This week at Astro 3303

This week at Astro 3303 Lecture 05, Sep 11, 2017 Pick up PE#5 Today: The Interstellar Medium (ISM) - Definition - Ingredients: HI, HII, H 2 - Physical Processes, Energetics - Diagnostic Tools - Link to Stellar and Galaxy Evolution Reading: Chapter 3.1-3.2 of textbook On the horizon: Wed Sep 27: 30-minute test

HW #2 Part III The Astrophysics Data System (ADS) is very powerful use the many options to your advantage for HW #2!

HW #2 Part III

HW #2 Part III The Astrophysics Data System (ADS) is very powerful use the many options to your advantage for HW #2!

Who am I B.A./M.S. University of Bonn & Max-Planck-Institute for Radio Astronomy, Germany Ph.D. 2007 University of Heidelberg & MPI for Astronomy, Germany NASA Hubble Postdoctoral Fellow & Senior Research Fellow at Caltech Cornell faculty since Fall 2012 Teaching at Cornell: Astro 2211, Astro 6516, 6525, 6590, 7620, Astro 3303(now) & undergraduate and graduate student advisor

Research The first galaxies in the universe Þ When and how do the first massive galaxies form? Þ What do their environments look like? Þ Under which physical conditions do they form stars? Þ What is the connection between the growth of supermassive black holes and the stars in their host galaxies? Galaxy formation and evolution through cosmic time Þ What is the connection between the interstellar medium content of galaxies (gas+dust) and the steep decline in cosmic star formation in galaxies since redshift 1-3 (~8-11 billion years ago)? Þ How important are major mergers vs. gas accretion in galaxy evolution? Gravitational lensing HFLS3 (z=6.34) gas dust 10,000 light years Þ A natural telescope and probe of small scales in distant galaxies Þ Constraints on fundamental cosmological parameters Ω(Mmol) [Msun Mpc -3 ] 10 8 10 7 Ω(mol) [Msun Mpc -3 ] 0 1 2 3 redshift

HFLS3 gas dust Recent discovery of most distant massive starburst galaxy HFLS3 (z=6.34): Almost as many stars at the Milky Way Similar total mass as the Milky Way 10,000 light years 40x more gas and dust 2000x more star formation and ~20x more star formation than extreme nearby starburst Arp 220 at a time when the infant universe had only ~6% of its present age HFLS3 (artist s rendition) Þ A pretty fiery cradle Riechers et al. 2013, Nature

Tools of the trade Typically observe cold gas and dust in distant galaxies Research thus uses many telescopes but especially radio/ (sub)millimeter interferometers such as the Very Large Array & ALMA ALMA submillimeter interferometer, Chile 50 12m telescopes + 16 7/12m telescopes Very Large Array, New Mexico radio interferometer 27 telescopes, 25m size rendered image

Spectral Energy Distribution of a Galaxy Aim: understand spectral energy distributions of galaxies. Virtually all continuum and spectral features at long wavelengths are due to the ISM

Basics The interstellar medium (ISM) is everything between the stars Gas: highly ionized to neutral to molecular Dust: solid particles Radiation Magnetic Fields Cosmic Rays Small fraction (few %) of baryonic mass of Milky Way Complex interaction w/ stars and large-scale dynamics

The Galactic Ecosystem The Milky Way is largely empty - typ. distance between stars: ~2 pc - heliosphere radius: ~235 AU stars occupy only 3x10-10 fraction of Milky Way The remaining 99.99999997% are filled by the ISM: - hydrogen, helium, + traces of metals - ionized (H II,...), neutral (H I,...), molecular (H 2,...) - gas phase or solid state (dust, ice,...)

