Star formation in the outskirts of galaxies

February 8, 2016
Star formation in the outer spiral regions of the galaxy NGC 4625 is seen in ultraviolet light (blue); these arms are nearly invisible in optical light, but have hot, newborn stars that radiate in the UV. A new study finds that the star formation processes in these outer regions generally resemble the processes at work in more normal, denser regions where molecular gas abounds. The atomic gas is traced in the radio (purple); optical starlight is red. Credit: NASA/JPL-Caltech/Carnegie Observatories/WSRT

Star formation environments can be roughly grouped into three types, categorized by the density of their gas (or more precisely, the projected "surface" density of the gas, which is easier to determine than the conventional volume density). In moderately high density regions, where the gas is primarily molecular in form rather than atomic, there is a strong correlation between the amount of star formation taking place and the density. This result is the basis for concluding that stars form from molecular material. In very high density regions like those found in merging and starbursting galaxies, the star-formation rates compared to the total mass of available material are even larger. In low-density regions there is little known about correlations between the total gas and star-formation activity.

Low-density regions, however, are important: they can cover very large spatial extents in the outer domains of galaxies, going well beyond the sizes defined by starlight in the optical. Recently, sensitive searches for molecular in these outer regions have been able to map that component, while ultraviolet surveys have spotted UV emission from up to four times farther out in galaxies than the nominal, optical radius. Since the UV is produced by hot young stars, the presumption is that there are new stars forming there. Does star formation in these outer regions correlate with the in the same way as higher density regions, or might perhaps the star formation process proceed differently?

CfA astronomer Linda Watson led a team of five colleagues to address these questions. They analyzed published observations of (a bright tracer of molecular material) in fifteen outer regions of the galaxy NGC 4625 where UV was spotted but which are faint in the optical, and derived the relationship between and gas . They found that in general the activity is consistent with the same physical processes at work in the brighter, inner regions of the galaxies, a finding that is somewhat reassuring for theorists. But they also spotted a few outer locations where something different was happening: much higher rates of stars were being formed. Molecular gas is a crude tracer for age (because it takes time to convert atomic material into molecular), and most of the regions in this study are estimated to be between about one-to-seven million years old. The scientists raise the possibility that evolutionary effects between these regions could be at work, and urge deeper carbon monoxide observations to facilitate a broader analysis.

Explore further: Inferring the star formation rates of galaxies

More information: "Testing the molecular-hydrogen Kennicutt–Schmidt law in the low-density environments of extended ultraviolet disc galaxies," Linda C. Watson, Paul Martini, Ute Lisenfeld, Torsten Boker, and Eva Schinnerer, MNRAS 455, 1807, 2016. adsabs.harvard.edu/abs/2016MNRAS.455.1807W

Related Stories

Inferring the star formation rates of galaxies

November 23, 2015

Our Milky Way galaxy produces on average a few new stars every year across the entire system. Massive young stars emit large amounts of ultraviolet radiation which heats the local dust, and so the star formation process results ...

Where do stars form in merging galaxies?

March 2, 2015

Collisions between galaxies, and even less dramatic gravitational encounters between them, are recognized as triggering star formation. Observations of luminous galaxies, powered by starbursts, are consistent with this conclusion. ...

A universal law for star formation

January 9, 2012

(PhysOrg.com) -- Star formation is studied by astronomers not only because it produces new stars and planetary systems. It also generates copious amounts of ultraviolet light that heats dust which in turn causes the birth ...

Astronomers suggest more accurate star formation rates

April 10, 2014

(Phys.org) —Astronomers have found a new way of predicting the rate at which a molecular cloud—a stellar nursery—will form new stars. Using a novel technique to reconstruct a cloud's 3-D structure, astronomers can estimate ...

Recommended for you

Fermi finds possible dark matter ties in Andromeda galaxy

February 21, 2017

NASA's Fermi Gamma-ray Space Telescope has found a signal at the center of the neighboring Andromeda galaxy that could indicate the presence of the mysterious stuff known as dark matter. The gamma-ray signal is similar to ...

Tune your radio: Galaxies sing when forming stars

February 21, 2017

A team led from the Instituto de Astrofísica de Canarias (IAC) has found the most precise way ever to measure the rate at which stars form in galaxies using their radio emission at 1-10 Gigahertz frequency range.

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

wduckss
not rated yet Feb 08, 2016
The collapse of gas = the stars of similar mass. In the universe there is a whole range of different mass stars (include centers of galaxies).
This a misleading should be deleted and look for the right solution.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.