Download Measuring Astronomical Distances

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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