How Galaxies Get Their Gas. Jason Tumlinson STScI Hubble Science Briefing December 9, 2010

Transcription

1 How Galaxies Get Their Gas Jason Tumlinson STScI Hubble Science Briefing December 9, 2010

2 Astronomy asks some Big Questions... 2 How Do Galaxies Form?

3 Three Puzzles 1. Why are some galaxies blue and star-forming forming while others are red and dead? 2. How does the Milky Way fuel its ongoing star formation? 3. Are all galaxies like the MW in this respect? 3

4 4 Puzzle #1 Why are some galaxies blue and star forming while others are red and dead?

5 5 M87 - Red and Dead Elliptical M81 - Blue and Star-forming Spiral

6 From decades of work in extragalactic astronomy, we have a pretty good idea of how galaxies look in starlight. 6

7 Puzzle #1: Colors of galaxies from SDSS Redder NYU VAGC: Blanton et al (2005) Bluer Fainter Absolute Magnitude Brighter 7

8 Puzzle #1: Colors of galaxies from SDSS Redder NYU VAGC: Blanton et al (2005) Red sequence Blue Cloud Bluer Fainter Absolute Magnitude Brighter 8

9 Galaxy Transformation Download movie at this URL: Movie Credits: Frank Summers (STScI), Chris Mihos (Case) & Lars Hernquist (Harvard) 9

10 10 Solution #1 Galaxy Transformation in mergers may convert Spirals into Ellipticals, but... why do ellipticals stop forming stars?

11 11 Puzzle #2 How does the Milky Way fuel its ongoing star formation?

12 12 Two Micron All Sky Survey (2Mass) Near-Infrared

13 Axel Mellinger PASP, 121,

14 14 Leiden/Argentine/Bonn Survey of Galactic Neutral Hydrogen (21 cm) - HI MHI = 2 x 10 9 M for the whole Galaxy

15 Galactic star-forming regions add up to ~2 M per year of new stars. 15 HST Image of the Carina Nebula

16 How to think like an Astronomer... = 2 billion / 2 = 1 Gyr The Milky Way is forming stars at about 2 solar masses per year. It has about 2 billion solar masses of gas to work with. So this rate can be sustained for about 1 billion years. BUT.. it has been happening for 10 billion years! So... there is probably some external source of star-forming fuel. 16

17 17 Puzzle #2 The Milky Way likely has its ongoing star formation fueled from some outside source. But what is the source, and how does gas get to the disk?

18 18 Lyman Spitzer, a founding father of HST, proposes that there might be gas outside the Galactic disk (1956) ApJ, 124, 20 Million degree coronal extragalactic gas may exist outside galaxies and be detectable via UV absorption!

19 John Bahcall, along with Spitzer, proposes that galaxies are in fact much larger than their optical disks, and are surrounded by halos of diffuse gas out to > 100 kpc (1969). Proposed that the Lyman-alpha forest then known arose in extended galaxy halos: R = 100 kpc versus the kpc then believed based on optical and radio measurements. 19

20 Kawata & Rauch (2007) 20

21 Kawata & Rauch (2007) 21

22 The Intergalactic Medium? Download movie at this URL: At select 3-D fly-through showing Temperature Evolution Oppenheimer & Davé

23 23 Leiden/Argentine/Bonn Survey of Galactic Neutral Hydrogen

24 High-velocity clouds - is this the Galactic Fuel? Radio (neutral hydrogen gas) map of the whole sky 24

25 25 METHOD Quasar Absorption Lines Detect Weak Absorption from diffuse gas along the sightline

26 26 METHOD Quasar Absorption Lines FUSE Å delta z ~ 0.1 STIS Å Critical factors for success: Sensitivity to weak lines, Spectral resolution to resolve components, Wavelength coverage to build up total pathlength, and Access to UV wavelengths to measure physical diagnostics.

