Star formation history in the SMC: the case of NGC602

Deep HST/ACS photometry of the young cluster NGC 602, located in the remote low density "wing" of the Small Magellanic Cloud, reveals numerous pre-main sequence stars as well as young stars on the main sequence. The resolved stellar content thus provides a basis for studying the star formation history into recent times and constraining several stellar population properties, such as the present day mass function, the initial mass function and the binary fraction. To better characterize the pre-main sequence population, we present a new set of model stellar evolutionary tracks for this evolutionary phase with metallicity appropriate for the Small Magellanic Cloud (Z = 0.004). We use a stellar population synthesis code, which takes into account a full range of stellar evolution phases to derive our best estimate for the star formation history in the region by comparing observed and synthetic color-magnitude diagrams. The derived present day mass function for NGC 602 is consistent with that resulting from the synthetic diagrams. The star formation rate in the region has increased with time on a scale of tens of Myr, reaching $0.3-0.7 \times 10^{-3} M_\odot yr^{-1}$ in the last 2.5 Myr, comparable to what is found in Galactic OB associations. Star formation is most complete in the main cluster but continues at moderate levels in the gas-rich periphery of the nebula.Comment: 24 pages. Accepted for publication in A

Past and present star formation in the SMC: NGC 346 and its neighborhood

In the quest of understanding how star formation occurs and propagates in the low metallicity environment of the Small Magellanic Cloud (SMC), we acquired deep F555W (~V), and F814W (~I) HST/ACS images of the young and massive star forming region NGC 346. These images and their photometric analysis provide us with a snapshot of the star formation history of the region. We find evidence for star formation extending from ~10 Gyr in the past until ~150 Myr in the field of the SMC. The youngest stellar population (~3 +/- 1 Myr) is associated with the NGC 346 cluster. It includes a rich component of low mass pre-main sequence stars mainly concentrated in a number of sub-clusters, spatially co- located with CO clumps previously detected by Rubio et al. (2000). Within our analysis uncertainties, these sub-clusters appear coeval with each other. The most massive stars appear concentrated in the central sub-clusters, indicating possible mass segregation. A number of embedded clusters are also observed. This finding, combined with the overall wealth of dust and gas, could imply that star formation is still active. An intermediate age star cluster, BS90, formed ~4.3 +/-0.1 Gyr ago, is also present in the region. Thus, this region of the SMC has supported star formation with varying levels of intensity over much of the cosmic time.Comment: 38 pages, 13 figures, 3 tables; AJ accepte

The Stellar Mass Distribution in the Giant Star Forming Region NGC 346

Deep F555W and F814W Hubble Space Telescope ACS images are the basis for a study of the present day mass function (PDMF) of NGC346, the largest active star forming region in the Small Magellanic Cloud (SMC). We find a PDMF slope of Gamma=-1.43+/-0.18 in the mass range 0.8-60 Mo, in excellent agreement with the Salpeter Initial Mass Function (IMF) in the solar neighborhood. Caveats on the conversion of the PDMF to the IMF are discussed. The PDMF slope changes, as a function of the radial distance from the center of the NGC 346 star cluster, indicating a segregation of the most massive stars. This segregation is likely primordial considering the young age (~3 Myr) of NGC346, and its clumpy structure which suggests that the cluster has likely not had sufficient time to relax. Comparing our results for NGC346 with those derived for other star clusters in the SMC and the Milky Way (MW), we conclude that, while the star formation process might depend on the local cloud conditions, the IMF does not seem to be affected by general environmental effects such as galaxy type, metallicity, and dust content.Comment: 26 pages, 7 figures, 1 table, accepted for publication in A

The Role of the Magnetic Field in the Interstellar Medium of the Post-Starburst Dwarf Irregular Galaxy NGC 1569

(abridged) NGC 1569 is a nearby dwarf irregular galaxy which underwent an intense burst of star formation 10 to 40 Myr ago. We present observations that reach surface brightnesses two to eighty times fainter than previous radio continuum observations and the first radio continuum polarization observations. These observations allow us to probe the relationship of the magnetic field of NGC 1569 to the rest of its interstellar medium. We confirm the presence of an extended radio continuum halo at 20 cm and see for the first time the radio continuum feature associated with the western Halpha arm at wavelengths shorter than 20cm. The spectral index trends in this galaxy support the theory that there is a convective wind at work in this galaxy. We derive a total magnetic field strength of 38 microG in the central regions and 10-15 microG in the halo. The magnetic field is largely random in the center of the galaxy; the uniform field is ~3-9 microG and is strongest in the halo. We find that the magnetic pressure is the same order of magnitude but, in general, a factor of a few less than the other components of the interstellar medium in this galaxy. The uniform magnetic field in NGC 1569 is closely associated with the Halpha bubbles and filaments. We suggest that a supernova-driven dynamo may be operating in this galaxy. The outflow of hot gas from NGC 1569 is clearly shaping the magnetic field, but the magnetic field in turn may be aiding the outflow by channeling gas out of the disk of the galaxy. Dwarf galaxies with extended radio continuum halos like that of NGC 1569 may play an important role in magnetizing the intergalactic medium.Comment: ApJ accepted. 56 pages, 14 figures (low resolution), 8 tables. Version with high resolution figures at http://www.astro.virginia.edu/~aak8t/data/n1569/ms.pd

