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s e c n a t is g n D i l r a u c s i a e m o M on r t As Sydney Sherman 2nd Year Graduate Student Department of Astronomy University of Texas at Austin Cosmological Distance Ladder Distant Galaxies Distant Stars That Explode Predictably (Type Ia Supernovae) Distant Stars That Vary Predictably (Cepheid Variables) Stars Near The Solar System Sun Earth The Ancient Universe Stars and other “heavenly bodies” move on transparent spheres The Earth is the center of the universe Aristotle 350 BC Need some creative geometry for this to almost work You can be much less creative with geometry if the Sun is the center of the solar system Copernicus 1473 - 1543 The Ancients Had Trouble With Parallax Stellar Parallax = The apparent motion of foreground stars relative to background stars July January January July First Parallax Measurement Measured by Friedrich Wilhelm Bessel in 1838 Determined that the star 61 Cygni has a parallax of 0.314” or ~700,000 AU The true value is 0.287”, but Bessel got pretty close! Modern Parallax ESA’s Gaia Mission is designed to get the distances to 1 million stars within 30,000 light years of the sun Gaia will make the most detailed map of our galaxy to date How Useful is Parallax? 1 AU θ d If θ = 1”, then d = 206,265 AU = 1 Parsec As the parallax angle gets smaller and smaller, errors dominate the measurement Parallax cannot be used to measure the distance to objects outside our galaxy Outside Our Neighborhood Brighter stars are not necessarily closer than fainter stars Need some way to relate apparent magnitude (a measured quantity) to absolute magnitude (a physical property) Reminder about the astronomical magnitude system: m M a few hundred pc 10 pc Cepheid Variables Absolute Magnitude (M) In the early 1900’s Henrietta Swan Leavitt began studying variable stars at Harvard She found the relationship between the period of variation and the luminosity of the star Leavitt’s Law Period Distance to Cepheids Observe Period of variability Apparent magnitude (m) Use Leavitt’s Law Convert observed period into absolute magnitude (M) Find Distance Relate absolute magnitude and apparent magnitude to calculate distance m - M = 5log(d) - 5 M P Type Ia Supernovae The ultimate standard candle! Brighter than cepheids Easily found in distant galaxies (out to 1 billion Parsecs) Very reliable luminosity Relate known luminosity, apparent magnitude, and distance Downside: They cannot be predicted Distances to Everything Else In 1920, Edwin Hubble noticed a clear relation between the recession velocity of galaxies and their distances recession velocity = H0 * distance Hubble studied the spectra of galaxies and found their distance using the doppler shift in their spectral lines Redshift Each chemical element emits or absorbs light at a specific wavelength These emissions or absorptions create “spectral lines” Redshifted Unshifted Blueshifted Spectroscopic Redshifts z = redshift Use software to locate easily identifiable emission/ absorption lines Use observed wavelength and expected wavelength to determine redshift This is very expensive! Take Data Measure the flux from an object at many wavelengths Flux Photometric Redshifts Wavelength Compare With Templates Calculate Redshift You already know the expected wavelength for the template You also know how much you had to move it to fit the data λobs 1+ z = λrest Getting Galaxy Photometric Redshifts Important to have data at many different wavelengths Different instruments are capable of observing at different wavelengths This requires us to use more than one instrument 1 Million Galaxies! NEWFIRM DECam Spitzer Herschel SHELA/HETDEX Survey Multi-wavelength data — better phot-z Both wide and deep — allows us to study distant galaxies in many environments Zoom In MultiWavelength Spitzer Space Telescope IR Survey DECam Blanco 4m, CTIO Optical Survey Optical NEWFIRM Mayall 4m, Kitt Peak IR Survey Herschel Space Telescope FIR Survey Infrared Photometric Redshift Templates are generated using theoretical models of galaxy spectral energy distributions. Data for a single galaxy Multi-wavelength data is compiled for each galaxy in the sample. Optical (DECam) IR (Spitzer) Results z = 2.6 Best-fit template Repeat this for all 1 million galaxies How Do We Know We’re Right? We don’t! Use statistics output by the software to determine the probability that we are correct. Good Not so good Can This Be Improved? Spectra are expensive to get, but photometric redshifts can be unreliable What if we get spectroscopic redshifts