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Interstellar Grains Much of our original knowledge of interstellar dust comes from a 0.60 m telescope dubbed IRAS, the Infrared Astronomical Satellite (1983). IRAS data: covered over 96% of the sky at 12, 25, 60, and 100 m provided our first view of the extragalactic sky at farIR wavelengths had uniform sensitivity limits: 0.5 Jy at 12, 25, 60 m; 1.5 Jy at 100 m experienced negligible Galactic extinction had a typical positional uncertainty of 4"x15" (1) In ½ hour, IRAS obtained more >5m data than all previous efforts combined. The IRAS mission lasted 10 months. In addition to cataloging ~250,000 point sources, IRAS detected 20,000 extended sources and diffuse emission from the entire sky. The COBE, WMAP, and Planck satellites subsequently studied the latter in more detail the 1990s, 2000s, and 2010s. Shameless plug ASTR 5470 Physics of the Interstellar Medium 1 Interstellar Grains Other important spacebased infrared telescopes (& don't forget groundbased 2MASS): Infrared Space Observatory Spitzer Space Telescope 0.60 m mid/farIR imaging mid/farIR spectroscopy 0.85 m mid/farIR imaging midIR spectroscopy WISE Akari 0.67 m near/mid/farIR imaging 0.40 m midIR imaging It's big! SOFIA 2.5 m mid/farIR imaging IR/submm spectroscopy JWST Herschel Space Observatory 3.6 m farIR/submm imaging farIR/submm spectroscopy 6.5 m midIR imaging launch: 2018? ASTR 5470 Physics of the Interstellar Medium WFIRST 2.4 m nearIR imaging nearIR spectro launch: 2025? 2 Interstellar Grains Why (stratosphere and) spacebased? The atmosphere both and in the infrared. ASTR 5470 Physics of the Interstellar Medium 3 Interstellar Grains Why (stratosphere and) spacebased? The atmosphere both absorbs and emits in the infrared. J H K L M L' Ks N Q gemini.edu/sciops/telescopesandsites/observingconditionconstraints?q=node/10789 ASTR 5470 Physics of the Interstellar Medium 4 Interstellar Grains If we model the dust distribution in the Milky Way as a uniform slab of thickness 2h, then we'd expect the dust emission to be proportional to the cosecant of the Galactic latitude b: f(IR) ∝ hcsc(b). This is consistent with observations (below figure). Interstellar dust is composed of clumps of atoms and molecules, much like common smoke, soot, fog, chalk dust, ... Interstellar dust is typically of size ~100nm good scatterer of optical light Emission by dust is often fit by a single Planck function times a dependent emissivity f ∝ B(Tdust) e.g., ∝ Dust can polarize light elongated and aligned 3050% of a galaxy's luminosity is stellar light reprocessed by dust Interstellar space is 106 times 'dirtier' than Earth's atmosphere 1 Earth volume = 1 pair of dice Binney & Merrifield 1998 ASTR 5470 Physics of the Interstellar Medium b 2h MW HI MW dust Shows that dust comes from the ISM! 5 Interstellar Grains From prior page: Emission by dust is often fit by a single Planck function times a dependent emissivity f∝B(Tdust) e.g., ∝ Dale+ 2012 Calculating a single dust temperature is very common in extragalactic work. In which situations do you think it is unwarranted? One application of the dust T is computing how much dust mass is required for the observed L and T, Mdust ∝ f/B(Td) . Frequently used to compute the dusttogas ratio, and compared to that of the Milky Way. ASTR 5470 Physics of the Interstellar Medium 6 Interstellar Grains The Milky Way is patchy extinction by dust I = Ioe for a<<, ∝4 (Rayleigh) for a>>, ∝0 (geometrical) what does this mean? Extinction is higher towards the stars appear . Thus the term 'reddening'. What about the 'blueing' of reflection nebulae?! a ~0.011m (particles are ~size of visual photons) Extinction is ~ 1 mag/kpc in solar neighborhood Early references: Struve 1847; Light scattering by small particles van de