Lecture Seven: The Milky Way: Gas

Transcription

1 Lectue Seven: The Milky Way: Gas Gas in the Milky Way Between the stas in the disc of the Milky Way thee is gas, which is the stuff fom which the stas wee oiginally made, and to which stas etun heavy elements fom thei nuclea buning pocesses. Almost all the gas in the Milky Way lies in the disc, although the mass of gas is only 10% of the mass in stas. Gas gives a galaxy its distinctive popeties, without gas the Milky Way would be an S0 galaxy, not a spial, as the disc would have no young stas and no spial patten. Only half the stalight fom the Milky Way escapes the galaxy, dusty intestella gas absobs the est. 7th May 2014 Spake & Gallaghe, chapte 1,2 1 2 Gas in the Milky Way Like the stas, the gas in the Milky Way is subject to gavity, which ultimately causes the densest gas to collapse and fom new stas. Howeve, unlike stas, the intestella gas is subject to additional foces like gas pessue, magnetic foces, and the pessue caused by Cosmic Rays. The gas is heated and ionised by stella adiation; it is shocked and set into motion by fast stella winds, violent supenovae explosions and passage though the spial ams - it is a complex envionment! Unlike stas, gas does not come in standad units of size. The mass of a clump of gas is not elated to its tempeatue, o any othe quantity we can measue independent of distance. So the distances to gas clouds ae vey uncetain, except fo vey unusual cases whee we know that the gas suounds a sta. When we see absoption lines fom the gas in the specta of stas, then we know the gas is infont of the sta. Fom gas in cicula obits in the Milky Way disc we can detemine kinematic distances. We can measue the adial velocity of the gas in all diections, and thus build up a pictue of how the gas is distibuted in the disc of the Milky Way. Gas in the Milky Way When all adiation eaches us without being absobed (optically thin emission), the mass of gas moving with a paticula velocity is popotional to the intensity of the adiation. Visible light is absobed by intestella dust, wheeas adio waves can tavel though. But sometimes in a lage disc of gas we can look though enough mateial that the adio waves fom distant gas ae patially absobed by gas close to us (optically thick emission). HI is optically thick in the inne pats of the Milky Way, fo example. Dense cool clouds ae often taced by molecula emission, at millimete wavelengths, of 12 CO, which ae nealy always optically thick, much molecula gas is thus hidden fom view. The most common molecule is H2, but because it is a symmetic molecule thee ae no stong emission lines, making it vey had to detect diectly. The next most abundant molecule is CO (thee is one CO molecule fo evey 10 4 of H2). Intestella gas is in motion on lage and small scales. Like stas, intestella gas clouds do not follow exactly cicula obits about the Galactic cente. They also have andom motions, typically ~5km/s fo molecula clouds, and 8-10km/s fo clouds of HI. 3 4

2 Neutal atomic hydogen: HI Neutal atomic hydogen: HI Hydogen is the most abundant element in the Univese and in the intestella medium (ISM) of the Milky Way. The 21-cm line is the esult of the magnetic inteaction between the electon and poton spins and so is a magnetic dipole tansition The cold intestella gas does not emit adiation at visible wavelength, but at adio wavelengths due to a hypefine line fom two closely spaced enegy levels in the gound state of the neutal H atom (HI). An HI atom with the spins aligned will spontaneously flip back to the lowe enegy, non-aligned, state afte sometime. The emission coefficient of this magnetic dipole is: A10 = whee µ 10 is the Boh magneton, the intinsic dipole moment of the electon: electons have spin angula momentum L = ~/2, classical adius e = e2 /(me c2 ), and chage e, so hypefine tansitions ae a consequence of coupling between nuclea spin and the magnetic field geneated by the obiting electon. µ 10 = The fequency of the cente of this line is defined fom quantum mechanics: 10 = 8 gi 3 me mp µ, 3hc ~ e me c Thus the emission coefficient fo the 21-cm line is 2 (RM c) = MHz and theefoe its adiative half-life is about 21 eg G 1 A10 = s 1 1/2 = A s 11 million yeas whee gi is the nuclea g-facto fo a poton, e2 /(~c) 1/ is the fine stuctue constant, and RMc is the Rydbeg fequency This is eally long! But thee s a lot of neutal hydogen out thee... This fequency coesponds to a wavelength, λ 21 cm and theefoe this line is often called the 21-cm line One of the ae occasions in astonomy whee a non-teestial phenomena was pecisely pedicted, by Henk van de Hulst in 1944 befoe it was actually obseved, in Fist Detection of HIMeasue flux density as function fequency Kootwijk Radio obsevations Fist Dutch RadioofTelescope, 6 Mapping the Milky Way in HI i.e. as function of velocity (Dopple effect) By measuing the adial velocity of the 21cm line and its intensity, we can measue the distance to and amount of HI in the Milky Way Integal of the obseved flux density ove Fequency (velocity) povides the total numbe of HI atoms along the line of sight: the column density NHI in cm-2 compae the obseved fequency of the line with MHz to get the adial velocity: z= obs em em = em obs and v = cz obs and integate the obseved intensity ove fequency to get the column density (NHI in cm2) of neutal hydogen Fist obsevations with the adio telescope nea Kootwijk Ewen & Pucell 1951, Natue, 168, 356 Mulle & Oot 1951, Natue, 168, 357 Oot, Ke & Westehout 1958 MNRAS, 118,

