Download Module 11.1.1: Galaxies: Morphology and the Hubble Sequence

Module 11.1.1: Galaxies: Morphology and the Hubble Sequence, Part 1 [slide 1] We now turn our attention to galaxies. [slide 2] First, we'll talk about morphology and classification. You can think of galaxies as the basic constituents of the universe, obviously distinct units when you look at the sky. And the reason for that is that they are over-­‐dense by a factor of 1,000 relative to the extrapolation of their large-­‐scale structure. And, as we covered earlier, this is due to the additional collapse due to the cooling of variants. One thing about galaxies is that they have a remarkably large range of properties, and yet there are some regularities among them. Both the wide range and regularities are indicative of different processes of galaxy formation and galaxy evolution. In some sense, they represent the fossil evidence of galaxy formation. To the first order, these differences in formative processes lead to the observable differences in galaxy morphology today. And, in addition to measuring cosmological parameters, understanding of galaxy formation -­‐ structure formation in general -­‐ is one of the key goals of modern cosmology. A couple of basic facts are that there are about 100 billion galaxies within the observable universe now, and their masses range from hundreds of million to maybe trillion solar masses, containing up to couple of hundred billion stars. [slide 3] The first thing that any empirical science would do is to catalog what's out there, and catalogs of galaxies have been basic data for astronomy for a long time. The very first one was due to Charles Messier in the 18th century. Messier cataloged a little over a hundred objects. We still call them “Messier”, or “M”, then their number, and about half of them are galaxies. The rest of them are different kinds of nebulae or star clusters. Following him, Herschel family has produced more extensive catalogs. And an astronomer, by the name of Dreyer, has recompiled those and published as the New General Catalog of Nebulae. Remember, people didn't know that galaxies were galaxies back then. And the acronym of this, NGC, is now commonly seen. You see a lot of objects; they're not just galaxies but star clusters, gaseous nebulae and so on. This was supplemented by a so-­‐called Index Catalog that extended things further to the southern hemisphere, and sometimes you see that designation “IC” and the number. In the 20th century, people started compiling more systematic collections of data on galaxies. Shapley and Ames were among the first, Tamman and Sandage followed. [slide 4] With the advent of the first systematic sky surveys, originally the photographic ones from Palomar Observatory, it became possible to do this in a systematic fashion and go much fainter. And the most notable of those is the Uppsala General Catalogue which has the designation UGC, and that catalogued all galaxies on the northern sky down to the apparent angular diameter for about 1 arcminute. Notice, this was the angular diameter selection and not magnitude selection, which is more common. UGC was extended to the southern sky using the southern sky photographic survey to ESO Uppsala Catalogue. In parallel, Vorontsov-­‐Vel'yaminov, in the Soviet Union, produced what he called Morphological Catalog of Galaxies. Zwicky also had a catalog of galaxies as well as clusters. Gerard de Vaucoleur and his collaborators compiled not simple catalogs that would list positions, magnitudes, maybe diameters, but everything that was known about Bright Galaxies at the time, and those were called Reference Catalogs, ending up with the third one of those. And those are sort of the last pre-­‐modern compilations of galaxian data. Nowadays, it's all done in fully automated objective fashion from digital sky surveys, and modern catalogs of galaxies contain tens to hundreds of millions of objects. [slide 5] So, after you catalog what's out there, the next step is the superficial morphology. This is sort of like biology was in Linnean days. You do not yet understand what is going on, but you see that there are differences. And indeed Edwin Hubble was the first one to address this in a somewhat quantitative fashion, and a lot of his early work is still relevant today. He proposed what's still used -­‐ as Hubble classification or Hubble types – in a popular book, The Realm of Nebulae. And that led to the famous tuning fork diagram, which I will show you in a moment. Hubble's classification was subsequently refined or extended by de Vaucouleurs, Van den Bergh, and others. But the basic things haven't really changed. These days, we actually classify galaxies in a more quantitative fashion using correlations and distributions of their physical properties rather than superficial appearance on pictures. Although, it turns out that superficial appearance or pictures of morphology does correlate with a number of real physical or astrophysical properties of galaxies. And we'll talk about that in more detail. Another thing you noticed about galaxies is that they all seem to have a limited subset of subsystems building blocks and, for example, bulges, which also look like elliptical galaxies and disks/spiral arms. It turns out that these subsystems in galaxies correspond to differences in stellar content and internal kinematics, age, star formation history, and so on. And so, maybe that would be a better way of classifying galaxies. [slide 6] So, here is Hubble's famous tuning fork diagram. Hubble noted that there is one obvious variable, which is the relative prominence of the bulge component – the central reddish component, and the disk with spiral arms, which is said to be fairly blue. So, he sorted galaxies from redder, the smoothest, to the bluer, which are also having more prominent disks and more prominent spiral arms. He split the spiral galaxies into two. Those that showed also bar-­‐like structure in the middle from which spiral arms originated and those that didn't. This turns out to be somewhat superficial, but it does have some physical meaning. So, Hubble thought that this was an evolutionary sequence that goes from left to right, from elliptical galaxies into spirals. And now, we think that it is the exactly opposite of that really happens, but Hubble had simply no other way of telling. And so, because of that, the elliptical galaxies, which are actually older, are called early types, according to Hubble. And spiral galaxies, which on average are younger, are called late types. It's a little confusing but it's just the way the usage is. Within the elliptical family, not knowing anything else about them, Hubble simply classified them by the apparent ellipticity in the sky. It turns out that, that actually doesn't correlate with anything whatsoever in terms of physical properties of ellipticals, but that's what he had. He classified spirals by the relative prominence of spiral arms – Sa's being the least prominent; Sc's being the most prominent spiral arms – that also correlates with relative importance of bulge versus disk. And then, there are galaxies that simply looked like scrambled eggs, and he couldn't classify them. He just called them irregular. Turns out, that's also a very heterogeneous group of things. Some of which are mergers of large galaxies; some of which are genuine dwarf galaxies with spotty star formation. But that wasn't known at that time. [slide 7] So, elliptical galaxies account for maybe up to 20% of all galaxies, and most of them are found in clusters or maybe dense groups, a relatively modest number in the general field. Hubble classified them according to the apparent ellipticity projected on sky, having no other physical properties to use. And subsequently, there were subcategories of ellipticals based on their luminosity. This time, giant ellipticals, central dominant galaxies and clusters, dwarf ellipticals, and so-­‐called dwarf spheroidals, which actually are a completely different family of galaxies, but more about that later. One of the Hubble criteria for calling something an elliptical galaxy is that they have the smooth appearance, meaning no spiral arms. They have no dust lanes obscuring them. They have no star formation. Turns out, actually, elliptical galaxies have all of that. They have a lot of gas except that gas tends to be X-­‐ray gas. They sometimes have dust lanes from spiral galaxies they gobbled up. And sometimes they have a little bit of star formation too. So Hubble sub-­‐typed according to the apparent ellipticity -­‐ E0's being the round ones, E7's being the most elliptical that he could see. Now, we think that E7's actually contain disks but without star formation. [slide 8] And here are a couple of examples of elliptical galaxies from our immediate extragalactic neighborhood, the Virgo cluster. M87, the central elliptical galaxy in Virgo, is on the left. A lot of little dots that you see around it are not stars. They're actually globular star clusters that belong to M87. M87 has about 10,000 globular clusters around it. To put this in perspective, Milky Way has about 150. M84, M86, two other big ellipticals in Virgo, are to the right. Remember, M is for Messier Catalog. Module 11.1.2: Galaxies: Morphology and the Hubble Sequence, Part 2 [slide 9] Now, between elliptical and spiral galaxies Hubble introduced the intermediate type S0's, which are galaxies that obviously had a disk but he couldn't see any spiral arms. Here's Sombrero galaxy, one of the famous examples. And indeed physically they are transitional in the sense that it's a large bulge with relatively small or rather unimportant disk, and disk that doesn't have a lot of active star formation. And just like other spiral galaxies, these S0's, or lenticulars, because their shape resembles a lens, could also contain bars. [slide 10] Among the spiral sequence, which really is two parallel sequences – one with bars one without, there is a trend from the earliest types, Sa's, to the latest types, Sc's, sometimes people call Sd's and even more extreme cases, in a sense that as you go along the sequence, the bulge becomes less important, the disk becomes more important, spiral arms become more prominent and more ragged as you go further. [slide 11] Here is a typical spiral galaxy, our immediately neighbor, Andromeda Galaxy or M31, which has two big companions, M32, small elliptical, and NGC205, which is so-­‐called dwarf elliptical, as well as lot of even smaller companions or asteroidals. Now, in this picture all the little dots that you see are actually stars in our own galaxy. [slide 12] M51 is another well-­‐photographed spiral galaxy. It seems to have something at end of its spiral arm. This is actually another galaxy, with which M51 is interacting – sort of its own magellanic cloud, except bigger and in a more spectacular state of merging. [slide 13] So, about half of all disk galaxies contain these bars, including the Milk Way. Ours is hard to see because of the dust in the disk but now, thanks to the infrared observations, we are pretty sure that we do have a bar. And what really are bars? They are a form of dynamical instability in rotating disks that forms spontaneously. Presence of dark halos actually can regulate against the formation of bars, so spiral disks are just borderline unstable, to formational bars. Now, bars are not static patterns. They actually do rotate as the disk rotates; unlike spiral arms, which really are not – they are density waves, rather than physical, distinct physical subsystems, about which we will talk about later. Bars can also serve as means of transferring gas from the outer disk to the middle, where it can ignite star formation or feed an active nucleus, if there is one. The reason for this is that this rotating tri-­‐axial potential creates instability and can soak up energy or angular momentum of the surrounding interstellar medium, thus allowing it to sink to the middle. [slide 14] You see, often spiral arms seem to originate at the ends of a bar, and that too has to do with dynamical instabilities, and mechanisms of formation of spiral arms. [slide 15] And then there are dwarf galaxies, which is something of a heterogeneous grab bag of things. Hubble put them at the end of his sequence, but there are really several completely different families in there. Dwarfs can be gas-­‐poor or gas-­‐rich. Gas-­‐rich would be things like very small spirals or genuinely galaxies with no specific shape, but plenty of gas igniting star-­‐formation. By large, unless they have a lot star formation, they have very low surface brightness, and so they are very hard to spot. And just like the majority of all stars are dwarves, the majority of all galaxies are also dwarves. Numerically, they are more important than traditional galaxies in the Hubble sequence, but they actually don't contain very significant amount of mass or luminosity. So we can think of these dwarf galaxies as food for larger galaxies: