IAU Division H

Interstellar Matter and Local Universe

The Local Universe

A working definition for IAU Division H


There is increasing interest in studying the local universe (near field) as distinct from the high redshift universe (far field). We now talk of near-field cosmology as distinct from far-field cosmology. But what constitutes "local" or "nearby" means different things to different research groups.

The Local Volume is defined by the distance (~10 Mpc) over which stellar populations in galaxies can be resolved by the Hubble Space Telescope. We can extend this definition to include Virgo (~15 Mpc) so as to cover the full range of galaxy environments, from voids to massive groups and clusters. In an era of ELTs, it will be possible to extend our definition of the Local Volume to even greater distances.


We adopt a sphere of radius ~15 Mpc centred at the Local Group as our definition of the Local Universe. This sphere encloses all galaxies that have accurate distances determined from the tip of the red giant branch and surface brightness fluctuation methods. This sphere is also known as the Local Volume and includes the Local Void as defined by Tully.

Within the Local Universe, we have a very good picture of the 3D distribution of galaxies, in particular, the Tully Catalogue of Galaxies. Detailed 3D flow models for all galaxies have become possible for the first time. This volume falls within the scope of the Local Universe considered by N-body, e.g. the CLUES collaboration.

We consider the Local Universe to be a useful and physically motivated working definition. Over this volume, we can consider all galaxies to be at roughly the same cosmic epoch. The local density field varies by a factor of about 100. Thus, large-scale evolutionary effects are more likely to be due to the underlying density field (i.e. environment) than to any variation in epoch. All galaxies with masses equivalent to the SMC or larger can be imaged in most wavelength bands (e.g. x-rays, infrared, radio). The most detailed and complete observations of galaxies will always come from this volume and will therefore continue to dominate studies of physical processes of the ISM and stars in galaxies.

Note that this volume of ~15 Mpc mostly excludes galaxies identified in all sky galaxy redshift surveys (2MASS, 2dFGRS, SDSS, 6dFGS, GAMA). On the other hand, studies of the detailed physics and chemistry of the ISM and star- and cluster formation in Division H can extend well beyond this 15 Mpc Volume, complementing the galaxy-wide physics and structure formation topics in Division J.

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