Digital Universe Atlas

Digital Universe Guide

• Contents
• Overview
  • What is the DU?
  • Getting Help
  • Digital Universe License
  • Acknowledgments
• Getting Started
  • Installing
  • Launching the Milky Way
  • Learning to Fly Partiview
  • Partiview's User Interface
  • Digital Universe Files
• Milky Way Atlas
  • Overview
  • Tutorial
  • Data Groups
  • Optimizing
• Extragalactic Atlas
  • Overview & Primer
  • Tutorial
  • Data Groups
  • Optimizing
• Appendix
  • Light-Travel Time
  • Parallax and Distance
  • Redshift and Distance
  • Electromagnetic Spectrum
  • Constants and Units
• Index

• Home
• Digital Universe
• Guide
• 2MASS Galaxy Catalog

2MASS Galaxy Catalog

The Two Micron All Sky Survey, abbreviated 2MASS and pronounced “two-mass,” is an image survey of the entire sky. Between 1997 and 2001, two automated 1.3-meter telescopes, one on Mount Hopkins, Arizona, and another on Cerro Tololo, Chile, scanned the sky in three near-infrared wavelengths resulting in over four million images and information on over 470 million objects. However, a survey of images does not yield distances to the objects identified within them.

To determine distances, we need the object's redshift. [The definition of redshift and its relationship to distance is discussed in “Redshift and Distance.”] The 2MASS Galaxy Redshift Catalog assembles redshift data from many surveys, including 2MASS, ZCAT, Sloan, 2dF, 6dF, and others, and includes redshifts derived from photometric (i.e., brightness) and velocity data.

Combining the 2MASS galaxies and these various redshift surveys results in the most complete all-sky census of galaxies, including more galaxies in the “zone of obscuration,” where the bright optical band of the Milky Way blocks our view of the galaxies beyond. The resulting catalog contains an impressive 1.14 million galaxies that surround the Milky Way.

The impact of 2MASS has been tremendous for astronomy and astrophysics. Not only has the survey enabled this galaxy data set, giving us one of the best pictures of the local Universe, but scientists are finding many new objects, including L-type and T-type dwarf stars (included in the dwarfs data group).

Why Use Infrared Telescopes?

Astronomers use infrared detectors to reveal parts of the Universe that remain hidden in visible light. These regions are embedded in dense clouds of gas and dust which glow brightly in visible light. Infrared light passes through these dense clouds, allowing astronomers to see objects within them. For example, we see “deeper” toward the Galactic center in infrared light, and when we look at the Orion Nebula in the infrared we see stars within the gas that we cannot see in visible light. Furthermore, Earth's atmosphere is relatively transparent to infrared light at the wavelengths used in the survey (J = 1.25 microns, H = 1.65 microns, and K = 2.17 microns), so the light passes freely to Earth's surface.

Data Variables

The sheer density and coverage of the 2MASS presents a challenge when trying to see the structure within the data set. For this reason, we have defined several variables to hone your view of these galaxies.

The most useful of these data variables are the proximity factors, prox2 and prox5. These numbers describe the likelihood that within a given volume surrounding a particular galaxy there exists another galaxy. The more galaxies in that volume, the higher the proximity factor. For example, prox2 is the proximity factor for a spherical volume with a radius of R = 2 million parsecs (or 6.52 million light-years). Similarly, prox5 is the factor corresponding to a radius R = 5 million parsecs (or 16.3 million light-years). Low values of the proximity factors indicate that the galaxy in question is not near other galaxies—that it's out on its own. The higher the proximity values, the more concentrated the area around the galaxy will be. And, since the radius is smaller for prox2, these concentrations will be more dense than those for prox5.

By default, only those 2MASS galaxies with prox5 < 0.7 are displayed, removing 85% of the galaxies. You might think this is crazy, but it's the only way to see the forest through the trees. If you type see all while 2MASS is active, you will see the full-up catalog of 1.14 million galaxies. Seeing the density of galaxies surrounding us is certainly impressive, but the vast number of galaxies prevent us from seeing any structure.

Two data variables describe the distance to each galaxy. One variable, called distMly, is the distance expressed in millions of light-years and ranges from about 2 million to 3,000 million (or 3 billion) light-years. For example, type thresh distMly 0 100 to see the 3,478 galaxies within 100 million light-years of the Sun. The Virgo Cluster is the most prominent cluster in view, while the smaller Fornax Cluster lies opposite the Milky Way from Virgo.

Similarly, because redshift is directly related to distance, we can use it to produce subsets of data. For example, thresh redshift 0 0.007 is approximately equal to the 100-million-light-year threshold we set above.

