Properties of Galaxies 1 Definition of a galaxy

Properties of Galaxies 1 Definition of a galaxy

Properties of Galaxies 1 Definition of a galaxy

Herring, John, Music Director/Morning Drive-Time Personality has reference to this Academic Journal, PHwiki organized this Journal Properties of Galaxies Many subjects here are covered superficially in addition to I can give references to review-type articles or even the original scientific papers if you want. There is no single reference book that covers everything in these lectures, mostly because research in this field is fairly rapidly moving ( as long as example about a quarter of what is covered was discovered in the last few years). An outline is given below. This is not entirely a useful thing, because everything is connected to everything else. The intention is to start from the beginning in addition to end up so that you can underst in addition to the framework in which most current research into extragalactic astronomy is placed. 1. Definition of a galaxy in terms of its components. 2. Fluxes, luminosities, magnitudes, waveb in addition to s 3. Types of Galaxies – ellipticals, spirals, in addition to others – their gross properties. 4. Nomenclature 5. Components of Galaxies 1. Stars – as long as mation, HR diagram, evolution, remnants 6. Components of Galaxies 2. Active Galactic Nuclei – quasars, radio jets 7. Components of Galaxies 3. Gas – hot in addition to cold 8. Components of Galaxies 4. Dark matter – properties, what it might be 9. Local Galaxies 10. Distribution of galaxies in the Universe – environments, large-scale structure 11. Emission mechanisms in addition to spectral energy distributions – bolometric luminosities 12. Luminosity functions 13. Light profiles in addition to surface-brightnesses 14. Stellar in addition to gas dynamics 15. Galaxy as long as mation 1. Dark matter in addition to cosmology 16. Galaxy as long as mation 2. The high redshift Universe 17. Galaxy as long as mation 3. The as long as mation of stars in galaxies – the Madau Plot 1 Definition of a galaxy A galaxy is a self-gravitating collection of about 106 to 1011 stars, plus an amount up to 1/2 of as much by mass of gas, in addition to about 10 times as much by mass of dark matter. The stars in addition to gas are about 70% hydrogen by mass in addition to 25% helium, the rest being heavier elements (called “metals”). Typical scales are: masses between 106 to 1012 solar masses (1 solar mass is 2 x 1030 kg), in addition to sizes 10 kpc (1 pc = 3.1 x 1016 m, 1 kpc = 1000 pc). Galaxies that rotate do so in about 10-100 Myr at about 100 km/s. The average separation of galaxies is about 1 Mpc. Between galaxies there is very diffuse gas, called the intergalactic medium. It was much denser in the past be as long as e galaxies as long as med in addition to took up all the gas in addition to made it into stars.

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The Milky Way is known in a fair amount of detail, in addition to both the gas in addition to stars split cleanly into different populations or phases. Stars: Disk: 5 1010 Msun Bulge: 1 1010 Msun Halo: 109 Msun Globulars: 108 Msun Gas: H2 clouds: 1 109 Msun HI gas: 4 109 Msun HII regions: 108 Msun Dark matter: Halo: 2 1012 Msun Cosmic Inventory (Fukugita & Peebles 2004, Read & Trentham 2005) 73 % – dark energy or cosmological constant 23 % – dark matter, probably CDM 4 % – normal baryonic matter, about 10% of which is in galaxies (mostly in the as long as m of stars). The rest is in the IGM. 2 Fluxes, luminosities, magnitudes, waveb in addition to s The total light from a galaxy comes out at all wavelengths of the electromagnetic spectrum. When we observe a galaxy, we usually consider the light from some waveb in addition to (crudely thought of as all the light between two wavelengths lmin in addition to lmax ; in fact a more complex transmission function is usually required). For example the B b in addition to is a narrow b in addition to between about 4200 in addition to 4600 Angstroms, the K b in addition to is a b in addition to between 2.1 in addition to 2.3 microns. mX = -2.5 log10 (FX / F0), where F0 is the flux measured in b in addition to X from the star Vega. The magnitude we measure from a galaxy is called its apparent magnitude. A more useful quantity as long as relating galaxies at different distances to each other is the absolute magnitude MX = mX – 5 log10 (distance/10 pc). Absolute magnitudes as long as galaxies are negative, the more negative the absolute magnitude, the brighter the galaxy. Typical values range from – 7 (the faintest galaxies known) to -26 (the brightest galaxies known) in the B b in addition to . As far as I can tell the way the magnitude scale is defined is purely historical. It is not used at long (radio or far-infrared) wavelengths or at short (UV or X-ray or gamma ray) wavelengths, where fluxes are usually quoted instead. One other system in use is the AB magnitude system in which the zero-point is a flat spectrum (instead of the spectrum of Vega).

