{ "cells": [ { "cell_type": "markdown", "metadata": {}, "source": [ "# Diversity of Galaxies" ] }, { "cell_type": "code", "execution_count": 1, "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Populating the interactive namespace from numpy and matplotlib\n", "Populating the interactive namespace from numpy and matplotlib\n" ] } ], "source": [ "from IPython.display import Image\n", "from IPython.display import Video, HTML, YouTubeVideo\n", "\n", "import pylab as pl\n", "%pylab inline\n", "conf = %config InlineBackend.rc\n", "conf[\"figure.figsize\"] = (6, 6)\n", "conf['savefig.dpi']=100\n", "conf['font.serif'] = \"Computer Modern\"\n", "conf['font.sans-serif'] = \"Computer Modern\"\n", "conf['text.usetex']=True\n", "\n", "width = 600\n", "%config InlineBackend.rc\n", "%pylab inline" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Galaxy morphology\n", "\n", "Galaxies can be broadly classified as spirals, ellipticals and irregular galaxies based on their morphology, and we saw a few examples of those in the previous chapter. Spiral galaxies have disky structure with stars which primarily go on circular orbits and show the existence of spiral arms (which is the origin of their name). When looked at face-on, they appear nice and round due to these orbits. But not all spiral galaxies are face on. \n", "\n", "- Can you think of a way to determine the angle of inclination? Which assumptions do you have to make? Can you think of how to test these assumptions?\n", "\n", "
\n", " Click for Answer \n", " \n", " Answer could be potentially written here.\n", "\n", "
" ] }, { "cell_type": "code", "execution_count": 2, "metadata": {}, "outputs": [ { "data": { "text/html": [ "" ], "text/plain": [ "" ] }, "execution_count": 2, "metadata": {}, "output_type": "execute_result" } ], "source": [ "Image(url=\"https://upload.wikimedia.org/wikipedia/commons/thumb/c/c5/M101_hires_STScI-PRC2006-10a.jpg/614px-M101_hires_STScI-PRC2006-10a.jpg\", width=width)\n" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "The motion in Elliptical galaxies is dominated by dispersion of the stars, rather than rotation unlike spirals. This gives them an ellipsoidal appearance. Some ellipticals also have some rotation as well which can cause some flattening.\n", "\n", "- Can you determine the inclination of elliptical galaxies from their photometry?\n", "\n", "\n", "
\n", " Click for Answer \n", " \n", " Answer could be potentially written here.\n", "\n", "
" ] }, { "cell_type": "code", "execution_count": 3, "metadata": {}, "outputs": [ { "data": { "text/html": [ "" ], "text/plain": [ "" ] }, "execution_count": 3, "metadata": {}, "output_type": "execute_result" } ], "source": [ "Image(url=\"https://en.es-static.us/upl/2012/10/heic0804a.jpg\", width=width)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Irregular galaxies, are well, irregular looking. Often they are associated with galaxies that are merging with each other.\n" ] }, { "cell_type": "code", "execution_count": 4, "metadata": {}, "outputs": [ { "data": { "text/html": [ "" ], "text/plain": [ "" ] }, "execution_count": 4, "metadata": {}, "output_type": "execute_result" } ], "source": [ "Image(url=\"http://abyss.uoregon.edu/~js/images/heic0615a.jpg\", width=width)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Often galaxies have different components which look resemble both spiral and ellipticals. Therefore, Hubble came up with a way of classifying galaxies based on the resemblance. He arranged galaxies along what is known as a tuning fork diagram. On the left hand side we have elliptical galaxies, which go from E0 to E7, with E0s which are round and galaxies getting progressively more flattened as we move to E7. The number is the closest integer to $10\\times(1-b/a)$ of the galaxies. The S0 galaxies (or lenticulars) have a bulge as well as a large disk around them. It can be very difficult to distinguish between E0s and S0s which are face-on, and similarly Ens from lenticulars which have some inclination in between. The detailed studies of isophotes and of the kinematics can help to distinguish between the two.\n", "\n", "The spiral galaxies occupy the two prongs of the fork. The upper prong has spiral galaxies (labelled Sa, Sb, Sc and so on), which have progressively smaller and smaller bulges. The lower prong has galaxies which have a prominent bar in addition.\n", "\n", "This Hubble tuning fork does not represent an evolutionary sequence, however. " ] }, { "cell_type": "code", "execution_count": 5, "metadata": {}, "outputs": [ { "data": { "text/html": [ "" ], "text/plain": [ "" ] }, "execution_count": 5, "metadata": {}, "output_type": "execute_result" } ], "source": [ "Image(url=\"http://cdn.sci-news.com/images/enlarge6/image_7280e-Hubble-Tuning-Fork.jpg\", width=width)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Galaxy luminosity/Stellar mass\n", "\n", "Galaxies also show a large variety in their luminosity (defined as the energy per unit time in a given wavelength band emitted by these galaxies). Some galaxies have luminosities as big as $10^{12}$ times the luminosity of the Sun ($L_{\\odot}$), while some have luminosities as small as a $1000 L_{\\odot}$. Large galaxy imaging surveys are very useful to get a census of galaxies. The smallest galaxies are the most abundant in terms of their number density, even though each of them outputs a very small amount of light. Large galaxies are quite rare.\n", "\n", "- Given this information, if I make a histogram of the number of galaxies as a function of luminosity, can you predict how it will look like?\n", "\n", "\n", "
