ASTRO 310: Galac/c & Extragalac/c Astronomy Prof. Jeff Kenney. Class 7 Sept 19, 2018 The Milky Way Galaxy: Gas: HII Regions

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1 ASTRO 310: Galac/c & Extragalac/c Astronomy Prof. Jeff Kenney Class 7 Sept 19, 2018 The Milky Way Galaxy: Gas: HII Regions

2 importance of HII regions one of main ISM phases great example for understanding physical processes in ISM par/cularly the interac/ons between par/cles and photons show how we can use observed photons (spectra) to learn about physical processes and physical condi/ons in galaxies photons from HII regions are excellent tracer of the star forma)on rate in galaxies

3 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 absorp/on Molecular medium (MM) Cold, dense, gravita/onally bound clouds CO and other emission lines Ref: Brinks 1990; Boulanger & Cox 1990; Kulkarni & Heiles 1988

4 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 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

5 The warm (T~104 K) ionized part of the ISM can be directly observed at op/cal wavelengths Many of the Messier objects (from his 1784 catalog) are gaseous nebula with op/cal light produced by emission lines. Stellar & Death Stellar Birth Due to interac/ons with newly- formed stars in regions of dense ISM (stars form from gas and then illuminate it) these are generally much brighter M16 Eagle Nebula (star- forming region) M42 Orion Nebula (star- forming region) Due to maker recently ejected from stars (stars eject gas and then illuminate it) these are generally much fainter M57 Ring Nebula (planetary nebula) M1 Crab Nebula (supernova remnant)

6 Hα emission from HII regions photons from the recombina/on of hydrogen Roseke nebula; Red is Hα emission more compact (younger) HII region Yellow/orange is Hα emission

7 Hα emission n=3- >2 recombina/on line typically occurs where gas has T~10 4 K and atoms can be ionized H atom Suppose H atom somehow gets excited and its electron is in a higher energy level (n>1) (e.g., by collision with another atom, or if it previously absorbed a photon) Aper a while the electron will spontaneously fall to a lower energy level, and the atom emits a photon as it does

8 HII regions form around newly formed massive stars, so trace an important stage in stellar evolu/on

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10 possible 3D view of HII region IC410 HST op/cal image 3D apparent parallax simula/on based on educated guess of 3D structure from earthsky.org

11 Milky Way in Hα Vela SNR Galaxy center SMC LMC Orion Mosaic of con/nuum- subtracted Hα images from the WHAM & SHASSA surveys most compact Hα sources are HII regions Finkbeiner 2003

12 Andromeda Galaxy M31 what wavelengths/filters are used to make this image?

13 Andromeda Galaxy M31 broadband op/cal image + Hα (red)

14 Andromeda Galaxy M31 most Hα in M31 arises from gas photoionized by hot massive stars (HII regions) Hα image young stars more spa/ally concentrated than old stars

15 Hα photons from HII regions are one of best quan,ta,ve tracers of the star forma)on rate in galaxies M51 in op/cal

16 Hα photons from HII regions are one of best quan,ta,ve tracers of the star forma)on rate in galaxies M51 in op/cal Hα traces star- forming regions in spiral arms M51 in Hα most Hα in M51 arises from gas photoionized by hot massive stars (HII regions)

17 nearby starburst galaxy M82 op/cal broadband image

18 nearby starburst galaxy M82 op/cal broadband image op/cal broadband image plus Hα image (red) Hα above disk arises from shock- excited gas in starburst ourlow

19 Broadband image showing stars

20 Broadband image showing stars Narrowband image showing Hα gas Hα shows shock- excited gas from galaxy- galaxy collision Kenney etal (2008)

21 DISCOVER Blogs / Bad Astronomy Galactic tentacles of DOOM October 7th, :37 PM by Phil Plait in Astronomy, Cool stuff, DeathfromtheSkies!, Pretty pictures 34 comments RSS feed Trackback >

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23 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

24 An idealized HII region: Stromgren sphere ionizing (UV) photons emiked by central star Assume: Pure H No dust Uniform density - > sphere Single star No evolu/on à steady- state solu/on In reality neither region is purely neutral or purely ionized, but a mix of both. In HII regions a large frac/on (>99%) of the gas ionized.