Hulst 1957 Definitions Q = opt/geom extinction efficiency scat/geom scattering efficiency Qs = Qa = abs/geom Q = Qs + Qa absorption efficiency total extinction efficiency = opt/a2 (for spheres) Can dust particles absorb? AV = extinction in V band (~550nm) AV/NHtot ~ dust/gas ratio It is found that AV/NHtot ~ mag cm2 a given mass of gas contains a characteristic amount of dust, irrespective of whether the hydrogen is in the form of H or H 2. ASTR 5470 Physics of the Interstellar Medium 7 Interstellar Grains What Do We Observe in the Milky Way? The Neutral Hydrogen View ASTR 5470 Physics of the Interstellar Medium 8 Interstellar Grains What Do We Observe in the Milky Way? The FarInfrared View COsmic Background Explorer 60240 m image Hey, what's that blue 'S' curve? Mathis 1990 Galactic extinction curves show peaks characteristic of a few distinct species of dust grains Classical grains of size are primarily responsible for the diffuse Galactic emission at long wavelengths ASTR 5470 Physics of the Interstellar Medium 9 Interstellar Grains IV = IVo10Av/2.5, NH = nHds = nHD for uniform density D I = Ioe V=AVln10/2.5 = AV 2.30/2.5 = AV0.921 = AV/1.086 and V = Qa2ndD = optndD and NH = nHD Now we can estimate the gastodust ratio V/NH= Qa2nd/nH nH/nd = Qa2NH/V dust = 4/3a3grainnd = dust mass/volume gas = mHnH gas/dust = = ¾ mH/a3grain Qa2 NH/V = 1.086 ¾ mH/a Q/grain NH/AV ASTR 5470 Physics of the Interstellar Medium 10 Interstellar Grains Take a~0.1m, grain~ 2 g cm3, mH = 1.67 1024 g Q varies with , but for optical and typical grain size, Q is of order 100. gas/dust = 130 ~ 102 Reference for gastodust ratios in spiral galaxies: Mayya & Rengarajan (1997), RemyRuyer+2014 Q=Qa+Qs This ratio is roughly what one expects if we sum up metaltohydrogen ratios for the solar neighborhood abundances (see table from Ch. 2 notes). (ni/nH) A.W.i1 ~ 102 2 What does this mean plot mean, and how can a crosssection be twice as large as the geometrical value? 1 5 10 x=2a/ ASTR 5470 Physics of the Interstellar Medium 11 Interstellar Grains Extinction The dimming of light A = (mm0) p.133 Binney & Merrifield 1998 Reddening or a general color excess is the difference between observed and intrinsic color E(XY) = [m(x)m(y)] – [m0(x)m0(y)] = AxAy Some standard filters for optical astronomy (nm) (nm) U 365 66 u' 356 60 B 445 94 g' 483 138 V 551 88 r' 626 138 R 658 138 i' 767 153 I 806 149 z' 910 137 The standard color excess quoted is E(BV). For much of the ISM, AV~3.1E(BV) (see Slide 15). ASTR 5470 Physics of the Interstellar Medium 12 Interstellar Grains Extinction curve A 4 The extinction curve broadly maximizes near 0.1m because there are many dust grains in the ISM of this size. The sharper features arise from absorption by other particular dust grain types. 4 Does AV scale with any parameter in this listing? Why doesn't this table obey the canonical 1 mag/kpc we learned about on Slide 7? A_V (mag) N_H (cm-2) n_H (cm-3) R (pc) T (K) M (Msun) Globule IR Cloud Dark Cloud Diffuse Cloud Cloud Complex 4 8e21 7000 0.3 10 30 30 6e22 40000 0.4 50 400 4 8e21 2000 1 10 300 0.2 4e20 20 5 80 400 4 8e21 200 10 10 30000 ASTR 5470 Physics of the Interstellar Medium 13 Milky Way Extinction (Draine 2003) Interstellar Grains RVAV/(ABAV) characterizes the slope of the extinction curve over optical wavelengths. Very large why? grains would produce RV whereas Rayleigh scattering (A∝4) would produce very steep extinction with RV1.2. A 'standard' Milky Way value is RV3.1. We discussed how AV/NH was essentially constant for every lineofsight. Work by a UWyo PhD shows that this is true to within a factor of 2 (below). Does the trend make sense? ASTR 5470 Physics of the Interstellar Medium14 Interstellar Grains Milky Way Extinction (Draine 2003) There are over 150 'diffuse interstellar bands' that