The HELLAS2XMM survey. V. Near-Infrared observations of X-ray sources with extreme X/O ratios

We present the results of deep near-infrared observations of 11 hard X-ray selected sources in the Hellas2XMM survey, with faint optical magnitude (R>24) and high X-ray-to-optical flux ratio. All but one of the sources have been detected in the Ks band, with bright counterparts (Ks<19) and red colors (R-K>5), and therefore belong to the ERO population. A detailed analysis of the surface brightness profiles allows us to classify all of the NIR counterparts. There are 2 point-like objects, 7 elliptical galaxies and one source with a disky profile. None of the extended sources shows any evidence for the presence of a central unresolved object tracing the putative X-ray emitting AGN. Using both the R-K colors and the morphological information, we have estimated for all the sources a ``minimum photometric redshift'', ranging between 0.8 and 2.4; the elliptical hosts have zmin=0.9-1.4. We computed the X-ray properties using these redshifts: most of the sources have NH>10^{22}, with unabsorbed X-ray luminosities up to 10^{45}erg s^{-1}. These objects therefore belong to the population of obscured (Type II) quasars and, from a statistical point of view, they turn out to be a non-negligible fraction (~10%) of the most luminous AGN. Selecting the high X/O sources for a follow-up study in the NIR is therefore a powerful technique aimed at studying at high redshift the hosts of Type II AGN. Overall, our results seem to indicate that the hosts are mostly elliptical galaxies at z~1, and that these near-IR bright objects would be among the most massive spheroids at these epochs.Comment: 15 pages, 8 figures. Accepted for publication in A&A. V2: minor typos correcte

Arthroscopic treatment of a osteochondritis dissecans of the femoral trochlea [Trattamento artroscopico di una osteocondrite dissecante della troclea femorale]

The juvenile osteochondritis dissecans of femoral trochlea is an uncommon lesion. Review of the literature reveals few reported cases of the medial or lateral trochlea. We report on 11-year-old child with a complete separate trochlea fragment. MRI and arthroscopy have shown a 2,5 7 2,2 cm osteochondral lesion. The fragment was reduced and synthesised by 3 absorbable pins using arthroscopic approach. At 6 months follow-up, the patient did not complain of knee pain, had full range of motion and returned to all the activities performed before the traumatic event. MRI and Athro-MRI showed healing of the lesion and continuity of the articular surface without any cleft between the fragment and the surrounding cartilage

Matrix-induced autologous chondrocyte implantation (MACI): high-resolution-MRI study at medium term follow-up.

Objective: Evaluation of articular cartilage repair after Matrix-induced autologous chondrocyte implantation (MACI) by high-resolution standard MRI and arthro-MRI. Material and methods: Thirty patients with a minimum of 5 years follow-up (mean 67 months, range 60 to 84 months) were evaluated with a high-resolution standard MRI and arthro-MRI (1.5-T unit, using GE, T2*, spin-echo T1, FatSat fast spin-echo DP and T2-weighted sequences). The results were classified according to the grading system described by Marlovits that considered the degree of defect repair in width and length, surface, structure and signal intensity of the repair tissue, and status of the subchondral lamina and bone. A score higher than 50 points (max: 100 points) was assumed as a successful repair. The results were then compared to the most common clinical rating scales (ICRS, Lysholm II, Tegner, Cincinnati) to evaluate the possible correspondence. Results: In 18 patients we observed a complete filling of the defect. Implant integration was evident in 17 cases. Intact surface was found in 18 cases. Arthro-MRI revealed 7 cases of superficial fibrillations and fissures. In 23 implants the signal was isointense and similar to native cartilage. Oedema or cysts were individuated in 9 cases. In 23 patients we recorded an overall value higher than 50 points. Four cases were associated with a clinical failure while 3 cases were not associated. Conclusions: High-resolution MRI provides a useful tool for studying articular cartilage repair tissue. Arthro-MRI can improve the sensibility of the study to detect any clefts between native cartilage and regenerated tissue and any superficial implant fibrillations. It has to be demonstrated if MRI can be predictive of future clinical failures. Only an higher number of cases and long term studies will allow to confirm these observations