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Post Info TOPIC: NGC6822
Blobrana


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NGC 6822
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Title: Star complexes and stellar populations in NGC 6822 - Comparison with the Magellanic Clouds
Authors: A. Karampelas (1), A. Dapergolas (2), E. Kontizas (2), E. Livanou (1), M. Kontizas (1), I. Bellas-Velidis (2), J. M. Vilchez (3) ((1) Department of Astrophysics, Astronomy & Mechanics, Faculty of Physics, University of Athens, (2) Institute for Astronomy and Astrophysics, National Observatory of Athens, (3) Instituto de Astrofisica de Andalucia (CSIC))
(Version v2)

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). Furthermore, the spatial distributions of different age stellar populations were compared with each other. 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 geometrical properties, an ellipse was fitted to every 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. 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.

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Blobrana


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RE: NGC6822
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Title: Diffuse Ionised Gas inside the dwarf irregular galaxy NGC 6822
Authors: A.M. Hidalgo-Gamez (ESFM-Ipn & Ia-Unam), A.Peimbert (IA-Unam)

We have studied the differences between the diffuse ionised gas (DIG) and the HII regions along a slit position in the local dwarf irregular galaxy NGC 6822. The slit position passes through the two most prominent HII regions: Hubble V and Hubble X. Important differences have been found in the excitation, ionisation, and [NII]6584/Ha and [SII]6717/Ha line ratios between the DIG and the HII locations. Moreover, the values of all the line ratios are not similar to those in the DIG locations of spiral galaxies but are very similar to the values in other irregular galaxies, such as IC 10. We also determined the rate of recombination using the HeI5875 line. Finally, we obtained a picture of the ionisation sources of the DIG. We consider that the leakage of photons from the HII regions might explain most of the line ratios, except [NII]/Ha, which might be explained by turbulence.

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Blobrana


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Title: The Nature of Infrared Emission in the Local Group Dwarf Galaxy NGC 6822 As Revealed by Spitzer
Authors: John M. Cannon, Fabian Walter, Lee Armus, George J. Bendo, Daniela Calzetti, Bruce T. Draine, Charles W. Engelbracht, George Helou, Robert C. Kennicutt Jr., Claus Leitherer, Helene Roussel, Caroline Bot, Brent Buckalew, Daniel A. Dale, W. J. G. de Blok, Karl D. Gordon, David J. Hollenbach, Thomas H. Jarrett, Martin J. Meyer, Eric J. Murphy, Kartik Sheth, Michele D. Thornley

We present Spitzer imaging of the metal-deficient (Z ~30% Z_sun) Local Group dwarf galaxy NGC 6822. On spatial scales of ~130 pc, we study the nature of IR, H alpha, HI, and radio continuum emission. Nebular emission strength correlates with IR surface brightness; however, roughly half of the IR emission is associated with diffuse regions not luminous at H alpha (as found in previous studies). The global ratio of dust to HI gas in the ISM, while uncertain at the factor of ~2 level, is ~25 times lower than the global values derived for spiral galaxies using similar modelling techniques; localised ratios of dust to HI gas are about a factor of five higher than the global value in NGC 6822. There are strong variations (factors of ~10) in the relative ratios of H alpha and IR flux throughout the central disk; the low dust content of NGC 6822 is likely responsible for the different H alpha/IR ratios compared to those found in more metal-rich environments. The H alpha and IR emission is associated with high-column density (> ~10^21 cm-²) neutral gas. Increases in IR surface brightness appear to be affected by both increased radiation field strength and increased local gas density. Individual regions and the galaxy as a whole fall within the observed scatter of recent high-resolution studies of the radio-far IR correlation in nearby spiral galaxies; this is likely the result of depleted radio and far-IR emission strengths in the ISM of this dwarf galaxy.

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Title: C stars in the outer spheroid of NGC 6822
Authors: S. Demers, P. Battinelli, E. Artigau

From a 2 x 2 degree survey of NGC 6822 researchers have previously established that this Local Group dwarf irregular galaxy possesses a huge spheroid having more than one degree in length. This spheroid is in rotation but its rotation curve is known only within ~15' from the centre. It is therefore critical to identify bright stars belonging to the spheroid to characterise, as far as possible, its outer kinematics. Researchers use the new wide field near infrared imager CPAPIR, operated by the SMARTS consortium, to acquire J, Ks images of two 34.8' x 34.8' areas in the outer spheroid to search for C stars. The colour diagram of the fields allows the identification of 192 C stars candidates but a study of the FWHM of the images permits the rejection of numerous non-stellar objects with colours similar to C stars.
They are left with 75 new C stars, their mean Ks magnitude and mean colour are similar to the bulk of known NGC 6822 C stars. This outer spheroid survey confirms that the intermediate-age AGB stars are a major contributor to the stellar populations of the spheroid. The discovery of some 50 C stars well beyond the limit of the previously known rotation curve calls for a promising spectroscopic follow-up to a major axis distance of 40'.

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