Chemical abundances (Solar) H = 1 He < 0.1 C,N,O ~ 3, 0.7, 5 x 10-4 Fe ~ 3 x 10-5 The chemical composition of the ISM typically is comparable to the elemental abundance of the Solar System Asplund et al. 2009

ISM Components Observationally distinct objects - H II regions - reflection nebulae - dark clouds - supernova remnants (SNRs) - molecular clouds More general classification in different phases: - cool molecular clouds - cool H I clouds - warm inter-cloud gas - hot coronal gas Heating by: - stellar photons, X-ray emission, cosmic rays (energetic, ~GeV photons) - dissipation of mechanical energy Cooling through: - variety of atomic and molecular lines - continuum emission (strongly dependent of state of the material)

Objects: HII Regions H II regions surrounding early-type (<B2, T eff >25,000 K) stars, emitting lots of photons beyond Lyman limit (13.6 ev) - ionized gas, bright visible nebulous objects - T~10 4 K - n~10 3-10 4 cm -3 for compact (~0.5 pc) regions like Orion, n=10 cm -3 for diffuse nebulae like North America Nebula Associated with star-forming regions and molecular clouds optical spectra dominated by H and He recombination lines; collisionally excited, (forbidden) optical lines from ions like [O II], [O III], and [N II] Orion NGC 7000 / NA Nebula strong sources of thermal radio emission (free-free) + infrared emission from warm dust

Objects: Reflection Nebulae Bluish nebulae reflecting light from nearby stars NGC 2023 e.g., NGC 2023 in Orion; emission around the Pleiades No radio emission, but infrared emission from warm dust present (less intense than from HII regions) Pleiades Illuminated by stars later than B1 (less emission short of Lyman limit) Either cloud material from which star was formed; or chance encounter (Pleiades!); sometimes ejecta of late-type stars (e.g., red rectangle) Red Rectangle

Objects: Dark Nebulae Dark bands across the Milky Way Barnard 68 Dark clouds range from tiny (0.01 pc) [Bok globules] m to 10s of pc for large clouds; - -- covering a large range in visual extinction A V Sometimes very faint reflected light + often bright at mid- and far-ir wavelengths Some even dark in mid-ir : Infrared Dark Clouds (IRDCs) Horsehead Nebula NGC 2023

Objects are Physically Related M20 (Triffid Nebula) Reflection Nebula Dust/ Dark Nebula HII Region/ Emission Nebula

Objects: Photon-Dominated Regions (PDRs) Optical dominated by HII, reflection, or dark clouds Orion Bar IR dominated by PDRs: Photon-Dominated Regions (originally called Photo-Dissociation Region) = transition zones between atomic and molecular gas near bright O,B stars 6-13.6 ev, far-uv photons dissociate and ionize molecular gas. Most photons are absorbed by dust, but some add to heating through photoelectric effect few hundred K PAH H 2 CO PDRs bright in IR dust continuum, far-ir atomic finestructure cooling lines + molecular lines Tielens et al. 1993

Objects: Supernova Remnants (SNRs) Crab Nebula Left-over ejecta from SN explosion About 100 SNRs visible in Milky Way Filamentary and shell-like structures (but some compact, e.g., Crab), emitting line radiation Bright at radio wavelengths due to synchrotron emission; bright in X-rays because of hot (10 6 K) gas Cygnus Loop

ISM Phases These objects really are prominent manifestations of the different phases of the ISM neutral / atomic ionized molecular Coronal (very hot ionized)

ISM Phases The state of Hydrogen determines the state of the ISM Molecular region H 2 Neutral region H 0 / HI Ionized region H + / HII

Phase: Neutral Atomic Gas Traced by H I 21 cm line or optical/uv absorption lines of a variety of elements against background stars Consists of: - cold, diffuse H I clouds (~100 K): Cold Neutral Medium (CNM) - warm inter-cloud gas (~5000 K): Warm Neutral Medium (WNM) Galactic distribution: everywhere! Hartmann et al. 1997

HI vs. Starlight NCG 6946 (to scale) stars HI 21cm

Phase: Ionized Gas traced through UV/optical ionic absorption lines, Ha (Balmer line) emission - Ha emission dominated by H II regions, but most ionized gas by mass resides in a huge, diffuse reservoir (10 9 M sun ) Warm Ionized Medium (WIM), density ~0.2 cm -3, temp. ~8000 K Ionized by what? Photons escaped from H II regions (mainly photoionization by OB stars) Min. energy rate: 3x10 5 kpc -2 s -1 (equiv. of one O4 star per kpc 2 ) Total energy required: 3x10 8 L sun Finkbeiner (2003)