are probably due to PAHs. They were discovered in 1922, but not a single one has been convincingly identified! ASTR 5470 Physics of the Interstellar Medium 15 Interstellar Grains Thermal Emission from Large Interstellar Dust Grains Dust absorbs photons, typically in the UV or visible since the crosssection is higher there, and then radiates (usually in the infrared — a dust grain cannot get too hot or it will evaporate!). ¼ ASTR 5470 Physics of the Interstellar Medium16 Interstellar Grains Blackbody reminder If the stars radiate as blackbodies, the main sequence profiles are defined by the effective stellar temperatures ASTR 5470 Physics of the Interstellar Medium 17 Interstellar Grains where emissivity per unit area per steradian = QemB = QabsB (by Kirchoff's Law) Suppose we have very big particles, e.g., planets or asteroids. Now in general, the l.h.s. of the radiative balance is dominated by optical/UV photons and the r.h.s. by the emission of infrared photons. Let us take the average visual albedo to be A, implying Qabs=1A, and the average emissivity in the infrared to be Qem= . Let us assume rapid rotation isothermal planet or asteroid. Using Wien's Law (peakT ~ 0.29 cm K) gives peak ~ 10m for T=279 K (in the midinfrared) Note that if the object were not rotating (and not thermally conductive), we have ¼ In other words, the effective radiating area is a2 rather than 4a2 for the subsolar point (i.e., at 'noon'). The moon could be an example. For a large object like an asteroid we would have Qem~1. However, for a dust particle in the infrared Qem=Qabs<<1 in general because the particle is much smaller than the wavelength. In the visible Qabs~1. All this means that small particles will be hotter than larger ones because they do not radiate very efficiently in the infrared. 18 What do we see emitted from dust? Interstellar Grains As we've already seen, the extinction curve toward stars is fairly smooth and exhibits a peak near 220nm, which is attributed to large PAH clusters. The bulk of the absorbed starlight heats large dust grains to temperatures ~20K, grains that are thinly spread throughout the ISM. These grains are responsible for the 'diffuse cirrus' emission (see Slide 9; of course those dust particles very near hot stars will be heated to much higher T). According to Wien's Law, the characteristic wavelength of emission is ~150m. fn In fact, up to 50% of the energy radiated in the Universe is emitted at IR wavelengths. cirrus spectrum Dole et al. 2006 150mm physics.uwyo.edu/~ddale/research/wysh/wysh1.gif swire.ipac.caltech.edu/swire/astronomers/strategies.html Galaxy formation & evolution processes contribute about 5% of the electromagnetic content of the Universe, with about half from starlight and half from dust ASTR 5470 Physics of the Interstellar Medium 19 Interstellar Grains All is not rosy with this simple picture of dust emission, however. In some cases dust is found which is much hotter than you would expect to find on the basis of thermal equilibrium calculations, e.g., the case of the reflection nebula NGC 7023. L*=104 Lsun d=0.15 pc Td = 17 K (QUV/QIR)1/4 where QUV and QIR are average UV and IR absorption and emissivity. For a plausible range of QUV and QIR, we'd expect Td = 70150 K. But this is not what is observed! If dust grains near NGC7023 are in thermal equilibrium with the surrounding interstellar radiation field, then the observations indicate Td >1000 K !! One possible explanation is that the observations are of reflected starlight — that light should be bluer than the stellar spectrum, but observations do not support this explanation (it isn't bluer). What is the answer? Stochasticallyheated small grains If an energetic photon strikes a very small particle, one composed of maybe 50 atoms (e.g., polycyclic