Galaxy morphology

Summary of Spitzer Results on Galaxy Morphology and Classification... 

Dwarf Spheroidal galaxies are the smallest and faintest galaxies known. They are typically dominated by old stellar populations (e.g. Sculptor and Sextans), but some of them (e.g. Fornax) exhibit more recent star formation episodes (2-8 Gyr ago). Analysis of the horizontal branch morphology shows that Red HB stars are more centrally concentrated than Blue HB stars which could be interpreted either as an age or a metallicity gradient or both ([1]). Only spectroscopic observations can unambiguously separate metallicity gradients and make a link with the kinematics. 

Fig. 1. The Hubble-Toomre diagram, relating the present morphologies of (large) galaxies and their (major) merger histories. Quantification of at least one merger and assembly history, for the Milky Way, is required to test the generality of these evolutionary paths.

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 Crab Nebula is the most noticeable object in our Galaxy, and the remnant of the famous Chinese guest star appeared in 1054AD, the best association between supernova remnants(SNR) and ancient guest stars. Before seventies, the Crab Nebula was considered as a special SNR with different morphology and physical features from that of most SNRs, Now more and more Crab-like SNRs have been detected(Weiler 1985). It is necessary to make a systematical investigation for the Crab-like SNRs, especially for those with central pulsars because they offer us more physical messages than others. 

Figure 2.4. Cluster contour maps superposed on optical images (see Jones Forman 1999 for details). The clusters illustrate the morphological classes described in the text and listed in Table 2.2. The last class, G , where the cluster emission is dominated by that from individual galaxies, is not shown. 

Einasto Dynamical and morphological properties of galaxy systems ... 

The astrophysical problem of justifying on theoretical grounds the morphology of galaxies (spiral and eUiptical, with their different content in stars and gas), their chemical evolution (initial rapid enrichment of metals, i.e., any element heavier than hydrogen and helium), and, finally, the attempt to trace a classification based on different physical aspects of the evolution, has been tackled by employing the approach of cooperative systems. In these models a scenario is proposed where the large-scale dynamics are related to the local microscopic interactions. At the same time a macroscopic description (e.g., the interplay of various phases, the metallicity) is derived by means of few (stochastic) variables. 

The mathematical structure of the models is their unifying background systems of nonlinear coupled differential equations with eventually nonlocal terms. Approximate analytic solutions have been calculated for linearized or reduced models, and their asymptotic behaviors have been determined, while various numerical simulations have been performed for the complete models. The structure of the fixed points and their values and stability have been analyzed, and some preliminary correspondence between fixed points and morphological classes of galaxies is evident—for example, the parallelism between low and high gas content with elliptical and spiral galaxies, respectively. 

The morphological classihcahon scheme introduced by Hubble (1926,1936), based on blue-light images, has been modihed periodically over the years (e.g., Sandage 1961 de Vaucouleurs 1959 Kormendy 1979 Buta 1995) but remains a fundamental method by which astronomers continue to sort and compare galaxies. While the method has proven very successful over much of the last century, its dependence on fundamental, physical properties intrinsic to the galaxies is indirect due to the complicated emission processes sampled in the E-band. 

The galaxies presented here are a small but representative subset of a sample of about 100 scheduled to be observed with Spitzer. These galaxies were taken from a complete sample from Ho et al. (1997). The sample observed to date with Spitzer nearly fully spans the classical morphological sequence (only Sa is missing). 

Two colors ([3.6] — [4.5] and [3.6] —[8.0]), as well as the 3.6 /xmbulge-to-disk-ratio, correlate well with traditional morphological type and hence can be regarded as a means of galaxy classification. 

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