1,725,291 research outputs found
Asymptotic giant branch variables in NGC 6822
No full textIncludes bibliographical references (p. 71-75).NGC 6822, a Local Group dwarf irregular galaxy, was observed in the nearinfrared JHKS bands using the Japanese-SAAO Infrared Survey Facility (IRSF) telescope over a period of about four years. An extensive search for large amplitude asymptotic giant branch variables over the observed 7.5' x 21.1' area of NGC 6822 found 162 probable variables. In particular, 30 Miras and 9 semiregulars have been identified and characterised
An Old Cluster in NGC 6822
Get PDFWe present spectroscopy of two clusters in the dwarf irregular galaxy NGC 6822. From these we deduce an age for cluster VII of 11 ^(+4)_(-3) Gyr and [Fe/H] = -1.95 ± 0.15 dex. Cluster VII appears to be an analog of the metal-poor Galactic globular clusters. Cluster VI is found to be much younger and more metal-rich, with an age of approximately 2 Gyr. Its derived metallicity, [Fe/H] ≈ -1.0 dex, is comparable to that of the gas seen today in NGC 6822. The existence of a metal-poor old cluster in NGC 6822 rules out models for the chemical evolution of this galaxy with significant prompt initial enhancement. We find that a star formation rate that is constant with time and is within a factor of 2 of the present star formation rate can reproduce the two points on the age-metallicity relationship for NGC 6822 over the past 10 Gyr defined by these two clusters
Long period variable survey in NGC 6822
No full textContext. The star-formation history of the intermediate-age population of the Local Group dwarf irregular galaxy NGC 6822 may have been influenced by the often discussed interaction with its HI cloud that might have produced the present polar-ring structure.
Aims. We investigate the period distribution of the Mira variables in the central region and the spheroid of NGC 6822.
Methods. We use the new wide-field near-infrared imager CPAPIR to acquire Ks images of a 32′ × 32′ area, which includes a good part of the eastern spheroid of NGC 6822.
Results. Periods could be determined for 45 cyclic and semi-regular variables brighter than ⟨ K ⟩ = 17. The vast majority of the variables have ages between 1 and 3 Gyr.
Conclusions. The period distribution of the long period variables may reflect the proposed interaction with the HI cloud that produces the polar ring
Photometric survey of the polar ring galaxy NGC 6822
No full textContext.We have previously established, from a carbon star survey, that the
Local Group dwarf irregular galaxy NGC 6822 is much larger than
its central bright core.
Aims. Four MegaCam fields are acquired
to survey a 2 area centred on
NGC 6822 to fully determine its extent and map its stellar populations.
Methods.Photometry of over one million stars is
obtained in the SDSS g', r', i' to three magnitudes below the
TRGB. RGB stars, selected from their magnitudes and colours, are used
to map the NGC 6822 stellar distribution up to a distance of 60 arcmin.
Results.We map the reddening over the whole area.
We establish that the stellar outer structure of NGC 6822 is elliptical
in shape, with and
a major-axis PA = 65°, contrasting with the orientation of
the HI disk. The density enhancement can be seen up to a semi-major axis
of making NGC 6822 as big as the Small Magellanic Cloud.
We fit two exponentials to the surface density profile of the spheroid,
and identify a bulge with a scale length of and an outer spheroid
with a scale length of .
We find intermediate-age C stars up to while demonstrating
that the SDSS filters are unsuitable to identify extragalactic C stars.