aromatic hydrocarbons), the particle may significantly heat up. ASTR 5470 Physics of the Interstellar Medium Sellgren, Werner, & Dinerstein 1983 20 Stochastic Heating of Small Interstellar Aromatic Grains For low Td we have the Debye law: Cv ∝ Td3 For high Td Cv = 3Nk why? Grain temp Interstellar Grains (6N = # degrees of freedom) Minutes Time The dividing line between low and high temperatures is the 'Debye Temperature' which is 200500 K for typical grain material. It is usually indicated by D. Suppose the dust becomes very warm when a photon hits it, so that Td>D. The main contribution to the heat capacity will be the high T component, so Cv=3Nk. Then h=3Nk Td. In NGC 7023, brightness temperatures of ~1000 K are seen from the dust. Supposing ~2000 Å (near the peak of a moderately warm star), N = h / 3kTd ~ 24 Approximately 24 'atoms' are present in the particle! A more careful analysis gives N=7090 atoms for silicate or graphite grains. Thus the physics of very small grains was born. Note that if we assume a lattice spacing of 3Å: N=4/3a3(1/3Å)3 a=3(¾ N / )1/3 ~ 3(¾ 80 / )1/3 ~ 8Å for the particle size 21 Interstellar Grains Grain temp Stochastic Heating of Small Interstellar Aromatic Grains – back of the envelope Minutes Time 22 Interstellar Grains Stochastic Heating of Small Interstellar Aromatic Grains – a realistic model A day in the life of 4 carbonaceous grains, heated by the local interstellar radiation field. abs is the mean time between photon absorptions (Draine 2003; Draine & Li 2001). Recall from our notes that PAHS smaller than 15Å are responsible for half of the heating of the neutral ISM, and grains between 15 and 100Å are responsible for the other half. Recall also that the diffuse cirrus is caused by 'classical' large grains of size 2000Å. ASTR 5470 Physics of the Interstellar Medium 23 Interstellar Grains Naphthalene Coronene C24H12 PAHs (simple ones) Phenanthrene Chrysene ASTR 5470 Physics of the Interstellar Medium 24 Interstellar Grains What Do We Observe in the Milky Way? The MidInfrared View ISO spectrum of: NGC 7023 (reflection nebula) M 17 (starforming/HII region) Cesarsky et al. 1996 Polycyclic Aromatic HydroCarbons ~50 Carbon atom planar structures with Hydrogen atom appendages Astrochemistry Lab Models Roelfsema et al. 1996 ASTR 5470 Physics of the Interstellar Medium 25 Interstellar Grains MidInfrared Emission Features Are Ubiquitous Such features are typically seen in: planetary nebulae, protoplanetary nebulae, reflection nebulae, H II regions, circumstellar envelopes, Galactic diffuse cirrus, external galaxies, Buicks, ... ASTR 5470 Physics of the Interstellar Medium 26 Interstellar Grains MidInfrared PAH Features (Draine 2003) The midinfrared features are attributed to vibrational modes (bending, stretching) of PAHs (e.g. Leger & Puget 1984). When H atoms are attached to the edge of an aromatic ring skeleton, the system will vibrate if triggered by an optical or UV photon. The bending modes can be 'mono' (no adjacent H), 'duo' (2 contiguous H), 'trio' (3 contiguous), or 'quartet' (4 contiguous). ASTR 5470 Physics of the Interstellar Medium 27 Interstellar Grains The Infrared Spectral Energy Distribution for normal galaxies of different star formation activity levels. Note the presence of midinfrared features (in addition to the farinfrared thermal emission). Dale+14 lambda 3.0m 9.7 18.0 11 20 3.3,6.2, 7.7,8.6, 11.2,12.0, 12.7,16.4, 17.1 identification ice (absorption) silicate (emiss/abs) silicate (emiss/abs) SiC silicate PAH PAH PAH PAH PAH References from ISO+Spitzer: Helou et al. 2000 Lu et al. 2003 Dale et al. 2001; 2002 Peeters et al. 2002 Smith et al. 2004; 2007 Sturm et al. 2000 ASTR 5470 Physics of the Interstellar Medium 28 Interstellar Grains The Infrared Spectral Energy Distribution