for a subset of a larger sample? We can then use machine learning to improve the photometric redshift estimates I will combine spectroscopic redshifts from the HETDEX survey, which cover a subset of my 1 million galaxy sample, to improve my phot-z estimates. HETDEX HETDEX = Hobby-Eberly Telescope Dark Energy Experiment UT led upgrade to the HET at McDonald Observatory allowed the telescope to observe a wider field of view This blind survey will get spectra for ~0.3 million galaxies in the SHELA field Machine Learning Training Set Has both photometry and spectra This sample is a subset of the whole survey Hoyle et al. 2015 Machine Learning Teach the computer to recognize traits that are best used to determine a specific property, in this case, redshift What to do with all these redshifts? Now that we have turned the 2D image into a 3D map, we can study galaxy properties in different environments at multiple epochs. The Power of SHELA 2 Extremely large area (24 deg ) Can probe many different environments Low Density High Density SFRD Star Formation Rate Density Clusters SFRD redshift Groups SFRD redshift Fields Madau and Dickinson, 2014 redshift The Big Questions How do galaxies build their mass in different environments? Is star formation suppressed or enhanced in the densest regions? What role does environment play in the assembly of the first galaxy clusters? …and many more! Just like Rome, the cosmological distance ladder wasn’t built in one day! Without the hard work of those who dared to believe the universe was larger than they could fathom, we wouldn’t be able to study distant galaxies today. Questions? References: Cosmic Distance Ladder - https://www.aavso.org/cosmic-distance-ladder Parallax - Parallax: The Race to Measure the Cosmos, Alan W. Hirshfeld Gaia - http://www.esa.int/Our_Activities/Space_Science/Gaia/Parallax Cepheid Variables - http://hyperphysics.phy-astr.gsu.edu/hbase/astro/cepheid.html Type Ia Supernova - https://ned.ipac.caltech.edu/level5/Branch2/frames.html Hubble’s Law - http://www.pnas.org/content/101/1/8.full.pdf?sid=eebffd1c-a66e-4687-be33-9b34e767cf4b Spectroscopic Redshift - http://skyserver.sdss.org/dr1/en/proj/basic/universe/redshifts.asp Dark Energy Camera - http://www.ctio.noao.edu/noao/content/dark-energy-camera-decam NEWFIRM/KPNO - http://ast.noao.edu/facilities/kpno Spitzer Space Telescope - http://www.spitzer.caltech.edu Herschel - http://herschel.cf.ac.uk HETDEX - http://hetdex.org Photometric Redshift - http://www.sedfitting.org/SED08/Welcome.html SFRD Plot - https://arxiv.org/pdf/1403.0007v3.pdf Machine Learning - https://arxiv.org/pdf/1410.4696v4.pdf Images: Aristotle - http://communicationtheory.org/aristotle’s-communication-model/ Copernicus - https://en.wikipedia.org/wiki/Nicolaus_Copernicus Heliocentric Model - http://astro.hopkinsschools.org/course_documents/history/copernicus_to_galileo/copernicus.htm Parallax - http://josefshomperlenblog.org/billion-star-mapper-takes-sky-snap/ Gaia’s Reach - http://www.nature.com/polopoly_fs/7.12820.1380647705!/image/Gaia's-reach.jpg_gen/derivatives/landscape_630/Gaia's-reach.jpg Gaia Spacecraft - http://blogs.esa.int/gaia/files/2013/07/Gaia_mapping_the_stars_of_the_Milky_Way.jpg Bessel - http://www.100ciaquimica.net/biograf/cientif/B/bessel.htm Levitt - https://en.wikipedia.org/wiki/Henrietta_Swan_Leavitt Harvard “Computers” - http://hea-www.harvard.edu/~fine/Observatory/all.html Type Ia Supernova - https://astrobites.org/2015/04/07/super-bright-supernovae-are-single-degenerate/ Supernova Light Curve - http://hyperphysics.phy-astr.gsu.edu/hbase/astro/snovcn.html Hubble Plot - http://www.pnas.org/content/101/1/8.full.pdf?sid=eebffd1c-a66e-4687-be33-9b34e767cf4b Hubble Spectra - http://www.phys.ncku.edu.tw/~astrolab/astro_course/homework/HobbleLaw.pdf Dr. Sheldon Cooper - http://bigbangtheory.wikia.com/wiki/File:Sheldon_Doppler_Effect.gif Redshift - https://en.wikipedia.org/wiki/Redshift Redshifted Spectra - http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/redshift.html EM Spectrum - http://imagine.gsfc.nasa.gov/Images/science/EM_spectrum_compare_level1_lg.jpg DECam - https://www.noao.edu/image_gallery/html/im0132.html Spitzer - http://www.spitzer.caltech.edu Herschel - http://www.davidreneke.com/farewell-to-herschel/herschel-2/ HETDEX - http://instrumentation.tamu.edu/hetdex.html 3D Map - http://www.sdss.org/press-releases/wp-content/uploads/2016/07/boss3dwedge.png SHELA Density Map - Jonathan Florez, private communication SFRD Plot - https://ned.ipac.caltech.edu/level5/March14/Madau/Figures/figure9a.jpg