Spectral energy distribution fitting with MagPhys package

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1 Spectral energy distribution fitting with MagPhys package Nikola Baran 22 September 2015 Exercise Description The goal of this exercise is to introduce the student to one of the important techniques used in today s astronomy - spectral energy distribution (SED) fitting. The exercise will be performed using Multi-wavelength Analysis of Galaxy Physical Properties (MAGPHYS) package 1 which consists of a large Fortran toolkit. The exercise consists of taking the output dataset of a series of observations performed using various filters, and implementing filter responses to obtain the best-fit spectral energy distribution from one of the available templates. It is required, upon completion, to write a report in the form of a short scientific paper. This report must contain all the information needed to reproduce the results: precise description of the dataset, methods used to obtain the results, parameter values etc. Also, the results must be described and interpreted. Student is encouraged to read the tutorials and lectures given at the official pages of MAGPHYS. Instructions that follow also contain the guidelines for the code, and advice on how to run different stages of SED fitting. Commands written here are to be adapted to the specific task given to the student. For clarity, they have been written in Courier font, and long commands have been split into multiple rows with one parameter per row. However, in console they should be input as one line. It is highly recommended that the student reads the documentation for the MAGPHYS package used in this exercise. To plot spectral energy distributions, an IDL routine will be provided. Thus, this exercise will introduce the student to programming in IDL(Interactive Data Language) - a very useful tool used in modern astronomy (and beyond). To view and edit the input and output files, any text editor can be used (e.g., vim, emacs). Text editor is an essential tool used by astronomers on a daily basis. For that reason, it can be a good investment to get acquainted 1 1

2 to more powerful ones like Notepad++ 2, Sublime 3 or jedit 4. Text editors like these allow for syntax highlighting for numerous programming languages, and offer a wide choice of plugins that turn them into true swiss-army-knives. Every text editor has its strengths and weaknesses, so the student is encouraged to find the most suitable one. Remember, the best tool is the one you know how to use well! 1 Introduction An object emitting electromagnetic radiation will inevitably imprint some of its properties into that radiation. Even an object as simple as a black body imprints a property - its temperature, into the radiation it emits. Objects we find in the universe are far more complex than a simple black body, and they will have a number of different emission and absorption mechanisms. Formation of new stars in galaxiesis one of them. When protostarsin a galaxyignite 5, and form young stars, they emit a lot of radiation in the ultraviolet and blue parts of the visible spectrum. On the other hand, old stars are red and faint. Some of them that collapse into supernovae will enrich their neighborhood with heavy elements, and they will send out powerful shock waves that emit synchrotron radiation in radio. It is interesting to note that galaxies that actively form stars, also have a high star death rate, meaning they turn into supernovae 6. Another type of interesting objects are active galactic nuclei (AGN). If the accretion of matter onto the central black hole of a galaxy is significant, it forms an active galactic nucleus. Often, these nuclei (or central engines ) outshine the host galaxy. Since accretion onto a black hole releases a lot of energy, these objects can accelerate charged particles to very high energies, which then emit synchrotron radiation visible in radio, X-ray, and beyond. Also, AGN heat up the surrounding gas and dust, giving them away in infrared and X-ray parts of the spectrum. Thus, using multi-wavelength data, we can construct a distribution of received energy (photons) as a function of wavelength. When we have such distribution, we compare it to a SED of a well studied object. If SEDs are similar enough, it is reasonable to assume that the observed object has the same characteristics to the one we compared it to. An SED of a well-known object is called a template, and it contains information on various properties of a galaxy like its stellar mass, star formation rate, specific star formation rate, dust luminosity etc. A set of templates is called a library. MAGPHYS can use various libraries, and in this exercise we will use a library from Bruzual&Charlot Ignition and burning in this context refer to the start and continuation of processes of nuclear fusion, and not to rapid oxidation. 6 This might seem counterintuitive at first, but keep in mind the timescales we observe are in the order of millions or billions of years. 2

