Digital Universe Guide - Hayden Planetarium

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186 4. THE EXTRAGALACTIC ATLAS 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. Data Variables for the 2MASS Group Number Name Description Units 0 distMly Distance light-years (millions) 1 redshift Redshift — 2 zcatalog Redshift source catalog — 3 prox2 Proximity factor for R = 2 Mpc — 4 prox5 Proximity factor for R = 5 Mpc — 5 rahrs Right Ascension hours 6 decdeg Declination degrees 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
4.3. EXTRAGALACTIC DATA GROUPS 187 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). Redshift Source Catalog ID zcatalog Catalog Name 1 Tully’s Nearby Galaxies 2 Six-degree Field Galaxy Survey 3 Sloan <strong>Digital</strong> Sky Survey 4 NASA/IPAC Extragalactic Database (NED) 5 ZCAT, which includes the CfA Redshift Survey 6 Lyon-Meudon Extragalactic Database (LEDA) 7 Two-degree Field Galaxy Survey 9 Photometric redshift Using these data variables will help you see the structure of our local <strong>Universe</strong> 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.
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The Digital Universe Guide Brian Ab
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Contents Contents 3 List of Tables
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List of Tables 2.1 Log and Linear F
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1.2. GETTING HELP 7 1.2 Getting Hel
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1.5. ACKNOWLEDGMENTS 9 1.5 Acknowle
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2.2. LAUNCHING THE MILKY WAY ATLAS
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2.3. LEARNING TO FLY PARTIVIEW 13 T
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2.3. LEARNING TO FLY PARTIVIEW 15 M
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2.4. PARTIVIEW’S USER INTERFACE 1
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2.5. DIGITAL UNIVERSE FILES 23 File
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3.2. MILKY WAY ATLAS TUTORIAL 27 3.
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3.3. MILKY WAY DATA GROUPS 67 Earth
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3.3. MILKY WAY DATA GROUPS 69 Data
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Page 145 and 146: 4 The Extragalactic Atlas The word
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Page 207 and 208: Appendix A Light-Travel Time and Di
Page 209 and 210: Appendix B Parallax and Distance On
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Page 215 and 216: Appendix D Electromagnetic Spectrum
Page 217 and 218: Appendix E Constants and Units In a
Page 219 and 220: Metric Prefixes These prefixes are
Page 221 and 222: INDEX 221 center command, 50, 61 cl
Page 223 and 224: INDEX 223 Earth’s radio signals,
Page 225 and 226: INDEX 225 irregular galaxy, 147, 17
Page 227 and 228: INDEX 227 entering, 21 every, 206 j
Page 229: INDEX 229 stellar distance uncertai
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