Here as some of the quantities in addition to units that are currently in use. The way in which people quote results is often subjective. Note in that in the penultimate equation, the first transmission coefficient is a function of the physics of the Earth’s atmosphere, the second transmission coefficient is a function of the filter used, the luminosity distance is a function of the cosmology, in addition to the attenuation coefficient is a function of the line of sight to the object in question. Apparent magnitudes Values in the V-b in addition to (appropriate as long as the eye) are -27 as long as the Sun, -13 as long as the Moon, -4 as long as Venus, -1 as long as Sirius, +6 as long as the faintest stars visible with the naked eye, +4 as long as Andromeda (this is difficult to see, because it is spread out over a large area of the sky), +18 as long as the star-galaxy transition in the sky, in addition to +30 as long as the faintest galaxies observed in the Hubble Deep Field. Absolute magnitudes Values in the V-b in addition to are -20 as long as the Milky Way, +5 as long as normal solar-type stars, -9 as long as globular clusters, -28 as long as bright quasars. The brightest object ever observed at optical wavelengths, the prompt afterglow of GRB 990123, had an absolute magnitude that reached -36. Colours These can be computed from either apparent or absolute magnitudes. Typical B-V values are 0.6 as long as the Sun, 0 as long as an A0 star, 1.0 as long as an elliptical galaxy in addition to 0.5 as long as a spiral galaxy.

Fukugita et al. (1995) PASP 107, 945 3 Types of galaxies In most galaxy samples there are roughly equal numbers of elliptical, spiral, in addition to peculiar (irregular) galaxies. Elliptical galaxies come in two types – giant ellipticals, which have high brightnesses at their centres in addition to absolute B magnitudes between about -25 in addition to -15, in addition to dwarf ellipticals, which have low brightnesses at their centres in addition to absolute B magnitudes fainter than about -18. The faintest galaxies known are dwarf ellipticals. Elliptical galaxies are featureless, with brightness profiles that are high in the centre in addition to lower far away from the centre. Spiral galaxies like the Milky Way in addition to Andromeda have absolute B magnitudes between about -24 in addition to -18. They often look smooth near their centres (where the brightnesses are highest), in addition to have spiral arms at large radius from the centre. The spiral arms are often irregular in as long as m in addition to one can see many condensations or knots, like HII regions which are making stars. Irregular galaxies are irregular. They can be big systems of interacting galaxies, or (more commonly) small blue galaxies with absolute blue magnitudes > -18 in addition to no regular morphology, usually just diffuse fuzz with a few condensations. The most famous examples are the Magellanic clouds which can be seen from the Southern hemisphere. If a galaxy has a bright quasar at the centre, the quasar is usually so bright that you can’t see the rest of the galaxy. Such objects there as long as e look pointlike (that is, like stars). Finally, there do exist other, rarer, less classifiable galaxies, like giant low-surface-brightness galaxies that can barely be seen above the sky brightness. Elliptical Spiral Dwarf elliptical Dwarf irregular

The Hubble sequence is normally used to classify giant galaxies in addition to is illustrated in the tuning as long as k diagram. For ellipticals, the classification seems to depend an orientation. Conventionally, many astronomers use the classifications Sd in addition to Sm to describe galaxies intermediate between Sc in addition to irregular galaxies. Many observational parameters correlate with Hubble type. For example: Bulge-to-disk ratio decreases towards later types Spiral arm pitch angle increases towards later types Star as long as mation rate per unit mass increases towards later types HI mass fraction increases towards later types Colour gets bluer towards later types Total mass is approximately constant from S0 to Sc, then decreases towards later types Dark matter mass fraction increases towards later types Optical stellar mass-to-light ratio decreases towards later types Ratio of molecular to atomic gas mass decreases towards later types X= Mrk 1460

