Galaxies luminosity evolution

The IRAS galaxies provided some of the earliest evidence from redshift surveys, and from source counts as a function of observed flux, that the spiral galaxy population has undergone evolution (ORS see Fig. 12.2). This result is analogous to similar evidence from source counts of radio galaxies and quasars, as well as quasar redshifts, and a correlation that has been observed between radio and infrared luminosity suggests that the evolution could be similar in both cases. Typical simple models for such evolution include luminosity evolution according to... 

The predictions of chemical evolution models can be tested in a cosmological context to study the galaxy surface brightness and size evolution as a function of redshift. Roche et al. (1998) have already done that and suggested that a size and luminosity evolution, as suggested by the inside-out scenario fits better the observations. 

Abstract. 1 present K band photometry of the objects in the spectroscopic redshift survey of [CoUess et al., 1990]. The absolute K magnitudes of the objects are consistent with the no-evolution or pure luminosity evolution models. The excess f t blue galaxies seen in the B band number counts at intermediate magnitudes are a result of a low normalization, and do not dominate the population until B 25. Extreme merging or excess dwarf models are not needed. 

Much attention has been paid recently to an apparent excess of faint blue galaxies observed in photometric surveys. When the models of the B band number counts are normalized at B=16, the data show an excess over the luminosity evolution models of a factor of 2 at B=22. ([Tyson, 1988], [Lilly et al., 1991]) However, the K band number counts do not show this same excess, ([Gardner et al., 1993]). The shape of the number-redshifr distribution of surveys conducted at 20 < B < 22.5 by [Broadhurst et al., 1988] and [CoUess et al., 1990] are fitted by the no-evolution model. The median redshifts of the data from these surveys, and deeper data of [Cowie et al., 1991] and [AUington-Smith et al., 1992] show no evolution as faint as B=24. Proposed explanations for the high B band number counts include massive amounts of merging at intermediate redshifts (z 0.4) ([Broadhurst et al., 1992]) and an excess population of dwarf galaxies which appears at these redshifts, but has dissipated or faded by the present epoch. ([Cowie et al., 1991])... 

The new data, while subject to the incompleteness of the 43 upper limits on the fiux, is also fitted by the no-evolution model in Figure 2. At these magnitude levels, pure luminosity evolution makes little difference in the model predictions, and the amount cannot be determined, but the effects of more extreme models such as merging or excess dwarfs are not evident in the data. Thus, in the region 20 < R < 22, the shape of the luminosity function of field galaxies is normal, both in the B band and in the K band. 

The accretion history of a parent galaxy is constructed using a semi-analytical code. The full phase-space evolution during each accretion event is then followed separately with numerical simulations [2]. Star-formation and chemical evolution models are implemented within each satellite. The star formation prescription matches the number and luminosity of present-day galaxies in the Local Group, whereas the chemical evolution model takes into account the metal enrichment of successive stellar populations as well as feedback processes. Below we present results of a sample of four such simulated galaxy halos, denoted as Halos HI, H2, H3 and H4. 

Many connections have been found between the luminosity peak, the shape of the light curve, evolution in the colour, spectral appearance, and membership of a galaxy of given morphology. However, after the first 150 days, uniformity takes over and all these objects fade in the same way and with the same spectrum. 

The observed terminal velocities of O-stars provide interesting information about the evolution of massive stars. This is shown in Fig. 5, which is taken from Garmany and Conti (1985) and displays vM vs. Tef for a sample of O-stars (luminosity classes between V and III) in the Galaxy, LMC and SMC. Two striking facts can be read off from Fig. 5 ... 

The next problem was to find internally constitent values of physical parameters of stellar populations of different age and composition. For this purpose I developed a model of physical evolution of stellar populations (Einasto 1971). When I started the modelling of physical evolution of galaxies I was not aware of similar work by Beatrice Tinsley (1968). When my work was almost finished I had the opportunity to read the PhD thesis by Beatrice. Both studies were rather similar, in some aspects my model was a bit more accurate (evolution was calculated as a continuous function of time whereas Beatrice found it for steps of 1 Gyr, also some initial parameters were different). Both models used the evolutionary tracks of stars of various composition (metallicity) and age, and the star formation rate by Salpeter (1955). I accepted a low-mass limit of star formation, Mo 0.03 Msun, whereas Beatrice used a much lower mass limit to get higher mass-to-luminosity ratio for elliptical galaxies. My model... 

The basic data for stochastic simulations of galaxies and their constituent populations and metallicity evolution is the initial mass function (IMF), which represents the mass distribution with which stars are presumed to form. Its derivation from the observed distribution of luminosity among field stars (refs. 57 and 58 and references therein) and from star clusters involves many detailed corrections for both stellar evolution and abundance variations among the observed population. The methods for achieving the IMF from the observed distribution are most thoroughly outlined by Miller and Scalo but can be stated briefly, since they also relate to an accurate testing of various proposed stochastic methods. It should first be noted that the problems encountered for stellar distributions are quite similar to those with which studies of galaxies and thdr intrinsic properties have to deal. 

The ultimate goal of any chemical evolution model is to account for the global and local metallicity within a galaxy, the gas and stellar mass distributions, and the stellar luminosity self-consistently. Thus, any discussion of abundances and chemical evolution should include a few words about observational determinations of gas and stellar masses and mass surface densities. 

Spectroscopy is the key to unlocking the information in starlight. Stellar spectra show a variety of absorption lines which allow a rapid classification of stars in a spectral sequence. This sequence reflects the variations in physical conditions (density, temperature, pressure, size, luminosity) between different stars. The strength of stellar absorption lines relative to the continuum can also be used in a simple way to determine the abundances of the elements in the stellar photosphere and thereby to probe the chemical evolution of the galaxy. Further, the precise wavelength position of spectral lines is a measure of the dynamics of stars and this has been used in recent years to establish the presence of a massive black hole in the centre of our galaxy and the presence of planets around other stars than the Sun. 

Studies of galactic evolution have focused on the comparison between the atomic (HI) and molecular (H2) gas properties and star formation rates as a function of environment, luminosity, and galaxy type. The general conclusions from these studies are as follows ... 

The radial luminosity profiles of disks in spiral galaxies csin be described by an exponential law and are thus characterized by two parameters, the central stnface brightness (/uo) and the scalelength (h). Freeman (1970) foimd /to to be nearly constant in his sample of galaxies observed in the B band. This Freeman law has of course serious implications for theories on galaxy formation and evolution. 

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