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courses:ast403:population-synthesis [2026/02/12 06:00] – shuvocourses:ast403:population-synthesis [2026/02/17 21:22] (current) – shuvo
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-===== Population Synthesis =====+====== Population Synthesis ======
  
 Population synthesis is a theoretical framework used to interpret the integrated spectra of galaxies as the sum of light emitted by their constituent stellar populations. Because the light of "normal" galaxies originates almost entirely from stars, and stellar evolution is relatively well-understood, astronomers can model a galaxy's spectral energy distribution (SED) by simulating its history of star formation and chemical enrichment. Population synthesis is a theoretical framework used to interpret the integrated spectra of galaxies as the sum of light emitted by their constituent stellar populations. Because the light of "normal" galaxies originates almost entirely from stars, and stellar evolution is relatively well-understood, astronomers can model a galaxy's spectral energy distribution (SED) by simulating its history of star formation and chemical enrichment.
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 In this equation, $S_{\lambda, Z}(t')$ represents the energy emitted per wavelength by a group of stars with initial metallicity $Z$ and age $t'$. This function accounts for the various evolutionary tracks stars follow in the Hertzsprung–Russell diagram (HRD) and their corresponding isochrones (positions of equal age in the HRD). In this equation, $S_{\lambda, Z}(t')$ represents the energy emitted per wavelength by a group of stars with initial metallicity $Z$ and age $t'$. This function accounts for the various evolutionary tracks stars follow in the Hertzsprung–Russell diagram (HRD) and their corresponding isochrones (positions of equal age in the HRD).
  
-[{{ :courses:ast403:spectral_evolution.jpg?600 | Fog1: Spectrum of a stellar population with solar metallicity that was instantaneously born a time $t$ ago; $t$ is given in units of $10^9$ years}}]+[{{ :courses:ast403:spectral_evolution.jpg?600 | Fig 1: Spectrum of a stellar population with solar metallicity that was instantaneously born a time $t$ ago; $t$ is given in units of $10^9$ years}}]
  
   * **Spectral and Color Evolution**:   * **Spectral and Color Evolution**:
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 //Mass-to-Light Ratio ($M/L$):// As stars age, the $M/L$ ratio increases because the total mass of the stars remains relatively constant while their total luminosity decreases. NIR filters (like the K-band) are often used as indicators of total stellar mass because the K-band $M/L$ ratio is less sensitive to age than blue bands. //Mass-to-Light Ratio ($M/L$):// As stars age, the $M/L$ ratio increases because the total mass of the stars remains relatively constant while their total luminosity decreases. NIR filters (like the K-band) are often used as indicators of total stellar mass because the K-band $M/L$ ratio is less sensitive to age than blue bands.
  
-[{{ :courses:ast403:color_evo.jpg?600 |Fig 2: (left) Evolution of colors between $0\le t \le 17 ×10^9$ yr +[{{ :courses:ast403:color_evo.jpg?600 | Fig 2: (left) Evolution of colors between $0\le t \le 17 ×10^9$ yr 
-for a stellar population with star-formation rate given by the experssion of $\psi(t)$, for five different values of the characteristic time-scale $\tau$ ($\tau=\infty$ is the limiting case for a constant star-formation rate) –Galactic center see solid curves. The typical colors for four different morphological types of galaxies are plotted. For each $\tau$, the evolution begins at the lower left, i.e., as a blue population in both color indices. In the case of constant star +for a stellar population with star-formation rate given by the experssion of $\psi(t)$, (right) The dependence of colors and $M/L$ on the metallicity 
-formation, the population never becomes redder than Irr’s; to achieve redder colors, $\tau$ has to be smaller. The dashed line +of the population. The typical colors for four different morphological types of galaxies are plotted. For each $\tau$, the evolution begins at the lower left, i.e., as a blue population in both color indices. In the case of constant star-formation, the population never becomes redder than Irr’s; to achieve redder colors, $\tau$ has to be smaller. }}] 
-connects points of t = 1010 yr on the different curves. Here, a Salpeter IMF and Solar metallicity was assumed. The shift + 
-in color obtained by doubling the metallicity is indicated by an arrow, as well as that due to an extinction coefficient of +
-E(B−V )= 0.1; both effects will make galaxies appear redder. (right) The dependence of colors and $M/L$ on the metallicity +
-of the population}}]+
  
   * **Key Diagnostic Features:**   * **Key Diagnostic Features:**
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 observations of galaxies at high redshift (and thus observations of galaxies at high redshift (and thus
 smaller age). smaller age).
 +
 +  * **More** 
 +  * Astrobite article about SED fittings: [[https://astrobites.org/2026/01/30/sed-fitting/]]
 +  * Various SED fitting softwares: [[http://www.sedfitting.org/Welcome.html]] 
 +
courses/ast403/population-synthesis.1770901238.txt.gz · Last modified: by shuvo
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