1 Components of Galaxies Gas The Importance of Gas Fuel for star formation (H 2 ) Tracer of galaxy kinematics/mass (HI) Tracer of dynamical history of interaction between galaxies (HI)
2 The Two-Level Atom Consider an atom comprised of only two levels, 1 & 2 n 2 atoms m -3 n 1 atoms m -3 When the atoms jumps from 2 to 1, it emits a photon of frequency ν 21 & energy hν 21, where h is the Planck constant (6.63x10-34 J. s).
3 Spontaneous Transition Probability from 2 to 1 is expressed in terms of the Einstein coefficient A 21 (s -1 per atom). The energy released per second per cubic meter through spontaneous decay is thus Stimulated Transition Probability is expressed in terms of the Einstein coefficient B 21 & the energy density U 21 (= I ν / c) such that the energy released per second per cubic meter per atom is, Similarly, B 12 represents the stimulated up transition probability.
4 The net number of absorptions per unit volume per second is then And thus the energy absorbed per unit volume per second is, Where κ ν is the absorption coefficient. Note that the source Function, S, is simply the ratio of the emission coefficient & the absorption coefficient j ν,
5 Given, I ν / c can be substituted for U 21 to get Balancing the absorption rate with the emission rate, In terms of Einstein coefficients, where the factor 4π comes from integrating over 4π steradian solid angle.
6 Rearranging the terms in A 21 And solving for I ν, In Thermodynamic equilibrium, Thus, given that the levels are populated as where g 1 & g 2 are statistical weights & T is the excitation temperature,
7 The Einstein coefficients are equal to.
9 Neutral Hydrogen A source of strong emission in galaxies is the 21 cm neutral hydrogen emission line. The line arises from the flip in the spin state of an electron in the ground state of neutral hydrogen.
10 Neutral Hydrogen, cont. Forbidden transition, with A 10 = 2.9x10-15 s -1. Transition rate lifetime is t 10 = 1.1x10 7 years Why is the signal strong? High HI column density (i.e., N HI = cm -2 ). I.e., if n 1 l is large, where l is the length along the line of sight containing HI, then h ν 10 n 1 l is large.
11 In galaxies, collisions dominate 21 cm transitions I.e., emitting clouds are in thermodynamic equilibrium. Determination of collision time: 1) collisional cross section σ = π (2x10-8 cm) 2 = 1x10-15 cm 2 2) atomic density N = 1 cm -3 The mean free path is Atomic gas in galaxies has T = 100K, thus the velocity v is, Thus, the time between collisions is
12 Most of the electrons in neutral hydrogen are in the F=1 state. This can be shown by comparing E = h ν / k T ex = 100 K (Typical HI T) Thus, where g F = 2F + 1. Since n 1 ~ 3n 0, then n HI = n 1 + n 0 = 4n 0
13 Optical Depth For HI at T = 100 K, h ν << k T. Thus, the spin states are populated where g 1 / g 0 = 3. Similarly, HI absorption occuring from transitions are proportional to Making the substitution n 0 = ¼ n H
14 The optical depth, τ, is thus, Where 1) N H = H column density 2) f(v) is a function that accounts for the contribution of 21 cm emission occuring at velocity v. f(v) is normalized such that, A useful form is where
