Learn · Other galaxies
Galaxy evolution — from gas to star systems
The spiral arms in a photograph are a snapshot from a history of thirteen billion years. To understand them you have to look beyond the stars to the gas they form from, to the vast halo everything is embedded in — and to the feedback with which stars and black holes put the brakes on their own galaxy.
The beginning: density fluctuations and halos
The cosmic microwave background shows the universe 380,000 years after the Big Bang as almost perfectly uniform — with density contrasts of only one part in a hundred thousand. Gravity amplifies such differences: denser regions attract matter, thinner ones empty out. First the dark matter, which feels only gravity, gathers into halos; the ordinary gas falls in after it, is heated by the fall and then has to radiate energy away in order to sink further. Where it can, it cools, condenses and forms the first stars — according to current observations only a few hundred million years after the Big Bang. Because the infalling gas almost always brings a little angular momentum it cannot shed, it collects in a rotating disk. That is the reason spiral galaxies exist at all. ESA: Structure formation
From small to large: hierarchical growth
In the standard cosmological model the small structures form first and the large ones from them — galaxies grow as halos merge and gas keeps streaming in along the filaments of the cosmic web. Every large galaxy therefore has a merger tree: dozens of smaller predecessors it has absorbed over time. The Milky Way is the best-studied case. In the motions of halo stars the Gaia spacecraft has found the remains of a galaxy that merged with the young Milky Way about ten billion years ago — “Gaia-Enceladus”, perhaps a quarter as massive as the Milky Way itself at the time. The Sagittarius dwarf galaxy is being torn apart right now and leaves a stellar stream around the whole Galaxy. And in M 31 a giant stellar stream betrays an encounter that did not happen long ago. None of these events can be seen directly in the image of a galaxy; they are found in the orbits and chemical composition of individual stars. Helmi et al. (2018): Gaia-Enceladus
“Cosmic noon” is long past: when the universe was three to four billion years old, on average ten times as many stars formed as today. The curve is schematic, following the measurements of Madau and Dickinson (2014); the Milky Way events are rough dates. Madau & Dickinson (2014)
The gas cycle
A galaxy is not a closed container. The Milky Way forms about one to two solar masses of stars per year, and at that pace its supply of cold gas would last only one to two billion years. That it has nevertheless been forming stars for ten billion years works only with resupply: gas streams in from the halo and from intergalactic space, often along thin filaments. At the same time supernovae and stellar winds blow gas out of the disk into the circumgalactic medium — a hot, tenuous gas envelope that can contain more mass than all the stars of the galaxy combined. Part of that gas later cools and falls back, enriched with the heavy elements of the previous stellar generation. This cycle is ultimately also where the elements planets are made of come from. Tumlinson et al. (2017): The circumgalactic medium
AI-generated illustration: gas supply to the disk and outflowing gas stand for the cycle of star formation and feedback. The flows are made visible for explanation; colours and proportions are illustrative. This image is AI-generated — not a photograph but a visualisation.
Feedback: stars and black holes adjust the dial
Without a brake, the gas of a young galaxy would turn almost entirely into stars in short order. The opposite is observed: only a small fraction of the gas in a halo ever ends up in stars. The reason is called feedback. In small galaxies the winds of massive stars and the explosions of supernovae suffice to heat gas and blow it right out of the shallow gravitational well — dwarf galaxies often lose their gas for good this way. In massive galaxies a different mechanism takes over: the central black hole. As long as it swallows gas, its surroundings shine as an active galactic nucleus and release enormous amounts of energy that keep the halo gas hot and prevent it from cooling and forming new stars. That the mass of the black hole is tightly linked to the mass of the surrounding central region hints that both grew together. Only with this feedback do simulations agree with the number and brightness of the observed galaxies. Kormendy & Ho (2013): Black holes and their galaxies
Blue, green, red: when star formation goes out
Plot colour against brightness for many galaxies and two clouds emerge. The blue cloud contains the disk galaxies with ongoing star formation; the red sequence the elliptical and lenticular galaxies in which it has ceased. Between them lies a sparsely populated green valley — and its sparseness says something important: the transition from blue to red is comparatively quick, one to two billion years, otherwise more galaxies would linger there. Whatever triggers the transition is called quenching. Massive galaxies do it from within, through their black hole. Smaller ones tend to lose their gas from outside — when falling into a galaxy cluster, where the hot cluster gas sweeps the cold disk clean. That is why ellipticals are common in cluster centres and spirals rare there. Strateva et al. (2001): Colour bimodality of galaxies
Without an encounter: internal evolution
Even a galaxy left alone for a long time does not stay as it is. Disks are dynamically active: they form bars, spiral arms and rings, and these structures redistribute angular momentum. A bar pushes gas inward, where it forms new stars in a nuclear ring or feeds a central black hole; over billions of years it can thus build up a small, disk-like “pseudobulge” that differs from the classical bulges created by mergers. This slow path is called secular evolution. NGC 6951 is a textbook case: a barred spiral with a ring of young star clusters only a few hundred parsecs across around an active nucleus. Our image shows the bar and the arms taking off from it; the ring lies beneath, unresolved in the bright centre. A galaxy can therefore grow on the outside through inflow while its centre rebuilds itself by an entirely different route. Kormendy & Kennicutt (2004): Secular evolution
What our images show — and leave open
A broadband image shows the distribution of starlight and, as pink knots, the sites of ongoing star formation. It does not show the gas, the halo, the motions or the age of individual regions. That NGC 6946 shows more star formation in its arms than M 31 does in hers matches the measured rates — the “Fireworks Galaxy” has delivered ten supernovae in the last century, more than any other known galaxy, a direct expression of its pace. But reading off the number of young stars from the brightness of a knot would be over-interpretation. And NGC 6951 and NGC 6952 designate the same object — a double entry in the catalogue, not a merging pair.
A few things worth knowing:
- The universe is past its peak. The star-formation rate has fallen by about an order of magnitude since cosmic noon. Most of the stars that will ever form already exist — even though galaxies like the Milky Way will carry on for billions of years.
- Big galaxies finished early. The most massive systems formed their stars earliest and stopped earliest; small galaxies are still active today. This “downsizing” contradicts the naive picture that big things take longest.
- Stars are not recycled, gas is. A star keeps its composition. But the gas it returns at the end is enriched with heavy elements — every generation starts with a little more carbon, oxygen and iron than the one before.
- The oldest stars of the Milky Way are almost as old as the universe. In the halo and in globular clusters there are stars of more than twelve billion years, formed before the disk even existed.
- A red system is not dead. Elliptical galaxies form hardly any new stars, but their old ones keep shining for many billions of years. “Red and dead” is jargon, not a description.
In my own sky
The gallery shows galaxies at different points of this story — from the star factory to the quenched dwarf:
