ASTRO 530: Galaxies Prof. Jeff Kenney

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1 ASTRO 530: Galaxies Prof. Jeff Kenney HI gas stars CLASS 14 October 26, 2015 ISM & HI in Galaxies 1

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?

2 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 equaqon derives from the quantum mechanics of the hyperfine transiqon and the assumpqon that the HI gas is opqcally thin. Mass depends on Distance squared Line flux is observed (integral over all velociqes) Flux density S(v) or F ν (janskys) 1 jansky = W m - 2 = erg s - 1 cm - 2

3 DetecQng 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

4 Neutral Hydrogen (HI): λ=21cm spectral line from spin- flip transiqon An H atom in upper energy state (parallel spins) will spontaneously change to lower energy state (anq- parallel spins), emifng a photon with λ=21cm in the process

orbital level Electron in ground state orbital level F=1 F=0

5 21cm HI spectral line from spin- flip transiqon 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

6 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

7 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 Qme much shorter than Qme it takes to emit 21cm photon) (very different from Hα spectral line (WIM) or X- Ray conqnuum emission (HIM), whose intensity depends on density squared)

The 21cm HI line is forbidden forbidden: collision Qme << spontaneous emission Qme Average excited H atom takes ~11 Myr to (spontaneously) make hyperfine transiqon which produces 21cm photon.

8 The 21cm HI line is forbidden forbidden: collision Qme << spontaneous emission Qme Average excited H atom takes ~11 Myr to (spontaneously) make hyperfine transiqon which produces 21cm photon. (This is average Qme for any 1 atom, the Qme will be different - - staqsqcal process) Einstein A coefficient for spontaneous emission of 21cm photon Collision Qme for H atoms in ISM is ~1000 yrs much shorter than 11 Myr. So collisions determine populaqon of energy levels. Most H atoms in upper energy level are collisionally de- excited before they can emit a photon. But a small frac0on 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

9 equaqon of radiaqve transfer (τ ν ) 0 I ν (0) I ν (τ ν )

10 equaqon of radiaqve transfer (τ ν ) 0 S ν (T) = in thermodynamic equilibrium, the source function = Planck intensity for low frequencies hν<<kt, use Rayleigh-Jeans limit of Planck intensity radio astronomers often express surface brightness I ν in terms of brightness temperature T b T b (τ ν ) = T b (0) T S

11 what is temperature in this equaqon? relevant fact is the number of parqcles in the upper and lower states this can be expressed in terms of an excitaqon temperature T ex or spin temperature T s even if gas not in TE

12 The Excita:on Temperature (T ex ) or spin temperature (T s ) is defined for a populaqon of parqcles via the Boltzmann factor. It saqsfies where n u and n l represent the number of parqcles in an upper (e.g. excited) and lower (e.g. ground) state, and g u and g l their staqsqcal weights respecqvely. Thus the excitaqon temperature is the temperature at which we would expect to find a system with this raqo of level populaqons. However it has no actual physical meaning except when in local thermodynamical equilibrium. The excitaqon temperature can even be negaqve for a system with inverted levels (such as a maser). In observaqons of the 21 cm line, the apparent value of the excitaqon temperature is ouen called the "spin temperature.

13 if opqcally thin i.e., τ << 1 and no background emission, i.e. T b (0) = 0 à T b (τ ν ) = τ ν T S but! n H so. T s dependence cancels out and T b depends only on N H = n H ds!!

14 Measuring HI column densiqes from 21cm line the HI column density N H (in atoms cm -2 ) can be determined directly from the integrated line brightness when τ << 1. N H

15 Measuring HI mass from 21cm line M H = m H N H da m H = mass of H atom A = area on sky, which depends on D =distance to galaxy

16 Measuring HI mass from 21cm line the HI mass (in M sun ) M H can be determined directly from the integrated line flux when τ << 1 This equaqon derives from the quantum mechanics of the hyperfine transiqon and the assumpqon that the HI gas is opqcally thin. Mass depends on Distance squared Line flux is observed (integral over all velociqes) NOTE: Some HI is optically thick (especially cold clouds the CNM) so the true total HI mass is higher than this formula predicts! Flux density S(v) or F ν (janskys) 1 jansky = W m - 2 = erg s - 1 cm - 2