15 Hydrogen Column Density The above expression can be rewritten & solved for hydrogen column density Or for τ << 1,
16 In terms of HI Mass, where, 1) d is in units of Mpc 2) S(v) dv is in units of Jy km s -1.
17 HI in Emission & Absorption HI is seen in both emission & absorption, depending on the optical depth & presence of background continuum source. Consider the equation of radiative transfer, Multiplying through by e -τ ν & rearranging the terms,
18 For an isothermal cloud in LTE, Sν = Bν = constant. Integrating yields, For this equation, 1) I ν (0) is the intensity of the background source 2) S ν is the intensity generated within the cloud. Typically, radio astronomers use temperatures instead of fluxes, where temperature is simply that given by the Planck function in the Rayleigh-Jean regime. Thus, for I ν & S ν, where T B = brightness temperature & T cloud = cloud temperature
19 Thus, How does the temperature measured change with τ? Consider a cloud observed in front of a background source of temperature T B (0) telescope Cloud Background Source
20 Molecular Gas Molecular hydrogen clouds are the sites of star formation Dust grain surfaces are the catalyst for H 2 formation The ratio of the number densities of dust grains to hydrogen atoms is n g / n H = The formation rate of H 2 is thus, where πa 2 is the grain cross section (a = 1x10-7 m) and For gas at a temperature of 100 K, v H = 1000 m s -1. Thus,
21 The destruction rate of H 2 is s -1. Specifically, 1) s -1 for N H2 < m -2 2) & s -1 for N H2 = m -2. Thus, the destruction rate is equal to the formation rate when, The cross section for photodissociation is σ PD = m 2. Thus, where l is the path length.
22 Thus, the condition required for significant shielding of dissociating radiation is, Thus, for a cloud with moderate hydrogen number density of n H ~ 10 8 m -3, the core can be molecular if the shell thickness is on the order of l ~ 1 pc.
23 CO as a Tracer of Molecular Hydrogen H 2 has no easily observed transitions 28 µm from space 2 & 12 µm only warm (T=100K) H 2 CO collisionally excited by H 2.
24 Collisional Excitation of CO The collisional coefficient is given by where 1) v H2 is the velocity of H 2 molecules 2) σ CO is the collisional cross section of CO 3) N CO is the number density of CO For significant emission from collisions, C = A J,J-1, where A J,J-1 is the Einstein coefficient for spontaneous emission. Thus,
25 The CO lines are optically thick, τ >> 1, thus N crit must be divided by the number of steps, N esc, it takes for a photon to random walk out of the molecular cloud, Thus, for CO(1 0), N crit > 3000 cm -3 (for τ << 1), and few 100 cm -3 (for τ >> 1).
26 The Mass of Molecular Hydrogen Clouds CO luminosity can be used to calculate the H 2 mass. The CO luminosity, L CO, of a molecular cloud is, where, 1) R = radius of the cloud 2) T CO v = flux of the CO emission line For a cloud in virial equilibrium, twice the time averaged kinetic energy KE is equal to the negative of the time-averaged potential energy W
27 Substituting in for v in the luminosity equation, Because, then, ½ Solving for M cloud yields,
28 It turns out that i.e., high density clouds are hot, low density clouds are cold. Thus, The proportionality constant is α.
29 What is α? (Scoville & Sanders, in Inter- Stellar Processes) L CO vs. dynamical mass in molecular clouds So, α = 4 M solar (K km s -1 pc 2 ) -1 Note the scatter!
30 Dynamical Mass Determinations In external galaxies, the virial theorem can be used to calculate the dynamical mass of a CO disk, & thus determine an upper limit on α. Dynamical mass is where, 1) v los = line of sight velocity 2) R is the radius of the disk 3) i is the inclination angle. Note that v los v true i
31 Properties of HI & H 2 in Galaxies The HI extent > optical extent of galaxies (2 3 R H at n = cm -2, where R H is the Holmberg Radius the radius at which Σ pg = 26.5 mags arcsec 2 ) Optical HI extent has a sharp cutoff (at n HI ~ cm -2 ) HI Many galaxies have HI holes in the central regions which correspond to the location of the stellar bulge. (Gilmore et al., pg 203)
32 Distribution vs. R HI Holes Some (like the Milky Way) have Molecular Rings (Scoville & Sanders 1987, in Interstellar Processes, pg 21)
33 HI, H 2, & Dust in the Milky Way (Gilmore et al., pg 68)
34 Counter-Example No HI hole No molecular ring (Young & Scoville 1991, ARAA, 29, 608)
