ASTRO 310: Galac/c & Extragalac/c Astronomy Prof. Jeff Kenney

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1 HI gas stars ASTRO 310: Galac/c & Extragalac/c Astronomy Prof. Jeff Kenney Class 18 November 5, 2018 HI Gas in Spiral Galaxies

2 HI in galaxies major mass component of ISM (there is also horer and colder gas) large part of gaseous reservoir of material for star forma/on (also H 2 ) Can detect it far from galaxy centers - - good for tracing orbital mo/ons and measuring total masses of galaxies - - leading to evidence for dark ma.er in galaxies in gas we see clear evidence for interac4ons - - both gravita/onal and ram pressure stripping

3 MAJOR PHASES OF ISM IN MILKY WAY PHASE OF ISM T(K) n (cm - 3 ) h (pc) f vol f mass Hot ionized medium (HIM) * * Warm ionized medium (WIM) Warm neutral medium (WNM) Cold neutral medium (CNM) Molecular medium (MM) T = temperature N = volume density h = ver/cal scale height f vol = volume filling factor f mass = ISM mass frac/on * HI phases Values given are representa/ve values for Milky Way. h and f vol are values within a few kpc of solar neighborhood; the inner ~kpc and outer galaxy are different. Ref: Brinks 1990; Boulanger & Cox 1990; Kulkarni & Heiles 1988

4 MAJOR PHASES OF ISM IN MILKY WAY PHASE OF ISM Traced by Hot ionized medium (HIM) Warm ionized medium (WIM) Warm neutral medium (WNM) Cold neutral medium (CNM) Coronal gas produced by supernovae HII regions around massive stars & throughout ISM Diffuse clouds & envelopes around molecular clouds Dense sheets & filaments; envelopes around molecular clouds X- Ray con/nuum emission Hα+other recombina/on lines; radio con/nuum emission HI in emission HI in emission & absorp/on * * Molecular medium (MM) Cold, dense, gravita/onally bound clouds CO and other emission lines * HI phases Ref: Brinks 1990; Boulanger & Cox 1990; Kulkarni & Heiles 1988

5 Milky Way in stars (NIR) and neutral gas (HI) Milky way galaxy in infrared (1.2, 1.6, 2.2µm)(J H K)(2MASS) Milky Way in HI (The Leiden/Argen/ne/Bonn Galac/c HI Survey) Map shows en/re sky! 180 o 180 o A disk component (not bulge) but some HI clouds extend up into the halo

6 Detec/ng atomic hydrogen with spectral line of HI H is most abundant element in universe (~74% by mass) I in HI means atom has all its electrons (only 1 in case of H) p e Electron in lowest (n=1) orbit

7 Neutral Hydrogen (HI): λ=21cm spectral line from spin- flip transi/on An H atom in upper energy state (parallel spins) will spontaneously change to lower energy state (an/- parallel spins), emirng a photon with λ=21cm in the process

8 21cm HI spectral line from spin- flip transi/on of neutral Hydrogen Hydrogen energy level diagram Electron in 2 nd orbital level Electron in ground state orbital level F=1 F=0 Electron in ground state orbital level is split into 2 hyperfine levels

9 Why 21cm HI line is important 1. It is easy to excite upper energy level Upper energy level is only E = 5.9x10-6 ev or T = E/k = 0.07 K above ground state!

10 Why 21cm HI line is important 1. It is easy to excite upper energy level Upper energy level is only E = 5.9x10-6 ev or T = E/k = 0.07 K above ground state! Tiny amount of energy required to excite upper state it is easy to do this with low energy collisions, which happen almost anywhere in universe!

11 Why 21cm HI line is important 1. It is easy to excite upper energy level Upper energy level is only E = 5.9x10-6 ev or T = E/k = 0.07 K above ground state! Tiny amount of energy required to excite upper state it is easy to do this with low energy collisions, which happen almost anywhere in universe! 21cm HI line is excellent probe of neutral hydrogen gas throughout universe

12 Why 21cm HI line is important 2. Intensity is good measure of amount of gas Intensity depends (mostly) on number of H atoms along the line of sight so it s a good measure of the mass of neutral Hydrogen along the line of sight

13 compare Hα recombina/on line from HII regions with 21cm line (=HI line) from HI regions