\n", " Click for Answer \n", " \n", " Answer could be potentially written here.\n", "\n", "
\n", "\n", "The luminosity of galaxies is the combined output of light from all its stars. Thus it is also related to the stellar mass of galaxies. However, there is quite a bit of scatter between the luminosity and stellar masses of galaxies. The mass to light ratio of galaxies is a function of their color. Galaxies which are actively forming stars have a smaller mass to light ratio than those which are not forming stars.\n", "\n", "- How can we measure the luminosity of a galaxy?\n", "- How does one define the color of a galaxy?\n", "- Can you think of how one can infer the stellar mass of a galaxy from its luminosity?\n", "\n", "\n", "\n", "
\n", " Click for Answer \n", " \n", " Answer could be potentially written here.\n", "\n", "
" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Sizes of galaxies\n", "\n", "Galaxies are quite fuzzy, which makes it quite difficult to assign a boundary to them. The sizes of galaxies are then defined in terms of the size of a galaxy which bounds a fixed fraction of the light of the galaxy. The half light radii is typically called the effective radius of a galaxy. In general, more luminous galaxies are bigger in size, but even then there is a considerable amount of scatter between the sizes of galaxies. This scatter then implies that galaxies with the same luminosity can have different surface brightness.\n", "\n", "We know that galaxy surveys have flux limits, but they also have surface brightness limits. This makes is harder to find galaxies which are very extended and faint.\n", "\n", "- How would you design a survey to detect galaxies with the faintest surface brightness?\n", "\n", "\n", "
\n", " Click for Answer \n", " Answer could be potentially written here.\n", "\n", "
\n", "\n", "The sizes of galaxies when combined with the kinematics can be used to infer dynamical masses of galaxies. The size of disk galaxies is set by the angular momentum of the gaseous disk from which the galaxies form. The angular momentum of the gas is often set by the underlying angular momentum of the dark matter halo in which these galaxies form." ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Gas masses\n", "\n", "In addition to the stellar mass component, galaxies also have a considerable amount of gas. The gas in the cold component is the reservoir for the star formation in galaxies. The cold gas fraction is typically high for star forming galaxies, while it is very low for ellliptical galaxies. The cold gas fraction expressed as $f_{\\rm cold}=M_{\\rm cold}/(M_* + M_{\\rm cold})$ expresses the integrated efficiency at which cold gas has been converted into stars.\n", "\n", "- Can you think of why one would require cold gas to form stars when stars have really hot interiors?\n", "\n", "
\n", " Click for Answer \n", " Answer could be potentially written here.\n", "\n", "
" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Color\n", "\n", "The colors of galaxies are related to the color of stars in the galaxy. Star forming galaxies have a lot of young massive OB star complexes which give them bluish appearance. These massive stars evolve very rapidly along the main sequence, and die off as supernovae. If star formation can sustain the fraction of OB stars relative to the less massive redder population of stars, then galaxy continues to look blue. Once star formation is quenched, the galaxy starts to be dominated by red stars. Spiral galaxies are blue in color, ellipticals are typically red in color.\n", "\n", "- Surprisingly, there also exist red spiral galaxies with large on-going star formation. Can you think of why these galaxies could appear red?\n", "\n", "\n", "
\n", " Click for Answer \n", " \n", " Answer could be potentially written here.\n", "\n", "
" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Environment\n", "\n", "Galaxies also live in different environments. Some reside in clusters of galaxies, while some reside in filamentary structures, and a few of them can also be found in very sparse void environments. The environment of galaxies can shape their properties as well as their evolution. The relative fractions of types of galaxies is a function of the environment. Elliptical galaxies like to reside in dense environments such as clusters, while spiral galaxies tend to reside in more isolated environments." ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "In the next chapter, we will see how these properties of galaxies can be measured, how they are correlated amongst themselves, and what drives some of these correlations." ] } ], "metadata": { "kernelspec": { "display_name": "Python 3", "language": "python", "name": "python3" }, "language_info": { "codemirror_mode": { "name": "ipython", "version": 3 }, "file_extension": ".py", "mimetype": "text/x-python", "name": "python", "nbconvert_exporter": "python", "pygments_lexer": "ipython3", "version": "3.7.5" } }, "nbformat": 4, "nbformat_minor": 2 }