35 Properties of HI & H 2 in Galaxies, cont. H 2, & not HI follows the distribution of dust & Hα emission in galaxies This is because dust is a catalyst for H 2 formation and stars form in molecular clouds
37 HI Profile HI Disk Inner part thin (half height ~ 500 pc) Outer part thick & warped (i.e., beyond the optical light) (B&M pg. 504)
38 HI Rotation Curves Useful tracer of mass in the galaxy halo (Galaxies & Cosmology, Combes et al., pg 71)
THE MILKY WAY GALAXY Type: Spiral galaxy composed of a highly flattened disk and a central elliptical bulge. The disk is about 100,000 light years (30kpc) in diameter. The term spiral arises from the external
Class #4 11 September 2008 Review Stellar evolution/nucleosynthesis/h-r diagrams Phases of the Interstellar Medium The Hydrogen Atom H-R diagram for 47 Tuc Evolution+nucleosynt hesis each box is a different
Galaxies Collection of stars, gas and dust bound together by their common gravitational pull. Galaxies range from 10,000 to 200,000 light-years in size. 1781 Charles Messier 1923 Edwin Hubble The distribution
Lecture 7: Molecular Transitions (2) Line radiation from molecular clouds to derive physical parameters H 2 CO (NH 3 ) See sections 5.1-5.3.1 and 6.1 of Stahler & Palla Column density Volume density (Gas
ASTR240: Radio Astronomy HW#3 Due Feb 27, 2013 Problem 1 (4 points) (Courtesy J. J. Condon & S. M. Ransom) The GBT (Green Bank Telescope, a steerable radio telescope roughly the size of a football field
Chapter 10 The Interstellar Medium Guidepost You have begun your study of the sun and other stars, but now it is time to study the thin gas and dust that drifts through space between the stars. This chapter
Reminders! Website: http://starsarestellar.blogspot.com/ Lectures 1-15 are available for download as study aids. Reading: You should have Chapters 1-14 read. Read Chapters 15-17 by the end of the week.
The relation between cold dust and star formation in nearby galaxies George J. Bendo (with the Herschel Local Galaxies Guaranteed-Time Surveys and the Herschel Virgo Cluster Survey) Outline Analyses before
Atomic Physics 3 ASTR 2110 Sarazin Homework #5 Due Wednesday, October 4 due to fall break Test #1 Monday, October 9, 11-11:50 am Ruffner G006 (classroom) You may not consult the text, your notes, or any
Relations between the Einstein coefficients Additional reading: Böhm-Vitense Ch 13.1, 13.2 In thermodynamic equilibrium, transition rate (per unit time per unit volume) from level 1 to level 2 must equal
Astronomy 421 Lecture 14: Stellar Atmospheres III 1 Lecture 14 - Key concepts: Spectral line widths and shapes Curve of growth 2 There exists a stronger jump, the Lyman limit, occurring at the wavelength
Chapter 19 Lecture The Cosmic Perspective Seventh Edition Our Galaxy Our Galaxy 19.1 The Milky Way Revealed Our goals for learning: Where are we located within our galaxy? What does our galaxy look like?
1/5/011 The Milky Way Galaxy Distribution of Globular Clusters around a Point in Sagittarius About 00 globular clusters are distributed in random directions around the center of our galaxy. 1 1/5/011 Structure
Physics of gas in galaxies. Two main parts: atomic processes & hydrodynamic processes. Atomic processes deal mainly with radiation Hydrodynamics is large scale dynamics of gas. Start small Radiative transfer
Lecture 28: Spiral Galaxies Readings: Section 25-4, 25-5, and 26-3 Key Ideas: Disk & Spheroid Components Old Stars in Spheroid Old & Young Stars in Disk Rotation of the Disk: Differential Rotation Pattern
Lecture 2: Molecular Clouds: Galactic Context and Observational Tracers Corona Australis molecular cloud: Andrew Oreshko Classification of Young Stellar Objects (YSOs) Spectral Index Hartmann: Accretion
The Interstellar Medium (ch. 18) The interstellar medium (ISM) is all the gas (and about 1% dust) that fills our Galaxy and others. It is the raw material from which stars form, and into which stars eject
Astronomy A BEGINNER S GUIDE TO THE UNIVERSE EIGHTH EDITION CHAPTER 14 The Milky Way Galaxy Lecture Presentation 14.0 the Milky Way galaxy How do we know the Milky Way exists? We can see it even though
Chapter 11 The Formation of Stars A World of Dust The space between the stars is not completely empty, but filled with very dilute gas and dust, producing some of the most beautiful objects in the sky.
