Powerful Ionizing
Sources in the
Nearby Universe
An international team of
astronomers using GMOS
on each of the Gemini telescopes has obtained the
first ever close-up images of
Lyman-alpha blobs (LABs) at
low redshifts of z = 0.3 (Figure 3). LABs may extend up
to 100 kiloparsecs, and emit
copious amounts of Lymanalpha radiation. They are
landmarks of massive galaxy
formation and have, so far,
only been found at high redshifts of about 1.5 or higher.
Gemini astronomer Mischa
Schirmer and collaborators
have shown that LABs may
still exist in the low redshift
Universe, 4 - 7 billion years
later than previously known,
based on far-ultraviolet measurements with the GALEX
satellite.
One of the biggest mysteries of LABs is their
ionizing power source. Various mechanisms
have been suggested, such as cold accretion
streams, hidden active galactic nuclei (AGN),
star bursts, and supernovae; however, many
LABs show no ionizing continuum source at
all. The researchers found weak AGN at the
cores of the discovered low-redshift LABs.
Their low redshifts allowed the astronomers
to study these objects in much more detail
than their high-redshift cousins.
The very luminous and extended nebulae observed require that the AGN must have been
in a very powerful state until a few 1,00010,000 years ago. Such episodic duty cycles
are typical for AGN, but are difficult to recognize otherwise because they last much longer
October 2016
than a human lifetime. One of the team’s main
results is that even a short burst of high AGN
activity is sufficient to power the LAB’s Lymanalpha emission for a very long time
This work is featured on the Gemini website
and is published in Monthly Notices of the
Royal Astronomical Society.
Nancy A. Levenson is Deputy Director and Head
of Science at Gemini Observatory and can be
reached at: [email protected]
Mischa Schirmer is a Science Fellow at Gemini
South and recently moved to the position of Assistant Scientist. He can be reached at:
[email protected]
GeminiFocus
Figure 3.
Gemini/GMOS images
of four of the new lowredshift Lyman-alpha
blobs, using g, r, and i
filters. From upper left to
lower right: J1505+1944,
J1455+0446, J1155–0147
and J0113+0106. These
objects are among the
most powerful [OIII]5007
emitters known in the
Universe, causing the
green color in these
optical images. Note that
the far-UV Lyman-alpha
radiation is not visible in
these optical images.
9