3 To decide which template fits best, MAGPHYS takes note of the χ 2 for each template it tries to fit. The one that is chosen as the best-fit, has the minimum χ 2, and it s characteristics are noted in the resulting file. Student is encouraged to study the χ 2 goodness of fit in general statistics literature. 2 MAGPHYS routines Before working with the routines, one should make a copy o the MAGPHYS folder in a folder with permissions to write and read. Also, in all.f90 routines, variables outfile, optlib, and irlib should be modified to suit the specific paths in use. 2.1 Compiling MAGPHYS is a set of Fortran routines. To run, these routines first need to be compiled. Popular compilers include ifort and gfortran. Since gfortran is free, and thus more common, we will proceed with using gfortran. From terminal, navigate to the folder MAGPHYS is copied to, and use the following commands to compile the required routines. gfortran get optic colors orig 1.f90 -o get optic colors orig 1 -O0 -g -fbacktrace -fdump-core -fcheck=all -ffixed-form -ffixed-line-length-132 or gfortran get optic colors orig 1.f90 -o get optic colors orig 1 -O2 -g -fbacktrace -march=native -ffixed-form -ffixed-line-length-132 Both of these commands should work with gfortran, and the student is free to choose one of them. Note that the numerous parameters should be written inline, and are required for the compiling of the Fortran routines written in a specific format. These should be repeated for routines named get infrared colors orig 1.f90, fit sed orig 1.f90,withget infrared colors orig 1,fit sed orig 1 set as output filenames instead of get optic colors orig Input As an input to SED fitting routines we need the following information: filters used in the survey, source brightness, and its redshift. The three files containing 3

4 those data are filters.dat, zlibs.dat, and observations.dat. The file filters.dat contains numbers that describe how much light at a given wavelength - a filter response curve. They are needed to properly reconstruct the actual SED of the source. The filezlibs.dat is a two column file containing the redshifts of sources we are about to perform SED fitting on. The file observations.dat contains source brightness as measured with each filter. Note the first line of the file contains names of filters. Filters can be named in any way, as long as they are described in filters.dat. 2.3 Running MAGPHYS routines Under Linux, one can use various shells. Shell is, in general, a user interface for access to an operating system s services. It can be either graphical or comandline. The Bourne-again shell (bash) is the shell usually default shell in Linux terminal. However, user can choose another shell. For the purpose of running MAGPHYS, we will use turbo-c shell (tcsh). In your home folder (i.e. /home/student5/) create (or edit if it already exists) a file named.tcshrc and put the following line into the file: setenv magphys /home/student5/sedfit/magphys note that this command shortens the path where MAGPHYS is installed, so adapt it accordingly. Now switch to turbo-c shell using the command tcsh Command bash switches back to the Bourne-again shell. From the tcsh run your.tcshrc file to set the enviroment variable for the routines 7 : source.tcshrc Since the routines should now be compiled, let s build optical color libraries: source get optic colors orig 1 and infrared color libraries: get infrared colors orig 1 and, finally, perform the SED fit: fit sed orig 1 7 The source command in tcsh is equivalent to the. command in bash. The. simbol at the beginning of the filename indicates a hidden file in Linux. 4

5 Figure 1: An example of the SED fit - luminosity vs. wavelength in logarithmic scale. The black line represents the best-fit to the observed points (red). The blue line is theoretical galactic SED without the absorption by interstellar medium. The value of reduced chi 2 for that fit is indicated in the top right. The lower part of the plot shows deviation of the fitted line from the observed points. Plot is created using IDL routine plotsed.pro. 2.4 Output Output ofthe MAGPHYS SEDfitting procedureare.sedand.fitfiles. Files have names corresponding to the numerical ID of the source as listed in the input files. Files with.sed extension contain the spectral energy distribution information, while those with.fit contain the information about the source derived from the best-fit template, like total stellar mass, star-formation rate etc. For details consult the official MAGPHYS documentation. It is often useful to plot the SED. For that purpose, a separate IDL routine with instructions will be provided. In the written report, try answering these questions: What does the best-fit solution tell us? What kind of galaxy does the source represent? Is it a passive, quiescent, galaxy, or a galaxy in which new stars actively forming? Is it some other type of object? Consult the literature, describe and interpret the results. Report your findings in the form described in Exercise Description. 5

Galaxies with Active Nuclei. Active Galactic Nuclei Seyfert Galaxies Radio Galaxies Quasars Supermassive Black Holes

Galaxies with Active Nuclei. Active Galactic Nuclei Seyfert Galaxies Radio Galaxies Quasars Supermassive Black Holes Galaxies with Active Nuclei Active Galactic Nuclei Seyfert Galaxies Radio Galaxies Quasars Supermassive Black Holes Active Galactic Nuclei About 20 25% of galaxies do not fit well into Hubble categories