23. The Milky Way Galaxy The Sun s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter in the Milky Way galaxy Density waves produce spiral
Interstellar Medium and Star Birth Interstellar dust Lagoon nebula: dust + gas Interstellar Dust Extinction and scattering responsible for localized patches of darkness (dark clouds), as well as widespread
Concepts: Properties of Electromagnetic Radiation Chapter 5 Electromagnetic waves Types of spectra Temperature Blackbody radiation Dual nature of radiation Atomic structure Interaction of light and matter
29:50 Stars, Galaxies, and the Universe Final Exam December 13, 2010 Form A There are 40 questions. Read each question and all of the choices before choosing. Budget your time. No whining. Walk with Ursus!
SECOND EDITION Astrophysics of Gaseous Nebulae and Active Galactic Nuclei Donald E. Osterbrock Lick Observatory, University of California, Santa Cruz Gary J. Ferland Department of Physics and Astronomy,
80 2 Observational Cosmology fluctuations, short-wavelength modes have amplitudes that are suppressed because these modes oscillated as acoustic waves during the radiation epoch whereas the amplitude of
Journal of Materials Science and Chemical Engineering, 2015, 3, 7-16 Published Online April 2015 in SciRes. http://www.scirp.org/journal/msce http://dx.doi.org/10.4236/msce.2015.34002 Photoionization Modelling
Black Holes in Hibernation Black Holes in Hibernation Only about 1 in 100 galaxies contains an active nucleus. This however does not mean that most galaxies do no have SMBHs since activity also requires
Opacity and Optical Depth Absorption dominated intensity change can be written as di λ = κ λ ρ I λ ds with κ λ the absorption coefficient, or opacity The initial intensity I λ 0 of a light beam will be
A100H Exploring the Universe: Discovering Galaxies Martin D. Weinberg UMass Astronomy email@example.com April 05, 2016 Read: Chap 19 04/05/16 slide 1 Exam #2 Returned by next class meeting
Answer Key Testname: MT1-333-12S 1) B 2) A 3) E 4) C 5) C 6) C 7) C 8) A 9) E 10) C 11) A 12) C 13) C 14) C 15) C 16) D 17) A 18) D 19) A 20) C 21) B 22) A 23) A 24) C 25) B 26) C 27) A star with apparent
Notes on Photoionized Regions Wednesday, January 12, 2011 CONTENTS: 1. Introduction 2. Hydrogen Nebulae A. Ionization equations B. Recombination coefficients and cross sections C. Structure of the hydrogen
School of Physics and Astronomy FACULTY OF MATHEMATICS & PHYSICAL SCIENCES Astrochemistry and Molecular Astrophysics Paola Caselli Outline 1. The formation of H 2 2. The formation of H 3 + 3. The chemistry
A100 Exploring the Universe: The Milky Way as a Galaxy Martin D. Weinberg UMass Astronomy firstname.lastname@example.org November 12, 2014 Read: Chap 19 11/12/14 slide 1 Exam #2 Returned and posted tomorrow
Chapter 23 The Milky Way Galaxy The Milky Way is our own galaxy viewed from the inside. It is a vast collection of more than 200 billion stars, planets, nebulae, clusters, dust and gas. Our own sun and
Astro-2: History of the Universe Lecture 13; May 30 2013 Previously on astro-2 Energy and mass are equivalent through Einstein s equation and can be converted into each other (pair production and annihilations)
Reading Quiz Clickers The Cosmic Perspective Seventh Edition Our Galaxy 19.1 The Milky Way Revealed What does our galaxy look like? How do stars orbit in our galaxy? Where are globular clusters located
Active Galaxies & Emission Line Diagnostics Review of Properties Discussed: 1) Powered by accretion unto a supermassive nuclear black hole 2) They are the possible precursors to luminous massive galaxies
