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Register a new userSimulating the recent star formation history in the halo of NGC 5128
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Simulated color-magnitude diagrams (CMDs) are used to investigate the recent star formation history in NGC 5128. The comparison of the simulations with the observed UV CMD for a field in the NE shell, where recent star formation is present, constrains the initial mass function (IMF) and the duration of the star formation episode. The star formation burst is still on-going or at most has stopped some 2 Myr ago. The look-back time on the main sequence is set by the incompleteness of the U-band observations at V~26, and is of the order of 50 Myr. The post main sequence phases have a look-back time up to 100 Myr, setting the maximum observable time for the star formation in this field. The comparison of the observed and simulated luminosity functions and the number of post main sequence vs. total number of stars favours models with active star formation in the last 100 Myr. The data are best fitted by an IMF with Salpeter slope (alpha=2.35), and are also consistent with slopes in the range of 2<~alpha<~2.6. They exclude steeper IMF slopes. The mean star formation rate for a Salpeter IMF in the range of masses 0.6<~M<~100 M_sun within the last 100 Myr is 1.6x10^-4 M_sun yr^-1 kpc^-2.
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We resolve stars of the nearest giant elliptical galaxy NGC 5128 using VLT with FORS1 and ISAAC. We construct deep U, V and Ks color-magnitude and color-color diagrams in two different halo fields (in the halo and in the north-eastern diffuse shell). In the outer, shell field, at ~14 kpc from the center of the galaxy, there is a significant recent star formation with stars as young as 10 Myr, approximately aligned with the prominent radio and x-ray jet from the nucleus of the host AGN. Ionized gas filaments are evident in ultraviolet images near the area where neutral HI and CO molecular gas was previously observed. The underlying stellar population of the halo of the giant elliptical is predominantly old with a very broad metallicity distribution. The presence of an extended giant branch reaching M_bol=-5 mag suggests the existence of a significant intermediate-age AGB population in the halo of this galaxy.
We used the direct CCD camera at the Magellan I telescope at Las Campanas Observatory and FORS1 at Antu VLT at ESO Paranal Observatory to image fields centered on the inner and outer optical filaments in the halo of NGC 5128. In the V vs. U-V color-magnitude diagrams we have identified young blue supergiants associated with these line-emitting filaments located between the inner radio lobe and the northern middle lobe. Around the outer filament stars as young as 10 Myr were detected. They are principally aligned with the direction of the radio jet, but a vertical north-east alignment along the edge of the HI cloud is also present. Young stars in the inner filament field are found inside the bright knots of photoionized gas and are strongly aligned in the direction towards the center of the galaxy at the same position angle as the inner radio jet. Fitting the Padova isochrones on UV color-magnitude diagrams we find that the blue stars around the inner filaments have ages similar to the ones around the outer filaments ~10-15 Myr and the same abundance of Z=0.004. The presence of young blue supergiants clearly shows that the bright blue knots in the north-eastern halo of NGC 5128 are associations of young stars with photoionized gas. The temperature of the brightest stars is T ~ 12000-16000 K, insufficient to account alone for the high excitation lines observed in the surrounding ionized gas. Thus the optical emission jet is principally seen due to its alignment with the radio structure of the AGN. The highly collimated star formation is present only in the north eastern halo of the galaxy, suggesting the interaction of the jet with the gas clouds deposited during the last accretion event as the preferred triggering mechanism. (abridged)
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.
We present new results on the star formation history of the dwarf irregular galaxy NGC 1569. The data were obtained with Hubble Space Telescopes NICMOS/NIC2 in the F110W (J) and F160W (H) near-infrared (NIR) filters and interpreted with the synthetic 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.
We have derived the star formation history of the Milky Way disk over the last 2 Gyr from the age distribution diagram of a large sample of open clusters comprising more than 580 objects. By interpreting the age distribution diagram using numerical results from an extensive library of N-body calculations carried out during the last ten years, we reconstruct the recent star formation history of the Milky Way disk. Our analysis suggests that superimposed on a relatively small level of constant star formation activity mainly in small-N star clusters, the star formation rate has experienced at least 5 episodes of enhanced star formation lasting about 0.2 Gyr with production of larger clusters. This cyclic behavior seems to show a period of 0.4+/-0.1 Gyr.
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