3 The Leiden-Agentina-Bonn (LAB) HI Suvey V = R 0 sin l V R V 0 R 0 Kinematic distances to gas in the Milky Way As we look acoss the MW, we see the HI gas take on a distibution of gas clouds with diffeent adial velocities These ae clouds at diffeent distances with diffeent angula speeds II I I III R<R0 R>R0 R>R0 R>R0 R>R0 Kalbela et al A&A R<R0 R = R 0 sin l V (R) =V + V 0 sin l Cloud A has the highest velocity: it sits at the tangent point along ou line of sight, so its adial velocity is maximized Clouds B & C have diffeent distances but the same velocity: they sit at the same adius fom the GC with the same velocity Cloud D is the futhest and has negative velocity because it sits at R>R The distibution of HI in the Milky Way The otation cuve of the Milky Way fom HI By mapping the MW in HI, we can then detemine the otation cuve V(R) This only woks fo cicula motion aound the Galactic cente: spial ams and the Galactic ba will distub the otation cuve by km/s o moe. Ba Spial am Open points: v0=200 km/s Cosses: v0=220 km/s Sun In the oute galaxy, we need to use othe taces, like young stas o thei associated hot o cold gas clouds GC N: Nothen HI (0º < l < 180º) S: Southen HI (180º< l < 360º) 11 12

4 The distibution of HI in the Milky Way The distibution of HI in the Milky Way The column density of neutal hydogen gas along some line of sight is defined: Z N HI = n HI ds los whee nhi is the numbe density of neutal hydogen (HI) atoms If the optical depth of the cloud, τ 1, then we can measue the column density fom the flux emitted by the cloud as Z apple N HI = Tb (v) v d K km s 1 cm 2 whee Tb(v) is the obseved 21cm bightness tempeatue, and the velocity integation extends ove the entie HI pofile. T b = I c 2 2k 2 h kt Kalbela et al. (2005): the Leiden-Agentina-Bonn Suvey The distibution of HI & H2 in the Milky Way based on kinematic distances, using CO to tace H2. N: Nothen HI S: Southen HI looking away fom the Galactic cente Spikes ae spial-am cossings Note that the distibution of HI in the Nothen and Southen MW is not the same M(HI)MW 4-8 x 10 9 M twice the mass of H2. almost all H2 but less than half HI lies within the sola cicle Molecula gas is piled up in a ing of adius 4 kpc. HI disc is thicke than molecula disc. HI can also be found fa above the plane of the disc. These ae high-velocity clouds of HI that ain down on the disc with velocities appoaching 100km/s. Some of this may be disc mateial which has been thown up by supenovae o winds fom hot massive stas, and now it is falling back. Some ae known to come fom extenal systems (e.g., Magellanic Steam)

5 HIGH VELOCITY CLOUDS HIGH VELOCITY CLOUDS HIGH VELOCITY CLOUDS Complex A d 8-10 kpc M 106 M!! Cohen Steam d 4-11 kpc M 106 M!! Complex C d 6-11 kpc M 107 M! Z ~ 0.15 Z!! GCP d kpc M 106 M!! Wakke et al. 2007, 2008; Tipp et al