they keep merging into the bigger ones. In the halo of the Milk Way, we see now remnants of at least one dwarf galaxy been consumed trough tidal interaction, the so-­‐called Sagittarius dwarf. Dwarf galaxies themselves are probably not the product of any significant merging with even smaller pieces. In some sense they are sort of the original building blocks of larger galaxies. [slide 16] So here is what Hubble called a dwarf irregular. This is the Large Magellanic Cloud, does look a bit irregular but actually what it is, it is a dwarf bar spiral in process of tidal disruption interacting with the Milk Way, and the obvious part is the bar. [slide 17] Also generally young star-­‐forming dwarf galaxies, I Zw 18 from Zwick's catalogue is shown here. We can think of them as almost like puddles of gas, which are just beginning to form stars for the first time or, at any rate, they haven't formed many stars in the past. So these are the closest things to young galaxies that we see near us. [slide 18] Now, what Hubble thought was dwarf elliptical and dwarf spheroidal are completely different kind of beasts. There are small ellipticals, the scale down versions of big ellipticals, and that's a separate story; M32 is an example of those. But even among the dwarves, there seem to be two classes of things. One is dwarf ellipticals, which really are probably a different family of objects all together. And dwarf spheroidals, so called because they belong to a galactic halo – usually they do have ellipticity – and those look just like a little peppering of stars atop of the background. They do not contain much in term of stars, but they turn out to contain lot of dark matter, and they're interesting from that viewpoint. [slide 19] More recently, thanks to the more sensitive detectors, it was realized that there are a lot of spirals out there, which have very, very low surface brightness disks, but nevertheless they do have disks. And the question was how many of those did we miss? Those are icebergs in the sky that we just see the bright central portion, but not the disk, which we really have to work hard to pull out of the noise. And the interesting thing about these galaxies is that they seem like normal spiral in any other way, in terms of the amount of gas, in terms of rotation curves, total masses, and so on, except they haven't made many stars and disks. And the reason for this is still unclear. One of the first large but low surface brightness disks is called Malin 1. The two panels in the lower right show a normal contrast picture and a really high contrast picture. [slide 20] So if you look at a distribution of surface brightness of disks, you'll see something like that. At the bright end, which here is on the right, are the traditional Hubble spirals. But then, there's an extension to the lower surface brightness of these newly found low surface brightness disks. In the past, because of selection effect, their existence just wasn't known, were hard to pull out of the noise, and it appeared as if there was a Gaussian distribution, with a characteristic central surface brightness, and that was called Freeman's Law. So just again this turned to be more or less selection effect. [slide 21] A different kind of irregular galaxies is, what we now know, really mergers of galaxies in progress. We talked about those when we talked about numerical simulations. Here is a couple of famous examples: Antennae, you see tidal tails; and Tadpole galaxy, where again there's a long tidal tail. [slide 22] And obviously since they represent at least two galaxies involved, they do not fit in any of the traditional morphological bins, but we now know that they are really important in the terms of galaxy evolution because galaxy merging is one the key process that are contributing to evolution of galaxies. [slide 23] A subset of those are so-­‐called polar ring galaxies, where merging of two spiral is not yet complete, and their angular momenta are orthogonal, so you see a ring remnant of one of them orthogonal to the disk of the other. This was puzzling at first, until people realized what was going on. [slide 24] So next time we will actually look into the trends of physical measurable properties along Hubble sequence and where do they really come from. Module 11.2: Trends Along the Hubble Sequence and Their Origins [slide 1] So far, we have reviewed the morphology of galaxies. But now let's take a look at whether that morphology actually correlates meaningfully with some other properties of galaxies or their formation history. [slide 2] First, let me critique the traditional approach to galaxy morphology. It is subjective. It is based on the appearance, visual appearance, at a particular wavelength – traditionally those would be blue light sensitive photographic plates. But now we have panchromatic view of the universe, and when you look at galaxies through different wavelength regimes, they look very different. Classification would presumably be very different depending on which wavelength regime was chosen. Now we happened to have chosen visible light and, in vicinity of visible light, if you look at bluer wavelengths – ultraviolet included – you'll be very sensitive to the star formation. So galaxies will be clumpier. There'll be more strong features due to the regions of star formation. If you look towards redder wavelengths, which are not so sensitive to star formation but reflect the bulk of the old stellar population, galaxies will look much smoother than that. [slide 3] So there are three major problems with the traditional morphological classification of galaxies: First it is subjective. It is based on just the visual appearance, looking at things and deciding whether some galaxies have more open spiral arms or not. However, these days we can actually process digital images of galaxies and define, in objective, consistent fashion some of those structural parameters. The second problem is that it's superficial. It really is based on just a visual appearance of a particular wavelength region and not on the actual physical properties that we'd be interested in, such as the galaxy mass for example. A more modern approach is to use correlations and clustering of different galaxies and properties in parameter spaces, and thus objectively define galaxy families using those correlations. Finally it is an incomplete classification. It was based on what was known, more or less, in Hubble's days. And it completely missed a major dichotomy