Lecture4 - Photometric Properties of Spiral Galaxies Bulges...

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Photometric Properties of Spiral Galaxies Disk scale length Central surface brightness (I d in BM) Bulges Luminosity profiles fit r 1/4 or r 1/n laws Structure appears similar to E’s, except bulges are more “flattened” (though bulges can be quite different from E’s dynamically) Disks Many are well-represented by an exponential profile I(R) = I o e -R/Rd (Freeman 1970) NGC 7331 Sb galaxy R-band isophotes
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Bulge dominates in center and again at very large radii (if bulge obeyed r 1/4 to large R) Disk dominates at intermediate radii •R d ~ 1 - 10 kpc (I-band; 20% longer in B-band - why?) •Disk in many spirals appear to end at some R max around 10 to 30 kpc or (3-5R d ) (R d ) ( μ R ) NGC 7331
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o (B-band) = 21.5 ± 0.5 (or μ oB ) Freeman’s Law (1970) - found that almost all spirals had disk surface brightness around this value Partly a selection effect since low-surface brightness (LSB) galaxies are harder to identify Many LSB disks identified since
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Lecture4 - Photometric Properties of Spiral Galaxies Bulges...

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