[madbadgalaxyman (Robert)]

I thought it might be interesting, as part of a general astronomy discussion, to explain something of the scientific rationale for the LVL survey and for other related studies of the Local Volume.


While an interesting byproduct of the LVL Survey is the production of numerous “pretty pictures” of galaxies that are aesthetically interesting, this was not actually why massive amounts of telescope time and massive amounts of data analysis were undertaken by the professional community.

The LVL Survey is part of a wide-ranging & ongoing imaging effort to find and characterize the entire contents of the “Local Volume”;
the objective of these studies of the Local Volume, by various related teams of astronomers, is to detect everything existing in a volume of space which is defined by a maximum distance of 10 Mpc (or 11Mpc, in some studies) from the Milky Way. (10Mpc = 32.6 million light years)

The “everything” that astronomers are trying to find in the Local Volume includes:
- all forms of matter and energy, whether dark or luminous, for instance gas in its various physical states, both inside galaxies and between the galaxies (ionized, neutral, various temperatures, various densities).
- all mass concentrations at various spatial scales (from the subatomic scale through to galaxy-sized)
- the amount and distribution of the mysterious “dark matter” which has so far been detected only as a result of its gravitational effects on stars and galaxies.
- any other mysterious (known and unknown) forces and objects that can be found in this nearby volume of space.

The Local Volume is the only “fair and unbiased” sample of objects that we can study, due to its nearness to us, because in more distant parts of the universe, even our biggest telescopes are unable to detect very small and/or very faint and/or very low surface brightness objects:

(1) We can detect galaxies in this nearby volume at extremely low levels of luminosity and surface brightness :
For instance, there are individual galaxies in I.Karachentsev's “Catalog of Neighbouring Galaxies” (which aims to be a nearly complete volume-limited sample of galaxies) which are similar to a mid-ranked globular cluster in their luminosity but which have an optical surface brightness that is 8 magnitudes lower than the characteristic surface brightness of the disks of luminous spiral galaxies.

[[ Q. But then why do astronomers call these tiny objects galaxies rather than globular star clusters?
A. Because one of these very small & extremely low luminosity galaxies would fly apart if it were not for the fact that it contains some dark matter (or if not dark matter, “some other thing that pulls on stars and gas”) in quantities much greater than the amount of luminous mass that is found in the galaxy. Globular clusters do not have any dark matter! ]]


(2) In the Local Volume, we can detect gas clouds at very low levels of luminous emission and cloud mass:
For instance, within the Local Volume, radio telescopes can detect & observe HI (cold neutral atomic gas) clouds of much lower masses than they can detect in more distant volumes of the universe. Intriguingly, in the nearest parts of the Local Volume, we can observe clouds of cold gas so small that they are being variously interpreted as:
- “High Velocity Clouds” which infall into a galaxy and which can thereby refuel star formation in a galaxy
AND / OR
- tiny “dark galaxies” which have minimal (or no) stars in them, but which are composed of enough dark matter to hold on to small amounts of gas.

Another example of the ability to detect low levels of emission in the Local Volume: in H-alpha and Far-ultraviolet imaging of this nearby region of space, (this is emission which traces the presence of the recent formation of massive stars, that is, it traces where the current star formation exists in any volume of space that we are interested in), we can detect star formation occurring at low levels in unusual places. For instance, it has been found that some S0 galaxies have star-forming rings or knots that are seen to be well outside the optical body of the system, and there have also been knots of hot & massive stars found within tidal tails and even within intergalactic gas clouds.


(3) Local Volume galaxies are near enough that we can do studies of individual stars and stellar populations, by resolving these nearby galaxies:
This enables an accurate assessment of the ages of the stars found within these galaxies. From this, the idea is to work out how star formation varied over time in a galaxy (the Star Formation History). We can figure out the relative proportions of stars of various ages that are found in a galaxy, and how some of the available interstellar gas was eventually turned into stars.


(4) In the Local Volume, we can do sensitive tests of how stars and galaxies and globular clusters are moved by gravity, or perhaps moved by other effects (known and unknown), thereby enabling estimates of the amount of matter, both dark and luminous, and of the forces acting on the matter that we detect:
From the observed orbital motions, and other motions, of various “test particles” at various scales (e.g. stars, globular clusters, galaxies, clusters of galaxies), it is possible to make calculations of how much “dark matter” is causing the observed motions; or it could be that some other effect such as “dark energy” or bulk flows of numerous galaxies or the expansion of the universe, is causing the motion of various objects in the Local Volume.
One interesting aspect of Local Volume studies is that while, for more distant galaxies, we are only able to measure the movement of an object in one direction, that is, we are only able to measure its radial velocity (its motion in our line of sight), for some nearby galaxies our telescopes can actually also detect the proper motion of the galaxy......proper motion is an observed motion across the apparent sky that corresponds to the tangential motion & the tangential (transverse) velocity of a galaxy at right angles to our line-of-sight. In everyday language, if we wait for long enough between observations, instruments like the Hubble Space Telescope can actually detect galaxies moving across our apparent sky.
(I am not kidding here. The proper motions of the galaxies LMC and M33 and IC10 have been measured, and there is an existing project to use the HST to measure the proper motions of several dwarf galaxies in the Local Group of galaxies)