14 how well does Hα emission trace the mass of hydrogen gas? A. the number of Hα photons is propor/onal to the total gas mass B. the number of Hα photons is propor/onal to the ionized gas mass C. the number of Hα photons depends on the ionized gas mass and the gas density D. the number of Hα photons depends on the ionized gas mass and the gas density and the temperature

15 how well does Hα emission trace the mass of hydrogen gas? A. the number of Hα photons is propor/onal to the total gas mass B. the number of Hα photons is propor/onal to the ionized gas mass C. the number of Hα photons depends on the ionized gas mass and the gas density D. the number of Hα photons depends on the ionized gas mass and the gas density and the temperature

16 Imagine galaxy disk filled with neutral hydrogen (HI), uniformly distributed There are newly formed stars ( ) within the gas disk, each with enough ionizing radia/on to ionize a lot of the surrounding gas This makes small regions of ionized gas around each of the massive stars: HII Regions HI HII

17 recall Homework #4 M HII = 126 M sun r* = 1.07 pc M HII = 12.6 M sun r* = 0.23 pc

18 but it means we can t use the number of Hα photons to trace the amount of ionized Hydrogen (unless we somehow know the density & temperature, which are hard to know)!

19 Why is 21cm HI line a good tracer of neutral hydrogen gas mass, but the Hα line a poor tracer of ionized hydrogen gas mass?

20 Hα : collision /me vs. spontaneous emission /me t coll (=t rec ) ~ 10 4 yr t em (=A - 1 ) ~ 10-8 sec depends on density & temperature. this is typical value for HII region. t em <<t coll Einstein A coefficient for spontaneous emission since t em <<t coll, atom in excited state will emit Hα photon before it has another collision frac/on of atoms that produce photons = frac/on of atoms in excited state, which depends on photoioniza4on rate & par4cle density and temperature more photoioniza/ons & more collisions (recombina/ons) à more Hα photons

21 HΙ : collision /me vs. spontaneous emission /me t coll ~ 10 3 yr t em (=A - 1 ) ~ 10 7 yr t em >>t coll since t em >>t coll, the vast majority of atoms in excited state will have another collision before it emits HΙ photon frac/on of atoms that produce photons = frac/on of atoms in excited state, doesn t depend on photons or par6cle density or temperature more collisions à same # HΙ photons

22 The 21cm HI line is forbidden forbidden: collision /me << spontaneous emission /me Average excited H atom takes ~11 Myr to (spontaneously) make hyperfine transi/on which produces 21cm photon. (This is average /me for any 1 atom, the /me will be different - - sta/s/cal process) Einstein A coefficient for spontaneous emission of 21cm photon Collision /me for H atoms in ISM is ~1000 yrs much shorter than 11 Myr. So collisions determine popula/on of energy levels. Most H atoms in upper energy level are collisionally de- excited before they can emit a photon. But a small frac4on of the excited H atoms manage to emit a photon before they have a collision. And this is enough to produce a 21cm emission line from a gas cloud that is strong enough to detect, since there are so many atoms in the gas cloud the number of HI photons produced is propor/onal to the number of H atoms in excited state = ¾ number of H atoms

23 Why 21cm HI line is important 2. Intensity is good measure of amount of gas Intensity depends (mostly) on number of H atoms along the line of sight so it s a good measure of the mass of neutral Hydrogen along the line of sight Intensity doesn t depend on gas density (collision /me much shorter than /me it takes to emit 21cm photon) (very different from Hα spectral line (WIM) or X- Ray con/nuum emission (HIM), whose intensity depends on density)

24 Measuring HI mass from 21cm line HI is major mass component of ISM: How can we measure the mass in this phase? What assumptions does this rely on? How well does this work? This equa/on derives from the quantum mechanics of the hyperfine transi/on and the assump/on that the HI gas is op/cally thin. Mass depends on Distance squared Line flux is observed (integral over all veloci/es) Flux density S(v) or F ν (janskys) 1 jansky = W m - 2 = erg s - 1 cm - 2

25 HI map and op/cal image of inclined spiral galaxy observe λ=21cm spectral line of with radio telescope

26 measuring rota/on veloci/es in spiral galaxy using doppler shis of spectral lines moving toward v rot v rot moving away λ ο λ HI in disk of inclined, rotating spiral galaxy

27 measuring rota/on veloci/es in spiral galaxy using doppler shis of spectral lines moving toward v rot v rot moving away λ λ blue λ ο