6 Molecula gas in the Milky Way Molecula gas in the Milky Way Molecula hydogen, H2, is symmetic and theefoe has no pemanent electic dipole moment. So despite the fact that H2 is the most abundant molecule in the Univese, it only adiates when shocked o iadiated to T 1000 K, while most H2 is at T~ K. CO is the second most abundant molecule in the galaxy (afte H2), with J=0-1 and J=1-2 tansitions at 2.6 mm and 1.3 mm espectively The minimum equied tempeatue to excite a molecule is Tmin~Eot/k A pola molecule has a non-zeo electic dipole moment and so will adiate due to both otational and vibational modes A otating molecule has an angula moment that is quantized in units of : Fo CO J=0-1 and J=1-2, the excitation tempeatues ae ~11 K and 17 K, espectively L = n~ = I! The citical density, at which collisional excitation is in equilibium with emission, fo CO J=1-2 is 700 cm-3, typical of giant molecula clouds in the MW whee I is the moment of inetia of the molecule and ω is its angula fequency of otation Because H2 is vey difficult to obseve diectly, we use CO to tace the molecula gas content of the Milky Way (and othe galaxies, too!) The enegy of the state with otational quantum level J is 2 J(J + 1)~, whee J = 0, 1, 2,... 2I Eot = And only tansitions between states J and J±1 ae pemitted: The emitted fequency of a tansition is then = We use a convesion between CO intensity and H2 mass called the XCO-facto, which is oughly constant in the Milky Way, but is highly unlikely to be univesal, and this uncetainty plagues extagalactic CO obsevations. J = ±1 ~J 2 2 meq whee m is the educed mass of the molecule and eq is its equilibium adius 21 Molecula gas in the Milky Way Molecula gas in the Milky Way Almost all CO in the MW lies within the Sola cicle CO distibution in Galactic coodinates Galactic Latitude Only 20% of HI is at R<R0 +3 Beam CO concentated with 4 kpc of GC, with a cental hole By associating CO in the oute Galaxy (R>R0) to young stella associations, we can use distances to these associations and the velocities of thei CO (i.e., thei cold molecula gas) to detemine the otation cuve of the Milky Way outside of the Sola cicle Usa Majo Cam CTA-1 G S212 W3 NGC7538 Cas A Tau-Pe-Au Complex e a W51 f Galactic Cente ol ec m Gem OB1 S147 Rosette λoi R i n g 24 P Fa ul a Ri a Cain a A m 1.0 log T mb dv (K km s 1 ) e s e u This is accuate enough to show that the otation of the Galaxy does not fall as a function of adius beyond the Sun s adius! +260 s A Vela CMa OB1 Caina ng +240 m S Maddalena s Mon Cloud Gem OB1 OB1 d in g A m +180 Noma CO adial velocity vs. l +200 Centauus n 6 p 12 A x O u t e E m A Mon R2 S. Oi Filament Oi A & B 30 FIG. 2. Velocity-integated CO map of the Milky Way. The angula esolution is 9 ove most of the map, including the entie Galactic plane, but is lowe (15 o 30 ) in some egions out of the plane (see Fig. 1 & Table 1). The sensitivity vaies somewhat fom egion to egion, since each component suvey was integated individually using moment masking o clipping in ode to display all statistically significant emission but little noise (see 2.2). A dotted line maks the sampling boundaies, given in moe detail in Fig W44 Aquila Rift Cyg X Cas A c W51 W50 Cyg OB7 p W3 e s e u s k 100 Lindblad Ring & Local Am P Mon OB1 CMa OB1 Gum Nebula Chamaeleon Oion Complex M S235 Vela Caina Nebula G317 4 R CA Scutum Sagittaius S147 Coal Sack W i R t Aquila South 12 3 LSR adial velocity ( km s 1 ) Cyg OB7 Cyg X Pegasus Maddalena s Cloud Lupus Aquila Rift t Laceta Pe OB2 2 Hecules Disk S147 S235 Ophiuchus Nuclea +2 Galactic Latitude Polais Flae Cepheus Flae Resolution LSR adial velocity ( km s 1 ) log T mb db 0.0 (K acdeg) FIG. 3. Longitude-velocity map of CO emission integated ove a stip ~4 wide in latitude centeed on the Galactic plane (see 2.2) a latitude ange adequate to include essentially all emission beyond the Local spial am (i.e., at v > 20 km s 1). The map has been smoothed in velocity to a esolution of 2 km s 1 and in longitude to a esolution of 12. The sensitivity vaies somewhat ove the map, since each component suvey was integated individually using moment masking at the 3-σ level (see 2.2)

7 The otation cuve of the Milky Way The otation cuve of the Milky Way If all gas is on cicula obits, and the mass is concentated at the cente of the Galaxy, then Keple s thid law says P 2 = 4 2 GM R3 and P = 2 V R 1/2 and so GM V = R But this isn t what we see! fom Honma & Sofue (1997) The otation cuve of the Milky Way We ll see in one of the next lectues that the cicula speed V at adius R is elated to the enclosed mass M(<R) by M(< R)= RV 2 G If V is close to constant with R, this implies that the mass must gow linealy with adius! This is why we think that thee is dak matte in the Milky Way Boomsma et al

8 M 51 Blue = HI (VLA, Rots et al.), White = optical Oosteloo, Fatenali & Sancisi