Galaxy classification Classification can be based on morphological

Galaxy classification • Classification can be based on – – morphological criteria color indices spectroscopic parameters (based on emission or absorption lines) the broad-band spectral distribution (galaxies with/without radio- and/or X-ray emission) • The morphological classification defined by Hubble is still the bestknown today, and it is based on optical observations. Schneider 2007

Elliptical galaxies • Elliptical galaxies have neither spiral arms nor a disk. • Stellar orbits in elliptical galaxies are oriented randomly. • Nearly elliptical isophotes • ~14% of galaxies are elliptical • Ellipticity ε ≡ 1−b/a, 0 ≤ ε ≤ 0. 7 (where a and b denote the semimajor and the semiminor axes) – Classification of ellipticals on the basis of ε: n=10 ε class En (e. g. E 0 circular isophotes) E 0 • E 6 Ellipticals span a wide range of masses and luminosities (6 orders of magnitude)

Types of ellipticals • Normal ellipticals. This class includes giant ellipticals (g. E’s), those of intermediate luminosity (E’s), and compact ellipticals (c. E’s), covering a range in absolute magnitudes from MB ~− 23 to MB ~− 15. • S 0 galaxies are often assigned to this class of early-type galaxies. (lenticular galaxies) • Dwarf ellipticals (d. E’s). These differ from the c. E’s in that they have a significantly smaller surface brightness and a lower metallicity. • c. D galaxies (central Dominant galaxy ). These are extremely luminous (up to MB ~− 25) and large (up to R ~1 Mpc) galaxies that are only found near the centers of dense clusters of galaxies. Their surface brightness is very high close to the center, they have an extended diffuse envelope, and they have a very high M/L ratio. • Blue compact dwarf galaxies. These “blue compact dwarfs” (BCD’s) are clearly bluer (with B−V between 0. 0 and 0. 3) than the other ellipticals, and contain an appreciable amount of gas in comparison. • Dwarf spheroidals (d. Sph’s) exhibit a very low luminosity and surface brightness. They have been observed down to MB ~− 8. Due to these properties, they have thus far only been observed in the Local Group. – Giant ellipticals are appreciably larger than spirals – Dwarf ellipticals are appreciably smaller than spirals – Dwarf ellipticals outnumber giant ellipticals by about 10: 1.

<SN> is the “specific frequency”, a measure for the number of globular clusters in relation to the visual luminosity “Normal” elliptical • “Perfect” r 1/4 fit • Has ring of HI • Has supermassive BH S 0 (lenticular) NGC 1275 – c. D in Perseus cluster From Carroll & Ostlie NGC 147 – d. E • a massive network of emission-line filaments • supermassive central BH Blue Compact Dwarf UGC 5497 Leo I – dsph

Brightness Profile - Effective radius • The brightness profiles of normal E’s and c. D’s follow a de Vaucouleurs profile or • re effective radius: F(r < re) = 1/2 Ftotal • Empirical law – no physical basis • The de Vaucouleurs profile provides the best fits for normal E’s • For E’s with exceptionally high (or low) luminosity the profile decreases more slowly (or rapidly) for larger radii. • The profile of c. D’s extends much farther out and is not properly described by a de Vaucouleurs profile except in its innermost part. Seeing problems NGC 4472

Brightness Profile in ellipticals of different class • The effective radius Re is strongly correlated with the absolute magnitude MB • The d. E’s and the d. Sph’s clearly follow a different distribution. • The surface brightness in normal E’s decreases with increasing luminosity, while it increases for d. E’s and d. Sph’s.

Composition of Ellipticals v Except for the BCD’s, elliptical galaxies appear red when observed in the optical, which suggests an old stellar population (but, metallicity). v It was once believed that ellipticals contain neither gas nor dust, but these components have now been found, though at a much lower mass-fraction than in spirals. v hot gas (~ 107 K) detected by its X-ray emission. v Hα emission lines of warm gas (~ 104 K) v cold gas (~100 K) in the HI (21 -cm) v CO molecular lines. v Many of the normal ellipticals contain visible amounts of dust, partially manifested as a dust disk. v The metallicity of ellipticals and S 0 galaxies increases towards the galaxy center, as derived from color gradients. Also in S 0 galaxies the bulge appears redder than the disk. Dust lane in Cen A

Characteristic optical spectrum (integrated) of elliptical galaxy – shows old population • strong absorption lines, due to metals in the stellar atmospheres of the low luminosity stellar population. • few to no emission lines A red colour can be due to high metallicity as well as old age!