25 HII region what is the II in HII? HI neutral (atomic) Hydrogen HII singly ionized Hydrogen

26 HII region with photoioniza,on & recombina,on HII region (ionized) HI region (neutral) UV photons (E>13.6 ev, λ<912a= µm) ionize H atoms, freeing electrons which subsequently recombine with some other H ion, producing recombina/on photons

27 Stromgren sphere: Steady- state solu/on photoioniza/on vs. recombina/on H atoms become ionized by absorbing ionizing UV photons (E>13.6 ev, λ<912a= µm) radiated by hot central star H + hν -> H + + e - e s and p s then undergo radia/ve recombina/on H + + e - - > H + hν in steady state, every photoioniza)on is balanced by recombina)on

28 Hα emission n=3- >2 recombina/on line typically occurs where gas has T~10 4 K and atoms can be ionized H atom Suppose H atom somehow gets excited and its electron is in a higher energy level (n>1) (e.g., by collision with another atom, or if it previously absorbed a photon) Aper a while the electron will spontaneously fall to a lower energy level, and the atom emits a photon as it does

29 Lyman con/nuum UV photons (E>13.6 ev, λ< µm) ionize H atoms (n=1-> free) recombina/on Photoioniza/on & recombina/on in H Hα Hα photon n=3- >2 λ= µm in red part of visible Electronic energy levels of Hydrogen

30 Stromgren sphere is idealized, steady- state model of HII region Steady- state: equate photoioniza,on rate to recombina,on rate

31 what determines photoioniza/on rate?

32 Stromgren sphere is idealized, steady- state model of HII region Steady- state: equate photoioniza,on rate to recombina,on rate photoioniza/on rate S * = # photoioniza/ons/sec S * # ionizing photons emiked per sec by star this is oversimplifica/on rate actually depends somewhat on energy of the photons star with L,T depends on T and L of star S* ionizing photons per sec the ioniza/on rate open called Q o by astronomers i.e., S * = Q o

33 Stromgren sphere is idealized, steady- state model of HII region Steady- state: equate photoioniza,on rate to recombina,on rate recombina/on rate = # electron recombina/ons /sec = (- dn e /dt) V HII V HII electron recombina/on rate per volume (# electron s - 1 cm - 3 ) volume of ionized region (cm 3 ) nega/ve, since when recombina/ons happen, the number of e s decrease this is change due to recombina/ons only, the TOTAL dn e /dt =0 in steady state!

34 Stromgren sphere is idealized, steady- state model of HII region Steady- state: equate photoioniza,on rate to recombina,on rate photoioniza/on rate = recombina/on rate S * = V HII (- dn e /dt) (# s - 1 ) (cm 3 ) ( # s - 1 cm - 3 ) assume ionized region extends to r * V HII = (4π/3) r * 3 S * = (4π/3) r * 3 (- dn e /dt) but what is recombina,on rate per volume?

35 what does recombina/on rate per volume dn e /dt depend upon?

36 how quickly does ionized hydrogen revert to neutral state? what is recombina/on rate dn e /dt? what does it depend on?

37 how quickly does ionized hydrogen revert to neutral state? what is recombina/on rate dn e /dt? what does it depend on? collision rate x probability of recombina/on for each collision

38 how quickly does ionized hydrogen revert to neutral state? what is recombina/on rate dn e /dt? what does it depend on? collision rate x probability of recombina/on for each collision collision rate depends on density of each colliding par/cle (n e and n p ) and how open they encounter each other à collision speed, or temperature

39 how quickly does ionized hydrogen revert to neutral state? what is recombina/on rate dn e /dt? what does it depend on? collision rate x probability of recombina/on for each collision collision rate depends on density of each colliding par/cle (n e and n p ) and how open they encounter each other à collision speed, or temperature separate out density dependence from temp dependence

40 how quickly does ionized hydrogen revert to neutral state? what is recombina/on rate dn e /dt? what does it depend on? collision rate x probability of recombina/on for each collision collision rate depends on density of each colliding par/cle (n e and n p ) and how open they encounter each other à collision speed, or temperature separate out density dependence from temp dependence probability of recombina/on for each collision: depends on cross- sec/on (probability) for recombina/on, which depends on collision speed/temperature

41 how quickly does ionized hydrogen revert to neutral state? what is recombina/on rate dn e /dt? what does it depend on? collision rate x probability of recombina/on for each collision collision rate depends on density of each colliding par/cle (n e and n p ) and how open they encounter each other à collision speed, or temperature separate out density dependence from temp dependence probability of recombina/on for each collision: depends on cross- sec/on (probability) for recombina/on, which depends on collision speed/temperature both collision rate and recombina/on probability depend on collision speed/ temperature combine into one term α electrons recombine at rate - dn e /dt = n e n p α(t e ) collision rate probability of recombina/on

42 but what is α(t e )? - dn e /dt = n e n p α(t e ) n e2 α(t e ) since n e n p for H but what is α(t e )? factor which includes both the probability (cross- sec/on) for recombina/on AND the par/cle speed (but not density) dependence of collision rate

43 but what is α(t e )? - dn e /dt = n e n p α(t e ) n e2 α(t e ) since n e n p for H but what is α(t e )? factor which includes both the probability (cross- sec/on) for recombina/on AND the par/cle speed (but not density) dependence of collision rate α(t e ) 2x10-13 (T e ) - 3/4 cm 3 s - 1 this is appx which hides physics of encounters with range of speeds, this appx OK for T~ ,000K (i.e., HII regions) exact value of α(t e ) depends on details in gas, can vary by factor of 2 or more

44 but what is α(t e )? - dn e /dt = n e n p α(t e ) n e2 α(t e ) since n e n p for H but what is α(t e )? factor which includes both the probability (cross- sec/on) for recombina/on AND the par/cle speed (but not density) dependence of collision rate α(t e ) 2x10-13 (T e ) - 3/4 cm 3 s - 1 this is appx which hides physics of encounters with range of speeds, this appx OK for T~ ,000K (i.e., HII regions) exact value of α(t e ) depends on details in gas, varies by factor of 2 or more Q: why does recombina,on rate decrease with increasing temp?

