Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW1Questions of the Day•How do we detect the presence of current star formation in galaxies?•How is neutral hydrogen distributed in galaxies?•How do star formation and density affect the transitions between different gas phases?•What are major differences between early and late type galaxies?•What is the Hubble sequence, and how do the properties of galaxies vary along it?1Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW2Gas & Dust in galaxies = “Interstellar Medium”This gas has a range of densities and temperatures:Cold & Dense = Molecular (H2) + DustWarm = Atomic! (HI) Hot & Diffuse = Ionized! (HII)2Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW3Molecular gas in the Milky Way…Dense Molecular CloudsYoung stars3Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW4O Stars destroy molecules and ionize Hydrogen• They produce almost all of the UV light necessary to ionize Hydrogen•Not many, but very destructive!B Stars are bright, but don’t produce much ionizing radiation4Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW5The Orion Nebula•Hot young O & B stars ionize and heat the surrounding gas.H2 ! HIIMolecular H Ionized H“HII Region”Star formation transforms the host molecular cloudDusty molecular gasIonized gasNew stars5Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXEagle Nebula: Hot young O & B stars eating away the molecular cloud from which they formed!Gas is still molecular in the columns…6Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW6O Stars live for only a few million yearsWhen these are gone……no stars are left to ionize the gas!If you see ionized gas, you know that there are recently formed O stars.7Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW7Ionized gas can be detected through “recombination” emission lines:_+1. Occasionally atoms “recombine”. Electron does not always recombine to the ground state__2. Electron drops down a level_+3. UV radiation quickly reionizes the atom8Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW8“Recombination” Radiation1. Recombination_+2. Decay____Some of these photons will be Balmer emission lines (H!)9Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW9The Rosette NebulaMost of the diffuse filametary red stuff is produced by H-alpha emission lines.HII Regions tend to look red10Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXVery colorful usually means emission linesIonized gas11Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW10HII RegionsPowered by O-stars12Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXHourglass nebula(not the true colors)Blue: OxygenGreen: H!Red: SulfurThere are other emission lines too…13Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW11We see “HII Regions” in other galaxies.Orion nebula in MWThe LMCO Stars = HII Region = H! emission lines14Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW12“Horsehead Nebula” in OrionWhich one is the: Young massive star? Molecular Gas & Dust? Emission lines from ionized gas15Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWX“Horsehead Nebula” in OrionLooks RED because H! emission line is so strong! H! is a good indicator of star formation!O Stars = HII Region = H! emission lines16Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXIf star formation stops, the O stars are gone within a few When these are gone……no stars are left to ionize the gas!Gas then “recombines” to form neutral atomic gas17Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXWays of measuring current star •H! emission line luminosity: – Detects the presence of HII regions where ionized gas is produced by young hot stars.•Ultraviolet luminosity:– Directly detects O-stars.•Far-Infrared (“FIR”) luminosity:– Detects the thermal radiation from dust which has been warmed by the young stars.18Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXFar-Infrared Radiation tracks Very long wavelengths -- Dust rarely heats up beyond 100K19Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW13Most of the gas in galaxies is neither ionized or molecular!Atomic Gas in the Milky Way!20Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW14Most of the HI is in the disk of spiral galaxiesOptical Image (stars & dust)Atomic Gas (neutral hydrogen = HI)21Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW15Atomic Hydrogen (HI) is detected through “21 cm radiation”.• Very low energy transition in very lowest energy state. Easy for any neutral Hydrogen gas to have electrons in the upper state of the first energy level.22Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXProduction of 21cm emission23Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW16The “Very Large Array” is one of the best places to map the distribution of atomic gas (HI)24Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW17The 21 cm line is EXTREMELY important for the study of galaxies!•Easy to detect with “radio telescopes”•Allows us to trace how the gas is moving (through the doppler effect). •Allows us to measure nearly the total mass of gas in a galaxy.25Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW18HI Maps of Galaxies often reveal suprises!The atomic gas is often much more extended than the stars.26Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW19HI Maps of Galaxies often reveal suprises!Atomic gas sometimes reveals connections between galaxies that were not apparent from stars alone!“Interacting Galaxies”27Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW20How are the three phases of gas inter-related?Atomic HIMolecular H2Ionized HIIGas is compressed, and coolsYoung massive O-stars form, and ionize the gasYoung massive stars die out, and electrons and nuclei recombine28Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW21Star formation can propagate! Supernovae from young stars compress resevoir of nearby atomic gas, forming denser molecular gas, seeding new SF!Neutral HIDenser H2Ionized HII29Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW2230Astronomy 101, Winter
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