how do galaxies form - physics and astronomy at...

How do Galaxies Form ?

Remember that galaxies with high redshifts are very far away:

cz ~ v = H0 d (for z << 1)

z = (λobs - λrest) / λrest

Because it takes up to billions of years for the light from distant galaxies to reach us. We see them not as they are, but as they billions of years ago.

We can study high-redshift galaxies to learn about galaxy evolution.

Tuesday, November 22, 2011

galaxies from 6-8 Gyr ago.

galaxies from 10-11 Gyr ago.

from Papovich et al. 2005, ApJ, 631, 101

HST images of ...


Galaxy EvolutionMajority of galaxies belong to clusters and groups of galaxies.

Density of galaxies in clusters is roughly 100x greater than that of stars in galaxies.

There is a higher probability of interactions and/or mergers between galaxies in regions of higher galaxy number density.

Interactions tend to increase the velocity dispersion (average velocities of stars in galaxies, squared), and probably destroy gaseous disks in late-type galaxies, causing

their stars to become elliptical-like.

Interactions also apply tidal effects on gas and stars. Causes gas to compress and form stars, often in immense bursts of star formation which can exceed the

“quiescent” star formation rate by 10-100 x !


How do Galaxies Form ?

Hierarchical Merger Model

Collisions and tidal interactions between merging fragments would disrupt some globular clusters and left some intact. In this model, the disrupted

systems would have led to the present distribution of field halo stars, while leaving intact globular clusters distributed throughout the spheroid.

Through many random mergers, there should be no net rotation of objects in the halo.

Model also predicts that some proto-Galactic fragments should still be out there. This explains significant number of small galaxies orbiting the Milky

Way and nearby Andromeda. These are surviving proto-Galactic fragments. Some are merging, like the Sagittarius dwarf.


Courtesy V. Springel


Andromeda galaxy and dwarfs


Andromeda galaxy and dwarfs

Tidal streams of previously merged bits


Galaxy Interactions


Simulation by Chris Mihos (ca. 1996)Tuesday, November 22, 2011

Simulation by Volker Springel (ca. 2006)Tuesday, November 22, 2011

1.7 Gyr1.5 Gyr1.4 Gyr

1.2 Gyr1 Gyr900 Myr

750 Myr

600 Myr450 Myr

300 Myr150 MyrT = 0 Myr

Elliptical Galaxies can form from gas-rich spiral galaxy

mergers.

These are simulations of a merger of 2 spiral galaxies

with 16,384 particles in each disk and 4096 particles in

each bulge.

The result is an elliptical galaxy.

See Hernquist 1993, ApJ, 409, 548.


Elliptical Galaxies can also grow from mergers of elliptical galaxies.

van Dokkum 2005, AJ, 130, 2647


Elliptical Galaxy Formation


Elliptical Galaxy Formation

Rings may be left over remnants of merged galaxies

NGC 3923 Malin & Carter, Nature, 285, 643, 1980


Active Galactic Nuclei

In 1908, Edward Fath (1880-1959) observed NGC 1068 with his spectroscope, which displayed odd (and very strong) emission lines.

In 1926 Hubble recorded emission lines of this and two other galaxies.

In 1943 Carl K. Seyfert (1911-1960) reported that a small fraction of galaxies have very, very bright nuclei that show broad emission lines

produced by atoms in high ionization states.

NGC 1068

OIII



Today, such objects are Seyfert galaxies.

Seyfert I galaxies have broad emission lines (1000-5000 km/s)

Seyfert II galaxies have narrow lines (<500 km/s)

NGC 1068 is a Seyfert II

NGC 1068

OIII


Mrk 1243 - Seyfert I (Osterbrock 1984, QJRAS, 25, 1)

Wavelength [Angstroms]

Flux

Flux


Mrk 1157 - Seyfert II (Osterbrock 1984, QJRAS, 25, 1)

Wavelength [Angstroms]

Flux

Flux


Active Galactic Nuclei“Mrk” means from the catalog of E. B. Markarian (1913-1985) who

produced a catalog of Seyfert galaxies in 1965.

Galaxies known to emit strongly in X-rays are Seyferts (type I’s have more X-rays than type II’s).

Other types of Active Galaxies include radio galaxies, quasars, and blazars.

Radio Galaxies

After WWII, science of radio astronomy took off. First discrete source of radio waves (other than the Sun) was Cygnus A. Below is a VLA image.

