A101 Slide Set:
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/edu
cation/resources-for-the-higher1
education-audience/
The Discovery
Visible
Light
Image
Ultraviolet
Image
(GALEX))
M83
Hot
young
stars
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 longer wavelength
UV, blue is “far” or shorter wavelength UV. Arrows point
to two of the spiral arm extensions discovered.)
Credits: Optical: R. Gendler; GALEX: NASA/JPLCaltech.
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. Ultraviolet-bright,
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.
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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 Ultraviolet. These stars are thus
prominent in GALEX UV images.
HST Resolution
GALEX Resolution
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.
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The Big Picture
Visible Light
Ultraviolet
Image
Image
(GALEX)
(SDSS)
Hot young
stars
The spiral galaxy NGC4656, 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. Credits: SDSS/NASA/JPL-Caltech/L.
Bianchi.
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 mostly assembled 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.
But GALEX has found populations of
young, hot stars in these extended
reservoirs around some spiral galaxies.
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How Does this Change our View?
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.
M83 in three views: in visible light, in GALEX’s UV image, and
enhanced to illustrate the distribution of its extended disk of lowdensity gas where the new young stars were found. Credits: R.
Gendler/NASA/JPL-Caltech/NRAO/AUI/NSF/MPIA.
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!
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Resources
First Press release / Image releases
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:
Thilker, D. et al. 2005 ,Astrophysical Journal, 619, L79.
..
Gil de Paz, A., et al. 2005 Astrophysical Journal Letters, 627, L29
First comprehensive paper on the "Extended UV Disks”:
Thilker et al. 2007, Astrophysical Journal Suppl., 173, 538.
GALEX/SDSS Images:
.
Bianchi, L., 2011, Astrophys. Space Sci., 335, 51.
A recent review:
Bianchi, L. 2015, in upcoming book "From the Realm of the Nebulae to the
Society of Galaxies”, Springer, in press.
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Young Galaxies Grow
BONUS CONTENT
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Bonus Content
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.
Hot young stars
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 NGC0404 in the center of the image.
Credits: NASA/JPL-Caltech/DSS.
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