for normal galaxies of different star formation activity levels. Note the presence of midinfrared features (in addition to the farinfrared thermal emission). lambda 3.0m 9.7 18.0 11 20 3.3,6.2, 7.7,8.6, 11.2,12.0, 12.7,16.4, 17.1 identification ice (absorption) silicate (emiss/abs) silicate (emiss/abs) SiC silicate PAH PAH PAH PAH PAH References from ISO+Spitzer: Helou et al. 2000 Lu et al. 2003 Dale et al. 2001; 2002 Peeters et al. 2002 Smith et al. 2004; 2007 Sturm et al. 2000 ASTR 5470 Physics of the Interstellar Medium 29 Interstellar Grains Q: What is the physical significance of the different farinfrared shapes below? Q: Can you make a quantitative prediction for the characteristics of the farinfrared hump? Q: Why don't the midinfrared features change in wavelength as well? Q: Do you think the infrared spectrum helps or hinders photometric redshift efforts? Kennicutt et al. 2003 ASTR 5470 Physics of the Interstellar Medium 30 Interstellar Grains Q: Do you think the infrared spectrum helps or hinders photometric redshift efforts? Spitzer infrared color tracks from z=0 to1 A photometric redshift is a redshift estimated according to the observed colors and a suite of galaxy SED templates. Ideally, a single template at a single redshift will be the unique solution. In practice, different templates at different redshifts can provide equally reasonable fits. Dale+01 f Galaxy SED z filter filter filter l ASTR 5470 Physics of the Interstellar Medium 31 Interstellar Grains Challenges to photometric redshift efforts: optical spectrum is variable according to star formation history optical spectrum can be reddened and attenuated due to dust extinction having the proper suite of template SEDs (e.g., AGN add additional confusion) farinfrared peaks at different wavelengths for different dust temperatures midinfrared feature (PAHs) can be minimal/absent for particularly hard radiation fields Thus, ideally one will have photometry from a large number of wavelengths, in order to have the best chance of constraining the redshift. f Galaxy SED z filter filter filter l ASTR 5470 Physics of the Interstellar Medium 32 Interstellar Grains http://lsst.org/files/img/ColorZ.gif ASTR 5470 Physics of the Interstellar Medium 33 Interstellar Grains A kcorrection is the correction made to an observed flux or color to account for the redshifting of the SED with respect to the fixed filter(s) bandpass. In the submillimeter there is a unique twist, a 'negative' kcorrection. The 850m flux stays relatively fixed for all observable redshifts, making this wavelength particularly powerful for carrying out cosmological surveys of dusty galaxies. Galaxy SED f z filter 850mm l ASTR 5470 Physics of the Interstellar Medium 34 Interstellar Grains The InfraredtoRadio correlation is one of the tightest correlations known for galaxies (see previous IRRadio SEDs on p. 28). References: Helou, Soifer, RowanRobinson 1985 Condon 1992 Yun, Reddy, Condon 2001 The correlation relies on the trapping of supernovaeejected electrons within galaxy magnetic field lines, and is defined as q = log IR/Radio It is a global correlation, but we are exploring it on small scales with Spitzer and Herschel. e s spiraling around interstellar magnetic field linessynchrotron radiation ASTR 5470 Physics of the Interstellar Medium Yun, Reddy, Condon 2001 35 Interstellar Grains The InfraredtoRadio correlation We understand the basic outline of the physics of this correlation. Massive stars are responsible for both heating dust grains which reemit in the infrared, and for kicking out energetic electrons in supernovae phenomena which spiral around galaxy magnetic field lines producing synchrotron radiation. However, though all stars can contribute to dust heating not all stars go supernovae. Moreover, the mean