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QSO Absorption Lines and Cosmological Simulations: The Quest to Understand Galaxies. - Galaxies form in the cosmic web - They accrete IGM gas , form stars, and deposit energy/metals back into IGM - Extended metal enriched “halos” are observed from z =0 to z =4 . Chris Churchill - PowerPoint PPT Presentation


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Chris ChurchillNew Mexico State

- Galaxies form in the cosmic web- They accrete IGM gas, form stars, and deposit energy/metals back into IGM- Extended metal enriched halos are observed from z=0 to z=4

Observational data of these halos are underutilized for constraining galaxy formation physics in cosmological simulations -how to do it?

QSO Absorption Lines and Cosmological Simulations: The Quest to Understand Galaxies

June 20, 10111This image is from a hydrodynamic cosmological simulation that uses an adapted tree refinement method to employ high resolution in regions where the gas and stars are changing rapidly, while allowing the regions that are evolving slowly to be modeled at lower resolution. The zoom in box is shows the gas density (no stars are shown) centered on a galaxy; the density scale (cm^-3) is given in the legend. Note the filamentary structure out of which the galaxy forms. Though not shown, stars form in the high density regions. Individual stars cannot be modeled, instead stars particles represent a population of stars formed in groups. The stars are modeled to evolve and supernovae explosions (including the outward force and heating of their energy). These stars pollute the gas with heavy elements like carbon, nitrogen, carbon, and oxygen. So, the complete star-gas cycles taking place in galaxies is modeled.The universe is about 13.6 billion years old

The universe is expanding and its geometric curvature is flat

The universe comprises (add to 100%) 70% dark energy 26% dark matter 4% baryonic matter (normal stuff)

How do we know there is dark energy?

About 6 billion yrs ago, the expansion rate of the universe changed from deceleration to acceleration; dark energy acts like a negative pressure and it began to dominate later in the life of the universe

How do we know there is dark matter?

If we add up all the mass in stars within any random galaxy and measure the velocities of the stars, we would deduce that the galaxies should fly apart (become unbound); star must contribute only 10% to the galaxys mass Getting acquainted with the universedark matter is 85% of all matter baryonic matter is 15% of all matterA brief inventory of the universe is presented here. The important thing to take away here is that normal matter (called baryonic matter) comprises only 4% of the universes total mass-energy density. Thus, everything we can directly study is quite literally the tip of the cosmic iceberg! As for dark matter, it is indirectly detected only through its gravitational influence on the normal matter. The normal matter interacts with light, and light is the only physical quantity that astronomers can measure. Dark matter does not interact with light. The normal matter is gravitationally attracted to the dark matter, which comprises 85% of all matter. Not only do we deduce dark matter in individual galaxies, but we also deduce that it is distributed throughout galaxy groups and galaxy clusters.

The dark energy is a relatively new discovery. It is also an indirectly deduced phenomenon. Actually, Einstein introduced this negative pressure component as the cosmological constant in his General Theory of Relativity, but he later retracted it and then called its introduction his biggest blunder. Once again, it looks as of Albert was on the money. Dark energy is deduce from observed brightness of supernovae (which are used as standard candles) in very distant galaxies (up to 10 billion light years away). In 1996-1997, their brightness was observed to be lower than was predicted by a cosmological model that has no cosmological constant. So, we needed to reinsert the cosmological constant into the model. Constraints from the WMAP satellite and the measured brightness of the distant supernovae are combined and we deduce that this so-called negative pressure (dark energy) comprises 70% of the mass-energy density of the universe. 2How do we know any of this?

Astronomers are a breed of historian with lots of extra hubris. Sandra Faber

Studying the history of the universe is something like trying to piece together the history of the earths climate by placing dixie cups out in the rain and then studying the few water drops collected Paul Hodge

Astronomers are handicapped experimental physicists, called observers.

