Introduction and Overview

1 Introduction and overview

1.1 The goal of the course (i) Astronomical observations of the properties of galaxies: how big, material content, and structure. (ii) Physical processes involved in understanding the properties of galaxies.

1.2 What are galaxies • A galaxy is a dynamically bound system of many stars • A galaxy like the MW contains ∼ 100 billion stars like the Sun • Typical size: A diameter of about 20 kpc; 1 kpc = 3×1018cm ∼ 3, 000light − years • Dynamically bound: stars are bound to the system and cannot fly apart • Thus a galaxy can exist for a very long time • Milky Way is one of billions of galaxies in the universe • Different galaxies have different properties. • Most of the visible stars in the universe are in galaxies

1.3 Galaxies are beautiful and sometimes strange objects 1.4 Put things in some order: morphology classiﬁcation From the left to the right, galaxy morphology is said to change from early to late type. In the Hubble sequence, galaxies are classiﬁed into four broad classes. (i) Elliptical galaxies: These have smooth, almost elliptical isophotes and are divided into sub-types E0, E1, · · ·, E7, where the integer is 10×(a−b)/b, with a and b the lengths of semi-major and semi-minor axes. (ii) Spiral galaxies: These have thin disks with spiral arm structures. They are divided into two branches, barred spirals and normal spirals, according to whether or not a recognizable bar-like structure is present in the central part of the galaxy. On each branch, galaxies are further divided into three classes, a, b and c, according to the following three criteria: • the fraction of the light in the central bulge;

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Fig. 1.1. The Hubble Deep Field (HDF), an exposure of about 10 days with the Hubble Space Telescope of a patch of sky one arc minute across. Galaxies with measured redshifts are marked. [Courtesy of Mark Dickinson (the Space Telescope Science Institute) and Judy Cohen (Keck Telescope)]

• the tightness with which spiral arms are wound; • the degree to which the spiral arms are resolved into stars, HII regions and ordered dust lanes. (iii) Lenticular or S0 galaxies: This class is intermediate between ellipticals and spirals. Like ellipticals, lenticulars have a smooth light distribution with no spiral arms or HII regions. Like spirals they have a thin disk and a bulge. They may also have a central bar – they are then classiﬁed as SB0. (iv) Irregular galaxies: These objects have neither a dominating bulge nor a rotationally symmetric disk and generally have a patchy appearance dominated by a few HII regions and lacking any obvious symmetry. (v) Peculiar galaxies: These are the ones with very strange appearances which cannot be put in the Hubble sequence. Introduction and overview 3

Fig. 1.2. Examples of diﬀerent types of galaxies. From left to right and top to bot- tom, NGC 4278 (E1), NGC 3377 (E6), NGC 5866 (SO), NGC 175 (SBa), NGC6814 (Sb), NGC 4565 (Sb, edge on), NGC 5364 (Sc), Ho II (Irr I), NGC 520 (IrrII). [Pho- tographs from the Carnegie Atlas, courtesy of A. Sandage]

1.5 Properties of galaxies along the Hubble sequence 1.6 Properties of individual galaxies • The number of stars in a galaxy, i.e. the luminosity of a galaxy (bright vz faint) • The distribution of stars in a galaxy, i.e. the morphology of a galaxy (elliptical, spiral, irregular, etc.) • The stellar population: i.e. mostly young stars or old stars (blue vz red) • The amount of gas contained in a galaxy (gas rich or gas poor) Some trends:

• spiral galaxies are on average bluer than elliptical galaxies • spiral galaxies are richer in cold gas 4

Fig. 1.3. Two prominent examples of peciliar galaxies. The left shows the peculiar galaxy known as the Antennae, a system exhibiting prominent tidal tails (the left inlet) and signature of a recent merger of two spiral galaxies. The right panel shows the peculiar galaxy known as the Cartwheel and companions, with the two inlets showing the images of a segment of the ring and the inner object, respectively. [Courtesy of NASA and Space Telescope Science Institute]

(NORMAL SPIRALS)

(ELLIPTICALS) Sa Sb Sc Im

E0 E3 E6 S0

SBa SBb SBc IBm

(BARRED SPIRALS)

Fig. 1.4. The schematics of the Hubble sequence of galaxy morphologies. [Adapted from R.G. Abraham (1998, astro-ph/9809131)]

• spectra of spiral galaxies contain emission lines, while those of ellipticals on absorption lines

1.7 Galaxy distribution in space • Mean separation between galaxies: few Mpc, much larger than the typical size of a galaxy, ∼ 20kpc. • Since most of the visible stars in the universe are in galaxies, the number density of stars within a galaxy is about 10 million times higher than the mean number density of stars in the universe as a whole. Thus galaxies are well-deﬁned astronomical identities. Introduction and overview 5

Fig. 1.5. Galaxy properties along the Hubble morphological sequence based on the RC3-UGC sample (after Roberts & Haynes 1994). Filled circles are medians, open ones are mean values. The bars bracket the 25 and 75 percentiles. Properties −1 plotted are LB (blue luminosity in erg s ), R25 (the radius in kpc to an isophote −2 of 25B mag arcsec ), MT (total mass in solar units within a radius R25/2), MHI (HI mass in solar units), MHI/LB, ΣT (total mass surface density), ΣHI (HI mass surface density), and B − V (B − V colour).

1.8 Why do we want to study galaxies? • Galaxies are bright, long-lived and abundant, can be observed in large numbers over cosmological distance and time scales. Good tracers of the structure and evolution of the universe. • As houses for stars and planets, their properties must be understood in order to understand stars and planets (or even extraterrestrial intelligence).

1.9 Put this course in a broad context • Cosmology: Since we are dealing with events on cosmological time and length scales, we need to understand the space-time structure on large scales. • Initial conditions for galaxy formation: These were set by physical processes in the early universe which are beyond our direct view, and which took place under conditions far diﬀerent from those we can reproduce in earth-bound laboratories. 6

Fig. 1.6. Spectra from ultraviolet to near-infrared for galaxies of diﬀerent types. As we move from ellipticals to late-type spirals, the blue continuum and emission lines becomes systematically stronger. For elliptical, bulge and SO galaxies, which lack hot young stars, most of the light emerges from longest wavelengths, where one sees absorption lines characteristic of cool K stars. In the blue, the spectrum of early type galaxies show strong H and K absorption lines of calcium and the G band, characteristic of solar type stars. Such galaxies emit little light at wavelength shorter than 4000A˚ and have no emission lines. By contrast, late-type spirals and starbursts emit most of their light in blue and near-ultraviolet. The light is produced by hot young stars, which also heat and ionize the interstellar medium to produce strong emission lines.

• Physical processes: physics is valid everywhere in the universe: gravi- tation, thermodynamics, electrodynamics, atomic, nuclear and particle physics, quanum physics

1.10 Topics to be covered in this course • Material content of galaxies, i.e. formation and evolution of stars, the integrated light from the stellar population of an individual galaxy, and the properties of interstellar medium. • The intrinsic structural properties of individual galaxies (size, luminosity, morphology, kinematics, etc) and the distribution of galaxies with respect to these properties. Introduction and overview 7

Fig. 1.7. The spatial distribution of galaxies as seen in a 4◦ slice projected onto the redshift/right-ascension plane. The galaxies shown here are part of the 2 Degree Field Galaxy Redshift Survey or 2dFGRS (Colless et al. 2001).

Fig. 1.8. Elliptical galaxies are preferentially found in high-density regions. 8

• The formation and evolution of galaxies.