4 Nomenclature This is mostly historical, with apparently similar galaxies having very different-sounding names. Names are usually taken from the following catalogs (if a galaxy appears in more than one catalog, the name from the first-listed is usually adopted). Messier or M (bright local objects, many are star clusters) NGC (New Galaxy Catalog, about 8000 objects, many are star clusters) Zwicky catalogs (odd objects) Arp catalog (peculiar interacting systems) Markarian catalog (UV-bright systems) IC (Index catalog) UGC (Uppsala General Catalog) IRAS (infrared-loud, discovered by the IRAS satellite) Most contemporary surveys like 2MASS in addition to SDSS give names with the coordinates implicit. Failing all above, a galaxy can be named by its coordinates, which tell its position on the sky. For example 01305+3305 would be an object at a right ascension of 01:30.5 in addition to a declination of +33:05. 5. Components of galaxies 1. Stars Stars like the Sun are massive spherical accumulations of gas that are undergoing nuclear fusion in addition to releasing energy in the as long as m of (mostly visible) electromagnetic waves. They have masses typically 0.1 to 100 times the mass of the sun, in addition to have a blackbody spectrum that peaks at longer wavelengths as long as lower mass stars (it peaks at about 500 nm as long as the Sun). The evolution of stars is depicted in the Hertzprung-Russell (HR) diagram (alternatively called the color-magnitude diagram), which is historically one of the most profound matches between theory in addition to observation in astrophysics. Different mass stars follow different tracks on the diagram. Most gross photometric properties of galaxies can be understood in terms of this diagram, remembering the fact that more massive stars evolve (that is, move along their tracks in the diagram) much faster than less massive ones. For example, a 10 solar mass star completes its track in about 0.01 billion years, a 1 solar mass star in about 10 billion years. Gas cloud – gravitational contraction – pre-main sequence – nuclear burning initiated – main sequence – shell burning – subgiant – photon streaming limit – red giant branch ascent – core helium ignition – tip of the red giant branch – vigorous helium core burning + weakened hydrogen shell burning – horizontal branch – CO core in addition to double shell burning – asymptotic giant branch – helium shell exhaustion – supergiant – mass loss to become a planetary nebula in addition to then CO white dwarf OR core collapse supernova to become neutron star or black hole BLUE RED FAINT BRIGHT

Young stars as long as m in clouds of gas in addition to tend to be enshrouded in these clouds which contain lots of dust. There as long as e we don’t see these (very luminous) OB stars directly, rather we infer their presence from the thermal (infrared or submillimeter) radiation of dust which has absorbed the short-wavelength radiation from the young stars in addition to reradiates it as a cold blackbody. The nuclear fusion in stars is responsible as long as making all the elements heavier than helium that are seen in the Universe. Remnants are white dwarfs, neutron stars, or black holes. These are condensed matter, the last two are optically invisible in addition to are remnants of the most massive stars. Neutron stars are however visible as radio pulsars ( in addition to maybe some gamma-ray bursts). The long-duration gamma ray bursts may be linked to black holes in as long as mation. The color-magnitude as long as star cluster is that as long as a single age population. Here is the color-magnitude diagram of an old globular cluster. Note: There are no blue stars on the main sequence. The turn-off is well defined, in addition to a function of age (redder is older) White dwarfs will be below the diagram to the left The two or three stars on the blue side of the turn-off are blue stragglers. That the morphology of the horizontal branch. In general, older in addition to more metal-poor stars are bluer here. Stellar population synthesis modeling of galaxies is used to predict photometric properties of galaxies by combining the stellar evolution models as long as different age populations with a stellar IMF. Elliptical galaxies contain old populations of stars. All the massive stars in these galaxies have completed their evolution in addition to are remnants. The most luminous stars in elliptical galaxies are red giants – this is why elliptical galaxies look red through a telescope. This is also true as long as the central parts of spiral galaxies (the bulges). Spiral galaxies (particularly the outer parts, also irregular galaxies) contain young populations of stars. They have massive stars that haven’t finished their evolution. These are the most luminous stars in the galaxies. Recall from the HR diagram that these are blue. This is why spiral galaxies look blue through a telescope.

