lecture 16: deep space astronomy 1143 – spring 2014

Lecture 16:

Deep Space

Astronomy 1143 – Spring 2014

Key IdeasLuminous 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 awayGalaxies, 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 CandlesLook 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 SupernovaeExplosions 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

Lum

ino

sity

Issues with Type Ia SNHappen 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 SupernovaeIn 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 OrbitsDisk Stars:

• Ordered circular orbitsconfined 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

Rotation Speed

Lum

inos

ity

Measuring Motions of Stars

Faber-Jackson Relation

Luminosity

Spr

ead

in s

peed

s

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

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’s Law

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

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 3949You 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 SurveyDedicated 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 UniverseShows homoegeniety and isotropy of Universe

The Cosmological Principle TodayModern 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)