1. 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 Instructor’s 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/

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 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

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 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.

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.

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 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.

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

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