Near-IR observations of NGC 6822: AGB stars,  distance, metallicity and structure

Cioni, M. R. L.; Habing, H. J.

doi:10.1051/0004-6361:20041608

Cited by 58 publications

(99 citation statements)

References 50 publications

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“…All selected targets are brighter than 17.45 mag in K 0 and have (J − K) 0 > 0.74 mag, although redder stars were targeted preferentially. Cioni & Habing (2005) detected the RGB tip (TRGB) at K s = 17.10 ± 0.01 mag, while Sibbons et al (2010) report that it varies across the galaxy by ΔK = 1.36 mag, with an average value of K = 17.48 ± 0.26 mag. Davidge (2003) adopted a distance modulus of 23.49 mag based on the Cepheid and RGB tip measurements of Gallart et al (1996b), and measured the onset of the RGB tip in K band near K = 17 mag.…”

Section: Observations and Data Reductionmentioning

confidence: 99%

Spectra probing the number ratio of C- to M-type AGB stars in the NGC 6822 galaxy

Kacharov

Rejkuba²,

Cioni

2012

A&A

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Aims. We calibrate spectroscopically the C-versus (vs.) M-type asymptotic giant branch (AGB) star selection made using near-IR photometry, and investigate the spatial distribution of the C/M ratio in NGC 6822, based on low resolution spectroscopy and near-IR photometry. Methods. We obtained low resolution multi-object spectroscopy with the VIMOS instrument at the ESO VLT of ∼800 stars in seven fields centred on NGC 6822. The spectroscopic classification of giant stars in NGC 6822 and foreground dwarf contaminants was made by comparing more than 500 good quality spectra with the spectroscopic atlas of Turnshek et al. (1985). The sample of spectroscopically confirmed AGB stars in NGC 6822 is divided into C-and M-rich giants to constrain the C vs. M AGB star selection criteria based on photometry. The larger near-IR photometric sample is then used to investigate the C/M ratio gradients across the galaxy.Results. We present the largest catalogue of near-IR photometry and spectra of AGB stars in NGC 6822 with 150 C-stars and 122 M-stars. Seventy-nine percent of the C-stars in our catalogue are redder than (J − K) 0 = 1.2 mag, and 12% are brighter than K 0 = 16.45 mag and bluer than (J − K) 0 = 1.2 mag. The remaining 9% are mixed with the M-type AGB stars, 88% of which have colours (J − H) 0 > 0.73 mag and (J − K) 0 between 0.9 mag and 1.2 mag. The remainder are mixed with dwarfs and C-type stars. The foreground dwarfs have preferably colours (J − H) 0 < 0.73 mag (95%). Using the proposed criteria, we estimate that the overall C/M ratio of the galaxy is around 0.8 with a spread between 0.2 < C/M < 1.8. These results suggest that the metallicity index [Fe/H] is between −1.2 dex and −1.3 dex according to the different calibrations and that there is a significant spread of about 0.4 ÷ 0.6 dex. We also discuss age rather than metallicity variations that could explain the C/M ratio trends.

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Section: Observations and Data Reductionmentioning

confidence: 99%

Spectra probing the number ratio of C- to M-type AGB stars in the NGC 6822 galaxy

Kacharov

Rejkuba²,

Cioni

2012

A&A

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“…Lee et al (1993) and Salaris & Cassisi (1998) gave (m − M) 0 = 23.46 ± 0.08 and 23.71 ± 0.14 using the RGB tip brightness as a distance indicator; Gallart et al (1996) estimate (m − M) 0 = 23.49 ± 0.08 using UBVRI photometry of eight Cepheids; Cioni & Habing (2005) using near-infrared (NIR) photometry of AGB stars found (m − M) 0 = 23.34 ± 0.12; Gieren et al (2006) found (m − M) 0 = 23.312 ± 0.021 through NIR photometry of Cepheids in the innermost region of NGC 6822. More recently, significant differences among the distance estimates to NGC 6822, based on the use of the classical Cepheid period-luminosity relation, have been extensively investigated by Feast et al (2012).…”