17 cold vs. opqcally thin HI revealed by comparing HI absorpqon vs. emission spectra mostly CNM both WNM & CNM HI absorption spectra are very sensitive to temperature (shows more cold HI) HI emission spectra are ~uniformly sensitive to optically thin HI at all temps (not temp sensitive) HI absorption + emission spectra toward/ right next to source Dickey+83 some peaks seen in both abs + em optically thin HI some peaks only in absorption cold HI

18 2 different phases of HI Cold Neutral Medium (CNM) T~ K, n~50 cm - 3, opqcally thick, small filling factor small cold clouds (undercounted in emission) Warm Neutral Medium (WNM) T~7000 K, n~0.5 cm - 3, opqcally thin, large filling factor intercloud medium relaqon between HI emission intensity and HI column density & mass assume opqcally thin HI but some HI (esp CNM) is opqcally thick so we are undercounqng it!

19 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 = verqcal scale height f vol = volume filling factor f mass = ISM mass fracqon * HI phases Values given are representaqve 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

20 MAJOR PHASES OF ISM IN MILKY WAY PHASE OF ISM descrip:on 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 conqnuum emission Hα+other recombinaqon lines; radio conqnuum emission HI in emission HI in absorpqon Molecular medium (MM) Cold, dense, gravitaqonally bound clouds CO and other emission lines Ref: Brinks 1990; Boulanger & Cox 1990; Kulkarni & Heiles 1988

21 Key ISM facts Molecular medium mostly in self- gravitaqng discrete clouds (but also some diffuse non- self- gravitaqng molecular gas) Rest of phases in global pressure equilibrium (P=knT; compare product of n and T) Most of gas mass in disk galaxies in MM+CNM+WNM - > observe with HI, CO Most of gas mass in luminous E s in HIM - > observe with X- Rays WIM not much gas mass, but good tracer of ongoing star formaqon

22 Surface brightness & flux: resolved and unresolved sources

23 Surface brightness & flux: resolved and unresolved sources

24 Surface brightness & flux: resolved and unresolved sources

25 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

26 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/ArgenQne/Bonn GalacQc HI Survey) Map shows enqre sky! 180 o 180 o A disk component (not bulge) but some HI clouds extend up into the halo

27 HI intensity = HI surface density OpQcal starlight HI gas vs. opqcal in NGC 3521 HI velocity HI velocity dispersion HI associated with disk, not with bulge Walter etal 2008 THINGS VLA

28 HI vs. opqcal in NGC 2403 HI more extended than stars Walter etal 2008 THINGS

29 HI vs. opqcal in NGC 2841 HI more extended than stars HI distribuqon in outer disk, beyond most of stars, is ouen warped Walter etal 2008 THINGS

Total HI on opqcal Warped HI disk in Sb NGC 4013 2 HI velocity channels on opqcal making

of galaxy is almost all gas, very few stars - > star formaqon inefficient in part of disk

30 Total HI on opqcal Warped HI disk in Sb NGC HI velocity channels on opqcal making a sort of 2D slice through 3D warp Bo~ema 1995, 1996 Outer disk highly warped Warped part of galaxy is almost all gas, very few stars - > star formaqon inefficient in part of disk that is warped Warps: not well understood, may result from gas accreqon or Qdal interacqon

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

32 OpQcal vs. HI in Sc M101 to same scale HI much more extended than opqcal - > Gas inefficient in forming stars in outer parts of galaxy Strong spiral structure in gas Outer disk lopsided probably due to recent accreqon Kamphuis etal 1991

33 Holes in HI disks numerous small holes large 1- sided holes straight arm segments can be caused by Qdal interacqon Holes in HI distribuqons in some galaxies Walter etal 2008 THINGS Due to energy from star formaqon (supernovae and OB stellar winds) or accreqon/minor mergers gas punching through the disk

HI holes in M101 disk: 2- sided superbubble 2-sided HI bubble PV diagram through bubble Kamphuis+91 WRST HI HI contours on optical SN 1951h center of HI bubble expanding HI shell

