Stellar population modeling and SED fitting 

Fitting the Spectral Energy Distributions of galaxies

Because electromagnetic radiation is the most significant carrier of information from distant galaxies, modeling and fitting of the spectral energy distribution (SED) is one of the most important astronomical tools. Ideally one would like to combine information from all energy / wavelength regions. On the other hand, the physical processes that cause emission are very diverse. As a result from a Lorentz workshop on "Fitting the SEDs of galaxies", I have compiled a webpage that contains links to stellar population models and fitting tools, mostly for the optical to infrared range. I gladly accept requests to add further links about tools I may have missed. 

Differential Stellar Population Models

Different formation and evolution of galaxies produce different patterns in the distribution of element abundances in the stars of those galaxies (e.g. Mg vs. Fe abundance). Historically, most measurements of the abundance ratios have relied on "indices", encoding the depths of specific absorption lines. This is because it has been very challenging to create stellar population models that would predict the spectra of galaxies to sufficient precision over a large wavelength range. In Walcher et al. (2009) we presented new, differential stellar population models that solve this longstanding problem. Specifically, we use semi-empirical models (e.g. Bruzual & Charlot or Vazdekis) as calibrators for solar metallicity, solar abundances. The purely theoretical models by Coelho et al. (2007) are then used to predict the effect of varying [Fe/H] and [α/Fe] over the full spectral range. We show in the paper that our model is accurate over the wavelength range from 4828 to 5364 Å.



Galaxy surveys using imaging spectroscopy


A panoramic view of galaxy
physical properties

The Calar Alto Legacy Integral Field Area survey (CALIFA, Sanchez et al., 2012) has obtained spatially resolved spectroscopic information of a sample of > 600 galaxies of all Hubble types in the local universe (0.005 < z < 0.03), which are covered over their full optical extent and over the full optical wavelength range (see Walcher et al., 2014 for a complete description of the sample). The CALIFA team has published articles covering a large range of subjects in galaxy evolution, such as the ionization conditions in the interstellar medium, abundance gradients in galaxies, detailed archeology of the stellar populations in galaxies, the effects of galaxy interactions on kinematics and star formation properties, baryonic fractions in galaxies as a function of galaxy mass, the environmental properties of supernova explosions, locations of Wolf-Rayet stars in galaxies, and more. 



The relation between the mass of the central supermassive black hole MBH and the velocity dispersion of its host spheroid σ is fundamental for our understanding of galaxy evolution and its relation to their nuclei. Correspondingly much work has been invested in determining accurate MBH masses. Surprisingly little has been done on standardizing the other axis, i.e. σ measurements. These values are often derived from various long-slit datasets at different physical radii of the galaxy and no homogeneous definition has been given. I have led a series of proposal to remedy this situation by using MUSE@VLT, PMAS@CAHA and archival A3D data to obtain kinematic maps out to 1 effective radius of all galaxies with a secure black hole mass. We now have data in hand for ∼70 galaxies out of a total of ∼80 galaxies with existing or planned secure black hole masses.

The data will be used to derive the velocity and velocity dispersion maps σ of all target galaxies. The primary aim is to provide a homogeneous dataset that allows to define the most relevant σ measurement for the MBH – σ relation. We will – purely empirically – test different measures of σ (e.g. over different radii) and then study the slope, zero-point and scatter of the respective relation. This will allow us to quantify which of the definitions minimizes the scatter in the MBH – σ relation – presumably an expression of the closest physical link. 


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