Section: Introductionmentioning

confidence: 99%

Distance and reddening of the Local Group dwarf irregular galaxy NGC 6822

Fusco

Buonanno

Bono

et al. 2012

A&A

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On the basis of a new photometric analysis of the Local Group dwarf irregular galaxy NCG 6822 based on observations obtained with the Advanced Camera for Surveys onboard the Hubble Space Telescope, we have obtained a new estimate of the extinction of two fields located in the southeast region of the galaxy. Because of significant differences in the distance estimates to NGC 6822 available in literature, we decided to provide an independent determination of the distance to this galaxy based on an updated and self-consistent theoretical calibration of the tip of the red giant branch brightness. As a result we newly determined the distance to NGC 6822 to be equal to (m − M) 0 = 23.54 ± 0.05, and compared our measurement with the most recent determinations of this distance.

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“…Owing to this dredge-up, AGB evolution can be classified according to atmospheric abundance, i.e., starting with oxygen-rich (C/O < 1) M-giant stars that evolve into carbon-rich (C/O > 1) C stars (Nowotny et al 2001). These two types of AGB stars can be used to estimate the metallicity of the parent galaxy because the ratio of C stars to M-giants (C/M) is anti-correlated with the metal abundance (Battinelli & Demers 2005;Cioni & Habing 2005;Cioni 2009). The AGB stars are among the brightest and coolest stars of the intermediate-age stellar population; accordingly, they appear mostly in near-infrared color−magnitude diagrams (CMDs).…”

Section: Introductionmentioning

confidence: 99%

Near-infrared properties of asymptotic giant branch stars in nearby dwarf elliptical galaxy NGC 205

Jung

Kim

et al. 2012

A&A

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Aims. We investigated the distribution of resolved asymptotic giant branch (AGB) stars over a much larger area than covered by previous near-infrared studies in the nearby dwarf elliptical galaxy NGC 205. Methods. Using data obtained with the WIRCam near-infrared imager of the CFHT, we selected the AGB stars in the JHK s color−magnitude diagrams, and separated the C stars from M-giant stars in the JHK s color−color diagram. Results. We identified 1, 550 C stars in NGC 205 with a mean absolute magnitude of M Ks = −7.49 ± 0.54, and colors of (J − K s ) 0 = 1.81 ± 0.41 and (H − K s ) 0 = 0.76 ± 0.24. The ratio of C stars to M-giant stars was estimated to be 0.15 ± 0.01 in NGC 205, and the local C/M ratios for the southern region are somewhat lower than those for the northern region. The (J − K s ) color distributions of AGB stars contain the main peak of the M-giant stars and the red tail of the C stars. A comparison of the theoretical isochrone models with the observed color distribution indicates that most of the bright M-giant stars in NGC 205 were formed at log (t yr ) ∼ 9.0−9.7. The logarithmic slope of the M Ks luminosity function for M-giant stars was estimated to be 0.84 ± 0.01, which is comparable with dwarf elliptical galaxies NGC 147 and NGC 185. Furthermore, we found that the logarithmic slopes of the M Ks luminosity function for C and M-giant stars are different to places, implying a different star formation history within NGC 205. The bolometric luminosity function for M-giant stars extends to M bol = −6.0 mag, and that for C stars spans −5.6 < M bol < −3.0. The bolometric luminosity function of C stars is unlikely to be a Gaussian distribution and the mean bolometric magnitude of C stars is estimated to be M bol = −4.24 ± 0.55, which is consistent with our results for dwarf elliptical galaxies NGC 147 and NGC 185.

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Near-IR observations of NGC 6822: AGB stars, distance, metallicity and structure

Cited by 58 publications

References 50 publications

Spectra probing the number ratio of C- to M-type AGB stars in the NGC 6822 galaxy

Spectra probing the number ratio of C- to M-type AGB stars in the NGC 6822 galaxy

Distance and reddening of the Local Group dwarf irregular galaxy NGC 6822

Near-infrared properties of asymptotic giant branch stars in nearby dwarf elliptical galaxy NGC 205

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