<body bgcolor="#ffffff" text="#000000" link="#0000ff" vlink="#0000ff"> <p> <center><table cellpadding=10 border=10 bgcolor=aqua><tr><td><center> <h2>Normal Galaxies<p></h2><h3> <center>Chapter 24, Galaxies, Building Blocks of the Universe</td><td> <img width=500 src="http://hendrix2.uoregon.edu/~imamura/123/lecture-3/galaxiesHST.jpg"> </td></tr></table></center> <h3> <p> Galaxies are clusters of stars, gas, dust, and dark matter whose emission is dominated by the light from its stars; their spectra are sum of the individual stars. Galaxies serve as markers in our study of the structure of the Universe. They are interesting in their own right, however. Hubble developed the following <i>morphological</i> based on their appearance) classification scheme for galaxies. Basically, Hubble formed the classes of <a href="http://hendrix2.uoregon.edu/~js/ast123/lecture-3/NGC4881.gif">ellipticals</a>, <a href="http://hendrix2.uoregon.edu/~js/ast123/lecture-3/NGC4639.gif">spirals</a>, and <a href="http://hendrix2.uoregon.edu/~js/ast123/lecture-3/NGC1365.gif"> barred spirals</a>. In addition, there are galaxies which have disks but no arms, S0s (lenticulars), and amorphous looking things which he referred to as <a href="http://hendrix2.uoregon.edu/~js/ast123/lecture-3/ngc2363.gif">Irregulars</a>. Schematically, the <font color=magenta> <i>Hubble Tuning Fork</i> </font> diagram looks like <p> <center> <img src="http://hendrix2.uoregon.edu/~imamura/123/lecture-2/ HubbleTuningFork.jpg"> </center> <p> The preceding is <font color=magenta>not</font> thought to be an evolutionary scheme. There, however, appears to be evolution between the different Hubble classes as a result of collisions between galaxies. The Milky Way galaxy is usually classified as an <i>SBb</i> galaxy in the Hubble scheme. <p> Despite the fact that the Hubble Sequence is based only on the appearance of galaxies (morphology of galaxies), several physical properties of galaxies vary smoothly along the sequence. We have, <p> <h3> <pre> little gas and dust <----------------------> lots of gas and dust mainly Pop II stars <----------------------> Pop I & II stars Reddish <----------------------------------> Bluish little ongoing star formation <------------> star formation large bulge <------------------------------> small bulge tight arms <---------> loose arms </pre> </center> <p> <p><hr> <p> <center> Origin of Elliptical and Spiral Galaxies </center> <p> The basic idea is that either an elliptical galaxy or spiral galaxy will form depending upon when star formation occurs in the galaxy formation process. Galaxies are thought to form from the collapse of low-density gas clouds. If the gas turns into stars during the early stages of the process, then we essentially have a bunch of BBs collapsing to form the galaxy. Because stars are small and they are far apart, they don't collide in the formation process. This allows the stars to maintain roughly their initial <i>shape</i> and to settle into a roughly spherical form. In this case, they become elliptical galaxies. <p> If the gas does not turn into stars quickly, then we have a system of collapsing gas clouds. The gas clouds are much larger than are stars and collide much more readily during the formation stage. The collapsing material thus <i>runs</i> into opposing material as it tries to pass through the equatorial plane (for an analogous situation, consider the <a href="http://hendrix2.uoregon.edu/~imamura/122/images/sform_diag.jpg"> star formation process</a>). The collisions do not allow the collapsing material to pass through each other and the material is forced to settle into a disk. After the disk forms, star formation begins in earnest and a spiral galaxy is produced. <p><hr> <p> <center> Distribution of Galaxies In Space </center> <p> <center>Extragalactic Distance Ladder</center> <p> <center> Clustering Scales </center> <p><table cellpadding=10 border=10 bgcolor=aqua><tr><td> <img src="http://hendrix2.uoregon.edu/~imamura/121/lecture-3/jworlds.jpg"></td> <td><h3>Solar System</a> ===> ~ 0.5 light day</td></tr> <tr><td> <img src="http://hendrix2.uoregon.edu/~js/ast123/lecture-3/NGC4639.gif"></td> <td><h3>Galaxy Sizes (NGC 4639)</a> ===> 0.1 Mly to 1 Mly</td></tr> <tr><td> <img src="http://hendrix2.uoregon.edu/~js/ast123/lecture-3/M31.gif"></td> <td><h3>Local Group (Andromeda, M31) ===> ~ 3 Mly </td></tr> <tr><td> <img src="http://hendrix2.uoregon.edu/~soper/ImGalaxies/virgo.jpg"></td> <td><h3>Clusters (Virgo cluster) ===> ~ 10 Mly </td></tr> <td></td> <td><h3>Clusters of Clusters (Super-Clusters) ===> ~ 300 