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

Kacharov, N.; Rejkuba, M.; Cioni, M. R. L.

doi:10.1051/0004-6361/201117383

Cited by 18 publications

(83 citation statements)

References 55 publications

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“…All three regions were compared with the chosen library spectra where possible. The standard spectra used for comparison were selected from the libraries of Danks & Dennefeld (1994) and Kacharov et al (2012), both of which can be obtained through the VizieR database.…”

Section: Automated Spectral Classificationmentioning

confidence: 99%

“…The spectral library of Kacharov et al (2012), which contains spectra of 546 sources at a resolution of 12-13 Å in the wavelength range of 5000-10 000 Å was particularly useful, as it is the largest spectral library of AGB sources in NGC 6822. It includes M-type giants of subclass M, M0, M0.5, M1, M2, M3, M4, M5, M6 and M6.5, S-type giants of subclass S0 and S4, and C-type giants of subclass C, C3.2, C5.5, C6.5 and C8.2; as well as foreground K-type (dK, dK7) and M-type (dM, dM0, dM1, dM1.5, dM2, dM3, dM3.5, dM4, dM4.5, dM5, dM6) dwarf stars in the direction of NGC 6822.…”

Section: Automated Spectral Classificationmentioning

confidence: 99%

“…It includes M-type giants of subclass M, M0, M0.5, M1, M2, M3, M4, M5, M6 and M6.5, S-type giants of subclass S0 and S4, and C-type giants of subclass C, C3.2, C5.5, C6.5 and C8.2; as well as foreground K-type (dK, dK7) and M-type (dM, dM0, dM1, dM1.5, dM2, dM3, dM3.5, dM4, dM4.5, dM5, dM6) dwarf stars in the direction of NGC 6822. Unfortunately, owing to the effects of severe fringing at redder wavelengths, Kacharov et al (2012) suggest that the useful wavelength range of their spectra is limited to 5000-7800 Å. We did not rely on their data beyond ∼7500 Å.…”

Section: Automated Spectral Classificationmentioning

confidence: 99%

“…We did not rely on their data beyond ∼7500 Å. This meant that we were only able to compare features in regions 1 and 2 when matching our spectra to those of Kacharov et al (2012).…”

Section: Automated Spectral Classificationmentioning

confidence: 99%

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Spectral classification of photometrically selected AGB candidates in NGC 6822

Sibbons

Ryan

Napiwotzki

et al. 2015

A&A

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Context. The ratio of C-and M-type asymptotic giant branch (AGB) stars is commonly used to estimate the metallicity of extragalactic populations. Sources in the AGB population must therefore be accurately classified as either C-or M-type. Spectroscopic data are presented for candidate C-and M-type AGB stars, previously classified using JHK photometry, in the Local Group dwarf galaxy NGC 6822. Aims. This paper aims to evaluate the success of the JHK classification criteria used in order to determine the level of error associated with this method, and to refine the criteria for future studies. The success rate of a second independent method of source classification, the CN-TiO method, is also examined. We also review the validity of the 4 kpc radial limit imposed in our previous work. Methods. Spectra of 323 sources, distributed across an area of 2 deg 2 , were taken using the AAOmega multi-fibre spectrograph on the Anglo-Australian Telescope and have been classified using an automated classification system and spectral standards from the literature. Nearly half (135) of these sources were selected in common with a photometric catalogue that relied on the CN-TiO method.Results. Within this sample we were able to classify 158 sources, including 82 C-type giants and one anomalous M-type giant, all members of NGC 6822, and 75 foreground K-and M-type dwarf sources. All but three of the giant sources are located within 3 kpc of the galactic centre. Using this spectroscopic sample, new JHK photometric criteria for the isolation and classification of C-and M-type AGB stars have been derived. The error rate in the CN-TiO method, arising from stars incorrectly classified as C-type, has been estimated to be ∼7%. Conclusions. Based on the new JHK classification criteria, revised estimates of the global C/M ratio, 0.95 ± 0.04, and iron abundance, −1.38 ± 0.06 dex, are presented for NGC 6822.

