Lecture 16: Deep Space Astronomy 1143 – Spring 2014

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

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

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.

Virgo is not very far away

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)

Distance Ladder

Type Ia Supernovae

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

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

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

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.

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

Speeds in Ellipticals & Spirals

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

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

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)

Tully-Fisher Relationship Luminosity Rotation Speed

Measuring Motions of Stars

Faber-Jackson Relation Spread in speeds Luminosity

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

Distance Ladder

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.

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.

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

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.

Example – NGC 3949 You measure a line of H at a wavelength of 658.05 nm. You know this line has a laboratory value of 656.3 nm. What is the distance to this galaxy? By the way, we think our Galaxy looks very much like this!

Example

Example

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!

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.

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 ~500-600 Mpc Relative distances are good, but the absolute scale is only known to ~10%

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

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)