January 10, 2013
Measuring Cosmic History With The Farthest Supernova Yet
April Flowers for redOrbit.com - Your Universe Online
Have you ever wished for a time machine to be able to travel to the distant past? What about a "Wayback Television Set" that would allow you to watch an entire month of ancient prehistory in real time?
Rubin announced a Type Ia supernova with a redshift of 1.71 that dates back 10 billion years in time. SN SCP-0401 is exceptional for its detailed spectrum and precision color measurement, which is unprecedented in a supernova so distant.
"This is the most distant supernova anyone has ever found for doing dependable cosmology," says Rubin, a member of the international Supernova Cosmology Project (SCP) based at Berkeley Lab. "The most important unanswered question we have about the nature of dark energy is whether it varies over time — whether it affects the expansion of the universe differently in different eras. With SN SCP-0401, we have the first example of a well-measured supernova sufficiently far away to study the expansion history of the universe from almost 10 billion years ago."
According to astrophysicist Saul Perlmutter, who is the head of SCP, a faculty senior scientist at Berkeley Lab's Physics Division and a professor of physics at the University of California, Berkeley, "Imagine you're channel surfing and you come across live news coverage of an exploding star — and then you see the dateline that says it's July 22nd, 9,947,989,219 BCE. By August 9 the supernova is at its brightest and starts to fade, but you get to watch the whole thing — even though, before the news could ever reach your TV, our solar system had to form, and then our planet, and intelligent life had to evolve on Earth."
There are special advantages to life-at-the-scene coverage. Because their colors can't be accurately measured, the light curves of most supernovae with redshifts above 1.5 are either incomplete or not cosmologically useful. Perlmutter says that our imaginary Wayback TV broadcast, however, "has enough high-resolution information to allow us to confidently compare this ancient supernova with much more recent astronomical events."
Wayback TV, unfortunately, is imaginary. The high resolution details of SN SCP-0401 is not, however. The findings of the SCP's study were reported in a recent issue of the Astrophysical Journal.
A Type Ia supernova, unlike the majority of supernovae that begin as stars whose cores collapse, begins as a white dwarf star borrowing mass from a companion star. When it reaches critical mass, it erupts in a titanic thermonuclear explosion. Type Ia's are not identical, but they are more similar in brightness than any other type of supernova, making it possible to accurately correct their variability for comparison.
Astronomers use Type Ia supernovae as "standard candles" for measuring cosmic distances. The dimmer a Type Ia appears, the farther away they are and this distance can be accurately measured. The redshift of a Type Ia is a direct gauge of how much the universe has expanded since the supernova exploded.
Comparing the distance and redshift for a sufficient number of Type Ia's over a long expanse of time allows for accurate measurement of the history of expansion. Two competing teams who announced the discovery in 1998 independently found this accelerating expansion, propelled by dark energy. Because of this discovery, the 2011 Nobel Prize in Physics was divided between Saul Perlmutter and Brian Schmidt and Adam Riess of the competing High-z Supernova Search Team.
SN SCP-0401 was first spotted in 2004 as part of a supernova survey conducted by the Supernova Cosmology Project using the Hubble Space Telescope (HST). The two teams were sharing alternate scans of the sky.
"We were looking for supernovae so far away that they really required the HST, not just to detect them but so we could separate them from their host galaxies, in hopes of getting clean spectra," said Rob Knop, now of Quest University Canada, who headed the SCP's search strategy in 2004. Knop also wrote the software that identified likely candidates. Knop and colleague Rachael Gibbons of Vanderbilt University, following an SCP tradition of nicknaming candidates after composers, named the new contender "Mingus" after the jazz composer.
Because it was so distant, Mingus skirted the edge of invisibility like trying to see a firefly all the way across the U.S. Another challenge is that Mingus is red — possibly its intrinsic color but more likely due to the very high redshift. The researchers can only tell which for sure with an accurate spectrum.
The Hubble's Advanced Camera for Surveys (ACS) captured the initial spectra. The ACS is equipped with a grism (a combined diffraction grating and prism) that collects light from the target object and from nearby objects as well.
"The ACS grism data suggested the most likely match was a Type Ia supernova at redshift 1.7 — very distant and very old — but by itself the ACS could not establish the spectral features which could confirm this," says SCP member Andrew Fruchter, of the Space Telescope Science Institute.
A different grism-equipped camera, the Wide Field Camera 3 (WFC3) — installed on the Hubble in 2009 — solved the problem by capturing some of Mingus's host galaxy.
Fruchter says, "The new WFC3 data pinned down the host's redshift at 1.713 — a match to the first estimate for Mingus." He adds that "each new instrument on HST has enabled new discoveries. It's exciting to see the Hubble continue to make breakthroughs even as we are building its successor."
Rubin derived a spectrum model that allowed him to compare its spectrum with published spectra from other Type Ia's to confirm that Mingus really was a Type Ia. This model also compared Mingus with core-collapse supernovae, as these objects would appear at redshift 1.713. Mingus's spectrum was almost a perfect match with the best-fit Type Ia spectrum, giving Rubin's model an over 90-percent chance of its identity. Mingus was confirmed as a Type Ia and with confirmation got its more official-sounding designation, SCP-0401.
"To be able to directly compare different Type Ia supernovae, we have to fit their light curves — the time it takes the supernova to reach maximum brightness and the time it takes that brightness to fall off," says Rubin. "We also have to be able to compare the brightness of the different colors during this process, in order to calibrate the supernova."
Perlmutter remarked that such calibration "can be a lot like trying to match a particular shade of house paint when you've got a thousand color chips to compare, maybe more." For Mingus, calibration meant finding exactly the right shade of red. Both the ACS camera and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), also carried aboard the Hubble, measured the brightness in different colors over time.
Our measurements of dark energy's possible time variation begins to sharpen with the cosmological picture from SCP-0401, marking a first step towards the precision measurements that will require observations of many more Type Ia supernovae as far away as SCP-0401.
"Hubble is our best bet to find and measure similar distant supernovae. Luckily Hubble has a few good years left," concluded Rubin.