27 27 FUSE s Map of Hot Oxygen Surrounding the Milky Way (2003) June 24, 1999

28 28 FUSE s Map of Hot Oxygen Surrounding the Milky Way (2003) June 24, 1999

29 29 FUSE s Map of Hot Oxygen Surrounding the Milky Way (2003) June 24, 1999

30 Two-thirds of the sky is covered with ionized gas! 30

31 31 Solution #2 So... the Milky Way appears to obtain ionized low-metallicity gas from the IGM, which first passes through a hot, diffuse Galactic corona.

32 32 Puzzle #3 But... is this generally true of galaxies at large? And does it relate to galaxy properties in general?

33 M87 - Red and Dead Elliptical M81 - Blue and Star-forming Spiral Why do spiral galaxies continue to form stars in gas-rich disks, while ellipticals are red and dead? How do star-forming galaxies do it for billions of years without exhausting their gas? Our hypothesis: star-forming galaxies are fed by accretion of cold gas clouds through their gaseous halos, while the halos of massive galaxies destroy infalling clouds before they can enter and form stars. 33

34 Our hypothesis: star-forming galaxies are fed by accretion of cold gas clouds through their gaseous halos, while the halos of massive galaxies destroy infalling clouds before they can enter and form stars. 34 M87 - Red and Dead Elliptical M81 - Blue and Star-forming Spiral T > million K T > 500,000 - million K Why do spiral galaxies continue to form stars in gas-rich disks, while ellipticals are red and dead? How do star-forming galaxies do it for billions of years without exhausting their gas?

35 Three Puzzles Motivate New Observations 35 Gaseous Halo - what happens here? 20 kpc

36 STS125-SM4 launch May 11,

37 37 Cosmic Origins Spectrograph COS Installation May 16, 2009 STS125-SM4 launch May 11, 2009

38 HST Cycle 17 Large Program - How Galaxies Acquire their Gas Background light source (QSO) STIS data (from 2002) 39 galaxies to be observed in 134 HST orbits (13 read and dead, 26 star-forming) Absorption by galaxy halo gas 38

39 COS Spectra (1 of 39) 39

40 COS Spectra (1 of 39) LLS: Tumlinson et al. (2010, in prep) DLA: Meiring et al. (2010, in prep) HI = Hydrogen I OVI = Oxygen VI DLA = Damped Lyman-Alpha LLS = Lyman Limit System 40

41 Ionized Gas Absorption in COS Data 41 HI OVI HI OVI

42 Three Absorption Line Systems - Which Galaxies? SDSS Image (Sloan Digital Sky Survey) z = QSO z = 0.46 z = Target Galaxy z =

43 Three Absorption Line Systems - Which Galaxies? SDSS Image z = QSO z = 0.46 B1: z = 0.356? WFC3 Image B2: z=0.356? z=0.114? (DLA) z = Target Galaxy z = A: z =

44 Distance to Galaxy (kpc) 44

45 45

46 A hint that Red galaxies are different! 46

47 Redder Bluer Fainter Absolute Magnitude Brighter 47

48 Hits Misses No Coverage N = % of galaxy halos are covered with ionized gas! Very similar to the Milky Way, which has 67% 48

49 Using HST/COS, we have addressed the problem of ionized gas in galaxy halos: 1. Hot, ionized gas traced by OVI exists in L* halos at 80+/-15% coverage. Just like the MW! 2. We find that OVI covers 95 % of blue galaxies ; only 3 of 8 red galaxies show it. Thus the dichotomy of galaxy colors appears in their gaseous halos as well. 3. It appears that hot gas in galaxy halos knows about galaxy type and/or star formation. WHY? 4. We are investigating both infall and outflow physical models; this is a work in progress. 49

50 50 Solution #3 So... it appears that other galaxies, in fact just about all other galaxies, might be accreting star forming fuel just as the Milky Way does: as warm, low metallicity gas ionized by the galaxy s s stars.

51 Unfinished business (Cycle 18) 51

52 The Hubble Space Telescope... and the new Cosmic Origins Spectrograph probe the invisible low-metallicity gas flowing into galaxies to form stars, as never before.

53 Thanks to you for listening!