Astronomy
Permanent link for this communityhttps://hdl.handle.net/2022/12958
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Browsing Astronomy by Subject "galaxies: dwarf"
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Item Deep ultraviolet luminosity functions at the infall region of the coma cluster(The American Astronomical Society, 2012) Salim, S.; Hammer, D.M.; Hornschemeier, A.E.; Smith, R.; Jenkins, L.; Mobasher, B.; Miller, N.; Ferguson, H.We have used deep $\textit{GALEX}$ observations at the infall region of the Coma cluster to measure the faintest ultraviolet (UV) luminosity functions (LFs) presented for a rich galaxy cluster thus far. The Coma UV LFs are measured to $M_{UV} = –10.5$ in the $\textit{GALEX}$ FUV and NUV bands, or 3.5 mag fainter than previous studies, and reach the dwarf early-type galaxy population in Coma for the first time. The Schechter faint-end slopes ($\alpha \approx –1.39$ in both $\textit{GALEX}$ bands) are shallower than reported in previous Coma UV LF studies owing to a flatter LF at faint magnitudes. A Gaussian-plus-Schechter model provides a slightly better parameterization of the UV LFs resulting in a faint-end slope of $\alpha \approx –1.15$ in both $\textit{GALEX}$ bands. The two-component model gives faint-end slopes shallower than $\alpha = –1$ (a turnover) for the LFs constructed separately for passive and star-forming galaxies. The UV LFs for star-forming galaxies show a turnover at $M_{UV} \approx –14$ owing to a deficit of dwarf star-forming galaxies in Coma with stellar masses below $M_{*} = 108 M_{\odot}$. A similar turnover is identified in recent UV LFs measured for the Virgo cluster suggesting this may be a common feature of local galaxy clusters, whereas the field UV LFs continue to rise at faint magnitudes. We did not identify an excess of passive galaxies as would be expected if the missing dwarf star-forming galaxies were quenched inside the cluster. In fact, the LFs for both dwarf passive and star-forming galaxies show the same turnover at faint magnitudes. We discuss the possible origin of the missing dwarf star-forming galaxies in Coma and their expected properties based on comparisons to local field galaxies.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 Globular cluster systems of spiral and S0 galaxies: Results from WIYN imaging of NGC1023, NGC1055, NGC7332, and NGC7339(The American Astronomical Society, 2012) Young, M.D.; Dowell, J.L.; Rhode, K.L.We present results from a study of the globular cluster (GC) systems of four spiral and S0 galaxies imaged as part of an ongoing wide-field survey of the GC systems of giant galaxies. The target galaxies - the SB0 galaxy NGC1023, the SBb galaxy NGC1055, and an isolated pair comprised of the Sbc galaxy NGC7339 and the S0 galaxy NGC7332 - were observed in BVR filters with the WIYN 3.5m telescope and Minimosaic camera. For two of the galaxies, we combined the WIYN imaging with previously published data from the $\textit{Hubble Space Telescope}$ and the Keck Observatory to help characterize the GC distribution in the central few kiloparsecs. We determine the radial distribution (surface density of GCs versus projected radius) of each galaxy's GC system and use it to calculate the total number of GCs ($N_{GC}$). We find $N_{GC}$ = 490 ± 30, 210 ± 40, 175 ± 15, and 75 ± 10 for NGC1023, NGC1055, NGC7332, and NGC7339, respectively. We also calculate the GC specific frequency (N GC normalized by host galaxy luminosity or mass) and find values typical of those of the other spiral and E/S0 galaxies in the survey. The two lenticular galaxies have sufficient numbers of GC candidates for us to perform statistical tests for bimodality in the GC color distributions. We find evidence at a high confidence level (>95%) for two populations in the $B - R$ distribution of the GC system of NGC1023. We find weaker evidence for bimodality (>81% confidence) in the GC color distribution of NGC7332. Finally, we identify eight GC candidates that may be associated with the Magellanic dwarf galaxy NGC1023A, which is a satellite of NGC1023.Item 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.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 $z \approx 0.33$.