Phase: Molecular Gas Traced through CO lines emission at millimeter wavelengths (CO J=1-0: 2.6 mm) Concentrated in Giant Molecular Clouds - size ~40 pc, mass >4x10 5 M sun, density ~300 cm -3, temperature ~10 K - but: large range in properties, and complex substructure Self-gravitating Pressure from turbulence and magnetic fields important Sites of Star Formation >200 molecular species detected - H 2 most common (but no lines from cold ISM) - CO/H 2 abundance ~10-4 10-5 updated from Dame et al. 2001

Milky Way: Gas Distribution neutral - flat distr. out to 18 kpc - thin: 100s pc to 1 kpc - warped disk ionized - filamentary, mostly along disk - likely along spiral arms but: difficult to determine from within the plane molecular - concentrated in 3 kpc molecular ring - thin: ~75pc

Phase: Coronal Gas Coronal gas: very hot, tenuous gas pervading the ISM - temperature ~ 10 5.5 10 6 K Hot Ionized Medium (HIM) - density ~ 4 x 10-3 cm -3 traced through highly ionized species, e.g., C IV, S VI, N V, O VI in absorption against background stars; also: thermal free-free emission, radiative recombination, UV, X-ray lines Fills most of the Galactic halo; disk is less clear Heated and ionized by SN shocks; Sun in Local Bubble; Galactic fountain fills the Milky Way halo

Hot Ionized Medium: X-ray emission 10 6 K gas emits at X-ray wavelengths HIM difficult to detect in other galaxies Challenge: contamination by X-ray binaries => need spatial resolution to resolve apart, e.g., Chandra vs. XMM

Phase Structure of the ISM Molecular Medium Cold Neutral Medium Warm Neutral Medium Warm Ionized Medium Hot Ionized Medium Density (cm -3 ) 10 2-10 5 4-80 0.1-0.6 ~0.2 10-3 -10-2 Temperature (K) 10-50 50-200 5500-8500 ~8000 10 5-10 7 Scale height (pc) ~70 ~140 ~400 ~900 >1000 Volume filling factor <1% ~2%-4% ~30% ~20% ~50% Mass fraction ~20% ~40% ~30% ~10% ~1% - Molecular clouds: gravitationally-bound sites of star formation; main tracer: CO - CNM: HI clouds and filaments; main tracers: HI 21 cm absorption, UV/optical absorption - WNM: envelopes of molecular clouds, HII regions; main tracer: HI 21 cm emission - WIM: ~90% of HII in ISM; main tracer: low surface brightness Ha emission - HIM: buoyant, hot disk gas, escapes to halo through bubbles/fountains (?) hot corona, main tracers: OIV, NV, CIV absorption, OVI and X-rays for highest temp.

Complex interaction between different phases Hot Ionized Medium Warm Ionized Medium Warm Neutral Medium Cold Neutral Medium Molecular Medium E. van Dishoeck

Additional Ingredients: Interstellar Dust absorption, scattering, reddening, extinction, polarization, infrared emission size distribution: n(a) ~ a -3.5 from 3000 Å to 5 Å; at 1000 Å: 10-13 grains/h atom 1% of gas mass mass dominated by large grains, surface by small grains! Much C, Si, Mg, Fe, Al, Ti, Ca (=refractory elements) locked up in dust: depletion Grains >100 Å in radiative equilibrium with the interstellar radiation field at 15 K; hotter near bright stars Grains <100 Å are flash-heated and show 10-25 µm emission

Additional Ingredients: Large Molecules Polycyclic Aromatic Hydrocarbons (PAHs) bridge gap between very small grains and molecules Prominent mid-ir bands (e.g., IRAC/Spitzer!) excited by UV photons in spite of small fraction (10-7 relative to H), locking up 10%-15% of carbon NCG 6946 also: Diffuse Interstellar Bands (DIBs): initially discovered in 1922 by Heger ~250 known, but unknown carriers! DIBs (Jenniskens & Desert)

ISM: Mass Budget Milky Way: M * ~ 7 x 10 10 M sun M gas ~ 7 x 10 9 M sun (HI + H 2 ) E. van Dishoeck

Energy Sources: Radiation Fields Interstellar radiation field (ISRF) - sum of CMB (radio/fir), thermal emission from dust (IR), cool stars + OB stars (VIS, UV), coronal gas (FUV and shorter) Tielens 2005 Shape and strength differs from location in the Galaxy and in other galaxies (e.g., near OB stars, in AGN, and in the early Universe)!