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Section: Automated Spectral Classificationmentioning

confidence: 99%

Section: Automated Spectral Classificationmentioning

confidence: 99%

Section: Automated Spectral Classificationmentioning

confidence: 99%

“…We did not rely on their data beyond ∼7500 Å. This meant that we were only able to compare features in regions 1 and 2 when matching our spectra to those of Kacharov et al (2012).…”

Section: Automated Spectral Classificationmentioning

confidence: 99%

See 2 more Smart Citations

Spectral classification of photometrically selected AGB candidates in NGC 6822

Sibbons

Ryan

Napiwotzki

et al. 2015

A&A

View full text Add to dashboard Cite

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“…We note that spectroscopic data are necessary for a more accurate identification of M-giant and C stars. However, Battinelli & Demers (2009) discussed that many C stars probably have a bluer color than the (J − K) 0 = 1.4 limit, and Kacharov et al (2012) indeed found in their spectroscopic data of NGC 6822 that several of the C stars in NGC 6822 have a bluer color than this limit. Thus, our color limit for the C stars is reasonable compared with the other conditions.…”

Section: Color−color Diagrams and The Selection Of The C Stars Of Ic mentioning

confidence: 96%

Near-infrared photometric properties of asymptotic giant branch stars in the dwarf irregular galaxy IC 1613

Chun

Jung

Kang

et al. 2015

A&A

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Aims. We investigated the photometric properties of asymptotic giant branch (AGB) stars of the dwarf irregular galaxy IC 1613 and its metallicity. Methods. We used near-infrared JHK s photometric data obtained with the WIRCam array on Canada-France-Hawaii Telescope (CFHT). The optical gi imaging data of a previous study were cross-matched with ours to distinguish the stellar populations in the galaxy and to select the AGB stars. We also separated the C stars from the M-giant stars in the JHK s color−color diagram. Results. We identified 140 C stars and 306 M-giant stars in IC 1613. The (J − K s ) color histogram of AGB stars shows the main peak of M-giant stars and the red tail of C stars. The broad color distribution of M-giant stars and the isochrones in the color magnitude diagram indicate that IC 1613 has a wide range of intermediate-age stars. We also know that most C stars were formed over the past 2−8 Gyr according the M bol -age relation. The C/M ratio of 0.46 ± 0.05 was estimated and [Fe/H] abundance of −1.23 ± 0.06 was derived from the C/M ratio. Radial distributions of C/M ratio and metallicity show the shallow trend of the negative metallicity gradient from the center. The local C/M ratio also indicates that the HI cavity has higher C/M values than the bright HI spot and HI gas region. However, the low [Fe/H] value near the central region of the galaxy is likely to be caused by the change in age of the dominant stellar population according to the radial distance, not the real metallicity variation. It might also be that relatively younger and metal-rich C stars are concentrated in the center of the galaxy, while the M-giant stars were homogeneously distributed from the center to the borders of the galaxy.

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The AGB population of NGC 6822: distribution and the C/M ratio fromJHKphotometry

Sibbons

Ryan

Cioni

et al. 2012

A&A

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107

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Context. NGC 6822 is an irregular dwarf galaxy and part of the Local Group. Its close proximity and apparent isolation provide a unique opportunity to study galactic evolution without any obvious strong external influences. Aims. This paper aims to study the spatial distribution of the asymptotic giant branch (AGB) population and metallicity in NGC 6822. Methods. Using deep, high quality JHK photometry, taken with WFCAM on UKIRT, carbon-and oxygen-rich AGB stars have been isolated. The ratio between their number, the C/M ratio, has then been used to derive the [Fe/H] abundance across the galaxy. Results. The tip of the red giant branch is located at K 0 = 17.41 ± 0.11 mag and the colour separation between carbon-and oxygenrich AGB stars is at (J − K) 0 = 1.20 ± 0.03 mag (i.e. (J − K) 2MASS ∼ 1.28 mag). A C/M ratio of 0.62 ± 0.03 has been derived in the inner 4 kpc of the galaxy, which translates into an iron abundance of [Fe/H] = −1.29 ± 0.07 dex. Variations of these parameters were investigated as a function of distance from the galaxy centre and azimuthal angle. Conclusions. The AGB population of NGC 6822 has been detected out to a radius of 4 kpc giving a diameter of 56 arcmin. It is metal-poor, but there is no obvious gradient in metallicity with either radial distance from the centre or azimuthal angle. The detected spread in the TRGB magnitude is consistent with that of a galaxy surrounded by a halo of old stars. The C/M ratio has the potential to be a very useful tool for the determination of metallicity in resolved galaxies but a better calibration of the C/M vs. [Fe/H] relation and a better understanding of the sensitivities of the C/M ratio to stellar selection criteria is first required.

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

Cited by 18 publications

References 55 publications

Spectral classification of photometrically selected AGB candidates in NGC 6822

Spectral classification of photometrically selected AGB candidates in NGC 6822

Near-infrared photometric properties of asymptotic giant branch stars in the dwarf irregular galaxy IC 1613

The AGB population of NGC 6822: distribution and the C/M ratio fromJHKphotometry

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