free path for UV photons is only ~100 pc whereas the diffusion length scale for cosmic ray electrons is 12 kpc (after this distance the electrons typically become entangled in galaxy magnetic fields). Thus, there must be some sort of global averaging to get the global infraredtoradio ratio to be so remarkably constant among galaxies. Also surprising is that this constancy holds for a wide range of galaxies. It holds for galaxy samples exhibiting a variety of metallicities, magnetic field strengths, masses, grain populations, and star formation rates. An interesting question is what occurs on smaller scales within galaxies, scales for which an even larger diversity in metallicities, magnetic fields, dust grains, and star formation rates might be explored? ASTR 5470 Physics of the Interstellar Medium 36 Interstellar Grains The InfraredtoRadio correlation Locally, the infraredtoradio relation is higher in spiral arms and other sites of star formation; HII regions have an excess of infrared emission relative to the radio continuum emission. Left to right: 1. 24m map 2. q70=70m/22cm (FIR vs synchrotron) 3. q24=24m/22cm (MIR vs synchrotron) Murphy+06 ASTR 5470 Physics of the Interstellar Medium 37 Interstellar Grains The InfraredtoRadio correlation Aperture masks overlaid on 24m images nucleus: cyan innerdisk: red outerdisk: yellow arm: blue interarm: green (Murphy et al. 2005) ASTR 5470 Physics of the Interstellar Medium 38 Interstellar Grains The InfraredtoRadio correlation q70 grouped by different environments within each galaxy. Solid lines indicate the expected trend if the galaxy disk was characterized by a constant radio surface brightness. Murphy06 ASTR 5470 Physics of the Interstellar Medium 39 Interstellar Grains The InfraredtoRadio correlation List four properties of a dwarf irregular galaxy that has recently undergone a burst of star formation. Be prepared to discuss how each property could affect the infraredtoradio ratio. 1. 2. 3. 4. ASTR 5470 Physics of the Interstellar Medium 40 Interstellar Grains Much of the PAH emission we observe in the Milky Way is stimulated by the absorption of optical photons with energy of order 1 eV. But the strong midinfrared PAH features are at ~10m and thus involve photons being released with energy ~0.1 eV. Explain how optical photons can be responsible for PAH features—where does all the 'leftover' energy go? It is impossible for an isolated electron to absorb a photon and conserve both momentum and total relativistic energy. Is it possible for an isolated atom to absorb a photon? The work function of a typical grain is 5 eV. Photons that escape from H II regions have energies at least as high as 11 eV. If a typical “classical” grain is 100 nm across, how ionized can the grain typically become in an 11 eV radiation field? ASTR 5470 Physics of the Interstellar Medium 41 Homework Recap: Interstellar Grains 8. Emission and absorption: cooling and heating a) Why isn't the absorption of a photon by an atom or molecule in an interstellar cloud necessarily a heating event? It deposits energy into the cloud, but what else must happen for it to heat the cloud? b) Why isn't the emission of a photon by an atom or molecule in an interstellar cloud necessarily a cooling event? Answers a) Consider a 3 eV photon (Joe) that leaves his star (Sally) and heads out for his morning work of exciting hydrogen. Consider a 13.7 eV photon (Jim) that heads out into a 104 K HII region (Sue). Consider Jim exploring a 50 K HI region (Marta). b) Consider Joe, the 3 eV photon again. Consider an 8 eV photon (Jerry) that leaves a Hydrogen atom within Marta, and runs into a neutral iron atom (Susette). ASTR 5470 Physics of the Interstellar Medium 42 Weisz+2011 Interstellar Grains ASTR 5470 Physics of the Interstellar Medium 43