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تسجيل مستخدم جديدThe Star Formation History of NGC 6822
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Ted K. Wyder
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Images of five fields in the Local Group dwarf irregular galaxy NGC 6822 obtained with the {it Hubble Space Telescope} in the F555W and F814W filters are presented. Photometry for the stars in these images was extracted using the Point-Spread-Function fitting program HSTPHOT/MULTIPHOT. The resulting color-magnitude diagrams reach down to $Vapprox26$, a level well below the red clump, and were used to solve quantitatively for the star formation history of NGC 6822. Assuming that stars began forming in this galaxy from low-metallicity gas and that there is little variation in the metallicity at each age, the distribution of stars along the red giant branch is best fit with star formation beginning in NGC 6822 12-15 Gyr ago. The best-fitting star formation histories for the old and intermediate age stars are similar among the five fields and show a constant or somewhat increasing star formation rate from 15 Gyr ago to the present except for a possible dip in the star formation rate from 3 to 5 Gyr ago. The main differences among the five fields are in the higher overall star formation rate per area in the bar fields as well as in the ratio of the recent star formation rate to the average past rate. These variations in the recent star formation rate imply that stars formed within the past 0.6 Gyr are not spatially very well mixed throughout the galaxy.
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We investigate the star formation threshold in NGC 6822, a nearby Local Group dwarf galaxy, on sub-kpc scales using high-resolution, wide-field, deep HI, Halpha and optical data. In a study of the HI velocity profiles we identify a cool and warm neut
ral component in the Interstellar Medium of NGC 6822. We show that the velocity dispersion of the cool component (~4 km/s) when used with a Toomre-Q criterion gives an optimal description of ongoing star formation in NGC 6822, superior to that using the more conventional dispersion value of 6 km/s. However, a simple constant surface density criterion for star formation gives an equally superior description. We also investigate the two-dimensional distribution of Q and the star formation threshold and find that these results also hold locally. The range in gas density in NGC 6822 is much larger than the range in critical density, and we argue that the conditions for star formation in NGC 6822 are fully driven by this density criterion. Star formation is local, and in NGC 6822 global rotational or shear parameters are apparently not important.
We characterize the star formation in the low-metallicity galaxy NGC 6822 over the past few hundred million years, using GALEX far-UV (FUV, 1344-1786 A) and near-UV (NUV, 1771-2831 A) imaging, and ground-based Ha imaging. From GALEX FUV image, we def
ine 77 star-forming (SF) regions with area >860 pc^2, and surface brightness <=26.8 mag(AB)arcsec^-2, within 0.2deg (1.7kpc) of the center of the galaxy. We estimate the extinction by interstellar dust in each SF region from resolved photometry of the hot stars it contains: E(B-V) ranges from the minimum foreground value of 0.22mag up to 0.66+-0.21mag. The integrated FUV and NUV photometry, compared with stellar population models, yields ages of the SF complexes up to a few hundred Myr, and masses from 2x10^2 Msun to 1.5x10^6 Msun. The derived ages and masses strongly depend on the assumed type of interstellar selective extinction, which we find to vary across the galaxy. The total mass of the FUV-defined SF regions translates into an average star formation rate (SFR) of 1.4x10^-2 Msun/yr over the past 100 Myr, and SFR=1.0x10^-2 Msun/yr in the most recent 10 Myr. The latter is in agreement with the value that we derive from the Ha luminosity, SFR=0.008 Msun/yr. The SFR in the most recent epoch becomes higher if we add the SFR=0.02 Msun/yr inferred from far-IR measurements, which trace star formation still embedded in dust (age <= a few Myr).
We present HST WFPC2 and STIS imaging of the low metallicity galaxy NGC 6822, performed as part of a study of the young stellar populations in the galaxies of the Local Group. Eleven WFPC2 pointings, with some overlap, cover two regions, extending ov
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color-magnitude diagram method. The galaxy experienced a complex star formation (SF) activity. The best fit to the data is found by assuming three episodes of activity in the last 1-2 Gyr. The most recent and strong episode constrained by these NIR data started ~37 Myr ago and ended ~13 Myr ago, although we cannot exclude the possibility that up to three SF episodes occurred in this time interval. The average star-formation rate (SFR) of the episode is 3.2 Msun yr-1 kpc-2, in agreement with literature data. A previous episode produced stars between 150 Myr and 40 Myr ago, with a mean SFR about 2/3 lower than the mean SFR of the youngest episode. An older SF episode occurred about 1 Gyr ago. All these SFRs are 2-3 orders of magnitude higher than those derived for late-type dwarfs of the Local Group. In all cases an initial mass function similar to Salpeters allows for a good reproduction of the data, but we cannot exclude flatter mass functions. These results have been obtained adopting a distance of 2.2 Mpc and a reddening E(B-V)=0.56. A larger distance would require younger episodes and higher SFRs. We have explored some possible scenarios using the astrated mass in the best fit model, in order to constrain the past star formation history. We cannot rule out a low rate in the past SF but we can safely conclude that the last 1-2 Gyr have been peculiar.
The star formation histories of four fields within the Local Group dwarf irregular galaxy NGC 6822 are presented. Each of the fields was imaged by the WFPC2 aboard the {it Hubble Space Telescope} and were used to obtain $VI$ color-magnitude diagrams
for each field reaching $Vsimeq26$. The magnitude of the tip of the red giant branch and the red clump were used to determine distances to NGC 6822 that are consistent with previous ground-based measurements. The distance, extinction and star formation history were also determined by fitting the entire color-magnitude diagram in each field. The distances from these fits are consistent with the other determinations within the estimated errors once the systematic effects of uncertainties in the age-metallicity relation are taken into account. The extinction varies among the four fields from approximately the foreground Galactic value to $approx 0.4$ mag higher in $V$ and roughly correlates with the $60micron$ surface brightness. The star formation histories in the four fields are similar for ages $gtrsim 1$ Gyr and are relatively constant or somewhat increasing with time. These old star formation rates are comparable to that expected from the typical gas surface densities at these galactocentric radii and suggest that no large scale redistribution of gas or stars is required to account for the inferred star formation rates. Three of the fields show a drop of a factor of $sim 2-4$ in the star formation rate about 600 Myr ago while the remaining field centered on the bar shows an increase.
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