Galaxies

A galaxy is a gravitationally bound system of stars, gas, dust and dark matter. They span seven orders of magnitude in mass, and their observed properties are tightly correlated in ways that constrain how they formed.

Classification

Hubble’s morphological scheme remains the working vocabulary, though its implied evolutionary sequence was wrong.

The scheme was drawn as a “tuning fork” and once interpreted as an evolutionary sequence from ellipticals to spirals. That reading is backwards. Ellipticals are generally the end state, produced by mergers of disc galaxies that destroy the ordered rotation and exhaust or expel the gas.

The Milky Way is a barred spiral, roughly SBbc, with about 10¹¹ stars, a stellar disc some 30 kpc across, and the Sun about 8.2 kpc from the centre.

Rotation curves

If a galaxy’s mass followed its light, orbital velocity beyond the visible disc would fall as $v \propto r^{-1/2}$, exactly as planetary velocities fall with distance from the Sun.

Measured rotation curves do not do this. They rise and then stay flat far beyond the optical edge, implying that enclosed mass keeps growing with radius:

Vera Rubin and Kent Ford established this across many spiral galaxies in the 1970s, building on earlier indications from Zwicky’s work on galaxy clusters. The inference is that galaxies are embedded in extended dark matter haloes containing several times the mass of their visible components. See Dark matter and dark energy.

A related empirical regularity, the Tully-Fisher relation, ties a spiral’s luminosity to its rotation velocity as roughly $L \propto v^4$. It is tight enough to serve as a distance indicator, and any successful theory of galaxy formation has to reproduce it.

Structure

The Milky Way’s components have distinct kinematics, ages and compositions, which is itself evidence about the assembly history.

Gaia’s astrometry transformed this picture by measuring positions, distances and motions for nearly two billion stars. It revealed that a large fraction of the stellar halo comes from a single major merger, now called Gaia-Enceladus, around 10 billion years ago. The Milky Way’s halo is substantially the shredded remains of an earlier galaxy. See Space telescopes.

Evolution

The hierarchical picture is that small structures form first and merge upward, which follows from cold dark matter cosmology and is now well supported.

Quenching, the cessation of star formation, is less settled. Galaxies divide observationally into a star-forming blue cloud and a passive red sequence with relatively few objects in between, implying that the transition is fast. Candidate mechanisms are energy injection from an active nucleus that heats or expels the gas, stripping of gas as a galaxy falls into a cluster, and simple exhaustion of the supply. Which dominates at which mass remains an active question.

Active galactic nuclei

Most large galaxies host a central supermassive black hole, and when it accretes material it can outshine the entire stellar population.

The energy source is gravitational rather than nuclear, and it is far more efficient. Accretion onto a black hole converts of order 10% of rest mass energy into radiation, against 0.7% for hydrogen fusion, which is why a region the size of the solar system can outshine a galaxy of 10¹¹ stars.

The unified model holds that most of these differences are viewing angle rather than physics. An obscuring dusty torus surrounds the accretion disc, so a face-on view shows the broad emission lines from fast-moving gas close in, while an edge-on view hides them. Blazars are the same objects seen down the jet, with apparent brightness boosted by relativistic beaming.

Accretion has a natural ceiling. The Eddington luminosity is the point at which radiation pressure on infalling material balances gravity, at roughly $3.3 \times 10^4$ solar luminosities per solar mass of black hole. Sustained accretion much above it drives the material away, which limits how fast a black hole can grow and makes the existence of billion-solar-mass quasars within the first billion years a genuine puzzle.

Coevolution

Black hole mass correlates tightly with the velocity dispersion of the host galaxy’s bulge, the M-sigma relation, over several orders of magnitude. The black hole is a tiny fraction of the galaxy’s mass and its gravitational influence extends only a small distance, so direct gravitational coupling cannot explain this.

The favoured explanation is feedback: the energy released during accretion heats and expels the surrounding gas, cutting off both further accretion and star formation. The black hole and the bulge regulate each other. This is why active galactic nuclei appear in galaxy formation models as a necessary ingredient rather than a curiosity, and it links the smallest scales in a galaxy to the largest.

References

• Binney, J. and Tremaine, S. Galactic Dynamics, 2nd ed., Princeton University Press, 2008.
• Mo, H., van den Bosch, F. and White, S. Galaxy Formation and Evolution, Cambridge University Press, 2010.
• Rubin, V. C. and Ford, W. K. “Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions”, ApJ, 159:379, 1970.
• Helmi, A. et al. “The merger that led to the formation of the Milky Way’s inner stellar halo and thick disk”, Nature, 563:85, 2018.
• Kormendy, J. and Ho, L. C. “Coevolution of Supermassive Black Holes and Host Galaxies”, Annual Review of Astronomy and Astrophysics, 51:511, 2013.