~2 arcmin

Redshift of Cygnus A is z=0.057, which from Hubble’s Law gives a distance of 240

Mpc. Brightest radio source is well beyond the Milky Way !


Radio Jets come from nuclei.

Centaurus A, visual and radio emission


Active Galactic NucleiQuasars

As radio telescopes increased numbers of sources in late 1950s, astronomers began identifying them in optical images.

In 1960 Thomas Matthews and Allan Sandage found a m=16 mag object matching 3C 48 (3C= “Third Cambridge Catalog” of radio

sources) with an emission line spectrum that could not be identified.

Sandage said, “The thing was exceedingly weird”.

In 1963, a similar spectrum was seen in 3C 273.

Optically, they looked like point sources (like stars?!) and not like galaxies.

The became known as quasi-stellar radio sources = Quasars.

Later astronomers recognized the emission lines as Balmer Hydrogen lines, but redshifted to incredible velocities, z=0.158 for

3C 273, or v ~ cz = 47,000 km /s !

3C 48 has z=0.367, or a radial velocity of 0.303 c !


The spectral lines of quasar 3C 273 has z = 0.158. This is one of the nearest and brightest quasars (as far as

apparent magnitude goes).



Quasars Luminosities

Calculate the luminosity of 3C 273. The apparent magnitude is V=12.8 mag. The modern day distance for its redshift is 620 Mpc.

MV = V - 5 log10(d / 10 pc) = -26.2 mag.

Using MSun = +4.82 for the absolute magnitude, we can estimate 3C273’s visual luminosity:

LV = 100(Msun-MV)/5 L⊙ = 2.6 x 1012 L⊙ = 1039 W.

Bolometric luminosities of Quasars range from 1038 to 1039 W, this is more than 100 times the output of a galaxy like the Milky Way !



Currently, Quasars have been identified with redshifts z > 6 !

Many of these come from the Sloan Digital Sky Survey (SDSS).

You book quotes that there are 520 quasars with z > 4. At z = 4 the recessional velocity is 0.92 c !

To determine distances at such large redshifts requires geometrical considerations (more on this when we do cosmology).

Effectively, the fractional change in wavelength due to the redshift is the same as the fraction change in the size of the Universe (recall

the Universe is expanding!)

z = (λobs - λrest) / λrest = (Robs - Remitted)/Remitted

Where Robs is the size of the Universe when the photon is observed and Remitted is the size of the Universe when the photon was emitted.



What powers Quasars and AGN ?

Most likely candidate is supermassive blackholes accreting material at a substantial fraction of the Eddington Limit.

Quasars are point sources even in HST images, implying the regions emitting the intense luminosity are < 0.1 kpc.

HST studies of QSOs show that they have host galaxies with tell-tale signs of mergers.



Bahcall 1995, ApJ, 479, 642



What powers Quasars and AGN ?

Most likely candidate is supermassive blackholes accreting material at a substantial fraction of the Eddington Limit.

Quasars are point sources even in HST images, implying the regions emitting the intense luminosity are < 0.1 kpc.

HST studies of QSOs show that they have host galaxies with tell-tale signs of mergers.

Variability studies show that the luminosity of Quasars can vary by a factor of 2 within days.

Some objects (mostly Blazars) vary by factors of 10-100 over short timescales.

Variability gives as estimate of the size of the emission region because the region must be connected by the speed of light


Active Galactic NucleiVariability gives as estimate of the size of the emission region because the region must be connected by the speed of light.

R = c Δt (1 - v2/c2)1/2 = c Δt /γFor Δt = 1 hr, taking γ=1 (can only be larger, making R smaller)

R = 1.1 x 1012 m = 7.2 AU (between Jupiter and Saturn).

Considering that the luminosity is >100 than the Milky Way, this is an incredibly small size !

Recall that there is a maximum luminosity before an object will blow itself apart due to radiation pressure. This the Eddington Limit.

L < LEd ≈(1.5 x 1031 W) x (M /M⊙)

For L = 5 x 1039 W you can solve for the mass, which would be M > 3.3 x 108 M⊙.

Finding such a large mass in such a small space is clear evidence for a supermassive black hole. The mass for an object with a

Schwarzschild radius =7.2 AU (above) is M = 3.7 x 108 M⊙.


Effects of an AGN on gas in two gas-rich galaxies.Di Mateo, Springel, et al., http://www.mpa-garching.mpg.de/


http://www.mpa-garching.mpg.de

http://www.mpa-garching.mpg.de

how do galaxies form - physics and astronomy at...

Documents