45 but what is α(t e )? - dn e /dt = n e n p α(t e ) n e2 α(t e ) since n e n p for H but what is α(t e )? factor which includes both the probability (cross- sec/on) for recombina/on AND the par/cle speed (but not density) dependence of collision rate α(t e ) 2x10-13 (T e ) - 3/4 cm 3 s - 1 this is appx which hides physics of encounters with range of speeds, this appx OK for T~ ,000K (i.e., HII regions) exact value of α(t e ) depends on details in gas, varies by factor of 2 or more Q: why does recombina/on rate decrease with increasing temp? A: although collisions more frequent, e s much less likely to recombine at high speeds (less likely to s/ck )

46 radius of stromgren sphere S * = (4π/3) r * 3 (- dn e /dt) S * = (4π/3) r * 3 n e n p α(t e ) BUT! n p ~ n e ~ n H (nearly all H, and nearly all ionized) n H = # H nuclei = n p + n HI (H ions + H atoms) but not many neutral atoms inside HII region so n p >> n HI - > r * = [3S * / 4πn H2 α(t e ) ] 1/3 radius of stromgren sphere

47 real structure of HII regions Ioniza/on frac/on Vs. radius r * In REALITY there is not a sharp transi/on from ionized to neutral, but a thin shell in which the ioniza/on state gradually transi/ons. (we will ignore this detail in this class ) Thickness of transi/on zone given by mean free path of ionizing photon.

48 r * = [3S * / 4πn H2 α(t e ) ] 1/3 radius of stromgren sphere Q: why does r* drop with increasing density?

49 r * = [3S * / 4πn H2 α(t e ) ] 1/3 radius of stromgren sphere Q: why does r* drop with increasing density? A: not just the same mass of ionized gas in smaller volume, but less mass!

50 r * = [3S * / 4πn H2 α(t e ) ] 1/3 radius of stromgren sphere Q: why does r* drop with increasing density? A: not just the same mass of ionized gas in smaller volume, but less mass! recombina/on rate goes WAY UP with higher density, so not as many total ions in steady state!

51 r * = [3S * / 4πn H2 α(t e ) ] 1/3 radius of stromgren sphere Q: why does r* drop with increasing density? A: not just the same mass of ionized gas in smaller volume, but less mass! recombina/on rate goes WAY UP with higher density, so not as many total ions in steady state! the Hα luminosity of the HII region doesn t depend on the density even though the mass does! this is important, because it means we can use the Hα luminosity coming out of the HII regions to trace the star forma/on rate, one of the most important measures of galaxy evolu/on!

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53 Which stars produce HII regions? What frac/on of the ionizing radia/on produced by stars in a typical galaxy comes from massive stars with M>10 M sun? A. 10% B. 50% C. 90% D. 99% E. all of the above F. it depends

54 Which stars produce HII regions? What frac/on of the ionizing radia/on produced by stars in a typical galaxy comes from massive stars with M>10 M sun? A. 10% B. 50% C. 90% D. 99% E. all of the above F. it depends

55 Lyman con/nuum UV photons (E>13.6 ev, λ< µm) ionize H atoms Only blackbodies hoker than T>20,000K emit enough ionizing photons to make HII region OB stars have T>20,000K Main sequence OB stars have large luminosity White dwarf OB stars hot enough but small size means small luminosity L = 4πR 2 σ SB T 4 for stars MS OB stars don t live long so any MS OB star is young & therefore formed recently

56 cluster age main sequence star life/me 20,000K 3000 K ** main sequence stars hoker than 20,000K have life/mes < 10 Myr SO photons arising ~only from hot MS stars must be tracing only YOUNG stars! ** we can use the luminosity arising from HII regions as measure of 56 the star forma)on rate!!

57 Star Forma/on Rate from Hα MSFR (>10 M sun )(M sun yr - 1 ) = L(Hα)/11.7x10 41 erg s - 1 star forma,on rate (SFR) of massive stars (M * >10M sun ) only Assumes Case B recombina/on & Kroupa & Weidner (2003) or Chabrier (2003) IMF SFR (tot) = 6.3 MSFR = L(Hα)/1.86x10 41 erg s - 1 total SFR for stars of all masses is several,mes larger.. although we can t directly measure the forma,on of lower mass stars! NOTE: number of Hα photons emerging from HII region doesn t depend on density of gas in HII region (or size of HII region) it depends only on the photoioniza/on rate S * 57