1. Lecture 16: Deep Space
Astronomy 1143 – Spring 2014

2. Key Ideas Luminous Standard Candles -- distances to distant galaxies
Type Ia Supernovae
Tully-Fisher Relation
Faber-Jackson Relation
Must be calibrated using nearby galaxies with Cepheid & other distance indicators
Hubble Parameter :H0
Present-day rate of expansion of the Universe.
Cosmological Redshifts: new way to get distances

3. Observing galaxies far, far away
Galaxies, gas clouds, and other phenomena that are billions of light-years away offer a “time machine” We see them (their properties, their expansion rate) as they were billions of years ago Want to see the history of the Universe? Observe things far away! Must get distances to know how far away and to get physical properties

4. The Distance Problem (again!)
Cepheid P-L relation is good but limited:
Limit ~30 – 40 Mpc (Hubble Space Telescope)
Very laborious to use (100’s of HST orbits)
Only works for Spiral or Irregular galaxies
Only practical out to the Virgo Cluster
This is only just next-door in cosmic terms
Need other methods to estimate very large cosmic distances.

5. Virgo is not very far away

6. Extending the Distance Scale: Luminous Standard Candles
Look for bright standard candles found for both Spiral and Elliptical galaxies
Type Ia Supernova explosions
Velocity-Luminosity relations for galaxies
Many other properties used
Calibrated by:
Cepheid Period-Luminosity distances
Nearby similar objects (from other steps)

7. Distance Ladder

8. Type Ia Supernovae

9. Type Ia Supernovae
Explosions of white dwarfs that get too massive Can outshine the rest of the stars in a galaxy, at least for a little while Identified as Type Ia SN by their spectra Measure spectra + lightcurve = standard candle

10. Standard Candle Not all Type Ia have the same peak luminosity But
Less luminous ones get fainter faster
Can correct for range in luminosity
No Type Ia within parallax, need to calibrate with other methods
Luminosity

11. Issues with Type Ia SN
Happen once every ~100 years in a single galaxy. Can’t get a distance to every galaxy this way.
Need to take lots of images
Find them during their explosions
Measure their lightcurves to determine their luminosity
Brightness comes along for the ride
Brightness could also be affected by dust
Not completely standard or calibrated

12. Calibrating Type Ia Supernovae
In other words, we need to figure out how far away this galaxy is (using Cepheids or the method to be discussed next), then we measure how luminous Type Ia SN are. Apply that knowledge to more distant SNe.

13. Galaxy Luminosities as Standard Candles
Method:
Assume distant galaxies are like nearby ones.
Use correlations between luminosity & distance-independent properties of galaxies
Compute luminosity distances using the entire galaxy.
Distance-independent property – speeds of stars in galaxy
Higher speeds=more mass=more stars =higher luminosity

14. Speeds in Ellipticals & Spirals

15. Stellar Orbits Disk Stars: Spheroid Stars:
Ordered circular orbits confined to a plane
Same orbit direction
Speeds similar at a given radius
Spheroid Stars:
Disordered elliptical orbits at all inclinations
Prograde & retrograde orbits
Wide ranges of speeds

16. Doppler Shifts and Speeds in Galaxies
Measure the fastest speeds from stars heading more towards you and more away from you Measure Doppler shifts for light that you know its laboratory wavelength very well Example: 21 cm line of hydrogen

17. Specific Techniques Tully-Fisher Relation for Spirals:
Galaxy Luminosity - Rotation Speed relation
Measure rotation speed from Doppler-shifting of 21cm radio emission (distance independent)
Fundamental Plane Relation for Ellipticals:
Faber-Jackson relation
Measure absorption-line widths (from Doppler-shifts of individual stars) from spectra (distance independent)

18. Tully-Fisher Relationship
Luminosity
Rotation Speed

19. Measuring Motions of Stars

20. Faber-Jackson Relation
Spread in speeds
Luminosity

21. The Bottom Line Variety of techniques get used
Mix and match to seek consistent results
Some methods work better in spirals vs. ellipticals
Sometimes you get lucky and a Type Ia SN goes off in a galaxy with a Cepheid-based distance
All rely on previous steps
Argue endlessly about the details

22. Distance Ladder

23. Hubble’s Law
v = recession velocity in km/sec
d = distance in Mpc
H0 = expansion rate today (Hubble Parameter)
In words:
The more distant a galaxy, the faster its recession velocity.

24. Hubble Parameter: H0 Measures the rate of expansion today:
H0 = 72 ± 8 km/sec/Mpc
Based on Hubble Space Telescope observations of Cepheids in nearby galaxies to calibrate distant galaxy indicators
H0 is hard to measure:
Recession speeds are easy to measure from the shifts of spectral lines.
But distances are very hard to measure.
Galaxies also have extra motions.

25. Cosmological Redshifts
All galaxies (with very few exceptions) are receding from us.
Recession is quantified in terms of the “cosmological redshift” of the galaxy, z
For galaxies nearby, we can write

26. This formula is only valid for relatively nearby galaxies.
Redshift Distances
For nearby galaxies, redshift (z) is directly proportional to the distance through the Hubble Law:
z = redshift
c = speed of light
This formula is only valid for relatively nearby galaxies.

27. Example – NGC 3949
You measure a line of H at a wavelength of nm. You know this line has a laboratory value of nm. What is the distance to this galaxy? By the way, we think our Galaxy looks very much like this!

28. Example

29. Example

30. Example -- Andromeda
Andromeda has a radial velocity of 266 km/s approaching the Milky Way Hubble’s equation is completely inappropriate. Milky Way and Andromeda are part of the Local Group, gravitationally bound. The distance between Andromeda and the Milky Way is getting smaller not larger Peculiar velocities complicate the picture!

31. Redshift Distances (cont’d)
Limitations:
Value of H0 is only known to ~10%
Random motions of galaxies affects measurements of z for nearby galaxies.
At large distances, the conversion between z and distance is much more complicated because of the changes in the expansion rate of the Universe.
Astronomers use cosmological redshift as a surrogate for distance, especially for more distant galaxies.

32. Mapping the Universe
Map the distribution of galaxies using their cosmological redshifts.
Largest maps include miliions of galaxies
Reveals sheets and filaments of galaxies surrounding great voids.
Depth is ~ Mpc
Relative distances are good, but the absolute scale is only known to ~10%

33. Sloan Digital Sky Survey
Dedicated 2.5-m telescope in New Mexico
Making images of 1/4 of the sky in 5 colors:
Accurate positions and photometry for a few 100 Million stars, galaxies, and quasars.
Redshift Survey:
1 Million galaxy redshifts
100,000 quasar redshifts
Deep 3D map of large part of local Universe
Shows homoegeniety and isotropy of Universe

35. The Cosmological Principle Today
Modern observations bear out large-scale homogeneity & isotropy on average:
Large-scale galaxy surveys
Cosmic Microwave Background
Greeks: geocentric Universe
Present-day:
Earth orbits Sun
Sun is a non-descript star ~8 kpc from center of Milky Way
Milky Way is one of billions of observable galaxies
On the other hand, we are the center of our observable Universe (but so is every other galaxy)