Lecture3 - Photometric Properties of Galaxies To measure...

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To measure the brightness distribution of galaxies, we must determine the surface brightness of the resolved galaxy. -measured in magnitudes/arcsec 2 ( μ I , μ B , μ R , etc.) Surface brightness is magnitude within 1 square arcsecond and is independent of distance since light falls as 1/d 2 , but the area subtended by 1 sq arcsec increases as d 2 . (however, cosmological dimming of 1/(1+z) 4 causes higher z galaxies to have lower surface brightnesses) Photometric Properties of Galaxies 15 20 25 30 radius μ B Much of the galaxy structure is fainter than the sky which must be accurately subtracted. Night sky at 22.7
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Surface brightness profiles are produced by azimuthally averaging around the galaxy along isophotes - lines of constant brightness. These are projected SB profiles. Seeing effects on SB profiles - unresolved points are spread out due to effects of our atmosphere – these effects are quantified by the Point Spread Function (PSF) -makes central part of profile flatter -makes isophote rounder Profiles and isophotes for galaxies observed with seeing conditions characterized by a Gaussian PSF of dispersion σ
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Surface photometry and deprojecting galaxy images What can we infer about the 3-d luminosity density j(r) in a transparent galaxy from its projected surface-brightness distribution I(R)? If I(R) is circularly symmetric, j(r) may be spherically symmetric: More on this in BT 4.2
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Elliptical Galaxies (and bulges of Spirals) μ B R 1/4 μ B ~ yR 1/4 I ~ 10 -0.4 μ B I ~ 10 -0.4yR1/4 I(R) = I e 10 {-3.33[(R/Re)1/4-1]} I(R) = I e exp{-7.67[(R/R e ) 1/4 -1]} “deVaucouleurs law” (1948) or “r 1/4 law” R e = effective radius containing 50% of luminosity R e = (a e b e ) 1/2 (factor of 3.33 chosen to make this so) -for major,minor axis I e = surface brightness at R e I o = I e 10 3.33 = 2138I e (central flux)
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I(R) = I o {[1+(R/R c ) 2 ] -1/2 - [1+(R T /R c ) 2 ] -1/2 } 2 radius where I=1/2 I o R T =cR c King models (1966) are a theoretically-based family of models derived
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