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ASTR 5610, Majewski [SPRING 2016]. Lecture Notes

ASTR 5610 (Majewski) Lecture Notes


INTRODUCTION TO THE THE LOCAL GROUP


OUR PLACE IN THE UNIVERSE: ZOOMING INTO THE LOCAL GROUP

Most of this course will focus on the Local Group. Let's place the Local Group within its context in the local Universe.

We have seen that CDM models of the universe predict a "soap-sudsy" universe, with matter concentrated into strands.

What do we actually observe?


CENSUS OF THE LOCAL GROUP

Extent of the Local Group

Here is yet another representation of the Local Group and several neighboring groups to the Local Group (the Maffei group to left, NGC3109 in foreground, and Sculptor group to right).

From the Encyclopaedia Galactica, http://www.astro.utu.fi/EGal/elg/ELG3Dl.html. The units shown are in kpc, and the green/blue vertical lines show height above/below the supergalactic plane. Other lines show projections on different supergalactic cardinal planes. Red dot sizes correspond to relative distance from the viewer. See this website for the individual galaxy identifications and a larger version of this image.
The following table (circa 2000) gives a more conservative listing (in order of luminosity) of "Local Group" galaxies, using a 1 Mpc distance as a limit. This table does not include the newly discovered, ultra-faint luminosity dwarf satellites of the Milky Way and Andromeda, and the issue of whether M31 or the Milky Way is more massive/brighter is still unresolved.

From Sparke & Gallagher, Galaxies in the Universe.
The definition of "Local Group" is somewhat fuzzy, and expanding the definition to 1.6 Mpc (as done in, for example, Mateo's 1998 review in ARAA, 36, 435) includes another handful of known galaxies, including members of the NGC3109 subgroup.

One could actually defined LG membership in a variety of ways:



What's In the Local Group?

You will learn in detail about morphological classification in ASTR 5630: Extragalactic Astronomy.

My goal here is to give a broad description of what galaxy types are in the Local Group -- resolved systems we can presently hope to study in detail -- and to indicate which galaxy types are not well represented nearby.

The following image summarizes vividly the distribution of types of galaxies in the Local Group, "weighted" by their relative luminosity/surface brightness.

Local Group galaxies shown to the same linear and surface brightness scales. By B. Bingelli, from http://www.usm.uni-muenchen.de/people/saglia/dm/galaxien/alldt/node11.html.
The above figure demonstrates qualitatively the distribution of galaxy types by surface brightness, size and total luminosity described quantitatively below:

Distribution of galaxy types by their central surface brightness and total luminosity. By B. Bingelli, taken from Sparke & Gallagher, Galaxies in the Universe.

Spheroidal/elliptical characteristics by Kormendy, taken from Sparke & Gallagher, Galaxies in the Universe

The above parameter spaces demonstrate some interesting aspects of the overall brightness/size distribution of stellar systems.


As seen in the table above and the Bingelli figure, the Local Group contains:


Galaxy Types and Luminosity Function by Environment

It is found that when one integrates over large volumes of the universe, the distribution of numbers of galaxies as a function of brightness, the luminosity function, follows something like a Schechter function:

where L*, Φ*; and α are free parameters fit to the data.

In magnitudes, the function looks a little hairier:

An example of a Schechter function (for absolute magnitudes in the R band) looks like this:

From Sparke & Gallagher, Galaxies in the Universe.

Some points:

If our detailed analysis of stellar populations in resolved galaxies is presently limited to Local Group, how well does the LG represent the universe of galaxies?


What the Local Group Does Not Have Represented

Environmental effects probably explain what kinds of galaxies in the universe are not well represented in the LG.


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All material copyright © 2003,2006,2008,2010,2012,2014,2016 Steven R. Majewski. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 551 and Astronomy 5610 at the University of Virginia.