Energy Sources: Magnetic Fields Traced by polarization, Faraday rotation, Zeeman splitting and H I 21 cm and OH Important energy & pressure source; sometimes controls gas dynamics 5 μg in Solar neighborhood; 8 μg in 4 kpc molecular ring around MW center inside molecular clouds: ~ 30 μg at n=1x10 4 cm -3 Morris 2014

Energy Sources: Cosmic Rays High energy (>100 MeV nucleon -1 ) particles; significant contribution to ISM energy density (2 ev cm -3 ). - relativistic protons 1..10 GeV - 10% He - 1% metals, electrons Originate from SNe Give rise to gamma rays when interacting with ISM Particles follow often Galactic magnetic field Low energy, ~100 MeV CR important for heating, ionization of ISM; but difficult to measure: exclusion by heliosphere

Energy Sources: Kinetic Energy support HI, molecular clouds against gravity important to shape the ISM OB super-bubbles, from SNe, from stellar outflows leads to turbulence through Rayleigh-Taylor and Kelvin-Helmholtz instabilities Decays through shock waves heating

Multi-phase ISM Pressure equilibrium Expanding SN shell Thermal pressure: P = n k b T i.e., P/k b = n T McKee & Ostriker 1977

Birthplace of stars: Molecular Clouds Ophiuchus Giant Molecular Cloud (by Loke Tan)

Zoom-in to a massive star forming region Zoom-in Milky Way M16 (Eagle) M17 (Horseshoe) Hale-Bopp M8 (Lagoon) Jupiter Credit: W. Keel

Massive SF Region Eagle Nebula (M16) credit: T.A. Rector & B.A. Wolpa

Eagle Nebula Eagle Nebula (M16) Credit: J. Hester & P. Scowen

Eagle Nebula Eagle Nebula (M16) Credit: J. Hester & P. Scowen

Zooming in further size of our solar system Credit: J. Hester & P. Scowen Eagle Nebula (M16)

Eagle Nebula Star-Forming Molecular Clouds Embryonic stars are formed inside the dense molecular clouds.. The molecular clouds are slowly eroded by the UV radiation from nearby hot stars photo-evaporation. The especially dense gaseous globules the EGGs are revealed. When the EGGs are eventually eroded away, the proto-stars emerge. M-16 Eagle Nebula Molecular Clouds EGGs Evaporating Gaseous Globules UV radiation from hot star

In the beginning... Stars are born deep in very cold dark (optically thick) clouds; not visible at optical wavelengths. Infrared telescopes can penetrate through these clouds and witness the first signs of life from a protostar

A star+disk appears...

Later, a proto-planetary disk remains around the Star ALMA image of dust emission

Giant Molecular Clouds Molecules are easily dissociated by ultraviolet photons from hot stars. - can only survive within dense, dusty clouds, where UV radiation is completely absorbed Largest molecular clouds are called Giant Molecular Clouds (GMCs): Temperature 10-20 K Diameter 15 60 pc UV emission from nearby stars destroys molecules in the outer parts of the cloud; is absorbed there. Molecules survive Cold, dense molecular cloud core Total mass 100 1 million solar masses HI Cloud

MCs: complex cloud structure: higher densities higher extinction lower temperatures faster chemical reactions shielding from radiation increased survival

Molecular Cooling is Complex

Molecular Cooling is Complex 1.3 mm spectrum of Arp 220: 28% of the broad-band flux is due to molecular lines Þ What is the continuum flux of a galaxy at (sub)mm wavelengths? Martin et al. 2011

Known Interstellar Molecules