New Evidence for inside-out formation of galaxy disks

In galaxies, stars are constantly forming, where some galaxies show more active star formation than others. Disk-dominated galaxies such as our own Milky Way have now been studied by Jing Wang, Guinevere Kauffmann, Roderik Overzier and Barbara Catinella at the Max-Planck Institute for Astrophysics (MPA) in collaboration with colleagues from other institutes to gain new insight into galaxy formation and evolution. The results of the team’s study support the picture of ”inside-out“ galaxy formation, where the active star forming regions gradually move towards the outer regions of the galaxy disk as gas with ever higher angular momentum is accreted.

Fig. 1: SDSS and GALEX images of one galaxy in the HI sample with a strong colour gradient. Left: SDSS g,r,i colour composite image; right: NUV map. In the UV light a bright ring is visible, which is not evident in the optical image.

Fig. 2: Relation between the colour gradient and HI mass fraction. In each stellar mass bin (circles with the same size), galaxies with higher HI mass fraction have stronger colour gradients, meaning that the galaxies become bluer towards the edge.

Fig. 3: SDSS images of two galaxies in the sample. The left image shows a typical galaxy with low asymmetry and the right image a typical galaxy with high asymmetry. The study showed that that the asymmetry of the galaxies is connected more closely to the total star formation than the HI mass fraction.

The fraction of available baryons locked up in stars in galaxies such as our own Milky Way is only around 20 percent. Simple physical considerations predict that most of these baryons should cool, accrete and form stars. Up to the present day, the galaxies accrete additional material in the form of gas from the external environment as some observational evidence shows. In particular, neutral hydrogen (HI) cloud complexes, HI-rich dwarfs in the vicinity of spiral galaxies, extended and warped outer layers of HI in spiral galaxies, and lopsided galaxy disks have all been cited as evidence for on-going gas accretion in nearby spiral galaxies. However, the total amount of gas accreting in this way is much too low to sustain star formation at the observed rates of 2-3 solar masses per year in these spirals. Astronomers have attempted to look for gas accretion in other forms, e.g. in an extended, hot gaseous corona surrounding the galaxy, in the form of ionized gas at intermediate temperatures, or in tiny clouds of neutral gas (where a ”tiny“ cloud for astronomers has a mere one thousand to one hundred thousand solar masses). But in spite of these on-going searches, conclusive evidence for gas accretion remains elusive.

If we cannot observe gas accretion directly, can we at least hope to observe its effects? Semi-analytical models for the formation of disk galaxies, their chemical evolution and star formation are commonly based on the ”inside-out“ picture in the context of Cold Dark Matter cosmologies. In this scheme, gas in the dark matter halo surrounding the galaxy cools, falls onto the galaxy, and fuels star formation in the disk. As the gas conserves its angular momentum and gas accreted at late times has a higher specific angular momentum, it settles in the outer regions of the galaxy. In this picture, galaxies with recently acquired gas from their halo therefore should have not only unusually high gas mass fractions but also young, star-forming outer disks.

To test this picture, the MPA scientists performed a statistical study of the colours, star formation rate and neutral hydrogen (HI) gas fractions in galaxies. The sample of HI-rich, nearby galaxies (see Fig. 1) was selected based on a combination of data from the Arecibo Legacy Fast ALFA survey (ALFALFA) and the GALEX Arecibo SDSS Survey (GASS).

Galaxies with more gas are in general bluer and show more active star formation. The new study now showed that an increasing HI content also leads to a colour gradient across the galaxy disk: Galaxies with larger HI fractions have bluer, more actively star-forming outer disks compared to their inner parts (see Fig. 2). This means that the outer regions of HI-rich galaxies are younger. HI-rich galaxies also appear larger in blue light than in red light.

These results are indeed consistent with the ”inside-out“ picture of disk galaxy formation. They furthermore provide indirect evidence for the idea that disk galaxies continue to grow through gas accretion in the local Universe. The study also showed that there is no intrinsic correlation between the HI fraction and the measured asymmetry of the optical light of the galaxy. This suggests that the gas was most likely accreted smoothly and not in discrete units (see Fig. 3).

Jing Wang, Guinevere Kauffmann, Roderik Overzier and Barbara Catinella

Further reading

Wang, J., Kauffmann, G., Overzier, R., Catinella, B. et al., "The GALEX Arecibo SDSS survey: III Evidence for the Inside-out Formation of Galactic Disks", 2011, MNRAS, 412, 1081 linkPfeilExtern.gif(