A101 Slide Set: Young Galaxies Grow

Transcription

1 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 cation/resources-for-the-highereducation-audience/ 1

2 The Discovery Visible Ultraviolet Light Image Image (GALEX)) M83 Hot young stars 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. 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. 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. 2

3 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. 3

4 The Big Picture Visible Ultraviolet Light 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. 4

5 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! 5

6 Resources First Press release / Image releases Scientific articles First discovery papers: Thilker, D. et al. 2005,Astrophysical Journal, 619, L79.. Ġil de Paz, A., et al 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 Young Galaxies Grow BONUS CONTENT 7

8 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. 6