17 Galaxies & Dark Matter

1. Dark Matter in the Universe

Galaxy Rotation Curves

  • (a) By observing orbital velocities at different distances from the center of a disk galaxy (M64, the “Evil Eye” galaxy, 5 Mpc distant), astronomers can plot a rotation curve for the galaxy.

  • (b) Rotation curves for some nearby spiral galaxies indicate masses of a few hundred billion times the mass of the Sun.

  • The corresponding curve for our own Galaxy is marked in red for comparison.

  • Rotation curvers allow measurement of galaxy masses.

  • (a) In a binary galaxy system, galaxy masses may be estimated by observing the orbit of one galaxy about the other.

  • (b) The mass of a galaxy cluster may be estimated by observing the motion of many galaxies in the cluster and estimating how much mass is needed to prevent the cluster from flying apart.

Galaxy Mass Measurements

  • Galaxies need 3-10 times more mass than can be observed to explain their rotation curves.

  • The discrepancy is even larger in galaxy clusters; about 10-100 times.

  • The total mass needed is more than the sum of the dartk matter associated with each galaxy.

Dark Galaxy?

  • Astronomers speculate that some galaxies may be composed almost entirely of dark matter, emitting virtually no visible light.

  • (Right) Has the long stream of gas leaving galaxy UGC 10214 been torn out by a close encounter with a dark companion at bottom right?

  • Or is the companion simply hidden behind, or even in front of, the visible galaxy?

Galaxy Cluster X-Ray Emission

  • (a) X-ray image of Abell 85, an old, distant cluster of galaxies. The cluster’s X-ray emission is shown in orange. The green graphs display a smooth, peaked intensity profile centered on the cluster, but not associated with individual galaxies.

  • (b) The contour map of the X rays is superimposed on an optical photo, showing its X rays are strongest around Abell 85’s central supergiant galaxy.

  • This shows that the space between the galaxies within galaxy clusters is filled with superheated gas.

  • (c) Superposition of infrared and X-ray radiation from another distant galaxy cluster. The X rays are shown as a fuzzy, bluish cloud of hot gas filling the intracluster spaces among the galaxies.

  • (d) IR image of the central region, showing the richness of this cluster, which spans about a million parsecs.

Origin of the Gas

  • It is believed this gas is primordial, dating from the very early days of the Universe.

  • There is not nearly enough of this gas to be the needed dark matter in galaxy clusters.

2. Galaxy Collisions

  • The “Cartwheel” galaxy (Left image - upper left) appears to be the result of a collision that has led to an expanding ring of star formation moving outward through the galactic disk.

  • Four bands: IR (red) + Optical (green) + UV (blue) + X-ray (purple)

  • The offending galaxy might be one of the smaller objects at right.

Galaxy Encounter

  • This encounter between two spirals, NGC 2207 (left) and IC 2163, has already led to bursts of star formation in each.

  • Eventually the two will merge, but not for a billion years or so.

  • (a) The “Antennae” galaxies collided a few tens of millions of years ago.

  • The long tidal “tails” (black and white image at the left) mark their final plunge.

  • Strings of young, bright “super star clusters” (color image at center) are the result of violent shock waves produced in the gas disks of the two colliding galaxies.

  • (b) A computer simulation of the encounter shows many of the same features as the real thing, strengthening the case that we really are seeing a collision in progress.

3. Galaxy Formation and Evolution

  • (a) The present view of galaxy formation holds that large systems were built up from smaller ones through collisions and mergers, as shown schematically in this drawing.

  • (b) This photograph provides “fossil evidence” for hundreds of galaxy shards and fragments, up to 5000 Mpc distant.

  • (c) Enlargements of selected portions of (b) reveal rich (billion-star) “star clusters,” all lying within a relatively small volume of space (about 1 Mpc across).

  • Their proximity to one another suggests that we may be seeing a group of pregalactic fragments about to merge to form a galaxy.

  • The events pictured took place about 10 billion years ago.

Hubble Deep Field

  • Numerous small, irregularly shaped young galaxies can be seen in this very deep (i.e catching fainter, therefore distant objects) optical image (made with an exposure of approximately 100 hours).

  • Redshift measurements (numbers next to objects) indicate that some of these galaxies lie well over 1000 Mpc from Earth.

  • The field of view is about 2 arc minutes across, or less than 1 percent of the area subtended by the full Moon.

  • The most distant appear irregular, supporting the theory of galaxy formation by merger.

Starburst Galaxies

  • (a) Collision. This interacting galaxy pair (IC 694, at the left, and NGC 3690) shows starbursts now under way in both galaxies — hence the bluish tint.

  • Such intense, short-lived bursts probably last for no more than a few tens of millions of years—a small fraction of a typical galaxy’s lifetime.

  • (b) Black Hole Interaction. This infrared image of a pair of starburst galaxies (called Arp 107) shows numerous young star clusters arrayed as though along a pearl necklace.

  • (c) Collision. The peculiar (Irr II) galaxy NGC 1275 contains a system of long filaments that seem to be exploding outward into space.

  • Its blue blobs are probably young globular clusters formed by the collision of two galaxies.

Galactic Cannibalism

  • Here we have a dramatic glimpse of a large and massive galaxy under assembly by the merging of smaller, lighter galaxies.

  • This is the way most galaxies probably developed in the earlier universe—by means of a “bottom up” scenario that hierarchically built really big objects by merging star-rich building blocks.

  • This image captures a formative process that occurred about 10 billion years ago, only a few billion years after the Big Bang.

