Galaxy populations

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... 

Color-magnitude diagrams of galaxy populations can provide some information on the metallicity (or metallicity spread) of a stellar population, since features in the CMD, such as the color of the red giant branch, can vary with metallicity. Unfortunately these features also vary with age of the populations, so there is a degeneracy between age and metallicity in the CMD (and in composite colors). Systematic uncertainties may... 

The group identified a circular belt between 7 and 9 kpc1 from the centre of the galaxy. This zone (Fig. 11.6) consists of a population of stars which developed between 8 and 4 billion years ago it contains about 10% of all the stars in the galaxy, and around 57% of the stars in the habitable zone are older than our sun (Lineweaver et al., 2004). 

Multiperiodic SPB stars were found in the whole range of the V magnitudes in the searched area. Unlike in the Galaxy, these stars populate the region of early B or even late O-type stars. 

There are indications that the presence of two populations is a common feature of dSph galaxies. Our preliminary analysis of HB stars, Vhei and [Fe/H] measurements in the other galaxies in our sample (Fornax and Sextans dSph Battaglia et al., in prep) also shows very similar characteristics to Scl, especially in the most metal poor component. Pure radial velocity studies [15], [16] have also considered the possibility that kinematically distinct components exist in Ursa Minor, Draco and Sextans dSph galaxies. 

How will we identify the extra astrophysics required to reconcile the properties of CDM dark haloes with those of luminous galaxies We can start by developing knowledge of the evolutionary history of at least one place in at least one galaxy. We would be unlucky if that place were far from the norm alternatively, any theory that predicts such a history to be very unusual might be suspect -the galaxian Copernican principle. Kinematics and current spatial location are of course critical parameters, so that traditional stellar populations analyses are... 

That is, the straightforward interpretation of abundance data for Galactic field stars in terms of stellar populations is feasible only because the Galaxy apparently acquired its gas early, or at a rate which was well-matched to the star formation rate across the whole volume now sampled by local halo stars, and kept this gas well-mixed and because the stellar IMF is (close to) invariant over time and metallicity. Neither deduction was obvious, nor is the underlying physics understood. However, these two deductions apply so well they have become assumed authors use any violation to rule out some possible Galaxy merger histories, as in the Venn et al. analysis from which Figure 1 is taken. 

The halo globular cluster system also provides valuable information, since accurate distances, and hence reliable ages, can be derived. Mackey Gilmore (2004) recently acquired and compiled a new, nearly complete, internally consistent set of photometric studies of the globular cluster population in both the Milky Way and its satellite galaxies they deduce, from analysis of HB morphol-ogy, age, abundance and structural information, somewhat more relaxed limits... 

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. 

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. 

Abstract. We present preliminary iron abundances and a element (Ca, Mg) abundance ratios for a sample of 22 Red Giant Branch (RGB) Stars in the Sagittarius galaxy (Sgr), selected near the RGB-Tip. The sample is representative of the Sgr dominant population. The mean iron abundance is [Fe/H]=-0.49. The a element abundance ratios are slightly subsolar, in agreement with the results recently presented by [2]. 

Abstract. We present the results of a spectroscopic analysis of bright stars in the Carina dSph galaxy. We collected low-resolution FORS2 VLT spectra of ss 200 stars. Our spectroscopic targets have been selected among the evolved Carina stars, in particular we selected low-mass, old Red Giant, Asymptotic, and Horizontal Branch stars, as well as intermediate-age stars of the Red Clump. We present preliminary estimates concerning the radial velocities of old and intermediate-age populations. 

Fig. 5. Quantitative tests of numerical galaxy models are now becoming feasible. It is interesting to note how little apparent substructure is evident in the best conserved quantity, angular momentum, and how complex is the evolutionary history of a stellar population , such as the thick disk, in this model. This figure is from Abadi et al. 2003.

The epoch and mode of galaxy formation are not well known, but both quasars and star-forming galaxies are known with redshifts up to about 7, corresponding to an era when the expanding Universe was only 1/8 of its present size, and the emission-line spectra of quasars indicate a large heavy-element abundance (solar or more Hamann Ferland 1999), suggesting prior stellar activity. The first stars, on the other hand, known as Population IIP, would have been devoid of metals whether they differed from normal stars in other basic characteristics, notably their mass distribution, is not known, since no completely metal-free stars have been... 

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