The Universe of Galaxies: from large to small. Physics of Galaxies 2012 part 1 introduction

Transcription

1 The Universe of Galaxies: from large to small Physics of Galaxies 2012 part 1 introduction 1

2 Galaxies lie at the crossroads of astronomy The study of galaxies brings together nearly all astronomical disciplines: stellar astronomy: the formation and evolution of stars in galaxies gastrophysics : the behavior of and the interaction between gas in and between galaxies high and low energy processes: from dust to AGN cosmology: the formation and evolution of galaxies 2

3 And uses nearly all observational techniques... from low-frequency radio observations (LOFAR) through the radio, mm, sub-mm, infrared, optical, and UV bands to the X-ray and γ-ray bands 3

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5 Messier 51: UV, optical, and NIR 5

6 Arp 85 NGC 5195 SB0 pec LINER Messier 51: M51 NGC 5194 SA(s)bc pec Sy 2.5 Radio (HI and CO), NIR, mid-ir, and X-ray 6

7 Galaxies span a huge range in sizes and masses! From giant elliptical galaxies with masses of times the mass of our own Sun, typically living in large clusters of galaxies... 7

8 M86 8

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10 ...to spiral galaxies like our own Milky Way, with masses of times that of our own Sun, usually living alone or with one or two similarly-sized companions... 10

11 M101 11

12 ...to dwarf galaxies with masses of only a million or so solar masses, which (as far as we can tell) are always the satellites of bigger galaxies 12

13 Draco 13

14 Hercules 14

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16 ellipticals 15

17 spirals ellipticals 15

18 dwarfs spirals ellipticals 15

19 dwarfs spirals most of the ellipticals mass is here 15

20 dwarfs spirals most of the galaxies are here most of the mass is here ellipticals 15

21 How does this come about? Why are there many more small galaxies than big galaxies? 16

22 Galaxies are regular Galaxies follow, by and large, scaling relations (or laws) like stars --- the HR diagram is a kind of scaling law, in log L vs. log T eff the HR diagram arises from the fact that stars forget their initial conditions on short (hydrodynamic) timescales in stars, evolution is clean; formation is messy 17

23 Galaxies, however, do not forget their initial conditions scaling relations tell us about the initial conditions of galaxy formation as well as subsequent processes for galaxies, neither formation nor evolution are clean! 18

24 Structure and morphology of galaxies Galaxies are composed of two types of matter: Baryonic matter---the stuff we re all made of--- which composes roughly 15% of the matter (but only 4.4% of the energy density) in the Universe Dark matter---the vast majority of which is not baryonic---which composes the other 85% of the mass of the Universe (but only 24% of the energy density) 19

25 The ingredients of the Universe Dark energy Dark matter Gas Stars Energy 72.0% 24.0% Mass 0.5% 3.5% 20

26 These types of matter have different radial distributions: Baryons are concentrated (primarily) to the inner tens of kpc; Dark matter can extend to hundreds of kpc Why? Baryons DM: 85% of mass Dissipation: baryons can lose energy through radiation, but DM can t 21

27 Galaxy formation with cold dark matter In the very early Universe, small lumps ( fluctuations ) in the nearly uniform density of dark and normal matter provide seeds for the growth of structure Since only gravity acts on dark matter, the dark matter collapses into halos that trap the gas (normal matter or baryons ) inside 22

28 Galaxy formation with cold dark matter Because the dark matter moves slowly (remember that it s cold ), small halos form first......and then merge to make larger halos this is called hierarchical galaxy formation : small objects form early and merge to make bigger objects Here s a movie that shows this process in action... 23

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30 Galaxy formation with cold dark matter As the gas settles into the dark matter halos, it will cool by radiation that is, giving off light which will allow it to condense even more than the dark matter Eventually, stars form when the gas is cool and dense enough 25

31 So what happens next? As stars form from the gas inside the halos, they create metals through nuclear reactions inside the stars By tracing the evolution of the elements, we can infer when and how long it took for stars to form 26

32 The evolution of the elements After the Big Bang, only H, He, and Li are made in the Universe All heavier elements are made in stars 27

33 Elements made up of even numbers of protons like Mg, Ca, Na, Si, S are called α-elements This is because these are made of adding He nuclei ( α particles ) to lighter elements These are made only in the explosions of high-mass stars: type II supernovae Elements near iron (Fe) are made in either the explosions of high-mass stars or in the explosions of low(er)-mass stars: type Ia supernovae 28

34 It is important to realize that stars live a time inversely proportional to their mass the higher the mass of a star, the shorter its lifetime! This means that the elements are produced on different timescales α-elements are produced quickly (high-mass stars!) Fe elements take longer (low-mass stars!) 29

35 Since [Fe/H] a measure of the number of iron atoms per hydrogen atom increases with time as more stars form and die, it is a (very rough!) clock As time passes and [Fe/H] increases, eventually the low-mass stars with explode and the ratio of α- elements to iron elements ([α/fe]) will start to decrease 30

36 Plots of this trend of [α/fe] vs. [Fe/H] thus give a very powerful view into how long stars formed in galaxies 31

37 If we find that stars in the dwarf satellites of the Milky Way have the same elemental abundance pattern as the Milky Way itself, we gain confidence that the Milky Way s halo can be built from similar dwarfs in the past...and that the current dwarfs are the remnants of the galaxy formation process 32

38 So the idea is then that small halos host dwarf galaxies which then merge to make bigger halos hosting bigger galaxies Where are the leftovers from this process? 33