Some HI is in reasonably well defined clouds. Motions inside the cloud, and motion of the cloud will broaden and shift the observed lines!

Transcription

1 Some HI is in reasonably well defined clouds. Motions inside the cloud, and motion of the cloud will broaden and shift the observed lines Idealized 21cm spectra Example observed 21cm spectra

2 HI densities and column densities Basic result: if optically thin, the total emission intensity along the line of sight gives the column density, N HI. N HI = # of HI atoms per cm 2 in the line of sight. Example: 1cm 2 area

3 Recall for blackbody radiation in the radio regime (low frequencies): This is the Rayleigh-Jeans limit, so the intensity from a Planck spectrum can be written as We define the term brightness temperature, T B, as T B is defined like this for any radiation process, not only blackbody radiation.

4 This is a useful concept, since we can derive the HI column density for optically thin gas: Note: this is a useful alternative to eq Note 2: Units of constant: cm -2 /K km s -1

5 21 cm hydrogen in the Milky Way.

6 HI in the Coma Cluster, overlaid on X-ray contours HI properties of galaxies close to the cluster center are very different from those further out. In the center, HI disks are smaller than the stellar disks and sometimes they are very asymmetrical. This is probably due to ram pressure stripping. Many processes can affect the environmental evolution, such as tidal interactions and merging, starvation, and ICM-ISM interactions

7 HI in NGC4522 (Virgo cluster) overlaid on R-band. Stellar disk looks smooth and undisturbed, but the gas has been completely removed from the outer disk. In the inner part we see gas being accelerated toward the cluster mean central velocity.

8 Binary black hole in Most compact supermassive binary black hole pair Projected separation between black holes = 7.3 pc

9 VLBI Results Line blueshifted 700 ± 10 km/s from systemic Line redshifted 370 ± 10 km/s from systemic HI absorption profiles

10 Exercise Suppose we observe an optical depth of for the 300 km/s wide line. Assuming Tspin = 6000 K, (a) what is the HI column density? (b) If the HI is in a cloud shaped like a giant coke can 50 pc long and with a radius of 10 pc, what is the total HI mass of the cloud?

11 Astronomy 422 Lecture 2: The Interstellar MediumII

12 Key concepts: Review of Optical Depth and Radiative Transfer The Warm Ionized Medium: Emission measures Dispersion measures Strömgren spheres and HII regions

13 Emission and absorption in a gas Interaction between radiation and material Start with ignoring emission, and only care about effects of absorption. Define the optical depth as the amount of absorbed radiation:

14 Assume the slab of gas has some width, then we integrate across it: Integrate

15 Optical depth depends on a) characteristics of absorbing material, and b) the amount of absorbing material Define the absorption coefficient,, which contains info about a). Defined as optical depth per unit length. Now lets include also the fact that the gas will emit radiation. Emission per unit length is called emissivity,, and we write the total change of intensity as:

16 Divide by is also called the source function. Integration (use integrating factor method) gives Two extreme cases: Optically thick emission Optically thin emission

17 Approximations: We can often assume that S is independent of position in cloud independent of and can thus be moved outside the integral. A simple solution is achieved: Spectral line: a small region in frequency space where opacity 0. Here we are interested in the difference between the line and continuum intensity - subtract from What happens at small optical depths?

18 Small optical depths, no emission (S ν = 0): I cont = 500 mjy I line = mjy ΔΙ = 2.5 mjy ΔΙ/I = [1 e -τ ] 2.5/500 = τ τ = 0.005

19 The general form of the source function thus ought to be such that at large optical depths. Let us continue radiation transport and spectral lines, and the relation to observations: Excitation temperature

20 Look at the incoming radiation field,, which often can be characterized through a brightness temperature T B (as we talked about before). At mm wavelengths the most important background continuum source is the CMB with T B ~2.7K. Over large parts of the radio spectrum the RJ approximation: and we can use

21 The Warm Ionized Medium (WIM): Known since 1938 that ionized gas exists outside HII regions. HI almost completely ionized, n ~ 0.1cm -3, T ~ 8000 K Diffuse medium, occupying perhaps 20-30% of the ISM volume. How to observe the WIM: Although mostly ionized, ionizations and recombinations always occurring are: Thus, we can observe H emission lines from a small fraction of H which is temporarily recombined. # recombinations per cm 3 per sec = n e n p α where α is the recombination coefficient (cm 3 s -1 ).

