A101 Slide Set: Young Galaxies Grow Developed by the GALEX Team 1 Topic: Galaxies Concepts:...
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A101 Slide Set: Young Galaxies Grow Developed by the GALEX Team 1 Topic: Galaxies Concepts: Ultraviolet observations, galaxy formation, galaxy evolution,
A101 Slide Set: Young Galaxies Grow Developed by the GALEX Team
1 Topic: Galaxies Concepts: Ultraviolet observations, galaxy
formation, galaxy evolution, young stellar populations Missions:
GALEX, HST Coordinated by the NASA Astrophysics Forum An
Instructors Guide for using the slide sets is available at the ASP
website https://www.astrosociety.org/edu
cation/resources-for-the-higher- education-audience/
https://www.astrosociety.org/edu cation/resources-for-the-higher-
education-audience/
Slide 2
The Discovery 2 M83 Visible Light Image Ultraviolet Image
(GALEX) ) 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/JPL-
Caltech. When NASAs 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 galaxys 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. Hot young stars
Slide 3
How was the Discovery Made? 3 GALEX ResolutionHST Resolution
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. When GALEXs
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.
Slide 4
The Big Picture 4 Visible Light Image (SDSS)Ultraviolet Image
(GALEX) Hot young stars 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
stars in these extended reservoirs around some spiral galaxies. 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.
Slide 5
How Does this Change our View? 5 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 starsand galaxies can increase their starry
dimensions! M83 in three views: in visible light, in GALEXs UV
image, and enhanced 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.
Slide 6
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. 5
Slide 7
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. 6 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. Hot young
stars