  • The bigger image highlights a region at upper left (in the white box) and nicknamed the “Spiderweb” Galaxy.

  • The inset shows more clearly dozens of small galaxies about to merge into a single huge object, in fact altogether one of the most massive galaxies known.

Tidal Streams in the Milky Way

  • This illustration depicts the breakup and dispersion of the contents of an incoming star-rich galaxy companion captured by our Milky Way.

  • Such streams of stars, all in similar orbits and having much the same composition, are found in the halo of our Galaxy today.

  • Eventually, the smaller galaxy dissolves within the larger one much as other dwarf companion galaxies were probably “consumed” by our Galaxy long ago.

  • This simulation shows how interaction with a smaller galaxy could turn a larger one into a spiral (e.g over several 100 million years).

Galaxy Mergers

  • (Top) When comparably-sized galaxies come together the result is probably an elliptically shaped galaxy, as their original arms and disks do not likely survive the encounter.

  • (Bogttom) By contrast, if a large spiral absorbs a smaller companion, the probable result is merely a larger spiral, with much of its original geometry unchanged.

4. Black Holes in Galaxies

  • (Left) This galaxy is viewed in the radio spectrum, mostly from 21-cm radiation.

  • Doppler shifts of emissions from the core show that the gas clouds around the center obey Kepler’s third law perfectly and have revealed a slightly warped disk.

  • These enormous speeds imply that the cental object is very close to a massive object - a black hole.

  • (Right) The mass of the central black hole is well correlated with the mass of the galactic bulge, for those galaxies where both have been measured.

Binary Black Hole

  • Starburst galaxy NGC6240: (a) optical, (b) X-ray images

  • They show two supermassive black holes (the blue-white objects near the center of the X-ray image) orbiting about 1 kpc apart.

  • According to theoretical estimates, they will merge in about 400 million years, releasing an intense burst of gravitational radiation in the process.

Quasar Host Galaxies

  • Images of distant quasars.

  • They clearly show the young host galaxies in which the quasars reside

  • They imply that quasars represent an early phase of galactic evolution (but highly luminious).

  • Note that several of the quasars appear to be associated with interacting galaxies, consistent with current theories of galaxy evolution.

Quasar Era

  • Quasar Epoch ended 10 billion years ago.

  • All quasars we have seen are older than this.

  • Therefore most probably the black holes (at the centers of the galaxies) powering the quasars.

Galaxy Evolution

  • Some possible evolutionary sequences for galaxies:

    • It begins with galaxy mergers;

    • leading to the highly luminous quasars;

    • decreasing in violence through the radio and Seyfert galaxies;

    • ending with normal ellipticals and spirals.

  • The central black holes that powered the early activity are still there at later times; they simply run out of fuel as time goes on.

5. The Universe in Large Scales

Galaxy clusters

  • They join in larger groupings called superclusters.

  • Our Local Group is part of the Local Supercluster:

    • Virgo Supercluster (see left center of the picture).

    • Picture shows a 100 Mpc wide region.

    • It contains tens of thousands of galaxies.

    • It doesn't show "individual galaxies" but superclusters (smoothed contour plots).

Structures in the Large Scale

  • At the extend of 100–200 Mpc scale:

    • This slice covers 6° of the sky.

    • Each dot is a galaxy.

    • Covering 1732 galaxies.

  • At this scale:

    • Walls and Voids appear.

    • Galaxies and clusters are not randomly distributed.

    • They have filamentary structures.

    • They surround vast, nearly empty voids.


Larger Scales

  • At the extend of this scale, smaller scale structures (thin blue arc in the northern sky) are still shown.

  • But no sign of a larger scale structure.

    • Note that density of galaxies decreases close to the end of the scale.

  • At this scale:

    • 23,697 galaxies.

    • Range goes up to 1000 Mpc

    • Coverage is about 80˚ x 4.5˚ wedges of the sky.



Cluster Collision

  • Clusters of galaxies must also occasionally collide (see the image).

  • Catalog name 1E 0657-56, “bullet cluster”, about 1 billion parsecs away.

    • the galaxies themselves in white

    • hot X-ray-emitting gas from the intracluster gas in red

  • The blue color represents the dark matter within the two large clusters.

  • This might have been the most energetic collision in the universe since the Big Bang.

6. Gravitational Lensing

Quasars at Large Scales

  • This appeared at first to be a double quasar (distance is about 2 billion pc).

  • But it is two images of the same quasar.

  • Note bright two “blobs” at upper left and lower right; two objects at the center are unrelated foreground clusters.

This could happen via gravitational lensing:

  • Quasar can be studied (due to a time difference between the two paths)

  • The lensing galaxy can be studied (analyzing the bending of the light).

(a) The “Einstein Cross” (taken by HST, size is couple of arc seconds) a multiply-imaged quasar. Four images of the quasar surrounds the lensing galaxy at the center. (b) A simplified artist’s conception (Earth at right, the distant quasar at left).

  • This gravitational lensing shows more than a hundred faint arcs from very distant galaxies.

  • The wispy pattern spread across the foreground galaxy cluster (A2218; about a billion parsecs distant) resembles a spider’s web.

  • But it is due to gravitational field of the cluster which deflects the light from background galaxies and distorts their appearance.

  • By measuring the extent of the distortion the mass of the cluster can be estimated.

  • The galaxy cluster 0024 + 1654, at about 1.5 billion pc.

  • The reddish-yellow blobs, concentrated toward the center of the image, are members of the cluster.

  • The bluish looplike features are images of a single background galaxy.