34 HI holes in M101 disk: 2- sided superbubble 2-sided HI bubble PV diagram through bubble Kamphuis+91 WRST HI HI contours on optical SN 1951h center of HI bubble expanding HI shell associated with HI hole in spiral arm (2- sided kinemaqcally) d~1.5 kpc, M~3x10 7 M sun, v exp ~50 km/s KE~10 53 erg ~ 1000 SN due to energy from star formaqon (supernovae and OB stellar winds)

35 HI Hα Milky Way superbubble 7 kpc away 3 kpc high HI GBT, NRAO website

36 HI holes in M101 disk: large 1- sided hole area of disturbed HI PV diagram along red line M~10 8 M sun, v out ~150 km/s, KE~10 54 erg ~ 10,000 SN high- velocity HI associated with disturbed part of spiral arm (disturbed HI is kinemaqcally 1- sided) due to accreqon/minor merger? van der Hulst & Sancisi 1988

37 HI radial distribuqons Log Gas Mass Density in Msun/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 & Young 1989 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

HI radial distribuqons Log Gas Mass Density 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

38 HI radial distribuqons Log Gas Mass Density 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

39 HI radial distribuqons Sb spiral NGC 4192 Sc spiral M101 Log Gas Mass Density in Msun/pc^2 or Intensity Kenney & Young 1989 Kenney etal 1991 Radius (arcmin) 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 in many (but not all) galaxies

40 HI in Magellanic Clouds Evidence of Qdal interacqons much clearer in HI than stars because of larger extent of HI

41 NGC 5195 NGC 5194 Nearby classic spiral grand design galaxy M51 (=NGC 5194) is interacqng with another galaxy

42 Shallow opqcal image Deep opqcal image HI in Qdally interacqng galaxy pair M51/NGC 5195 HI map on opqcal HI map Evidence of Qdal interacqons much clearer in HI than stars because of larger extent of HI

43 Leo Triplet nearby small galaxy group

44 Leo Triplet HI in Loose groups Evidence of Qdal interacqons much clearer in HI than stars because of larger extent of HI Haynes, Giovanelli Arecibo telescope observaqons

45 CorrelaQon between HI mass and opqcal diameter S0- S0/a Sa- Sab Sb Sbc Sc Scd- Sm Irr Haynes & Giovanelli 1984 Good correlaqon between HI mass and opqcal diameter for Sa- Sm z~0 non- cluster spirals (1σ sca~er: factor of 2) Be~er correlaqon with opqcal diameter than luminosity, since HI not associated with bulges Disks of such galaxies are at similar evoluqonary state, in converqng gas into stars Typical values: Sc M(HI)/M(gas+stars) ~8% Sa M(HI)/M(gas+stars) ~4% Higher values for LSB & dwarf galaxies Higher values for z>0 galaxies Lower values for cluster galaxies

46 HI Deficiency in Clusters Use good correlaqon 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, but extends far out into outskirts, large range at all radii Solanes etal 2000

47 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

Cluster spirals with ram- pressure stripped HI gas tails Figure X. Virgo galaxies with long one- sided HI tails poinqng roughly away from M87 at cluster center.

48 Cluster spirals with ram- pressure stripped HI gas tails Figure X. Virgo galaxies with long one- sided HI tails poinqng roughly away from M87 at cluster center. Large panel shows X- Ray map of cluster, with galaxy tail direcqons indicated by arrows. Small panels show HI contour maps on greyscale opqcal images of galaxies. ProperQes suggest gas stripping during the galaxies first infall into the cluster. (Chung etal 2007).

49 ~100 kpc long ram pressure stripped tail of HI gas in cluster galaxy NGC 4388 (& M86) in Virgo cluster HI tail: WRST Oosterloo & van Gorkom 2005

Virgo spirals NGC 4522 HI on opqcal Ram pressure stripping of HI in cluster

stripped Was stripped In many cluster spirals, HI gas is LESS EXTENDED than stars

rps: gas not stars, gas removed from outside in, one- sided gas tails important

50 Virgo spirals NGC 4522 HI on opqcal Ram pressure stripping of HI in cluster spirals NGC 4330 Abramson etal 2011 Kenney etal 2004 IC3392 Chung etal 2009 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 DiagnosQcs for rps: gas not stars, gas removed from outside in, one- sided gas tails important star formaqon quenching mechanism in clusters: Changes spirals into earlier type spirals and S0 s