between giant galaxies – by giants I mean those that are usually seen in Hubble sequence – and dwarfs, which turn out to be largely a completely different set of beasts, and I'll show you that later. And they themselves may split in different categories. So that was completely missed by the original classification schemes. [slide 4] All right, so what does it mean? Properties of galaxies, including their appearance, are a product of their formative and evolutionary histories. And thus if we can interpret morphological trends, then we can learn something about galaxy formation and galaxy evolution. A lot can already be concluded from a very basic fact. Among the Hubble sequence galaxies, at least, there seem to be two dominant components: the elliptical-­‐like bulge component and a disk component. The relative permanence of these two determines a lot of properties of galaxies. The bulge/elliptical stellar components are older, and they're kinematically supported by random motions of stars. Stars have to somehow balance the potential wells in which they sit. And in elliptical galaxies and bulges, those motions are largely random, like molecules in a gas, so we call them pressure supported. In this galaxy, most of the kinetic energy is in a circular motion around the center. And very little kinetic energy is actually in random motions, though there is some. And so that's a major distinguishing characteristic. Note also that we'd been just talking about light and we already know that the dominant mass component is the dark matter. We think, with some justification, that dark matter is probably also pressure supported – random motions rather than a rotation. And also, disks are certainly distinct kinematical components. The spiral arms, which were used for the original classification, may be largely ornamental. They are interesting patterns, but they do not seem to really correlate very much with anything else. However, having said all that, there are still very important and significant trends along the Hubble sequence. [slide 5] As we go from the early types ellipticals and S0's towards later types of spirals – Sa, Sb, Sc – there are several trends. The average age of the stellar population decreases. The disks are younger, and the later Hubble types are even younger. The star formation rate decreases. There is almost none in elliptical and bulges, and more and more as you go towards the later types. Because of that, the color also changes, because young stellar populations are dominated by luminous blue stars and old stellar populations dominated by red giants. And so there would be trend from redder colors towards the bluer colors [along the sequence]. The gas content changes, or at least the neutral hydrogen gas changes. There is almost none in ellipticals and bulges except if it came from disks. And there is more and more hydrogen relative to the stellar mass as you go towards later types. Likewise, the components of the cold interstellar medium, including dust, are also increasing in their importance towards the later types. And finally, an important dynamical characteristic which tells us a lot about formative processes is that in the early types, most of the kinetic energy is in random motions. In the later types, most of the kinetic energy is in ordered, circular motions. [slide 6] So because of these trends, and what they really mean in a direct interpretation, Hubble classification persisted to this day. It's still useful, even though it's superficial, incomplete, and all that. It's still fairly useful because it does represent some important properties of galaxies. Not all, but some. However it does not represent others, for example, galaxy masses. You can't think of any more fundamental quantity than the total mass of a galaxy, and it is not contained at all in Hubble classifications. [slide 7] In fact, here are plots of the mean values of important characteristics of galaxies like radius, mass, luminosity, and mass to light ratios as functions of the Hubble type. As you can see, these plots are largely flat, meaning there is independence of these quantities on galaxy’s Hubble type, except for some, maybe a little bit towards the latest Hubble types. If the Hubble types were representative or indicative of, say, galaxy mass, there should have been great correlations here and there but there aren't any. So this is really probably the most fundamental problem with morphology based classification as opposed to, say, physical properties based classification. [slide 8] All of these trends can be interpreted in a simple way. They are really a sequence of the star formation histories of galaxies. Not just the star formation today (there is almost none in ellipticals and a lot in spirals) but integrated over the galaxy lifetimes. Put simply, early types -­‐ ellipticals, bulges -­‐ form most of their stars early on and then very little after, whereas disks tend to have much more extended star formation histories and it could be even flat, uniform star formation through the Hubble time. For some of them the late type disk, star formation may be even still increasing in time. Here is an interesting little fact: typical spirals, say like Milky Way, form stars at typical rates of several solar masses per year. And if you add up all of the hydrogen available for the star formation, there is usually about a billion solar masses or so. In other words, if spirals were to continue like this they'll burn through all of the available hydrogen in a billion years or less. So it would seem as if we were really living at a special time. More likely, there is a fresh supply of gas that comes in from the intergalactic medium and gets still accreted to galaxies. [slide 9] And here is, schematically, what I meant by the trend in star formation histories. If you plot, say, star formation rates as a function of time in ellipticals and bulges, there is a lot of activity early on, dies off very quickly, and there is hardly any after that. Whereas for disk galaxies, there may be little enhancement in the beginning, but by and large remains flat. Otherwise, as I mentioned, it can even be increasing. So this simple picture can explain all of the trends that we have seen so far. [slide 10] Now let's recall the concept of stellar populations as you probably remember from earlier astronomy studies. Those were introduced by Walter Baade, who noticed that there seemed to be two kinds of stars. There are younger stars that tends to be