Melbourne, J. and Williams, Benjamin F. and Dalcanton, Julianne J. and Rosenfield, Philip and Girardi, Léo and Marigo, P. and Weisz, D. and Dolphin, A. and Boyer, Martha L. and Olsen, Knut and Skillman, E. and Seth, Anil C. (2012) The Contribution of TP-AGB and RHeB Stars to the Near-IR Luminosity of Local Galaxies: Implications for Stellar Mass Measurements of High-redshift Galaxies. Astrophysical Journal, 748 (1). Art. No. 47. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20120418-101911732
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Using high spatial resolution Hubble Space Telescope WFC3 and Advanced Camera for Surveys imaging of resolved stellar populations, we constrain the contribution of thermally pulsing asymptotic giant branch (TP-AGB) stars and red helium burning (RHeB) stars to the 1.6 μm near-infrared (NIR) luminosities of 23 nearby galaxies, including dwarfs and spirals. The TP-AGB phase contributes as much as 17% of the integrated F160W flux, even when the red giant branch is well populated. The RHeB population contribution can match or even exceed the TP-AGB contribution, providing as much as 21% (18% after a statistical correction for foreground) of the integrated F160W light. We estimate that these two short-lived phases may account for up to 70% of the rest-frame NIR flux at higher redshift. The NIR mass-to-light (M/L) ratio should therefore be expected to vary significantly due to fluctuations in the star formation rate (SFR) over timescales from 25 Myr to several Gyr, an effect that may be responsible for some of the lingering scatter in NIR galaxy scaling relations such as the Tully-Fisher and metallicity-luminosity relations. We compare our observational results to predictions based on optically derived star formation histories and stellar population synthesis (SPS) models, including models based on the 2008 Padova isochrones (used in popular SPS programs) and the updated 2010 Padova isochrones, which shorten the lifetimes of low-mass (old) low-metallicity TP-AGB populations. The updated (2010) SPS models generally reproduce the expected numbers of TP-AGB stars in the sample; indeed, for 65% of the galaxies, the discrepancy between modeled and observed numbers is smaller than the measurement uncertainties. The weighted mean model/data number ratio for TP-AGB stars is 1.5 (1.4 with outliers removed) with a standard deviation of 0.5. The same SPS models, however, give a larger discrepancy in the F160W flux contribution from the TP-AGB stars, overpredicting the flux by a weighted mean factor of 2.3 (2.2 with outliers removed) with a standard deviation of 0.8. This larger offset is driven by the prediction of modest numbers of high-luminosity TP-AGB stars at young (<300 Myr) ages. The best-fit SPS models simultaneously tend to underpredict the numbers and fluxes of stars on the RHeB sequence, typically by a factor of 2.0 ± 0.6 for galaxies with significant numbers of RHeBs. Possible explanations for both the TP-AGB and RHeB model results include (1) difficulties with measuring the SFHs of galaxies especially on the short timescales over which these stars evolve (several Myr), (2) issues with the way the SPS codes populate the color-magnitude diagrams (e.g., how they handle pulsations or self-extinction), and/or (3) lingering issues with the lifetimes of these stars in the stellar evolution codes. Coincidentally these two competing discrepancies—overprediction of the TP-AGB and underprediction of the RHeBs—result in a predicted NIR M/L ratio largely unchanged for a rapid SFR, after correcting for these effects. However, the NIR-to-optical flux ratio of galaxies could be significantly smaller than AGB-rich models would predict, an outcome that has been observed in some intermediate-redshift post-starburst galaxies.
|Additional Information:||© 2012 American Astronomical Society. Received 2011 September 21; accepted 2012 January 18; published 2012 March 5. Authors B.F.W., J.D., P.R., and A.D. were partially supported by HST NASA grant GO-11719 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. L.G. and P.M. thank the support from contract ASI-INAF I/009/10/0.|
|Subject Keywords:||galaxies: fundamental parameters; galaxies: stellar content; Hertzsprung-Russell and C-M diagrams; stars: AGB and post-AGB|
|Official Citation:||The Contribution of TP-AGB and RHeB Stars to the Near-IR Luminosity of Local Galaxies: Implications for Stellar Mass Measurements of High-redshift Galaxies J. Melbourne et al. 2012 ApJ 748 47|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||18 Apr 2012 17:42|
|Last Modified:||26 Dec 2012 15:04|
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