mining and Technology
National Radio Astronomy Observatory Socorro
Paul P. van der Werf
Leiden University
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 961 1 18Subject Categories (SSIF 2011)
Meteorology and Atmospheric Sciences
Astronomy, Astrophysics and Cosmology
Atom and Molecular Physics and Optics
DOI
10.3847/1538-4357/ad0e64