Digital Universe Guide - Hayden Planetarium

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194 4. THE EXTRAGALACTIC ATLAS Data Variables The 2dF galaxy survey has the following data variables available (note that these are the same for the 2dF quasars and the Sloan galaxies and quasars): Data Variables for the 2dF and Sloan Surveys Number Name Description Units 0 Rmag R magnitude mag 1 redshift Object’s redshift — 2 Tlookback The Lookback Time Gyr 3 distMpc Distance Mpc 4 distMly Distance millions of light-years 5 radeg Right Ascension degrees 6 decdeg Declination degrees The zeroth data variable, Rmag, is the apparent R magnitude of the object. The R filter peaks at 658 nm, which is in the red end of the visible spectrum. In the 2dF galaxy survey, the magnitude has the range 11.69 ≤ Rmag ≤ 21.7 mags, so we’re looking at dim objects indeed. The redshift ranges from 0.0016 to 3.5, with a mean value of 0.118. The look-back time, how long ago an object emitted the light, should not exceed the age of the Universe, which is currently thought to be 13.7 billion years. The look-back time ranges from 0.02 billion to 11.7 billion years, meaning the oldest galaxies in these data date back 11.7 billion years. To see them, you must increase the brightness of the data set. Following these, we have two distance variables, one in megaparsecs and the other in light-years. We also include their position on the sky, which is useful in setting a threshold for the data if you want to display only one part of the data set. If you turn on the 20Gly group, you will see that most galaxies extend to about 2.5 billion light-years before the data become sparse. As mentioned in “The Distance Scale,” the distance for each galaxy reflects the present location of that galaxy, not where its light left from. For example, a galaxy around 3 billion light-years in the Atlas has a look-back time of 2.7 billion years; the galaxy was 2.7 billion light-years away when the light we observe today left the galaxy. This discrepancy comes about from the expansion of the Universe; the farther an object is from us, the larger this discrepancy becomes. Out to about 2 billion light-years, the 2dF galaxies show the large-scale structure of the Universe. Even in these thin slices, the pattern of clusters, connecting filaments, and voids is present in these data. Imagine if the 2dF were to look at the entire sky, we would be surrounded by galaxies that form this pattern. This is the goal of the Sloan Digital Sky Survey (SDSS), and we will discuss its latest
4.3. EXTRAGALACTIC DATA GROUPS 195 results next.
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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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Page 145 and 146: 4 The Extragalactic Atlas The word
Page 147 and 148: 4.1. EXTRAGALACTIC ATLAS OVERVIEW 1
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Page 205 and 206: 4.4. OPTIMIZING THE EXTRAGALACTIC A
Page 207 and 208: Appendix A Light-Travel Time and Di
Page 209 and 210: Appendix B Parallax and Distance On
Page 211 and 212: absolute magnitude M, can be calcul
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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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