in galaxy disks, and older stars that tend to be in bulges or galactic halos. And he called them population I and population II. So they differ in their ages and where they're found, and sometimes in metallicity and also their kinematics. [slide 11] But Baade's ideas can be extended. You can think of stellar populations as subsystems inside galaxies. They're characterized by their location, by their density distribution, by their kinematics, by their star formation history, by the resulting metallicity, and so on. And there's probably more than 2. For example, in the Milky Way alone, we can count at least 4. There is the old, but metal rich bulge. There is the old, but metal poor halo, much more extended,. There is the young stellar disk where stars are now being formed. And a bit older, and somewhat thicker disk composed of older stars. Again, note, dark matter is yet whole another issue here. [slide 12] How can we understand the connection between star formation history and dynamics? Well, schematically, it works like this. Stars are essentially mass points, and even in colliding galaxies -­‐ galaxy mergers -­‐ they are behaving like dissipationless systems. They just follow whatever potential there is. And they, in a sense, dynamically remember the dynamics of their birth. So if you start with a bunch of small galaxies merging together in a random fashion, then stars that used to belong to them will continue to move in random directions, and you'll end up with a stellar system that is supported by random motions which is just like elliptical galaxies. [slide 13] Now, consider collapse, not of galaxies already made of some stars, but just hydrogen clouds. They dissipate energy, but they cannot dissipate angular momentum. Therefore, they settle in the configuration that gives them minimum energy for the given amount of angular momentum, which is a flat, thin rotating disk. Now they make stars, and those stars then remember dynamics of their birth, and they continue moving in circular orbits. There is very little in terms of random motions. So in this way we can connect the dynamics of stellar populations or subsystem of these galaxies with the histories of their star formation. [slide 14] What about their metallicities? Remember that chemical enrichment of interstellar medium comes from stars themselves. They form stars, massive ones explode, disseminate heavier elements they cooked up into the interstellar medium, new stars are formed from that, and so on. Now if you have a really massive big galaxy, supernova ejecta will not be able to escape deep potential well, and they will mix in with the rest of the gas, which serves as a fuel for the next generation of stars, and the metallicity will gradually increase in time. On the other hand, if you have a very low mass host system -­‐ a little dwarf galaxy or protogalactic fragment -­‐ the potential well is not so deep, and the kinetic energy of supernova ejecta is sufficient to expel them out into the intergalactic space. So that little galaxy does not evolve chemically very much. It does a little bit but its metallicity remains low and doesn't change very much in time. [slide 15] So that's the qualitative picture, but let's try to put this in a quantitative footing. We would like to know about actual distributions and correlations of meaningful physical properties of galaxies that we can measure in well defined fashion, and those include first the density distribution. How are they distributed spatially? What is the density profile? Their kinematical profile, how is the kinematics changing as a function of radius? The relative importance of old and young stars, or rotating in random components? And the chemical abundances, and how they change? So by and large, structural properties or density distribution are obtained through photometry, or surface photometry which is spatially resolved photometry. Pretty much everything else -­‐ kinematics, metallicities, star formation rates -­‐ comes from spectroscopic measurements. And in order to interpret these measurements, we have to use various population synthesis models and dynamical models. Dynamical models can be assumed as toy models with some specific gravitational potential, and then [finding the density distribution that they predict, and that has to be consistent with the assumed gravitational potential]. Stellar population synthesis models, meaning what kind of a mix of different stars would be required to obtain spectrum as observed. Of course, because stars evolve in time, their mixture will evolve and the spectrum will evolve in time, and so we need galaxy evolution models like that. We have all of those and we'll talk about them a little later. [slide 16] So next we will turn to the structural properties of spiral galaxies. Module 11.3: Spiral Galaxies: Photometric Properties [slide 1] Let us now take a look at the structural or photometric properties of spiral galaxies. First, about spiral galaxies in general, they do have a lot of diversity in their structure and components. [slide 2] First of all, there is the degree in which their spiral arms are prominent, which was, of course, the basis of Hubble classification, as well as the ratio bulge to disk. And because bulge and disk have different stellar populations -­‐ bulge being an old one, disk being younger -­‐ there are all these differences in star-­‐formation rates and colors and so on. The disks, of course, have interstellar material called gas and dust, from which stars are forming. Also, spirals tend to avoid dense regions of the large-­‐scale structure, because this is where galaxy mergers can happen, or do happen, and those spirals have been turned into ellipticals. [slide 3] Recall the trends that we discussed about Hubble sequence. Spirals, of course, participate in all of them. Most of the trends are along the spiral branch, and they are listed here. But basically, as we go from early Hubble types towards the late ones, there is an increasing amount of star formation, increasing amount of gas, and increase in the disk to bulge ratio. All of this manifests itself in colors -­‐ they get bluer -­‐ and other measured photometric properties. [slide 4] So, let's look at individual subcomponents of spiral galaxies. First, there are the disks and they're characterized by exponential distribution. We'll show that in a moment. And there may be more than one kind of disk. There are thin disks within which star formation occurs. That's where the