Astronomy

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Browsing Astronomy by Author "Calzetti, D."

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    Direct oxygen abundances for low-luminosity LVL galaxies
    (The American Astronomical Society, 2012) Berg, D.A.; Skillman, E.D.; Marble, A.R.; Van Zee, L.; Engelbracht, C.W.; Lee, J.C.; Kennicutt, R.C.; Calzetti, D.; Dale, D.A.; Johnson, B.D.
    We present MMT spectroscopic observations of H II regions in 42 low luminosity galaxies in the Spitzer Local Volume Legacy survey. For 31 of the 42 galaxies in our sample, we were able to measure the temperature sensitive [O III] $\lambda$4363 line at a strength of $4\sigma$ or greater, and thus determine oxygen abundances using the "direct" method. Our results provide the first "direct" estimates of oxygen abundance for 19 of these galaxies. "Direct" oxygen abundances were compared to $B$-band luminosities, 4.5 $\mu$m luminosities, and stellar masses in order to characterize the luminosity-metallicity and mass-metallicity relationships at low luminosity. We present and analyze a "Combined Select" sample composed of 38 objects (drawn from a sub-set of our parent sample and the literature) with "direct" oxygen abundances and reliable distance determinations (based on the tip of the red giant branch or Cepheid variables). Consistent with previous studies, the $B$ band and 4.5 $\mu$m luminosity-metallicity relationships for the 38 objects were found to be$12 + log(O/H) = (6.27 \pm 0.21) + (- 0.11 \pm 0.01)M_{B}$and$12+log(O/H) = (6.10 \pm 0.21) + (- 0.10 \pm 0.01)M_{[4.5]}$with dispersions of $\sigma$ = 0.15 and 0.14, respectively. The slopes of the optical and near-IR L-Z relationships have been reported to be different for galaxies with luminosities greater than that of the LMC. However, the similarity of the slopes of the optical and near-IR L-Z relationships for our sample probably reflects little influence by dust extinction in the low luminosity galaxies. For this sample, we derive a mass-metallicity relationship of $12 + log\big(O/H\big) = \big(5.61 \pm 0.24\big) + \big(0.29 \pm 0.03\big)log \big(M_{\star}\big)$, which agrees with previous studies; however, the dispersion ($\sigma$ = 0.15) is not significantly lower than that of the L-Z relationships. Because of the low dispersions in these relationships, if an accurate distance is available, the luminosity of a low luminosity galaxy is often a better indicator of metallicity than that derived using certain "strong-line" methods, so significant departures from the L-Z relationships may indicate that caution is prudent in such cases. With these new "direct" metallicities we also revisit the 70/160 $\mu$m color metallicity relationship. Additionally, we examine N/O abundance trends with respect to oxygen abundance and B - V color. We find a positive correlation between N/O ratio and B - V color for $0.05 \lesssim B - V \lesssim 0.75: log (N/O) = (1.18 \pm 0.9\big) × (B - V) + (- 1.92 \pm 0.08)$, with a dispersion of $\sigma$ = 0.14, which is in agreement with previous studies.
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    Modeling the effects of star formation histories on H$\alpha$ and ultraviolet fluxes in nearby dwarf galaxies
    (The American Astronomical Society, 2012) Weisz, D.R.; Johnson, B.D.; Johnson, L.C.; Skillman, E.D.; Lee, J.C.; Kennicutt, R.C.; Calzetti, D.; Van Zee, L.; Bothwell, M.S.; Dalcanton, J.J.; Dale, D.A.; Williams, B.F.
    We consider the effects of non-constant star formation histories (SFHs) on $\text{H}\alpha$ and GALEX far-ultraviolet (FUV) star formation rate (SFR) indicators. Under the assumption of a fully populated Chabrier initial mass function (IMF), we compare the distribution of $\text{H}\alpha$-to-FUV flux ratios from ~1500 simple, periodic model SFHs with observations of 185 galaxies from the $\textit{Spitzer}$ Local Volume Legacy survey. We find a set of SFH models that are well matched to the data, such that more massive galaxies are best characterized by nearly constant SFHs, while low-mass systems experience burst amplitudes of ~30 (i.e., an increase in the SFR by a factor of 30 over the SFR during the inter-burst period), burst durations of tens of Myr, and periods of ~250 Myr; these SFHs are broadly consistent with the increased stochastic star formation expected in systems with lower SFRs. We analyze the predicted temporal evolution of galaxy stellar mass, R-band surface brightness, $\text{H}\alpha$-derived SFR, and blue luminosity, and find that they provide a reasonable match to observed flux distributions. We find that our model SFHs are generally able to reproduce both the observed systematic decline and increased scatter in $\text{H}\alpha$-to-FUV ratios toward low-mass systems, without invoking other physical mechanisms. We also compare our predictions with those from the Integrated Galactic IMF theory with a constant SFR. We find that while both predict a systematic decline in the observed ratios, only the time variable SFH models are capable of producing the observed population of low-mass galaxies $\big(M_{*} \lesssim 10^{7} M_{\odot}\big)$ with normal $\text{H}\alpha$-to-FUV ratios. These results demonstrate that a variable IMF alone has difficulty explaining the observed scatter in the $\text{H}\alpha$-to-FUV ratios. We conclude by considering the limitations of the model SFHs and discuss the use of additional empirical constraints to improve future SFH modeling efforts.