ASTR 1P02 Test 2, March 2017 Page 1 BROCK UNIVERSITY Test 2: March 2017 Number of pages: 9 Course: ASTR 1P02, Section 2 Number of students: 1193 Examination date: 4 March 2017 Time limit: 50 min Time of
Section: Stars Pages 32-38 Study Guide Chapter 2 Circle the letter of the best answer for each question. 1. What do scientists study to learn about stars? a. gravity c. space b. starlight d. colors COLOR
Chapter Six Spontaneous Emission, Stimulated Emission, and Absorption In this chapter, we review the general principles governing absorption and emission of radiation by absorbers with quantized energy
UNIVERSITY OF SOUTHAMPTON PHYS2013W1 SEMESTER 1 EXAMINATION 2012/13 GALAXIES Duration: 120 MINS Answer all questions in Section A and two and only two questions in Section B. Section A carries 1/3 of the
Chapter 15 The Milky Way Galaxy The Milky Way Almost everything we see in the night sky belongs to the Milky Way We see most of the Milky Way as a faint band of light across the sky From the outside, our
Prof. Jeff Kenney Class 10 October 3, 2016 3 reasons it was hard to figure out that we are in a Galaxy 1. it's big -- one needs sensitive telescopes to see (individual stars) across the Galaxy 2. we're
Interstellar Medium Physics 113 Goderya Chapter(s): 10 Learning Outcomes: A World of Dust The space between the stars is not completely empty, but filled with very dilute gas and dust, producing some of
AST-1002 Section 0459 Review for Final Exam Please do not forget about doing the evaluation! Bring pencil #2 with eraser No use of calculator or any electronic device during the exam We provide the scantrons
What tool do astronomers use to understand the evolution of stars? Groups indicate types of stars or stages in their evolution. What is plotted? How does an individual star move around the diagram? What
Lecture Outlines Chapter 24 Astronomy Today 8th Edition Chaisson/McMillan Chapter 24 Galaxies Units of Chapter 24 24.1 Hubble s Galaxy Classification 24.2 The Distribution of Galaxies in Space 24.3 Hubble
Radiative transfer etc Last time we derived the transfer equation dτ ν = S ν I v where I ν is the intensity, S ν = j ν /α ν is the source function and τ ν = R α ν dl is the optical depth. The formal solution
Our Galaxy Chapter 19 Lecture The Cosmic Perspective 19.1 The Milky Way Revealed What does our galaxy look like? What does our galaxy look like? How do stars orbit in our galaxy? Seventh Edition Our Galaxy
How Long do Stars Live (as Main Sequence Stars)? A star on Main Sequence has fusion of H to He in its core. How fast depends on mass of H available and rate of fusion. Mass of H in core depends on mass
The Physics of the Interstellar Medium Ulrike Heiter Contact: 471 5970 email@example.com www.astro.uu.se Matter between stars Average distance between stars in solar neighbourhood: 1 pc = 3 x 1013 km,
Lecture 18 - Photon Dominated Regions 1. What is a PDR? 2. Physical and Chemical Concepts 3. Molecules in Diffuse Clouds 4. Galactic and Extragalactic PDRs References Tielens, Ch. 9 Hollenbach & Tielens,
Chapter 9 The Formation and Structure of Stars The Interstellar Medium (ISM) The space between the stars is not completely empty, but filled with very dilute gas and dust, producing some of the most beautiful
Galaxy Simulators Star Formation for Dummies ^ Mark Krumholz UC Santa Cruz HIPACC Summer School August 6, 2010 The Challenge of Star Formation ~10 pc ~10 pc ~10 pc Like stars, star formation involves impossibly
Spatial distribution of stars in the Milky Way What kinds of stars are present in the Solar neighborhood, and in what numbers? How are they distributed spatially? How do we know? How can we measure this?