22 About half of all recombinations yield Hα photons. If H is more or less completely ionized, n e ~n p, and thus the Hα intensity n e2 α. We observe Hα emission along line of sight. If the length along the LOS is L, we can define the emission measure as: LOS n e L

23 Note: EM gives the average n e 2 along the LOS. The ISM is clumpy, and individual clumps can have different n e. LOS n e1 n e2 If atomic here, n e =0 L 1 L 2 Note 2: Astronomer's units: [n e ] = cm -3 [ds, L] = pc WIM: Hα EM typically <= 50 pc cm -6 HII regions: Hα EM typically <= ,000 pc cm -6

24 Hα is common, but there are also other optical emission lines used: NII 6548, 6583 Å SII 6716, 6731 Å OI 6300 Å OII 3727 Å OIII 5007 Å etc. Radiation ionizes these elements, and collisions excite them to higher levels. All these lines are useful diagnostics of physical conditions in the WIM, e.g., temperature, density and abundances.

25 Pulsar dispersion measure (DM) In ionized gas, the velocity of the radio waves depends on frequency, due to slowing down of photons by electrons. Analogous to index of refraction. Longer wavelengths are slowed more. We can thus learn about the ionized medium by studying pulse arrival times from pulsars.

26 Pulsar dispersion measure (DM) LWA1

27 Pulsar after de-dispersion LWA1

28 For two frequencies ν 1 and ν 2 : where L = distance to pulsar. We define the dispersion measure as: If L is known, then NB: use cgs units for this expression for DM.

29 Note: Can get <n e > only for directions along which a pulsar exists Gives no information about the ionized gas beyond the pulsar Units of DM usually [pc cm -3 ] Typically pc cm -3 is measured Example: For a DM = 100 pc cm -3, and L = 1kpc gives n e =0.1 cm -3.

30 HII regions Ionized gas near hot, young massive stars. Compared to the WIM, more well defined and denser. n e ~ cm -3. Example: Orion nebula. Red => Hα dominates emission. Why near massive stars only? Need 13.6eV Use Wien's law => 912 Å (UV) This is a B0 or B1 star. Thus, HII regions surround only O and B stars, and share their short lifetimes.

31 We can find the expected size of an HII region by assuming ionization equilibrium: ionization rate = recombination rate # recombinations per cm 3 per sec is n e n p α = n e 2 α If the region is spherical with a constant electron density, then is the # recombinations per sec over the whole HII region. If the star emits N ionizing photons per s, then

32 And we can solve for the radius: R S is the Strömgren radius. The sphere is called Strömgren sphere. Not always this simple NB, n e in cgs units cm -3

33 N comes from knowing spectral type of star, with a model atmosphere to predict the UV flux output. The expression on the previous slide assumes: 1 star - in reality there may be many all electrons are from hydrogen all ionizing photons are absorbed by hydrogen no ionizing photons escape - but probably 50% do - WIM Real HII regions are messy

34 Example: The Rosette Nebula In the center is a star cluster, NGC2244. Hα

35 Example O6 star with T eff = 45,000 K L = 1.3x10 5 L What is the Number of ionizing photons? What is the radius of the Stromgren Sphere?

36 Example O6 star with T eff = 45,000 K L = 1.3x10 5 L Assuming a blackbody spectrum, we use Wien's law: To simplify, assume all ionizing photons from star have λ=λ max, and the star only emits ionizing photons. Then, photon energy and thus

37 For H at temperatures and densities of HII regions, we use α=3x10-13 cm 3 s -1. Assuming also n e = 1000 cm -3, then:

38 Orion Nebula: HST. Well studied region with ongoing star formation.

39 Orion Nebula in IR showing Becklin-Neugebauer object The Orion radio zoo - PIGS, DEERS and FOXES

40 Schematic diagram: Balance of ionization and recombination sets up HII region. H + UV photon => H + + e - Neutral H gas (HI region) Cool gas Hot gas UV light from hot O star Ionized H gas (HII region) A few light years ~ 1 pc

41 Orion Nebula: Case study of star formation. A model of the association of the Orion Nebula, its molecular cloud and infrared sources. Dust Infrared cluster LOS Orion Nebula Ionized gas Trapezium cluster Dense molecular cloud 2 light years = 0.6 pc Less dense molecular cloud

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43 Strömgren spheres: properties

44 Next time: Hot Ionized Medium