Star formation histories of model ellipticals De Lucia et al. 2005

Colour-magnitude relation Abell 2218 The linear relation for the brighter galaxies indicates that most of the E and S 0 galaxies within the cluster were formed via the same mechanism and that this mechanism couples the colour of the stars formed within the galaxy to the final mass of the galaxy. http: //www. astro. lu. se/Education/utb/AST 314/col. Mag_intro. html#a 2218 CM

Metallicity – Magnitude Relation

On Elliptical Galaxy Formation • Elliptical galaxies show a remarkable uniformity in their photometric and chemical properties, • One of the strongest constraints being the mass-metallicity relation • The first proposed scenario of elliptical formation was the so-called monolithic collapse scenario (e. g. Larson, 1974). – ellipticals are assumed to have formed at high redshift as a result of a rapid collapse of a gas cloud. – This gas is rapidly converted into stars by means of a very strong burst – Galactic wind powered by the energy injected into the ISM by SNe and stellar winds. carries out the residual gas from the galaxies, thus inhibiting further star formation. – the mass-metallicity relation could be easily explained in terms of metallicity sequences, namely the more massive objects develop the wind later (due to their deeper potential wells) and, thus, have more time to enrich their stellar generations. • This scenario has been modified to take into account the increase of [Mg/Fe] abundance ratio in the stars as a function of galactic mass – Mg (by type II SNe, on short timescales), Fe (by type Ia SNe, on longer timescale), – the Mg/Fe-mass relation implies that the more massive objects should have formed faster than the less massive ones – Pipino & Matteucci (2004) implemented an infall term in the chemical evolution equation and found that most of the photo-chemical observables, including the Mg/Fe-mass relation can be reproduced in a scenario in which the more massive galaxies formed faster and with a much more efficient star formation process with respect to the low mass objects. – PM 04 suggested that a single galaxy should form outside-in, namely the outermost regions form earlier and faster with respect to the central parts

• • Dynamics of Elliptical Galaxies Why are E’s not spherical? Rotational flattening? (as with Earth at equator) – If that were the explanation the rotational velocity vrot, which is measurable in the relative Doppler shift of absorption lines, would have to be of about the same magnitude as the velocity dispersion of the stars σv that is measurable through the Doppler broadening of lines. – for the rotational flattening of an axially symmetric, oblate galaxy, we need: – for luminous ellipticals one finds that, in general, vrot <<σv, so luminous ellipticals are in general not rotationally flattened. – For less luminous ellipticals and for the bulges of disk galaxies, however, rotational flattening can play an important role • • The stars behave like a collisionless gas: elliptical galaxies are stabilized by (dynamical) pressure, and they are elliptical because the stellar distribution is anisotropic in velocity space. This corresponds to an anisotropic pressure The dynamics of the orbits are determined solely by the large-scale gravitational field of the galaxy pair collisions do not play any role in the evolution of stellar orbits >>age of Universe

The Faber–Jackson Relation • • the velocity dispersion in the center of ellipticals, σ0, scales with luminosity The Faber–Jackson relation specifies a connection between the luminosity and a kinematic property of elliptical galaxies. • Various other relations also exist between the parameters of elliptical galaxies (e. g. log Re – MΒ etc) Schneider 2007 • So, the inference is that there may well be a relation between the various parameters for which the dispersion is smaller than that of the Faber–Jackson relation: the fundamental plane. (e. g. Primary component analysis)

The fundamental plane The distribution of elliptical galaxies in the three-dimensional parameter space Projections of the FP onto different two parameter planes Schneider 2007

FP relation – virial theorem Virial theorem which agrees with the FP relation provided that i. e. if the M/L ratio increases with M

Introductory remarks for elliptical formation • • Elliptical galaxies are the most massive stellar systems in the local Universe and They appear to define a homogeneous class of objects with – uniformly old and red populations, – Negligible amounts of gas, – very little star formation. • 50 % or more of the stellar mass in the local Universe appears to be in early–type systems and bulges • Their deceptively simple appearance inspired a ‘classical’ formation scenario in which they form in a single intense burst of star formation at high redshifts (z ~ 5), followed by passive evolution of their stellar populations to the present day (Partridge & Peebles 1967; Larson 1975). • This so-called monolithic scenario successfully explains – the tightness of the fundamental scaling relations (the colour–magnitude relation and the Fundamental Plane) – the evolution of these relations as a function of redshift (Kodama et al. 1998; van De Lucia et al. (2005) Dokkum & Stanford 2003).

Other suggestions for elliptical galaxy formation Ellipticals are more complex than what the simple monolithic scenario predicts • Toomre & Toomre (1972) suggested that elliptical galaxies can form from major mergers of massive disk galaxies. • Detailed numerical simulations (Farouki & Shapiro 1982; Negroponte & White 1983) showed that the merger of two spiral galaxies of comparable mass can indeed produce a remnant with structural and photometric properties resembling those of elliptical galaxies. • In more recent years, a large body of observational evidence has been collected that demonstrates that interactions and mergers indeed represent a common phenomenon at high redshifts, and that these processes affect the population of elliptical galaxies in the local Universe. • Schweizer & Seitzer (1992) found evidence for bluer colours of elliptical galaxies with increasing morphological disturbance • Later studies using absorption–line indices have demonstrated that a significant fraction of cluster early–type galaxies has undergone recent episodes of star formation (Barger et al. 1996, Menanteau, et al. 2001; van de Ven, et al. 2003, Treu et al. 2002 etc) At least for a part of the elliptical galaxy population, a hierarchical formation scenario in which larger spheroidals are assembled relatively late from the merger of late–type galaxies of comparable mass. Such a bottom-up formation scenario is naturally expected for the structure formation process in cosmologies dominated by cold dark matter.