Starburst cycles in galaxies

Cosmological simulations of the evolution of cold dark matter (CDM) show that the dark matter in galaxy halos forms cuspy distributions - with inner profiles that are too steep compared with observations. This is commonly referred to as the "cuspy halo problem". One solution to this problem, that was proposed early on, is as a galaxy loses mass in the form of explosions this can lead to an irreversible expansion of the orbits of stars and dark matter near the centre of the halo. The very dense cusps would then be spread out over a wider area. These conclusions, however, were based on simple analytic arguments and it was not clear whether this mechanism could in fact produce central density profiles in close agreement with observations. Later gas-dynamical simulations of dwarf galaxies indeed demonstrated that repeated gas outflows during bursts of star formation could in principle transfer enough energy to the dark matter component to flatten 'cuspy' central dark matter profiles.

Nevertheless, it has remained unclear whether the energy requirements for flattening cuspy profiles are in line with the actual stellar populations and star formation histories of real low mass galaxies. In order to estimate how frequently starbursts occur as well as the amplitude range in star formation during a burst, it is necessary to analyze a large sample of galaxies that are intrinsically similar.

High quality spectra provide a number of stellar features that are extremely useful as diagnostics of the star formation history of a galaxy. A primary feature is the strong break at 4000 Angstroms, caused by the blanket absorption of high energy radiation from metals in stellar atmospheres. This break becomes strong once young, hot, blue stars have evolved off the Main Sequence. In addition, absorption lines from the Balmer series, which are strongest in stars of spectral type A-F, are a diagnostic of the contribution of stars of intermediate ages to the total luminosity of the galaxy. Finally, Balmer emission lines arise in large, low-density clouds of gas where very recently formed stars emit copious amounts of ultraviolet light that ionize the surrounding gas (predominantly hydrogen).

Used in concert, Kauffmann (2014) found that these spectral features allow one to clearly separate galaxies in three groups: those that are currently undergoing a burst of star formation, those that have formed their stars continuously and those that have experienced a burst in the past (Fig. 1). Applied to a large sample of galaxies from the Sloan Digital Sky Survey, the scientists were able to constrain the fraction of galaxies that were experiencing current starbursts, the mass of stars typically formed in these bursts, as well the duration of the starbursts. One could then investigate whether the burst frequency depended on the mass of the galaxy and whether starbursts were associated with changes in the internal structure of galaxies.

The analysis showed that the fraction of the total star formation rate in galaxies with ongoing bursts was a strong function of stellar mass, declining from 0.85 for the smallest galaxies in the sample to 0.25 for galaxies with masses close to that of the Milky Way. Also the burst mass fraction, the half-mass formation times and the burst amplitudes and durations could be constrained. Finally, the scientists found that the central stellar densities in bursting low mass galaxies are reduced compared to their quiescent counterparts.

These results are in remarkably good agreement with predictions of some of the recent hydrodynamical simulations and give further credence to the idea that the cuspy halo problem can be solved by energy input from multiple starbursts over the lifetime of the galaxy.

Guinevere Kauffmann