To parse these data on yet another parameter, the 2MASS galaxies include a redshift source catalog identifier called zcatalog. This data variable is an integer that describes the redshift source, allowing you to cull the component catalogs from the larger data set. Naturally, we have set selection expressions for these redshift sources (see below).

Using these data variables will help you see the structure of our local Universe through the overwhelming density of galaxies in this data set. Use these parameters creatively and in combination with one another to show any slice of galaxies you would like to see.

Selection Expressions

We have predefined several selection expressions in the extragalactic.cf file that make it easier to view the 2MASS galaxies. We summarize these in the table below.

The lss expression is the default setting when you launch the Extragalactic Atlas. It selects all galaxies with an R = 5 Mpc proximity factor greater than 0.7. This leaves only 15% of the total 2MASS galaxies, but allows you to see the large-scale structure of galaxy clusters and filaments.

The d100 and d372 settings allow you to see all galaxies within 100 million light-years and 372 million light-years, respectively. We chose 372 million light-years because it represents the same volume as the Tully data group.

To see galaxies with relatively good redshift data, use the zbest expression. This displays galaxies with high redshift confidence. As the distance increases, accurate redshifts become more difficult to measure, so the uncertainty in distance increases. However, redshifts less than 0.06 are known with higher certainty.

The clusters expression culls out clusters of galaxies, the most dense regions around the Milky Way.

The remaining expressions show portions of the data set based on the redshift source. Perhaps the most useful of these is the nophotoz expression, which removes all the galaxies that have a photometric redshift. Spectroscopic redshifts are more accurate than a photometric estimation, so these galaxies' distances are more accurate.

Luminosity Settings

In addition to the selection expressions, we have supplied several luminosity settings for the data group that enable you to emphasize aspects of the massive data set.

In Partiview, it is possible to define many luminosity settings using the lum command, each with its own scaling factor defined by the slum command. We typically use a constant luminosity for 2MASS (lum const 1), and this is the setting in effect when you launch the Digital Universe. However, to emphasize the structure of these data, we also supply a luminosity setting according to the R = 5 proximity factor, prox5 (lum prox5 0 1). This sets the luminosity of the points such that the regions of higher density appear brighter. You can toggle between these two states by typing lum prox5 and lum const. Use the Slum Slider to set separate slum factors for each.

Color Settings

Color is a helpful tool in visualizing data and plays an important role with these 1-million-plus galaxies. In the extragalactic.cf file, we've defined five color settings which are set according to the 2MASS color map file, 2mass.cmap.

The default color map uses the prox5 data variable. The low values receive an aqua color, while increasingly dense regions receive green, yellow, orange, and red for the most dense regions. Experiment with the other color settings, or try setting some of your own.

Radial Stretching

You may notice some radial stretching in the 2MASS galaxies, short lines of galaxies that appear to point toward Earth. In fact, this is common in all the galaxy surveys, including 2dF galaxies and Sloan galaxies. To emphasize the effect, you can display all the 2MASS galaxies then filter the galaxies by the prox2 factor

see all
thresh prox2 0.5 3

The data appear to form lines that point toward Earth. Astronomers call this radial stretching effect a “finger of god.” Throughout history, astronomers have shown just how ordinary we Earthlings are by disproving our position in the center of the Solar System, the Galaxy, and even the Universe. When, in the 1980s and 1990s long lines of galaxies pointing radially toward Earth were discovered, we had reason to believe we were special again. However, it was quickly apparent that this was an artificial result.

Astronomers realized these radial effects are a product of velocity dispersions that differ from the overall expansion of the Universe. Within a cluster of galaxies, an individual galaxy could be heading toward Earth (blueshifted), away from Earth (redshifted), or there may be no radial component at all in its motion (no shift in the light). These dispersions cause the distances of galaxies in clusters to be stretched radially. In fact, the mapping of large-scale structure is stymied by this effect, for the more concentrated the area, the more uncertainty there will be in the galaxy's distance.

For a more realistic view of the Universe, this stretching must be removed. For large clusters in 2MASS, the stretching has been removed, resulting in a statistically accurate representation of a galaxy cluster. In the mid-range densities, the radial stretching remains—you will see many long lines of galaxies that point toward Earth. As the density decreases, the stretching will not be as prominent.

Let's go back to our normal view for 2MASS by typing

see lss

Labels

The 2MASS data group has two sets of labels. One set is a local label, showing the names of individual galaxies when you fly close to them (with the default label size). Another set of labels in the 2MASSclusters.label file, identify galaxy clusters and structure. These labels are 20 times larger than the individual galaxies, so they will be visible from greater distances. Adjust the Labelsize and Labelminpixels Sliders to suit your label taste.

© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott

© 2001-2008 American Museum of Natural History.

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Last modified: 19 December 2007