The 2MASS Galaxies

Goals: Investigate the structure of the nearby Universe using 2MASS; Learn to manipulate the enormous 2MASS data group.

Before starting, turn on: galaxy, Tully

You will be using: 2MASS, galac, see, thresh, color, and lum commands

Adding to our knowledge of the nearby Universe, the Two Micron All Sky Survey (2MASS) has recently released the most complete picture of our cosmic surroundings. The data set consists of whopping 1.14 million galaxies within about 2 billion light-years. Let's explore what this data group reveals about the structure of the Universe.

Press the Home Button. You should be looking at the Milky Way with the Tully Virgo Cluster in the background. Turn off Tully and turn on the 2MASS group.

In the previous tutorial we introduced the elements of structure in the Universe: galaxies, filaments, and voids. With the 2MASS galaxy data, you see a more complete picture of the local Universe than what you see in the Tully group. In fact, within the same volume, 2MASS contains 32% more galaxies than Tully.

From your current view, you should see the Virgo Cluster and strands of multi-colored points. Like Tully, those galaxies in dense clusters are given the color red, but, unlike Tully, color in 2MASS describes galaxy density rather than the randomly assigned colors Tully assigned to filaments. In 2MASS, red regions have the highest density, followed by orange, yellow, green, and aqua for the lowest density regions.

Fly backward (Home and Virgo should move away from you) until you see the Fornax Cluster, then orbit from that point. We've lost the Milky Way, so turn on the galactic coordinates sphere (galac) to mark from where we came. You should see the large labels delineating the Virgo Cluster, Fornax Cluster, and you may possibly see the Centaurus and Hydra Clusters too. These are the nearest galaxy clusters to Earth.

Continue moving away from the Milky Way to see the entire data set, then adjust the brightness so the structure of the Universe pops out—the red clusters, orange and yellow filaments, out to the faint blue galaxies. Let's orbit around the galaxies. (You will need to adjust the brightness of these data using the Slum Slider throughout this tutorial.)

Seeing the Forest Through the Trees

First, let's see the entire 2MASS data set so we know what this picture really looks like without filtering. Type

see all

and all 1,140,931 galaxies are displayed. Here you have a sense of just how dense the Universe is. And, 2MASS only observed relatively bright galaxies. With this view, we no longer see the structure of the Universe so easily. To return to our filtered view showing the large-scale structure, type

see lss

The Proximity Factor

The lss selection is set using a data variable called the proximity factor. The 2MASS data come with two proximity factors, one for R = 2 Mpc and one for R = 5 Mpc (Mpc means million parsecs with 1pc = 3.26 light-years). These factors describe the density of space around a particular galaxy. Specifically, the R = 2 Mpc and R = 5 Mpc factors, called prox2 and prox5, respectively, are high when there are many galaxies within the specified radius, and they are low when a galaxy is by itself in space.

The sheer number and density of galaxies in 2MASS demands that we use creative ways to view these data. By default, we use the lss selection set in the extragalactic.cf file with the command only= prox5 > 0.7. This command tells Partiview to display only those galaxies with prox5 > 0.7, resulting in only 15% of total number shown. Removing 85% of the data may not seem logical, but allows us to see structure.

Dense Clusters

We have also defined a selection expression to display the dense clusters. Type the command

see clusters

and you will see the most dense regions of the nearby Universe. This selection is defined using the prox2 proximity factor in the command thresh prox2 1.3 3. Increase the label size to see the names of these clusters. Type see lss to return to the normal view.

Color Schemes

Type color and Partiview will report that colors are mapped to these galaxies using the prox5 data variable. We have defined a color map file and several color schemes to highlight various aspects of these data.

First, and most obvious, we can view the galaxies at a constant color. Type

color const

and the galaxies will change to an aqua color. Sometimes, a constant color is good for seeing structure and puts all the galaxies on common ground. Of course, you can change this constant color to any red-green-blue values you like, for example, make the points gray by typing

color const .5 .5 .5

The galaxies may also be colored by distance using either

color distMly

color redshift

These set the innermost galaxies to aqua, then continues to green, yellow, orange, and red for the farthest galaxies.

Coloring by proximity is the best way to highlight structure in the data set. The default color for the data group is

color prox5

which, as mentioned above, sets the color of each point based on the R = 5 Mpc proximity factor. We have also defined a similar color map for the prox2 proximity factor

color prox2

Both of these show dense regions in red, then, as the density decreases, the colors change to orange, yellow, green, and aqua.

Luminosity Settings

Normally, the 2MASS galaxies are given a constant luminosity, which is to say all points are given an equal intrinsic brightness. Of course, the actual brightness you see in Partiview is a function of the galaxy's intrinsic brightness plus your distance from that galaxy.

If you want to highlight the structure a little, you can assign luminosity as a function of the proximity value, so higher values of prox5 will have a higher intrinsic luminosity. We have preset such a scenario which you can see by typing

lum prox5

After issuing this command, you may see the galaxies go dim or bright; just adjust the Slum Slider (remember, each lum setting has its own slum value). This lum setting pumps up the brightness just a tad on the clusters and filaments so that they stand out a little. Return to the default constant luminosity with

lum const

so that all galaxies have equal intrinsic brightness.

The 2MASS galaxies have given us a comprehensive look at the structure of the nearby Universe. Next, we'll explore galaxy clusters more closely and take a look at the nearest superclusters.

Digital Universe Atlas

Digital Universe Guide

The 2MASS Galaxies