Spectral classification of photometrically selected AGB candidates in NGC 6822

© 2014 ESO. Reproduced with permission from Astronomy & Astrophysics. Content in the UH Research Archive is made available for personal research, educational, and non-commercial purposes only. Unless otherwise stated, all content is protected by copyright, and in the absence of an open license, permissions for further re-use should be sought from the publisher, the author, or other copyright holder.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 deg2, 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.Peer reviewe

The AGB population of NGC 6822: distribution and the C/M ratio from JHK photometry

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. This paper aims to study the spatial distribution of the asymptotic giant branch (AGB) population and metallicity in NGC 6822. 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. The tip of the red giant branch is located at K0 = 17.41 \pm 0.11 mag and the colour separation between carbon- and oxygen-rich AGB stars is at (J - K)0 = 1.20 \pm 0.03 mag (i.e. (J - K)2MAS S {\guillemotright} 1.28 mag). A C/M ratio of 0.62 \pm 0.03 has been derived in the inner 4 kpc of the galaxy, which translates into an iron abundance of [Fe/H] = -1.29\pm0.07 dex. Variations of these parameters were investigated as a function of distance from the galaxy centre and azimuthal angle. 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

The AGB population in IC 1613 using JHK photometry

Date of Acceptance: 12/08/2014Context. A member of the Local Group, IC 1613 is a gas rich irregular dwarf galaxy that appears to have formed stars continuously over the last 10 Gyr and is relatively independent of external influences from other galaxies. Aims. This paper aims to study the spatial distribution of the asymptotic giant branch (AGB) population in IC 1613 and its metallicity. Methods. Using WFCAM on UKIRT, high quality JHK photometry of an area of 0.8 deg2 centered on IC 1613 was obtained. The data have been used to isolate the C-and M-type components of the AGB population and using their number ratio, C/M, a global mean metallicity has been derived. The metallicity and the tip of the red giant branch magnitude (TRGB) have been studied as a function of distance from the galactic centre and as a function of azimuthal angle. Results. The TRGB has been found to be at K0 = 18.25 ± 0.15 mag. The colour separation between the C-and M-type components of the AGB population has been located at (J-K)0 = 1.15 ± 0.05 mag, giving a global C/M ratio of 0.52 ± 0.04 and from this an iron abundance of [Fe/H] =-1.26 ± 0.07 dex has been calculated. Conclusions. The AGB population has been detected out to a radial distance of 4.5 kpc in the de-projected plane of the galaxy. The measured TRGB is consistent with previous measurements and no significant variation is detected in the TRGB or in metallicity either with galactocentric distance or azimuthal angle.Peer reviewe

ASKAP and MeerKAT surveys of the magellanic clouds

The Magellanic Clouds are a stepping stone from the overwhelming detail of the Milky Way in which we are immersed, to the global characteristics of galaxies both in the nearby and distant universe. They are interacting, gas-rich dwarf galaxies of sub-solar metallicity, not unlike the building blocks that assembled the large galaxies that dominate groups and clusters, and representative of the conditions at the height of cosmic star formation. The Square Kilometre Array (SKA) can make huge strides in understanding galactic metabolism and the ecological processes that govern star formation, by observations of the Magellanic Clouds and other, nearby Magellanic-type irregular galaxies. Two programmes with SKA Pathfinders attempt to pave the way: the approved Galactic ASKAP Spectral Line Survey (GASKAP) includes a deep survey in H I and OH of the Magellanic Clouds, whilst MagiKAT is proposed to perform more detailed studies of selected regions within the Magellanic Clouds - also including Faraday rotation measurements and observations at higher frequencies. These surveys also close the gap with the revolutionizing surveys at far-IR wavelengths with the Spitzer Space Telescope and Herschel Space Observatory