1To Do TodayHand in Essay 5Where am I going this week?Dark Energy OverviewEssay 6Reading & prep for next timeGuided WritingTake 10 minutes to write down your thoughts about Essay 5, your preparation for it, and the end result.Where am I going?Hale 200 inch (5 meter) telescope2What are we looking for?What is the most common element in the Universe?What are we looking for?What is the most common element in the Universe?HYDROGEN!Andromeda Galaxy in Optical Light & Radio (Hydrogen) – same scale!What are we looking for? An atom of hydrogen will randomly give off a wave of radio energy (light!) every 10 million years. Then how can we possibly see them?ANSWER: LOTS OF ATOMS!The Milky Way contains 3 billion suns worth of hydrogen!Dwarf GalaxiesDetecting gas, not just starlightFainter objects that may contain lots of gasVery small dwarf galaxies that might be gas-rich.Are there “starless” galaxies?•Maybe they contain HI gas•They may also contain dark matter3Dwarf GalaxiesDetecting gas, not just starlightFainter objects that may contain lots of gasVery small dwarf galaxies that might be gas-rich.UGC 5288The Research QuestionThe ALFALFA survey has identified a sample of about 30 galaxies that are really nearby (within about 11 Mpc), are dwarfs, and have a lot of hydrogen relative to their size (about 10 million solar masses)Unanswered questions in dwarf galaxies:Why are there so few of them?Of the ones that are left, why do some of them manage to have lots of gas left over?How do low-mass dark matter halos form stars? Missing Satellite ProblemThe Problem: There are over an order of magnitude fewer observed dwarf satellite galaxies in the Local Group than the number of dwarf dark matter halos seen around giant halos in numerical simulations predicted by the ΛCDM cosmological model.Is ΛCDM wrong or have we just not found them?Perhaps we need to adjust the simulations (i.e. the characteristics of inflation or the nature of dark matter)Klypin et al 99 SimulationsFound: 281 satellitesKnown Local Group Members:~40Other ways to explain the mismatchDwarf galaxies are “invisible” (depleted of baryons) by z=0Dwarf galaxies are unable to survive to z=04Possible Sub-Halo Survival Inhibitors Tidal Disruption & Heating: When sub-haloes get too close to their parent galaxy they can be stretched or even cannibalized and suffer an increase in temperature. Feedback from Star Formation: Supernovae eject large amounts of gas, and given a small enough sub-halo, this ejection could have a significant effect on its evolution. Reionization: The gas in the early universe becomes very hot and difficult for gas to clump to form sub-haloes after reionization occurs.Possible Sub-Halo Survival InhibitorsCanis Major Dwarf (?)Why study Star Formation in Dwarf Galaxies? Gas-rich dwarfs lacking old stars appear “unevolved” and perhaps mimic star formation processes during the early universe. If dwarfs are building blocks for more massive galaxies, then old stellar populations in both should have similar properties. Also, oldest stars in massive galaxies must be as old as or younger than the oldest stars in dwarf galaxies. If cosmic reionization squelches star formation due to heating and gas-loss (as many cold dark matter models predict), then we should see a slowing of star-forming activity in the star formation histories of these dwarf galaxies. We haven’t been able to figure it out so far …5What am I doing?One way to trace star formation is to look and see how many “metals” there are in a galaxy.Supernovae make metalsStar formation makes supernovae!H-alpha is the easiest way for astronomers to trace the ionized hydrogen content of gas clouds. Since it takes nearly as much energy to excite the hydrogen atom's electron from n = 1 to n = 3 as it does to ionize the hydrogen atom, the probability of the electron being excited to n = 3 without being removed from the atom is very small. Instead, after being ionized, the electron and proton recombine to form a new hydrogen atom. In the new atom, the electron may begin in any energy level, and subsequently cascades to the ground state (n = 1), emitting photons with each transition. Approximately half the time, this cascade will include the n= 3 to n = 2 transition and the atom will emit H-alpha light. Therefore, the H-alpha line occurs where hydrogen is being ionized.6Optical spectrum from an HII regionAGC1002447Getting the spectrum of a dwarfStep 4 – Look at your raw spectrumif you're lucky, it should look something like this... Moving on . . . .Dark energy!Dark Energy OverviewWhat’s the main idea? How do we know it’s there?What affect does it have on the Universe?What could it be?How could we figure out what it is?What are the dark energy “problems”?The Main Idea8The Main IdeaRadiation Pressure in the early Universe caused it to expandGravity from the mass within the Universe should slow down that expansion to a stop.If there’s enough mass, the gravity should reverse the expansion.Instead, the Universe is expanding FASTER all the time, so there must be some energy that acts as “negative gravity” pushing it out.How do we know it’s there?Objects in the nearby Universe, which we see in the present time, are expanding away from us faster than objects far away, which we see as they were in the past.Expansion of the UniverseRedshift measurements tell us that all the galaxies are moving apart from each other . . .The universe is expanding!Why do we think the expansion is accelerated?9Expanding: 70 km/s/Mpc1 Mpc 1 Mpc 1 MpcOne second later . . . Expanding: 70 km/s/Mpc1 Mpc 1 Mpc 1 MpcOne second later . . . 1 Mpc + 70 km1 Mpc + 70 km1 Mpc + 70 km There are two things we are talking about here: the expansion rate, and the acceleration of the expansion rate, which is the rate of change of the expansion rate over time. It's just like distance, velocity, acceleration in basic physics: the distance is x, the velocity is a change in x over a change in time, and the acceleration is a change in velocity over a change in time. Today, in the Universe, we characterize the expansion with "Hubble's Constant": H_0= 70 km/s/Mpc. That's a weird unit, but what it means is that a region of empty space that is 1 Mpc(a "megaparsec -- about the size of the distance between galaxies) across is getting
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