51 HI distribuqons in EllipQcals E s NGC 5903 & NGC 5898 guess the HI distribuqon in these 2 ellipqcals Van Gorkom etal 1986

52 Peculiar HI distribuqons in EllipQcals accreted gas? E s NGC 5903 & NGC 5898 NGC 1052 Van Gorkom etal 1986 Van Gorkom etal ellipqcals: one with no HI, the other with a highly disturbed HI distribuqon

53 HI mass : early types vs. spirals Serra etal 2012 ATLAS 3D HI Serra etal 2012 sensiqve HI survey of early type galaxies 166 nearby (D<42 Mpc) E+S0 galaxies M K > Reasonable correlaqon between HI mass and opqcal luminosity for spirals, but poor correlaqon for E s & SO s Most Es & S0 s have li~le HI (undetected in most) but some have a lot HI gas in most E s & S0 s not closely related to stars - - consistent with external origin of HI in most E s and S0 s If disk galaxy has too li~le HI, it will likely be classified as S0 rather than spiral

HI surface density: early types vs. spirals ATLAS 3D HI Serra etal 2012 166 nearby (D<42 Mpc) early type galaxies (E + S0) M K >- 21.

54 HI surface density: early types vs. spirals ATLAS 3D HI Serra etal nearby (D<42 Mpc) early type galaxies (E + S0) M K > SensiQvity M HI = 5x10 6-5x10 7 M sun HI surface density generally much lower in E+S0 than spirals Low gas column density likely related to weak star formaqon in E+S0 s

HI morphologies in early type galaxies Serra etal 2012 (i) D (large discs) most of the HI rotates regularly and is distributed on a disc or ring larger than the stellar body of the galaxy (in half of

55 HI morphologies in early type galaxies Serra etal 2012 (i) D (large discs) most of the HI rotates regularly and is distributed on a disc or ring larger than the stellar body of the galaxy (in half of these, HI is misaligned with stars) (47% of detecqons). (ii) d (small discs) most of the HI rotates regularly and is distributed on a small disc confined within the stellar body of the galaxy (aligned with stars in most cases) (18% of detecqons). (iii) u (unse~led) most of the HI exhibits unse~led morphology (e.g. tails or streams of gas) and kinemaqcs (25% of detecqons). (iv) c (clouds) the HI is found in small, sca~ered clouds around the host galaxy (10% of detecqons).

56 HI distribuqons in early type galaxies: low density Serra etal 2012 At lower local galaxy densiqes, HI detecqons are more common and HI masses are larger. Ongoing (gentle?) accreqon more likely. HI distribuqons are more regular indicaqng that recent large interacqons are less common.

57 HI distribuqons in early type galaxies: high density Serra etal 2012 At higher local galaxy densiqes, HI generally not detected (ram pressure stripping and/or less accreqon), but if detected HI distribuqons are more irregular indicaqng recent interacqons

58 HI mass vs. local density for ETGs Large- scale density Local density Serra etal 2012 ATLAS3D Σ 10 and Σ 3 ; these are defined as the number density of galaxies contained within a 600- km s 1 - deep cylinder whose radius is equal to the distance from the tenth and third closest galaxy, respecqvely 40% of non- cluster ETGs galaxies detected in HI Only 10% of Virgo cluster ETGs detected in HI M(HI)/L K varies gradually with large- scale environmental density

60 HI absorpqon in halos of ETGs Lyα Thom etal 2012 Neutral HI in galaxy halos can absorb Lyman UV photons from background quasars, producing Lyα (& other) absorpqon lines (Lyα: n=1- >2 for Hydrogen) can detect MUCH lower HI column densiqes from absorpqon than emission!

61 HI absorpqon in halos of ETGs Thom etal 2012 Blue: late type galaxies Red: early type galaxies shows that low density neutral gas is common in galaxy halos out to ~ kpc HI absorpqon from column densiqes > cm - 2 common in halos of ETGs

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

HI gas stars ASTRO 310: Galac/c & Extragalac/c Astronomy Prof. Jeff Kenney Class 18 November 5, 2018 HI Gas in Spiral Galaxies HI in galaxies major mass component of ISM (there is also horer and colder