The Local Universe

Goals: Understand the distribution of galaxies around the Local Group.

Before starting, turn on: galaxy, local

You will be using: only= and thresh commands

If you're continuing from the previous tutorial, go to the next paragraph. If you're starting a new session, once the Extragalactic Atlas is up, pull away so that you can see all the data in the local galaxies and turn on the boxes to see the Local Group.

Exploring the entire data set, you may notice that most of the galaxies are in bunches about the size of the Local Group. These bunches typically have a large spiral or elliptical galaxy that attracts smaller galaxies.

Galactic Census

How many galaxies like the Milky Way are in the neighborhood? If you threshold the data according to type, you can see. A report on the Milky Way (place the mouse over the Milky Way and press the ‘p’ key) reveals that it has a type of 5. According to the table in “Local Galaxies,” this corresponds to an Sc spiral galaxy. Sc galaxies have small central bulges and loosely wound spiral arms. To see these galaxies in the data, type the command

only= type 5

Only eight galaxies remain and only two in the Local Group. Andromeda is an Sb type, having a larger central bulge and a tighter arm structure (there are three Sb galaxies). To see all the spirals, type the command

thresh type 1 8

Still only 48 galaxies out of about 250, amounting to 19% of the data. In this way, you can perform a census on the data and visually explore the results in the Galaxy Morphology Codes table in “Local Galaxies.”

The Brightest Galaxies

Now let's see the most luminous galaxies. Type the command

thresh lum 0.5 6

The lum data variable ranges from 0 to 5.7, with an average of 0.177. So with this statement, you are looking at the brightest galaxies. As you might expect, intrinsically bright galaxies are fewer in number than dimmer ones. Only 11 galaxies are bright enough to make this cut. They include the Milky Way, Andromeda, M33, M81, and the giant elliptical Centaurus A. Within a galaxy, the luminous stars are few and far between compared with the number of dimmer stars. Galaxies appear to follow this trend, as most of the nearby galaxies have low luminosities.

This picture of the nearby galaxies is the best science has to offer. Of course, there are many small, dimmer galaxies that we cannot see, even in our own backyard. Let's look at other galaxy surveys that map galaxies out to greater distances.

Digital Universe Atlas

Digital Universe Guide

The Local Universe