Media assistance: Helen Sim
+61-(0)2-9372-4251 (office)
+61-(0)419-635-905 (mob.)
hsim@aao.gov.au
Contact details and urls at end
10 September 2012
Stars "missing in action" now counted
Almost one in five exploding stars in nearby galaxies is simply not
seen, astronomers have determined. For galaxies further out, that
fraction doubles.
This finding clears the way for these stellar beacons to be used as a
good measure of how fast galaxies made stars earlier in the Universe's
history.
Key evidence for this "body count" of missing supernovae came from
detailed studies of the galaxy Arp 299, made with the 8.2-m Gemini
North telescope.
The work is reported in a paper, "Core-Collapse Supernovae Missed by
Optical Surveys", published online in the Astrophysical Journal
by a team including Dr Stuart Ryder of the Australian Astronomical
Observatory.
Massive stars live fast and die young, going out in a blaze of glory
as "core-collapse supernovae".
"Because they don't live long, only about 10 million years, the number
of massive stars that we can see being born should be essentially
identical to the number we see exploding," said the paper's lead
author, Dr Seppo Mattila of the University of Turku in Finland.
"The trouble has been, lots of these supernovae just seemed to have
gone missing," he said.
"We expected to see more."
Our own Milky Way Galaxy is a case in point.
Two to three supernovae should explode in our Galaxy every century.
But the last time anyone might-just might-have seen one directly was
in 1680.
In 2008, however, NASA's Chandra X-ray Observatory and the Very Large
Array radio telescope of the US National Radio Astronomy Observatory
found a supernova remnant about 140 years old near the centre of our
Galaxy.
"That's where the supernovae are mostly hiding-in the dusty central
parts of galaxies," said the AAO's Dr Stuart Ryder.
"That's where most stars in a galaxy congregate, and where most of
them die."
Because these regions are dusty and crowded, supernovae and their
remnants can only be excavated from them with high-resolution
observations-observations made with space telescopes, radio
interferometers, or adaptive-optics systems on ground-based telescopes
that observe in the near-infrared, and which can produce images as
sharp as can the Hubble Space Telescope.
To calculate galaxies' "supernova budgets", Dr Mattila's team drew
upon recent discoveries of supernovae made by other researchers, plus
their own observations made with the 4.2-m William Herschel Telescope,
the 2.6-m Nordic Optical Telescope, and the 8.2-m Gemini North
Telescope.
The team considered core-collapse supernovae in two kinds of galaxies:
those that are heavily obscured by dust but are otherwise "normal",
and the so-called luminous and ultraluminous infrared galaxies (known
respectively as LIRGs and ULIRGs).
In the nearby Universe, most of the star-formation goes on in "normal"
galaxies, while at higher redshifts (z ~ 1-2, corresponding to seven
to ten billion years ago in the history of the Universe) most of it
occurs in LIRGs and ULIRGs.
The galaxy Arp 299 is one of the best-studied examples of a LIRG and,
lying 150 million light-years away, it is also one of the closest. Dr
Mattila's team drew on previous searches for supernovae in this galaxy
and also conducted their own, in the process discovering one new
supernova (SN2010P) and confirming another (SN2010O).
For both "normal" galaxies and Arp 299, the team compared the numbers
of detected supernovae with the numbers predicted on the basis of both
radio and far-infrared luminosities. From this they estimated what
fraction of core-collapse supernovae must have been missed.
From the figure for Arp 299, and a model of how LIRGs and ULIRGs
evolve, they estimated how many core-collapse supernovae would be
missed in LIRGs and ULIRGs at higher redshifts.
The team's figures- Optical surveys miss about 20% of core-collapse
supernova in "normal" galaxies and up to a whopping 80% in LIRGs and
ULIRGs, they say.
Adjusting for the variation in galaxy type with redshift, it appears
that at a redshift of one (7.7 billion years ago), the fraction of
missed core-collapse supernovae rises to about 40%, and stays at
around that level to a redshift of two (10.3 billion years ago).
Dr Ryder stresses that there are "small number statistics" involved.
"Our estimates have significant uncertainties attached to them," he
said.
"A new Australian-built adaptive optics camera on the Gemini South
telescope, coming on-stream soon, will enable us to discover many more
supernovae, and to refine these estimates."
"But right now, for the first time we have a figure for total number
of core-collapse supernovae in the nearby Universe that matches well
with the star-formation rate. So we're on the right track."
Links
S.
Mattila et al. 2012 ApJ 756 111 doi:10.1088/0004-637X/756/2/111/
Also at http://arxiv.org/abs/1206.1314v3
(open access)
More information
Dr Stuart Ryder, AAO
sdr@aao.gov.au (travelling in Europe until early October 2012)
Dr Seppo Mattila, University of Turku, Finland
sepmat@utu.fi
office: +35 8 (0)2 333 8299
mobile: +35 50 331 6967