GBT26B-076

On the hunt for the second-most important galaxy in the Universe

Abstract

We propose a volume-limited survey of galaxies within ~15 Mpc for 22 GHz water megamaser emission, with the goal of finding "the next NGC 4258." NGC 4258 is the nearest and brightest disk megamaser system known, and its geometric distance carries disproportionate weight in setting the absolute scale of distances used in cosmology. This unique role makes NGC 4258 the most important galaxy in the Universe for cosmology, but it also presents a single point of failure that is no longer acceptable in the era of percent-level cosmological measurements and in the face of the "Hubble tension." Our strategy is deliberately simple: a comprehensive survey of galaxies out to ~15 Mpc. This approach avoids selection-based blind spots while ensuring that any detected system is near enough to enable an anchor-quality geometric distance measurement. Our survey will cover a volume roughly an order of magnitude larger than that containing NGC 4258 itself. Finding even one additional NGC 4258-like system would immediately enable consistency checks and harden the absolute distance scale against subtle systematics that dominate at the percent level.

Investigators

Name	Institution
Dominic Pesce *	Virginia, University of; Smithsonian Astrophysical Observatory
Cheng-Yu Kuo	Virginia, University of; National Sun Yat-sen University
Paola Castangia	Istituto Nazionale di Astrofisica
Frederic Courbin	Barcelona, University of
Chris Henkel	Max-Planck-Institut für Radioastronomie
Elisabetta Ladu	Istituto Nazionale di Astrofisica
Mark Reid	Center for Astrophysics | Harvard & Smithsonian
Andrea Tarchi	Istituto Nazionale di Astrofisica
Jim Braatz	National Radio Astronomy Observatory

* indicates the PI