Download Young Galaxies Grow - Astronomical Society of the Pacific

Young Galaxies Grow
Developed by the GALEX Team
Topic:
Galaxies
Concepts:
Ultraviolet observations,
galaxy formation,
galaxy evolution,
young stellar populations
Missions:
GALEX, HST
Coordinated by:
the NASA Astrophysics Forum
An Instructor’s Guide for using
the slide sets is available at the
ASP website:
https://www.astrosociety.org/
education/resources-for-thehigher-education-audience/
1
The discovery
The spiral galaxy Messier 83 (M83), seen first in
visible light, then in a GALEX ultraviolet image.
(In the GALEX image, yellow is “near” or longerwavelength UV, blue is “far” or shorter-wavelength
UV. Arrows point to two of the spiral arm extensions
discovered.) Credits: Optical: R. Gendler; GALEX:
• When NASA’s Ultraviolet (UV)
telescope GALEX looked at the
spiral galaxy M83, tenuous spiral
arms appeared much more
extended than seen in visible
images of the galaxy. Ultravioletbright, thin structures stretch to
almost five times the galaxy’s
optical radius.
• Another spiral galaxy showed a
similar extended Ultraviolet disk.
So astronomers began to look at
several such galaxies with GALEX;
one third appeared larger in
UV than in visible light. The UV
emission revealed young, massive
stars in galaxy outskirts where they
had not been detected before.
NASA/JPL-Caltech.
2
How was the discovery made?
• Ultraviolet light reveals young,
massive stars more easily than
other wavelengths, because
these stars have very hot surface
temperatures and their light is
mainly emitted in the UV. These
stars are thus prominent in GALEX
UV images.
• When GALEX’s far-UV sensitivity
and wide field capabilities
showed the extended UV
disks, astronomers used large
telescopes to follow up the
GALEX discovery. The Hubble
Space Telescope (HST), with
greater resolving power than
GALEX, confirmed the presence
of hot, young stars in the spiral
extensions.
Credits: NASA/JPL-Caltech; HST/D. Thilker et al.
3
The big picture
Ultraviolet Image (GALEX)
Hot
young
stars
The spiral galaxy NGC 4656, seen “edge-on,”
first in visible light, and then in a GALEX image.
The GALEX image shows UV emissions from hot,
young stars in the extended regions beyond the
optically visible disk.
• Spiral galaxies form from a spinning disk of
matter. Their brightest component is a flat,
dense disk, where most of their stars are
formed along spiral arms. Our Milky Way is
a spiral galaxy,
• Such disk galaxies assembled mostly in
earlier epochs, but they continue to form
stars today from remaining gas within the
spiral arms. They are also surrounded by
extended reservoirs of low-density gas
thought to be too sparse to clump and
form new stars.
• GALEX, however, has found populations
of young, hot stars in these extended
reservoirs around some spiral galaxies.
Credits: SDSS/NASA/JPL-Caltech/L. Bianchi.
4
How does this change our view?
M83 in three views: in visible light, in GALEX’s UV image,
and with radio emission added to illustrate the distribution of
its extended disk of low-density gas where the new young
stars were found. Credits: R. Gendler/NASA/JPL-Caltech/NRAO/
AUI/NSF/MPIA.
• The disks of gas from which spiral
galaxies form are very extended,
but are also very sparse in their
outer regions.
• Stars form when clouds of gas
and dust condense until nuclear
reactions ignite in their cores. The
density of gas beyond the bright
central galaxy was believed to
be too low for star formation to
occur.
• But GALEX has revealed that
under certain conditions (still to
be sorted out), even such thin
gas can condense and form
new stars — and galaxies can
increase their starry dimensions!
5
Resources
First press release and image releases about this result:
http://www.galex.caltech.edu/newsroom/glx2007-01f.html
http://www.galex.caltech.edu/media/glx2008-01r_img01.html
http://www.galex.caltech.edu/newsroom/glx2008-01r.html
Scientific articles:
First discovery papers:
“Recent Star Formation in the Extreme Outer Disk of M83,”
Thilker, D. et al. 2005 ,Astrophysical Journal, 619, L79.
http://adsabs.harvard.edu/abs/2005ApJ...619L..79T
“Discovery of an Extended Ultraviolet Disk in the Nearby Galaxy NGC 4625,”
Gil de Paz, A., et al. 2005 Astrophysical Journal Letters, 627, L29.
http://adsabs.harvard.edu/abs/2005ApJ...627L..29G
First comprehensive paper on the “Extended UV Disks”:
“A Search for Extended Ultraviolet Disk (XUV-Disk) Galaxies in the Local Universe,”
Thilker et al. 2007, Astrophysical Journal Suppl., 173, 538.
http://adsabs.harvard.edu/abs/2007ApJS..173..538T
GALEX/SDSS images:
“GALEX and star formation,” Bianchi, L., 2011, Astrophys. Space Sci., 335, 51.
http://link.springer.com/article/10.1007/s10509-011-0612-2
A recent review:
“From the Realm of the Nebulae to the Society of Galaxies,” Bianchi, L., 2015, Springer, in press.6
Young galaxies grow
BONUS CONTENT
7
• Spiral galaxies, like M83, Andromeda, and the Milky Way, were known to still be
forming stars in their disk, although not to the extent revealed by GALEX.
• Other types of galaxies, termed “elliptical” and “lenticular,” stopped forming
stars shortly after their initial assembly. Therefore, their stellar populations, usually
conspicuous, contain only stars of very old ages.
• But GALEX ultraviolet images revealed
rings of sparse young star groups around
some “old” galaxies, indicating that
stars are still forming in extended halos
of sparse gas surrounding these types of
galaxies as well.
GALEX image showing a wide ring of UV-emitting
(blue) regions, indicating the presence of hot
young stars, surrounding the old (yellow) lenticular
galaxy NGC 0404 in the center of the image.
Credits: NASA/JPL-Caltech/DSS.
8