interstellar material is. But they may be embedded into thicker disks, composed mainly of intermediate to old aged stars. The bulges are essentially little ellipticals in middle of spirals. Not all spirals have bulges; some simply have disks that go all the way to the middle, maybe rounded off, but no distinct elliptical-­‐like component. Now, unlike Baade's original idea of Population II stars that's supposed to be metal-­‐poor -­‐ stars in halo are metal poor -­‐ those in galactic bulge as well as in ellipticals are actually metal-­‐rich. They did result from a substantial amount of chemical evolution, but very early on. And again, there is the important difference in dynamics. The disks are rotationally supported against self-­‐gravity, and bulges are pressure supported -­‐ random motions. About half of all spirals contain bar-­‐like feature, and you've seen pictures -­‐ sometimes spiral arms begin at the ends of the bar. Bars are similar to bulges in their composition, but dynamically, they're distinct. In their very centers, some people argue that spirals contain an additional component concentric with the bulge itself, which can be very dense, sometimes may contain supermassive black hole -­‐ which may or may not be active, and there is usually some star formation. If you remember, numerical experiments and collisions of spiral galaxies, the gas tends to sink to the center because it loses energy. And if you accumulate a lot of gas with high density, it'll tend to make stars. In contrast to that, the most extended stellar component is the stellar halo. And that is composed out of metal-­‐poor stars, the kind of stars that make dwarf spheroidal galaxies. So, we believe that all of the stellar content of the galactic stellar halo is from disrupted, merged dwarf galaxies that have been torn apart by tidal forces. And they populate the halo. And there is lot of good experimental evidence for that. And of course, there is the dark halo. Dark halos are more extended than visible parts. There are some hints as to what their radial distribution could be, as well as their shape. They're probably triaxial ellipsoids. [slide 5] So, the way we can quantify distribution of visible material is through surface photometry meaning. But before we look at the distribution of stars, we first need to correct for the inclination effects, because face on spirals are more or less circular, so their apparent ellipticity in the sky tells you about what the inclination is. Then, we have to correct for the interstellar extinction, both in the galaxies themselves but also in the Milky Way and the direction that we look towards. Usually, these radial surface brightness profiles are obtained by averaging in circular or elliptical annuli. and so that tends to average over the spiral arms. As it turns out, typical surface brightness profiles of spiral disks are exponential. The projected surface brightness declines exponentially from the center out. And the e-­‐folding lengths or disk scale lengths are of the order of few kiloparsecs. [slide 5] So, if you measure a surface brightness profile of a galaxy, then you'll see something like this. If plotted on semi log plot, log surface brightness versus linear radius, an exponential looks like a straight line, and indeed that's what you see in the outer regions. There is extra light in the middle, and that's due to the bulge. And so, one can then fit an exponential disk plus the bulge component, using one of the elliptical galaxy-­‐fitting formulae that we will look at in the next chapter, and does decompose the light into bulge and to disk. [slide 6] So, this is now well-­‐established procedure and many galaxies have been studied in this way. Here are examples of a few. [slide 7] Now, here are some contour maps of the surface brightness distribution of spiral galaxies in the sky. And these are lines of equal brightness. And to anticipate something I'll show in a moment, they do seem to stop at some point. [slide 8] So, what about perpendicular structure? I mean, along the radius from center out, they're exponential. But as it turns out, the distribution of density in spiral galaxies perpendicular to the plane of the disk is also exponential. And the typical scale height is of the order of hundreds of parsecs, much shorter than the disk scale length of several kiloparsecs. [slide 9] So, one interesting thing about spiral galaxies is that their disks have cutoffs. They do have an edge. Past certain radius, there are no more stars. However, hydrogen, from which stars are made, usually extends further and the dark halo further yet. So, that suggests that disks may be forming from the inside out, in terms of star formation. [slide 10] So, if you measure the profile, you can take the integral under it and that gives you the total luminosity of the galaxy, or its two components, the bulge and the disk. Inclination correction tends to be the trickiest of them all. But, now we know how to do that. And the extinction correction depends on estimating the amount of dust along the line of sight. Because interstellar extinction causes reddening, absorbing more blue light than red light, using colors can give us indication how much reddening there is. And from that, how much of total absorption there is. [slide 11] Next time, we will talk about interstellar medium, gas in spiral galaxies. Module 11.4: Spiral Galaxies: Gas Content [Slide 1] So we have seen what stars do in spiral galaxies, but what about gas? This is a composite picture of a lot of neutral hydrogen images of a lot of different spiral galaxies, and you can see that in many cases you see spiral arms just fine. [Slide 2] Well, there are many components to the gas in spiral galaxies. There is cool, neutral hydrogen, which we denote as H-­‐Roman numeral one (HI). Then there is molecular gas, usually most of that is actually molecular hydrogen and then carbon monoxide, but there are hundreds, if not thousands, of others species of molecules out there, some of them very complex. And the importance of the gas is that it’s the fuel for star formation. You need to have cold gas, cold interstellar material clouds to collapse and be able to ignite nuclear fusion in the core. Now there is ionized gas as well, ionized by the ultraviolet radiation from young stars, and that's what makes those pretty pictures of nebulae and star formation regions. That, we can observe through optical spectroscopy or other forms of spectroscopy. But the neutral hydrogen is