A101 Slide Set:

Young Galaxies Grow

• Developed by the GALEX Team

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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-the-higher-education-audience/

The Discovery

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M83

Visible Light ImageUltraviolet 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 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.

Hot

young

stars

How was the Discovery Made?

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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 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.

The Big Picture

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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 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.

How Does this Change our View?

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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!

M83 in three views: in visible light, in GALEX’s 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.

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.

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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