Galaxies 626 Lecture 5 Galaxies 626 The epoch of reionization After Reionization After reionization, star formation was never the same: the first massive stars produce dust, which catalyzes H2 formation
Chapter 24 Galaxies Units of Chapter 24 24.1 Hubble s Galaxy Classification 24.2 The Distribution of Galaxies in Space 24.3 Hubble s Law 24.4 XXActive Galactic Nuclei XXRelativistic Redshifts and Look-Back
Copyright (2003) Geroge W. Collins, II 11 Environment of the Radiation Field... Thus far, we have said little or nothing about the gas through which the radiation is flowing. This constitutes the second
HII regions Massive (hot) stars produce large numbers of ionizing photons (energy above 13.6 ev) which ionize hydrogen in the vicinity. Detailed nebular structure depends on density distribution of surrounding
Spectral Energy Distribution of galaxies Paola Santini PhD in Astronomy, Astrophysics and Space Science A.A. 2013 2014 Key points lecture 1 Multiwavalength astronomy: 1. Gives a complete view of the galaxy
CHAPTER 28 STARS AND GALAXIES 28.1 A CLOSER LOOK AT LIGHT Light is a form of electromagnetic radiation, which is energy that travels in waves. Waves of energy travel at 300,000 km/sec (speed of light Ex:
Periodic Table of Elements Taking Fingerprints of Stars, Galaxies, and Other Stuff Absorption and Emission from Atoms, Ions, and Molecules Universe is mostly (97%) Hydrogen and Helium (H and He) The ONLY
Chapter 19 Lecture Chapter 19: Our Galaxy Our Galaxy 19.1 The Milky Way Revealed Our goals for learning: What does our galaxy look like? How do stars orbit in our galaxy? What does our galaxy look like?
The Milky Way Three Major Components Bulge young and old stars Disk young stars located in spiral arms Halo oldest stars and globular clusters Components are chemically, kinematically, and spatially distinct
Transfer Equation and Blackbodies Initial questions: There are sources in the centers of some galaxies that are extraordinarily bright in microwaves. What s going on? The brightest galaxies in the universe
3. Stellar Atmospheres: Opacities 3.1. Continuum opacity The removal of energy from a beam of photons as it passes through matter is governed by o line absorption (bound-bound) o photoelectric absorption
Galaxies Aim to understand the characteristics of galaxies, how they have evolved in time, and how they depend on environment (location in space), size, mass, etc. Need a (physically) meaningful way of
Lecture 14: Cosmology Olbers paradox Redshift and the expansion of the Universe The Cosmological Principle Ω and the curvature of space The Big Bang model Primordial nucleosynthesis The Cosmic Microwave
firstname.lastname@example.org Durham University David M Alexander Active Galactic Nuclei The Power Source QuickTime and a YUV420 codec decompressor are needed to see this picture. Black hole is one billionth
Figure 70.01 The Milky Way Wide-angle photo of the Milky Way Overview: Number of Stars Mass Shape Size Age Sun s location First ideas about MW structure Figure 70.03 Shapely (~1900): The system of globular
Journal Club Presentation on The BIMA Survey of Nearby Galaxies. I. The Radial Distribution of CO Emission in Spiral Galaxies by Regan et al. ApJ, 561:218-237, 2001 Nov 1 1 Fun With Acronyms BIMA Berkely
. Basic assumptions for stellar atmospheres 1. geometry, stationarity. conservation of momentum, mass 3. conservation of energy 4. Local Thermodynamic Equilibrium 1 1. Geometry Stars as gaseous spheres
Astronomische Waarneemtechnieken (Astronomical Observing Techniques) 1 st Lecture: 1 September 11 This lecture: Radiometry Radiative transfer Black body radiation Astronomical magnitudes Preface: The Solid
Lecture 25 The Milky Way Galaxy November 29, 2017 1 2 Size of the Universe The Milky Way galaxy is very much larger than the solar system Powers of Ten interactive applet 3 Galaxies Large collections of
What is a Galaxy? Galaxies A galaxy is a collection of billions of stars, dust, and gas all held together by gravity. Galaxies are scattered throughout the universe. They vary greatly in size and shape.
The HI 21 cm Line Hydrogen is the most abundant element in the interstellar medium (ISM), but the symmetric H 2 molecule has no permanent dipole moment and hence does not emit a detectable spectral line
Energy mosquito lands on your arm = 1 erg Firecracker = 5 x 10 9 ergs 1 stick of dynamite = 2 x 10 13 ergs 1 ton of TNT = 4 x 10 16 ergs 1 atomic bomb = 1 x 10 21 ergs Magnitude 8 earthquake = 1 x 10 26
AST242 LECTURE NOTES PART 7 Contents 1. HII regions and Ionization Fronts 1 1.1. The Strömgren Sphere 2 1.2. Early Evolution 3 1.3. Achieving Pressure equilibrium 3 1.4. Jump conditions on an ionization