Field star and cluster formation in resolved galaxies

Star formation is a complex phenomenon involving several physical processes such as turbulence, gravitational collapse, cooling, gravitational trigger. There are many open questions about star formation in galaxies: is it a continuous process or does it proceed by bursts? What triggers star formation internally or externally? How does star formation proceed in a low density, low metal content environment ? A program has been undertaken to cast light on the process of field and cluster formation in resolved galaxies (mainly LMC/SMC and LSBs) deriving the age distribution of a large number of clusters and comparing it with the field star formation. Recent surveys such as OGLE2, 2MASS, HST/ACS camera and the SAGE projects making use of the Spitzer mid-infrared observations have provided a large amount of data of extremely good quality in different bandpasses, that are ideal to study the process of star formation. The Large and Small Magellanic Clouds, believed to be interacting with each other, are ideal laboratories to study the process of field star and cluster formation in low metal content regions. In the SMC, the cluster age distribution supports the idea that clusters formed in the last 1 Gyr of the SMC history in a roughly continuous way with periods of enhancements. The two super-shells 37A and 304A detected in the HI distribution are clearly visible in the age distribution of the clusters: an enhancement in the cluster formation rate has taken place from the epoch of the shell formation. A tight correlation between young clusters and the HI intensity is found. The degree of correlation decreases with the age of the clusters. Clusters older than 300 Myr are located away from the HI peaks. Clusters and associations younger than 10 Myr are related to the CO clouds in the SW region of the SMC disk. A positive correlation between the location of the young clusters and the velocity in the dispersion field of the atomic gas is derived only for the shell 304A, suggesting that the cloud-cloud collision is probably not the most important mechanism of cluster formation.

Evidence of gravitational triggered episode due to the most recent close interaction between SMC and LMC is found both in the cluster and field star distribution (Chiosi et al. 2006). At older ages, the star formation rate of the field population in the SMC shows periods of enhancements at 3-4 Gyr and finally 6 Gyr. However it is relatively quiescent at ages older than 6 Gyr. This result suggests that at older ages, the tidal interaction between the Magellanic Clouds and the Milky Way was not able to trigger significant star formation events (Chiosi & Vallenari 2007).

We discover pre-main sequence candidates in the nebula N∼11 in the Large Magellanic Clouds using HST ACS photometry. The comparison of the Colour-Magnitude diagram with pre-main sequence tracks and the presence of Spitzer objects YSO I and II suggest that the star formation has been active for a long period in the region, from a few 10⁵ yrs to several Myr ago.

To better understand the star formation process and its triggering mechanism we are studying LSB galaxies, believed to be an extreme case of star formation in low metal content environments . Their formation appear to have followed a very different evolutionary history from high surface brightness galaxies (e.g., McGaugh 1992, Knezek 1993).  Their stellar populations, stellar masses, current star formation rates, and other properties appear to differ significantly from their high surface brightness counterparts, and little is known about their corresponding dust properties. A better understanding of these differences is needed in order to understand how they evolved to their present status. Many studies are consistent with a scenario where BLSB galaxies have on average a constant or increasing SFR with a ratio of young to old stars larger than found in high brightness spirals . However, different opinions are expressed in the literature, either suggesting that the SFR can be proceeding in frequent, small amplitude bursts . The study of HST CMDs in the LSB galaxy UGC 5889 show that the last 200 Myr the star formation proceeded in modest bursts at a rate of the order of 10^-2-10^-3 Mo/yr, with periods of extremely low SFR or even quiescence (Vallenari et al 2005). The rate derived from the present study for the last 20 Myr is in agreement with the Hα emission from the galaxy. The presence of a consistent population older than 200 Myr is indicated by the data.

People: A. Vallenari, E. V. Held, G. Bertelli, E. Nasi 

 Collaboration: E. Chiosi, A. Moretti (Padova Univ.), L. Rizzi (Institute for Astronomy, Hawaii, USA), D.J. Bomans (Bochum Observatory), I. Saviane, L. Schmidtobreick (ESO, Chile) 

 Publications: Vallenari (2008), MmSAIT, 79,448

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The Magellanic Bridge Stellar Population

The resolved stellar content of nearby galaxies can be investigated via detailed studies of their photometric, kinematic and spectroscopic properties. Ultimately, this allows to re-construct the finest details addressing the formation and evolution of these galaxies.

Our project concentrates on studying the stellar population associated to the HI gaseous bridge connecting the two Magellanic Clouds. This stellar bridge extends from the SMC and seems to terminate half way through to the LMC. We are currently analysing a wide-field photometric data-set (obtained through special narrow band filters) and centered on the tip of the stellar bridge. Our goal is to detect old (>2 Gyr) stellar populations, if any, by the means of identification of few Magellanic red giant stars (currently confused in the Galactic foreground contamination). The eventual detection of a ~>2 Gyr population in the Bridge implies that the 2 clouds have been "frozen" for at least 2 Gyr and that the bridge is not a recent ~60 Myr feature that has been triggered by the latest Magellanic-Milky Way (MW) encounter.

Our program thus addresses an essential and necessary ingredient for the simulations reconstructing the Magellanic and Magellanic-Milky Way past interactions. It is of particular interest to note that Momany & Zaggia (A&A 2005) have used proper-motion data to infer that the Magellanic Clouds may be at their first passage around the Milky Way and, are being un-bound to it. This idea goes against all current scenarios of the MC-MW interaction, nevertheless, it is interesting to note that very recent HST proper motion data (Busla et al. arXiv:astro-ph/0703196) favor a first MC-MW interaction.

People: Y. Momany, S. Zaggia, E.V. Held, M. Gullieuszik 

Collaboration: I. Saviane, G. Piotto (Padova Univ.), L. Bedin (Baltimore, USA)

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The VISTA near-infrared YJKs survey of the Magellanic System

 

The VISTA near-infrared survey of the Magellanic System (VMC) is one of the 6 public surveys approved by ESO to be conducted with the VIRCAM camera at the new 4m VISTAtelescope. It will cover 184 square degrees encompassing the D_25 of both Clouds as well as major features delineated in the distribution of stars and H I gas, in the YJKs pass bands. The survey depth - 6 magnitudes deeper than 2MASS - has been chosen to reach the oldest main sequence turn-off across the entire system, and will allow us to determine the spatially-resolved star formation history (SFH) with unprecedented quality and to construct a detailed 3D map of the system.

Researchers at OAPd are responsible for a crucial part of the survey: the realization of artificial star tests (AST) and the initial SFH-recovery analyses for all images. ASTs will be released as data products by the survey team. The preparation work has started using simulated data built with the TRILEGAL tool. The first real VMC images are expected to be available in mid-2009.

 

People: L. Girardi, S. Rubele 

 Collaboration: M.R. Cioni (Hertfordshire Univ.), L. Kerber (sao Paulo Univ.), VMC team 

 Publications:  Kerber et al. (2009), A&A accepted; Girardi et al. (2009), MNRAS Letters, in press; Cioni et al. (2008), PASA 25,121