September 16, 2003
Berkeley Lab Research News
Homing in on Dark Energy with Supernova Studies from Space

What is Host-Galaxy Extinction?
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Space Telescope Supernovae
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Host Galaxy Extinction
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Dark Energy

BERKELEY, CA Type Ia supernovae are among the best standard candles known to astronomy — objects whose distance can be determined because their intrinsic brightness is known or can be computed, just as the distance to a 100-watt bulb can be calculated by comparing its apparent brightness with its actual brightness.

Determining the expansion rate of the universe by comparing the brightness and redshift of far-off Type Ia supernovae therefore critically depends on accurate measurements of both.


Looking through dust in galaxies can make supernovae appear dimmer, but dust also makes their light redder — just as dust in our atmosphere makes sunsets appear red. (Galaxy image: 2MASS, with simulated SN)

One worrisome possible source of error in measuring distant supernovae has been host-galaxy extinction, the filtering effect of dust peculiar to the galaxy in which the supernova occurs. Dust occurs in our own galaxy too, but has been so extensively studied that it is of less concern in supernova distance measurements.

The concern is that distant supernovae appear dimmer not because of the accelerating effects of dark energy but, more prosaically, because of dust. There is a straightforward way to distinguish these effects, however, since dust normally reddens the light passing through it. Shorter, bluer wavelengths are absorbed and scattered more readily than longer, redder wavelengths.

"When you want to measure a supernova's brightness you can measure the light that was blue when it left, or the light that was red," says Greg Aldering, a member of the Supernova Cosmology Project and leader of the Nearby Supernova Factory program, which concentrates on studying the intrinsic properties of Type Ia supernovae. "Both measurements are valid, but what you want is to make sure you get the same answer on both sides of the spectrum. If the blue is fainter, you've got a dust problem."

Imagine supernovae as stoplights. Does the green light appear fainter because it's farther away (top) or because it has been absorbed and scattered by dust (bottom)? We can tell by comparing the brightness of the red light: if dust made the green light fainter, the red light will not be fainter by as much.

The extraordinarily high quality of photometric data from the 11 distant supernovae studied by the Hubble Space Telescope in this study allowed their intrinsic brightness to be determined and compared in both bands.

The study determined that no anomalous effects of host-galaxy extinction occur at great distance; distant supernovae are comparable to nearby supernovae in this respect, underlining their utility as standard candles.

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