best observed through the spin-­‐flip line, which I'll explain in a moment, at the wavelength of 21 centimeters. At that wavelength, radio waves go through the dust, and so we can see all the way through. So just as there are stellar disks, there are gas disks, and they also can show spiral arms. But one important thing is that hydrogen disks extend further than the optical ones, sometimes by a substantial factor. And because this spin flip line is so sharp and well defined, it can be used as an excellent tracer of velocities of gas, including rotation of galaxies, through the Doppler shift. [Slide 3] This is how it works. In a hydrogen atom, you have a proton and an electron. And there are 2 possibilities for their spins: to be aligned or anti-­‐aligned. One has a slightly higher energy level than the other and that transition corresponds to the photons of the wavelength of 21 cm. So, this is, where it counts. Now, this is not something that's easily achieved in a lab but can happen easily in densities and temperatures of interstellar medium. So there's plenty of it, and this 21-­‐cm line has been the main tool to probe interstellar material in these galaxies, near and far. [Slide 4] So here is a contour map of neutral hydrogen superposed on an optical image of this galaxy and it's obvious that hydrogen goes much further in visible stars. Now by observing Doppler shifts of the 21-­‐cm line we can see the velocity field. [Slide 5] And that looks like this. These are the lines of equal radial velocity and they look like this because on one side the disc is coming towards us and the other is going away from us. Because the gas goes so much further out than stars, we can measure rotation curves much further using this method than, say, through optical spectroscopy. [Slide 6] So here is a simple comparison of optical and hydrogen images of the famous spiral galaxy, M81 in the Virgo cluster, and you can see that gases nicely concentrated in spiral arms, which is, of course, where stars are made, although it tends to be a little more extended. [Slide 7] So that was the atomic gas. What about molecular gas? Aside from the hydrogen molecule, H2, the next most prominent one is carbon monoxide. Incidentally, all of the different molecular species tend to be found in same places, by and large. Now in the case of molecular clouds, which are colder, they get to be closer to the dust lanes and they're closer to the regions of star formation. You can see spiral arms outlined even better. But there are also new features: there are rings of molecular gas around galaxies, there are bars, and other things. [Slide 8] As you probably learned in astronomy before, the interstellar medium has multiple phases. I touched upon that a few minutes ago. First there is a cold interstellar medium and that's mostly the neutral hydrogen gas, temperatures ranging from typically 10's to 100 degrees Kelvin. So that's all of the cold gas, both atomic and molecular as well as dust. This is what makes stars in this cold galaxy. Next, there is warm interstellar material, typically with temperatures of thousands or tens of thousand of degrees. It's warmed up and ionized by the radiation by young stars, and that's where all these pretty nebulae come from. And finally, there is hot interstellar medium, which really mostly belongs to the galactic halo. It's the gas that's expelled from the disc through explosions of supernovae, and the shock waves of supernova remnants are what heat it up to this particular range. [Slide 9] Here, here is the typical spiral galaxy rotation curve, with its decomposition into the bulged disc and dark matter components. We spoke about those extensively when we talked about dark matter, so, I will not dwell on this much further. [Slide 10] And next, we will look at where does the spiral structure come from. Module 11.5: Spiral Galaxies -­‐ Density Waves [Slide 1] Finally, let us now see why spiral galaxies are spiral. [Slide 2] Obviously, spiral arms are the defining feature, and there are some important observational facts about that. Probably the most significant of them is that they're seen only in disks that do contain an interstellar material, the gas, which then causes the star formation. There are disks that do not contain much gas -­‐ those are the S0 galaxies. Another important clue is that they're seen in young stars, and since young stars tend to last less than a full rotational period, it suggests that, possibly, spiral arms are a transient phenomenon -­‐ that they actually last less than a typical rotation period of these galaxies. And, something that's a little tricky is that when you look at a spiral galaxy, you have an impression of a vortex, and that they rotate in a sense of you would see in, say, water going down the sink. They do, except that they move at half the angular speed of the disk. So the disk of the galaxy rotates in the same direction, as you'd infer from winding of the arms. And so do the spiral arms, but at half the [angular] speed. So relative to the disk, the spiral arms are actually moving in the other direction. They move as they’re scooping up the stars, counter intuitive to what you may think. [Slide 3] Now an important feature to remember here is that essentially all galactic disks have more or less flat rotation curves, meaning that the linear velocity is roughly constant as a function of radius, which means that the angular velocity goes as 1/radius. And so if you were to start with, say, straight features directly through the galactic center, due to the differential rotation it will naturally produce something that looks like segments of spiral arms. [Slide 4] Which actually presented a dilemma initially on, because if somehow spiral arms were constant, because of the differential rotation they'll keep winding and winding until they look like a really tightly wound spiral, and that's not what we see. [Slide 5] An interesting way to approach this is as follows. Imagine that stars move around the center of a galaxy in elliptical orbits, as roughly they do. But the subsequent orbits at larger radii are shifted a little bit. So it kind of looks like this. You start with one, then keep adding concentric, but tilted orbits. And by the time you're done, you can see that there is something that looks like two spiral arms. And in fact, this is roughly what happens, but the question is, why? [Slide 6] The answer why is the so-­‐called density wave theory, that was developed in the 1960s and 