28 measuring rota/on veloci/es in spiral galaxy using doppler shis of spectral lines moving toward v rot v rot moving away λ ο λ red λ

29 measuring rota/on veloci/es in spiral galaxy using doppler shis of spectral lines moving toward (blueshied) v rot v rot moving away (redshied) Δλ Δλ λ blue λ ο λ red λ

30 HI surface density map of spiral galaxy NGC 2903 (color à intensity = amount of HI gas) Line intensity (flux) gives HI surface density Σ HI (HI mass per area)

31 Approaching side blueshied Receding side redshied HI velocity map of spiral galaxy NGC 2903 Color= velocity of HI gas λo λ-λο λ Wavelength shi gives velocity by Doppler shi (λ-λο)/λ = v/c

32 Δλ Linewidth gives velocity dispersion by Doppler shi Δλ/λ = Δv/c

33 HI intensity = HI surface density Op/cal starlight HI gas vs. op/cal in NGC 3521 HI velocity HI velocity dispersion HI associated with disk, not with bulge Walter etal 2008 THINGS VLA

34 2 most important radio telescopes for HI studies of galaxies Very Large Array (VLA) New Mexico, US 27 dishes with 25m diameter interferometer Arecibo Telescope Puerto Rico 305m diameter Single dish

35 Sc galaxy M101 M101 is one of largest nearby spirals Sandage & Bedke

36 Op/cal vs. HI in Sc M101 to same scale op/cal (stars) HI (gas) HI much more extended than op/cal Kamphuis etal 1991

37 Why is HI gas more extended than stars in spiral galaxies? A. gas blown outwards by supernovae B. star forma/on inefficient in outer galaxy C. gas supported also by magne/c fields D. stars serle to center via mass segrega/on E. gravita/onal force is weak on small gas par/cles

38 Why is HI gas more extended than stars in spiral galaxies? A. gas blown outwards by supernovae B. star forma/on inefficient in outer galaxy C. gas supported also by magne/c fields D. stars serle to center via mass segrega/on E. gravita/onal force is weak on small gas par/cles

39 Op/cal vs. HI in Sc M101 to same scale op/cal (stars) HI (gas) HI much more extended than op/cal - > Gas inefficient in forming stars in outer parts of galaxy Kamphuis etal 1991

40 Op/cal vs. HI in Sc M101 to same scale op/cal (stars) HI (gas) HI much more extended than op/cal - > Gas inefficient in forming stars in outer parts of galaxy Strong spiral structure in gas - > need gas to form spiral structure Kamphuis etal 1991

41 Op/cal vs. HI in Sc M101 to same scale op/cal (stars) HI (gas) HI much more extended than op/cal - > Gas inefficient in forming stars in outer parts of galaxy Strong spiral structure in gas - > need gas to form spiral structure Outer disk lopsided > probably due to recent accre/on Kamphuis etal 1991

42 Holes in HI disks Walter etal 2008 THINGS Holes in HI distribu/ons in some galaxies Due to energy from star forma/on (supernovae and OB stellar winds) or accre/on/minor mergers gas punching through the disk

43 Holes in HI disks numerous small holes large 1- sided holes straight arm segments can be caused by /dal interac/on Walter etal 2008 THINGS Holes in HI distribu/ons in some galaxies Due to energy from star forma/on (supernovae and OB stellar winds) or accre/on/minor mergers gas punching through the disk

44 HI vs. op/cal in NGC 2841 HI more extended than stars HI distribu/on in outer disk, beyond most of stars, is oen warped Walter etal 2008 THINGS

45 Total HI on op/cal Warped HI disk in Sb NGC HI velocity channels on op/cal making a sort of 2D slice through 3D warp BoRema 1995, 1996 Outer disk highly warped Warped part of galaxy is almost all gas, very few stars - > star forma/on inefficient in part of disk that is warped Warps: not well understood, may result from gas accre/on or /dal interac/on

46 HI radial distribu/ons Gas Mass Density Σ in M sun /pc 2 or Intensity R band stars CO molecular gas 60 µm heated dust/ star formation Sc spiral M101 Hα --ionized gas/ star formation Kenney etal 1991 HI Radius (arcmin) Intensity of most things increase toward galaxy center but not HI Central HI hole or depression or plateau filled in with molecular gas in many (but not all) galaxies

