Lyman Break Galaxies, Dropouts, and Photometric Z
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Lyman Break Galaxies, Dropouts, and Photometric Z ----------------------------- Because galaxies are made of stars, we start with spectra of stars…. Hot O, B and A type stars. Steeply rising continua to the blue (Surf T ~ 10-30K). Dominated by absorption lines of Hydrogen (n=2 gr state).

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Lyman Break Galaxies, Dropouts, and Photometric Z-----------------------------Because galaxies are made of stars, we start with spectra of stars…


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Hot O, B and A type stars.

Steeply rising continua to the blue (Surf T ~ 10-30K).

Dominated by absorption lines of Hydrogen (n=2 gr state).

TheBalmer break at 3646angstrom marks the terminationof the hydrogen Balmerseries and is strongestin A-type stars. The break strength doesnot monotonically increase withage, but reaches amaximum in stellar populationsof intermediate ages (0.3 - 1Gyr).

For very high redshift galaxies, the Lyman break (n=1) may be used.


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Cool stars: F, G and K type.

Hydrogen less prominent. Ionized metals begin to appear (H and K lines of Ca II 3933, 3968A).

The 4000angstrom break arises becauseof an accumulation ofabsorption lines of mainlyionized metals. As theopacity increases with decreasingstellar temperature, the 4000angstrom break gets largerwith older ages, andit is largest forold and metal-rich stellarpopulations.


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Lyman Break Galaxies

Applying this to Galaxies

TheBalmer break and the4000 angstrom break areoften treated as onefeature, due to theirsimilar locations and thefact that they partiallyoverlap. However, the breaksoriginate from different physicalprocesses and behave differentlyas populations age.

Lyman break galaxies are typically starbursting (containing young hot stars) galaxies, and are easily found at great distances by drop outs in deep photometric searches

LyAlpharest=1215A, obs at 5000A



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Determining galaxy properties with broadband photometry

Best-fitting Bruzual & Charlot model (dark line) for SBM03#1: an exponentially decaying star formation rate with Tau = 300 Myr, viewed 640 Myr after the onset of star formation. The stellar mass is 3.4 x 1010 M.

For this galaxy, the redshift was already known..


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Multi-epoch star formation.

 The best-fitting two-component stellar population model (Salpeter IMF) for SBM03#1: a dominant 450-Myr population of mass 3.6 x 1010 M, with some ongoing star formation activity (a burst for the last 10 Myr involving 0.7 per cent of the stellar mass).


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Accuracy of photometric Z with/without spectra

Broadband: BVIzJHK, Light gray is synthetic fit to photometry only.

(NOTE: Red shift WAS known through spectral emission lines).

Dark grey model uses both Spectra (Pink markings) and photometry.




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Measuring Photometric Redshifts

Purple: fitting the optical-to-NIR broadband photometryby linear combinations ofempirical and theoretical galaxytemplates (Rudnick).

Green: fittingthe optical-to-NIR broadband photometryfrom the Bruzual &Charlot models

Pink: NIR continuum (pink spectra), withthe optical photometry.

Black dashed line: emission-line redshift, if present.


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