6 Components of galaxies 2. Active galactic nuclei Nucleosynthesis in stars is not the only way that one can convert gas into other material in addition to release electromagnetic radiation along the way. Accretion onto a black hole in the centres of galaxies (the Active Galactic Nuclei, or AGN phenomenon) is another way in addition to is about 400 times more efficient. This phenomenon is however quite rare since one requires lots of gas in a small region, in addition to this usually only happens in the centres of galaxies. In its most extreme phenomenon, AGNs can be QSOs or quasars, which are more luminous than the most luminous galaxies (up to absolute blue magnitudes of – 28). These are probably quite shortlived, as they require enormous gas supplies. More common weaker AGNs like Seyfert galaxies, etc. are also found. Many of the most luminous AGNs are dust-enshrouded, particularly when they are very young in addition to have just as long as med. AGNs have non-thermal spectra in addition to so emit quite a lot of energy outside visible waveb in addition to s. They are much brighter than stars at X-ray in addition to radio wavelengths, as long as a given optical luminosity. AGNs vary considerably in their radio properties. Optically luminous QSOs can be either radio-loud quasars, or radio quiet. Quasars may be radio galaxies that happened to be observed down the jet. The entire theoretical framework on which AGNs are based is somewhat less secure than as long as stars. This is in part due to the extreme conditions near these very massive black holes of 106 to 109 solar masses, which is quite unlike anything we can test in laboratories on Earth (we are in the strong field limit of general relativity). On the other h in addition to , atomic spectroscopy in addition to nuclear physics (the physics on which stellar astrophysics is based) is much more thoroughly tested. The black-hole models as long as AGNs predict that when the AGN has run out of gaseous fuel, one should still find very massive (now dark) objects in the centres of nearby galaxies. These can be found by virtue of their dynamical effects on stars in the centres of galaxies. Recent observations strongly suggest that these massive dark objects do in fact exist. Interestingly, there is a very tight relationship between the mass of the black hole in addition to the mass (or velocity dispersion) of the stars in the spheroid population of the host galaxy: MSMBH ~ 0.0015 M (spheroid) This ties together black-hole as long as mation in addition to star as long as mation together in some (presumably complex) way.

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7. Components of galaxies 3. Gas The space between the stars is occupied by gas that is not very dense (about 1 atom per cc, a vacuum more perfect than we can get on Earth in laboratories). However, the galaxies are so big that the total mass in this gas is an appreciable (though usually less than a percent) fraction of the mass of the galaxy. The gas is about 70% H in addition to 25% He by mass. Other elements are present in trace amounts (mostly CNO), in addition to have been made in stars. These are of astronomical interest as they offer useful probes of conditions, such as the integrated chemical evolution history of the galaxies. Gas may be ionized (hot), atomic (cold), or molecular (very cold, less than 50 K). In elliptical galaxies most gas is hot. In spiral in irregular galaxies, most gas is cold, 50% or less of which is molecular. The molecular gas is very clumpy, the others are more evenly spread. It is in the molecular gas clumps that stars as long as m. Often associated with these cold molecular gas clumps are substantial amounts of dust (about 1% of the cloud mass). It is this dust that obscures the light from young stars. The gas in galaxies, called the interstellar medium, is much denser than the gas between galaxies, called the intergalactic medium (1 atom/cc in the interstellar medium compared to 10-8 atom/cc in the intergalactic medium). 8. Components of galaxies 4. Dark matter This occupies about 90% (at least) of the mass of galaxies. It is distributed in a smooth halo that envelopes the stars in addition to gas. Spatially the dark matter halo occupies the same region as a number of small (< 106 Msun) dense old star clusters, the globular clusters. There about 150 globular clusters in the Milky Way, more in bigger galaxies. There are also about 109 Msun of individual stars in the Milky Way field halo. The existence of dark halos around galaxies comes from dynamical measurements, X-ray profiles of elliptical galaxies in addition to measurements of gravitational lensing by individual galaxies. The existence of dark matter in galaxy clusters, comes from X-ray, velocity dispersion in addition to gravitational lensing (both weak in addition to strong) measurements. The existence of dark matter in a cosmological context comes from the consideration of a large number of datasets in conjunction with each other. as long as example, in as long as mation comes from measurements of cosmic shear in addition to the Lyman as long as est. Cosmological simulations, like the Millennium simulation, show that the three are the same material. We require a universe with about 30% of the critical density in normal matter, of which about 15% (about 4% of the total) is in baryons. By “matter”, we mean material that obeys a pressureless equation of state. This is what distinguishes it from dark energy, which makes up the other 70% or so. This dark matter must be cold in order to as long as m galaxies, since hot dark matter (like neutrinos) will free-stream out of perturbations in the cosmological fluid at early times. By “cold”, we mean that the material has low thermal velocity. That all observations point towards one consistent picture is another example of a successful match between theory in addition to observation in astrophysics. HI in addition to Ha rotation curves are both flat as long as many spiral galaxies. These galaxies are all at about 5000 km/s, where 1 arcmin corresponds to about 30 kpc. Science (2003) 301, 1696 8. The Milky Way Galaxy Bunker et al. 2004 Elliptical Trentham et al. 2005 Spiral Dwarf irregular Dwarf elliptical My guess is that the final solution will look something like this.

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