Astronomy
Permanent link for this communityhttps://hdl.handle.net/2022/12958
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Browsing Astronomy by Subject "galaxies: abundances"
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Item Average metallicity and star formation rate of Ly$\alpha$ emitters probed by a triple narrowband survey*(The American Astronomical Society, 2012) Nakajima, K.; Ouchi, M.; Shimasaku, K.; Ono, Y.; Lee, J.C.; Foucad, S.; Ly, C.; Dale, D.A.; Salim, S.; Finn, R.; Almaini, O.; Okamura, S.We present the average metallicity and star formation rate (SFR) of $Ly\alpha$ emitters (LAEs) measured from our large-area survey with three narrowband (NB) filters covering the $Ly\alpha, \big[O II\big]\lambda3727$, and $\text{H}\alpha+\big[\text{N} II\big]$ lines of LAEs at $z = 2.2$. We select 919 $z = 2.2$ LAEs from Subaru/Suprime-Cam NB data in conjunction with Magellan/IMACS spectroscopy. Of these LAEs, 561 and 105 are observed with KPNO/NEWFIRM near-infrared NB filters whose central wavelengths are matched to redshifted $\big[\text{O} II\big]$ and $\text{H}\alpha$ nebular lines, respectively. By stacking the near-infrared images of the LAEs, we successfully obtain average nebular-line fluxes of LAEs, the majority of which are too faint to be identified individually by NB imaging or deep spectroscopy. The stacked object has an Hα luminosity of $1.7 × 10^{42} erg s^{–1}$ corresponding to an SFR of $14 M_{\odot} yr6{–1}$. We place, for the first time, a firm lower limit to the average metallicity of LAEs of $Z \gtrsim 0.09 Z_{\odot} \big(2\sigma\big)$ based on the $\big[\text{O} II\big]/\big(\text{H}\alpha+\big[\text{N} II\big]\big)$ index together with photoionization models and empirical relations. This lower limit of metallicity rules out the hypothesis that LAEs, so far observed at $z \sim 2$, are extremely metal-poor $\big(Z < 2 × 10–2 Z_{\odot}\big)$ galaxies at the $4\sigma$ level. This limit is higher than a simple extrapolation of the observed mass-metallicity relation of $z \sim 2$ UV-selected galaxies toward lower masses $\big(5 × 108 M_{\odot}\big)$, but roughly consistent with a recently proposed fundamental mass-metallicity relation when the LAEs' relatively low SFR is taken into account. The $\text{H}\alpha$ and $\text{Ly}\alpha$ luminosities of our NB-selected LAEs indicate that the escape fraction of $\text{Ly}\alpha$ photons is $\sim12%-30%$, much higher than the values derived for other galaxy populations at $z \sim 2$.Item 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.Item $\text{H}\alpha$ dots: A catalog of faint emission-line objects discovered in narrowband images(The American Astronomical Society, 2012) Kellar, J.A.; Salzer, J.J.; Wegner, G.; Gronwall, C.; Williams, A.During a wide-field narrowband $\text{H}\alpha$ imaging survey, we noted the presence of numerous isolated emission-line point sources in the data. These objects could represent ultra-low-luminosity galaxies at low-redshift (detection via $\text{H}\alpha$), isolated extragalactic H II regions associated with the galaxy targeted by the original observation, or background galaxies or QSOs where strong emission lines (most often [O III] $\lambda$ 5007) redshift into our narrowband filter. We have carried out a systematic search for these "$\text{H}\alpha$ dots" in over 200 15 × 15 arcmin fields. To date we have cataloged 61 candidate emission-line sources in roughly 11.7 deg$^{2}$. The sample has a median R magnitude of 19.5, and detects objects as faint as $R$ = 22.6. Follow-up spectroscopy reveals that ~85% of the candidates are bona fide emission-line objects, with roughly 60% of the real sources being lower-redshift objects (detection via $\text{H}\alpha$) and 40% being higher-redshift objects detected via [O III] emission or some other emission line. Here we present the results of our initial survey and follow-up spectroscopy. We use our sample to study the properties (including star-formation rates and metal abundances) of low-luminosity star-forming galaxies in the nearby universe and of low-metallicity star-forming galaxies at $z \approx 0.33$.