Mly</td></tr> <tr><td> <img src="http://hendrix2.uoregon.edu/~imamura/123/lecture-3/slice.jpg"></td> <td><h3><i>Great Wall</i> and <i>Voids</i> ===> 600 & 300 Mly (~ 0.6 and 0.3 Bly)</td></tr></table> <p><hr><p> <table cellpadding=10 border=10 bgcolor=aqua><tr><td> <h3><center>Redshift, Look-Back Time, and Distance</center><p> Because light travels with a finite speed, c = 300,000 kilometers per second, the light which we receive from distant objects must have left the objects sometime in the distant past. In the table to the right (taken from the text), we show the relationship between the redshift, current distance of the object and the <i>look-back time</i> for the object. Note that the Universe was smaller when the light left the distant objects. <td><table bgcolor=pink cellpadding=10 border=10 <tr><td><h3>Redshift</td><td><h3>v/c</tde><td><h3>Distance</td> <td><h3>Look-Back Time</td></tr> <tr><td><h3>0.000</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>0.010</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>0.025</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>0.050</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>0.100</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>0.200</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>0.250</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>0.500</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>0.750</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>1.000</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>1.500</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>2.000</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>3.000</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>4.000</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>5.000</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>6.000</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>10.00</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>50.00</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>100.0</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> <tr><td><h3>infinity</td><td><h3>0.000</td><td><h3>0 Ly</td><td><h3>0 y</td></tr> </table> </td></tr></table> <p><hr><p> <table bgcolor=aqua cellpadding=10 border=10><tr><td> <img src= "http://hendrix2.uoregon.edu/~imamura/123/lecture-1/cmbcobewmap.jpg"></td> <td><h3><center>Structure Formation</center><p> There is large scale structure in the Universe. An important problem is the question of <i> Where and when did this structure form?</i> Although the cosmic microwave background radiation (CMBR) is very <i>smooth</i> ---> the Universe was very smooth at the time of formation of the CMBR (~380,000 years after the birth of the Universe), the Universe did show considerable structure even at the time of the formation of the CMBR. This means that the Universe started to generate the currently observed structure of the Universe in the time before the formation of the CMBR (the Epoch of Recombination) (<a href="http://hendrix2.uoregon.edu/~imamura/123/lecture-3/ BigBangtillNOW.flv"/> computer simulation of structure formation</a>).</td><tr></table> <p><hr> <p> <center>Galaxies in Collision</center> <p> Galaxies are often times found in rich clusters and because the Universe was smaller in the past, collisions between galaxies are likely to have been more important in the past. Collisions still occur today and will occur in the future. For example, <table cellpadding=10 border=10><tr><td> <img width=400 src="antennae_1.jpg"></td> <td><a href="antennae_simulation.mp2"> <img width=500 src="antennae_hst_collision_2.jpg"></a><p><center> Antennae Galaxy</center></td> </tr></table></center> <table cellpadding=10 border=10><tr><td> <a href="CartWheelEvol.mp2"> <img width=500 src="cartwheel.gif"></a> <p><center>Cartwheel Galaxy</center></td> <td><img width=450 src="ngc-4676-when-mice-collide.jpg"> <p><center> Mice Galaxy</center></td> </tr></table></center> <p><hr><p> <table cellpadding=10 border=10><tr><td><h3><center>Milky Way and Andromeda Collision<p></center>The Milky Way and Andromeda (distance 2.1 million light years) are the largest galaxies in the <i>Local Group</I>. Andromeda is approaching the Milky Way and is expected to interact in ~2-3 billion years. The first collision will not disrupt the galaxies but they will re-interact eventually merging in less than 10 billion years.</td><td> <a href="andromedaandmilwy.mpg"><img src="M31.gif"></a> </td></tr></table></center> <p><p> <center> <a href="http://hendrix2.uoregon.edu/~imamura/123/astro.123.html"> Return to Home Page</a> </center>