47 HI radial distribu/ons Log Gas Mass Density Σ in M sun pc - 2 or Intensity H 2 HI Sb spiral NGC 4192 Kenney & Young 1989 Starlight (B) Radius (arcmin) HI much more extended than stars (& nearly all other baryonic things) in non- cluster spirals Central HI hole or depression or plateau filled in with molecular gas (H 2 ) in many (but not all) galaxies

48 NGC 5195 (M51b) NGC 5194 (M51a) Nearby M51 system is classic grand design spiral galaxy M51 (=NGC 5194) interac/ng with another galaxy NGC 5195

49 Shallow op/cal image Deep op/cal image HI in /dally interac/ng galaxy pair M51a+b HI map on op/cal HI map Evidence of /dal interac/ons much clearer in HI than stars because of larger extent of HI

50 Leo Triplet nearby small galaxy group

51 Leo Triplet HI in Loose groups Evidence of /dal interac/ons much clearer in HI than stars because of larger extent of HI Haynes, Giovanelli Arecibo telescope observa/ons

52 Correla/on between HI mass and stellar mass (or op/cal diameter) S0- S0/a Sa- Sab Sb Sbc Sc Scd- Sm Irr Haynes & Giovanelli 1984 Good between HI mass and diameter (or stellar luminosity or stellar mass ) for non- cluster spirals at z~0 (1σ scarer: factor of 2) Disks of such galaxies are at similar evolu4onary state, in conver4ng gas into stars Typical values: Sc M(HI)/M(gas+stars) ~8% Sa M(HI)/M(gas+stars) ~4% Higher values for low mass & LSB galaxies Higher values for z>0 galaxies Lower values for cluster galaxies

53 HI Deficiency in Clusters Use good correla/on between M HI and D opt for isolated spirals to define HI deficient DEF HI = log M HI (expected) log M HI (actual) Many cluster spirals are deficient in HI HI deficiency highest in cluster centers Solanes etal 2000

54 HI imaging of 50 spiral & peculiar galaxies in Virgo Cluster VIVA survey (Chung etal 2007,2009) VLA HI maps on X- ray HI maps blown up by 10x

55 Ram pressure stripped tail of HI gas in cluster galaxy NGC 4388 (& M86) in Virgo cluster HI tail: WRST Oosterloo & van Gorkom 2005 HI in galaxy: VLA VIVA Chung etal 2009

56 Cluster spirals with ram- pressure stripped HI gas tails Virgo galaxies with long one- sided HI tails poin/ng roughly away from M87 at cluster center. Large panel shows X- Ray map of cluster, with galaxy tail direc/ons indicated by arrows. Small panels show HI contour maps on greyscale op/cal images of galaxies. Proper/es suggest gas stripping during the galaxies first infall into the cluster. (Chung etal 2007).

57 Ram pressure stripping of galaxy gas Galaxies moving through gas (from ICM or IGM or ISM of other galaxy) will experience a ram pressure force which may push the ISM gas out of the galaxy. Gas with surface density Σ ISM may be stripped if the ram pressure ρ ICM v 2 exceeds the gravita/onal restoring force per area Σ ISM dφ/dz, or ram gravita/onal pressure > force per area ρ ICM v 2 > Σ ISM dφ/dz ρ ICM v r.p.s. is important for large & small galaxies in clusters, and for dwarf galaxies which are satellites of larger galaxies

58 Simula/on of galaxy s gas being ram pressure stripped as it orbits through cluster gas: tail extends over much of orbit (AMR, FLASH) simula/on by Elke Roedigger

59 Why ram pressure strips gas not stars Imagine star and gas cloud, each with same mass M, but star is much denser with smaller projected area A Gravita/onal force & ram pressure on each is the same, but ram pressure force is larger for big diffuse thing since it has larger area F grav vs. F ram = P ram A F ram STAR F ram GAS CLOUD M, A star M, A cloud Σ ISM = M/A cloud F grav F grav Ram pressure wind P ram = ρv 2

60 Virgo spirals NGC 4522 HI on op/cal Ram pressure stripping of HI in cluster spirals NGC 4330 Abramson etal 2011 Kenney etal 2004 IC3392 Being stripped Was stripped In many cluster spirals, HI gas is LESS EXTENDED than stars Gas in cluster spirals gets stripped by ram pressure from ICM gas, stars unaffected Changes spirals into earlier type spirals and (maybe) S0 s Chung etal 2009