We collect light in huge light buckets, try and organize the light by color, intensity, etc, apply the laws of atomic and gravitational physics- and

ka-chow(!) we have a little understanding of 4% of the the universe

AND, FOLKS WHAT A 4% IT IS!As an astronomer, your tools are knowledge of physics and photon counts. From the pattern of light we observed, we apply the laws of physics to deduce the composition, temperature, velocities, etc of the matter that emitted (or absorbed) the light. So, in a very real sense, all astronomical knowledge rests on the laws of physics being unchanging throughout cosmic time and space. All our knowledge is based upon deductions; astronomers do not, they cannot, make direct measurements of the matter in the universe. 3

Tips of the ice bergsSome examples of beautiful galaxies. Note the dark areas which can be seen in silhouette; they are cool gas clouds that absorb light. These dark clouds are where new stars will form. A typical spiral galaxy rotates about once every 250 million years. Our Milky Way Galaxy is a spiral (perhaps with a small bar- see 2nd galaxy down on the left). Our sun is located about 2/3rd the way out in the disk.

Also note that galaxies can appear pathological in their shapes or the gas is distributed in odd patterns. This is due to interactions between galaxies; if they pass close enough to one another, they can become gravitationally bound and eventually merge together. When galaxies collide, the stars actually never collide, only the gas collides. Thus, the gas usually falls to the center or the merging system while the stars stream in tidal tails. When the gas collides, it forms new stars (see bottom right, which shows the inner region of a merging system).4

Galactic Pathology is a transient train wreck phenomena

This movie (top panel) was motivated to help understand the merging system known as the mice (bottom left panel). To set up the simulation, the galaxy masses, rotation speed and direction, and approach vectors had to be estimated. Also, the viewing angle had to be chosen. So, there is a bit of educated guess work involvedbut if the simulation accurately models the merging system, then it gives us confidence that these input quantities correctly characterize the system. This helps us understand the physics of merging galaxies. In this case, not only do we learn the initial conditions of the merger, but we also deduce that the mice are infalling and on their second passage with each other! When playing the movie, try to estimate when the simulation matches the mice. In practice, astronomers analyze the detailed motions of the stars in the mice and quantitatively compare them to the motions of the stars in the simulations. It is very detailed business.5

These galaxies started their lives as gaseous halos before any stars ever form!

The gas (4% of the universe) gravitationally follows the formation of dark matter halos (85% of matter), which formed due to localized gravitational instabilities as the universe expands

These instabilities were seeded by quantum fluctuations created in the first fraction of a second of the Big Bang; the distribution of galaxies in the universe and the fluctuations in the cosmic microwave background provide the mass spectrum of the fluctuations (we know it well!)

All sky CMB fluctuationsCosmic web distribution of galaxiesHow do galaxies actually formThe large scale structure of the universe, meaning the relative positions of galaxies over all cosmic space, is literally an fossil imprint of the distribution of dark matter in first seconds after the Big Bang. When the universe was microscopic in size, matter was not created uniformly within it, but with small fluctuations higher and low density. As the universe expanded, the regions of relatively higher density gravitationally attract each other, and become more pronounced while regions of relatively lower density became less and less dense. The normal matter, which formed into galaxies, followed along and so galaxies formed in the regions of increasing dark matter density. This pattern of how galaxies are distributed throughout the universe is shown on the right. The fluctuations were also frozen into the temperature variations of the cosmic microwave background (CMB) light (see image on the left, which is an all sky picture). This light was emitted in a big flash when the universe was about 380,000 yrs old. The CMB was emitted in a very short burst at the moment in time when the universe cooled down enough for free electrons to become bound to protons, forming hydrogen atoms for the first time. Regions of higher density are red and of lower density are blue (though we measure temperature variations).6We put the dark matter fluctuation spectrum in as the initial condition in a cosmological simulations code, with which we model gravitational forces, star formation, stellar feedback from supernovae explosions, and the build up of chemical species heavier than helium, and model the expansion of the universe and let er rip we also put in the trace baryonic (normal) matter and let it follow the dark matter and we get- galaxies

and yes, we get out disk/spiral galaxies that rotate and elliptical galaxies, and al the train wrecks you could ask for

and we can track the b


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