1970s by Lin, Shu, Toomre, Kalnajs and others, and the upshot is that spiral arms are density waves in differentially rotating disks. If you were, say, to drop a rock in a pond it will have circular waves going away from the center. But galaxies are not stationary, and if you were to make a perturbation in a differentially rotating disk like the disks of spirals, what you get is not circular waves but spiral waves. The reason why they persist at all is resonant motions. You can decompose the motion of any star around the center of a galaxy to first approximation as circular orbit with some perturbation. And the perturbation can again be approximated as, say, a star wobbling around at radius, in other words, making circular orbits around some hypothetical node on the central part of the orbit. That resembles epicycles from the Ptolemaic theory of the Solar system. And in fact those are called the epicycles. So if you have the exact match in the numbers of orbits around the center of an epicycle and its motion around the center of a galaxy, you will have amplification, and that is exactly what's happening. So if you look at angular frequencies, omega (not to be confused with the density parameter), and then look at the epicyclic frequency (again angular frequency then divided by integer number, which is usually small one, 1 or 2 or something), then the first two resonances are called the Linblad resonances and they actually are the radii in which that particular rotational frequency occurs from which -­‐ or to which -­‐ spiral arms extend. Since usually the 1st harmonic or the first resonance is the strongest one, this is why we mostly see two arm spirals, although we do see, for example, four arms spirals and so on. So another way to phrase this is that the spiral arms are density waves, which are really resonances of perturbations in these differentially rotating disks. [Slide 7] So remember the orbit crowding diagram? That is more or less what happens here where an elliptical orbit is can be really be decomposed as circle plus an epicycle that makes exactly one turn as the big one turns around, and so that causes the elliptical shape. Now, the orbit crowding really implies that there is going to be a density pileup, and that's exactly what spiral arms are. Now, these waves are moving relative to the underlying disk gas and stars. And so, as the waves hit this material, they are compressing it. Compressed gas is liable to make stars, which is why we see star formation associated with the spiral arms. [Slide 8] So this is why star formation can be triggered by the spiral density waves. That's not the only way in which we can trigger star formation in disks, but obviously it works. And there'll be stars made of all different masses, but the most massive stars, as you probably know, are the most luminous ones and they live the shortest. So it's the shortest lived, most luminous stars, which haven't had chance to drift away from the wave before they explode. That will delineate the pattern of spiral density waves. [Slide 9] When you look at the bluer wavelengths, which are more [dominated by] the radiation from young, hot, luminous stars, you see very prominent spiral density patterns. If you go to the redder wavelengths, say, near infrared where the light is dominated by the older red giant stars, the spiral arms are still prominent -­‐ the density wave is still there -­‐ but not nearly as much, or not nearly as sharp, as you would see in a blue or ultraviolet light. [Slide 10] So schematically, you'd expect things to look like this. There is the spiral density wave that moves, relative to the underlying disk, in the opposite way of what you may think from, say, water going down the sink. In other words, the arms are scooping up the material, so the leading edge of the waves is the inner part of the spiral. This is where the density wave compresses the gas (molecular clouds) and makes dust lanes. And then, inside of them, there'll be star formation. So you expect to see dark lanes on the leading edge, which is the backside of the spiral arms, followed immediately by regions of star formation, and then kind of a more diffused, bluer stellar population as you go towards the trailing edge. [Slide 11] And now let's look what that looks like in the real life. So this is a picture from the Hubble Space Telescope of, I believe, M101, a nearby spiral, and this is exactly what you see. The inner part, which is the leading edge of the spiral arm, is where you see the dust lines. And then the red dots and blobs that you see in the dust lines, or immediately past them, are the regions of young star formation. Those are H alpha nebulae ionized by the young stars. The young stars burn out through the dust, dissipate, and then you see luminous, blue, stellar light. And then it kind of fades away as you go towards the trailing edge of the spiral. So the theory predicts exactly this. [Slide 12] To summarize, spiral arms are density waves that occur in differentially rotating stellar disks. They will compress gas, and that will lead to the star formation at the edges where the gas enters the spiral density wave. Stars themselves will simply pass through a density wave, just like water molecules pass nicely through the waves in water. And this dynamical theory is very successful in explaining the global properties of spiral galaxies, but it's not perfect. First of all, it doesn't say why were there waves to begin with. Some sort of a disturbance has to happen, and one possibility is that encounters between galaxies create such a disturbance. That's entirely possible. Another part, which is a little more difficult, is that not all spirals are perfect arm spirals. They're detached spiral arms, spurs, and things like that. So additional mechanisms might be responsible for the creation of such patterns. [Slide 13] Now whereas the theory predicts exactly what we should see in, say, a two arm grand design spiral, as they're called, there are other types of disk galaxies, spiral so to speak, in which the patterns are much more diffuse and they're called flocculent galaxies. They almost have no spiral arms, but they certainly do have patches of a star formation, and even maybe little segments [of spiral arms]. They may be caused by a different phenomenon, namely a differential stretching, just like we addressed at the beginning of the lecture. [Slide 14] So that's it for these galaxies. Next, we will start talking about elliptical galaxies.