Conclusions.NGC 6822 is a unique Local Group
galaxy with shape and structure suggesting a polar ring
configuration. Radial velocities of carbon stars have indeed
demonstrated that there are two kinematical systems in NGC 6822
Star complexes and stellar populations in NGC 6822
No full text10 pages, 7 figures, 4 tables.-- Pre-print archive.Aims. The star complexes (large scale star forming regions) of NGC 6822 were traced and mapped and their size distribution was compared with the size distribution of star complexes in the Magellanic Clouds (MCs). The spatial distributions of different age stellar populations were compared with each other.Methods. The star complexes of NGC 6822 were determined by using the isopleths, based on star counts, of the young stars of the galaxy, using a statistical cutoff limit in density. In order to map them and determine their geometric properties, an ellipse was fitted to each distinct region satisfying this minimum limit. The Kolmogorov-Smirnov statistical test was used to study possible patterns in their size distribution. Isopleths were also used to study the stellar populations of NGC 6822.Results. The star complexes of NGC 6822 were detected and a list of their positions and sizes was produced. Indications of hierarchical star formation, in terms of spatial distribution, time evolution and preferable sizes were found in NGC 6822 and the MCs. The spatial distribution of the various age stellar populations has indicated traces of an interaction in NGC 6822, dated before 350 50 Myr.This work was supported
by the Special Account for Research Grants of the National and Kapodistrian
University of Athens.Peer reviewe
Star complexes and stellar populations in NGC 6822
No full textAims. The star complexes (large scale star forming regions) of NGC 6822 were traced and mapped and their size distribution was compared with the size distribution of star complexes in the Magellanic Clouds (MCs). The spatial distributions of different age stellar populations were compared with each other.Methods. The star complexes of NGC 6822 were determined by using the isopleths, based on star counts, of the young stars of the galaxy, using a statistical cutoff limit in density. In order to map them and determine their geometric properties, an ellipse was fitted to each distinct region satisfying this minimum limit. The Kolmogorov-Smirnov statistical test was used to study possible patterns in their size distribution. Isopleths were also used to study the stellar populations of NGC 6822.Results. The star complexes of NGC 6822 were detected and a list of their positions and sizes was produced. Indications of hierarchical star formation, in terms of spatial distribution, time evolution and preferable sizes were found in NGC 6822 and the MCs. The spatial distribution of the various age stellar populations has indicated traces of an interaction in NGC 6822, dated before 350 ± 50 Myr. © 2009 ESO
CHANDRA OBSERVATION OF NGC 6822
No full textChandra observed the nearby dwarf galaxy NGC 6822. There are 70 sources in the Chandra field, two of which are fairly extended and likely background clusters of galaxies. A third source was resolved by Chandra that has the same size and position as a known supernova remnant. The suspected variability detected from this source in previous missions is shown to be in error. Themajority of the remaining 67 sources are consistent with background sources. Of the 61 sources detected above a completeness limit of 10 events, we estimate that 9 4 8 are associated with the galaxy, including both systematic and statistical errors. We compare the X-ray positions of all sources with various catalogs and Hubble Space Telescope data and offer tentative identifications for several. Based on the mass and star formation rate of NGC 6822, we expect only about 10 sources, mostly high-mass systems
The Stellar population and interstellar medium in NGC 6822
No full textWe present a comprehensive study of the stellar population and the interstellar medium in NGC 6822 using high-quality H I data (obtained with the Australia Telescope Compact Array) and optical broadband and narrowband data (obtained with Subaru and the Isaac Newton Telescope). Our Halpha observations are an order of magnitude deeper than previous studies and reveal a complex filamentary network covering almost the entire central disk of NGC 6822. We find hitherto unknown H II regions in the outskirts of NGC 6822 and the companion galaxy. The old and intermediate-age stellar population can be traced out to radii of over 0.6d (>5 kpc), significantly more extended than the H I disk. In sharp contrast, the distribution of the young, blue stars closely follows the distribution of the H I disk and displays a highly structured morphology. We find evidence for an older stellar population in the companion galaxy; the current star formation activity, although likely to have been triggered by the interaction with NGC 6822, is not the first star formation episode in this object. We show that the properties of the giant kiloparsec-sized hole in the outer H I disk of NGC 6822 are consistent with it being formed by the effects of stellar evolution
NGC 6822: SFH of its Intermediate-Age Population
No full textWe have monitored, during four years, a 32′× 32′ area of the Local Group dwarf galaxy NGC 6822. Observations lead to the discovery of 30 Miras and 24 semi-regular and irregular variables. When relating the periods of Miras to their age, we find a continuous star formation during the last 3 Gyr. This result is basically consistent with the hypothesis that the intermediate-age population of NGC 6822 was born following the “polar ring” formation
The globular cluster system of NGC 6822
Get PDFWe present a comprehensive analysis of the globular cluster (GC) system of the Local Group dwarf irregular galaxy NGC 6822. Our study is based on homogeneous optical and near-IR photometry, as well as long-slit spectroscopic observations which are used to determine new radial velocities for six GCs, two of which had no previous spectroscopic information. We construct optical-near-IR colour-colour diagrams and through comparison to simple stellar population models infer that the GCs have old ages consistent with being 9 Gyr or older, while their metallicities are in the range between -1.6 ≲ [Fe/H] ≲ -0.4. We conduct a kinematic analysis of the GC population and find tentative evidence for weak net rotation of the GC system, in the same sense as that exhibited by the underlying spheroid. The most likely amplitude of rotation is ≈10 km s-1, approximately half the magnitude of the observed velocity dispersion. Finally, we use the GCs to estimate the dynamical mass of NGC 6822 within ~11 kpc and we formally find it to be in the range between (3 and 4) × 109 M⊙. This implies an overall mass-to-light ratio in the range of ~30-40 and indicates that NGC 6822 is highly dark-matter-dominated. The mass and the corresponding mass-to-light ratio estimates are affected by various additional systematic effects due to limitations of the data and the model that are not necessary reflected in the formal uncertainties
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