Star formation in the central regions of galaxies

Massive star formation in the central regions of spiral galaxies plays an important role in the dynamical and secular evolution of their hosts. Here, we summarise a number of recent investigations of the star formation history and the physical conditions of the gas in circumnuclear regions, to illustrate not only the detailed results one can achieve, but also the potential of using state-of-the-art spectroscopic and analysis techniques in researching the central regions of galaxies in general. We review how the star formation history of nuclear rings confirms that they are long-lived and stable configurations. Gas flows in from the disk, through the bar, and into the ring, where successive episodes of massive star formation occur. Analysing the ring in NGC 7742 in particular, we determine the physical conditions of the line emitting gas using a combination of ionisation and stellar population modelling, concluding that the origin of the nuclear ring in this non-barred galaxy lies in a recent minor merger with a small gas-rich galaxy.Comment: Invited contribution, to appear in "Mapping the Galaxy and other galaxies", Eds. K. Wada and F. Combes, Springer, in pres

Broad-band photometric colors and effective temperature calibrations for late-type giants. II. Z<0.02

(Abridged) We investigate the effects of metallicity on the broad-band photometric colors of late-type giants, and make a comparison of synthetic colors with observed photometric properties of late-type giants over a wide range of effective temperatures (T_eff=3500-4800 K) and gravities (log g=0.0-2.5), at [M/H]=-1.0 and -2.0. The influence of metallicity on the synthetic photometric colors is generally small at effective temperatures above \~3800 K, but the effects grow larger at lower T_eff, due to the changing efficiency of molecule formation which reduces molecular opacities at lower [M/H]. To make a detailed comparison of the synthetic and observed photometric colors of late type giants in the T_eff--color and color--color planes, we derive a set of new T_eff--log g--color relations based on synthetic photometric colors, at [M/H]=-0.5, -1.0, -1.5, and -2.0. While differences between the new T_eff--color relations and those available from the literature are typically well within ~100 K, effective temperatures predicted by the scales based on synthetic colors tend to be slightly higher than those resulting from the T_eff--color relations based on observations, with the offsets up to ~100 K. This is clearly seen both at [M/H]=-1.0 and -2.0, especially in the T_eff--(B-V) and T_eff--(V-K) planes. The consistency between T_eff--log g--color scales based on synthetic colors calculated with different stellar atmosphere codes is very good, with typical differences being well within \Delta T_eff~70 K at [M/H]=-1.0 and \Delta T_eff~40 K at [M/H]=-2.0.Comment: 20 pages, 11 figures, A&A accepte

Photometric colors of late-type giants: theory versus observations

To assess the current status in the theoretical modeling of the spectral properties of late-type giants, we provide a comparison of synthetic photometric colors of late-type giants (calculated with PHOENIX, MARCS and ATLAS model atmospheres) with observations, at [M/H]=0.0 and -2.0. Overall, there is a good agreement between observed and synthetic colors, and synthetic colors and published Teff-color relations, both at [M/H]=0.0 and -2.0. Deviations from the observed trends in Teff-color planes are generally within \pm 150K (or less) in the effective temperature range Teff=3500-4800K. Synthetic colors calculated with different stellar atmosphere models typically agree to ~100K, within a large range of effective temperatures and gravities. Some discrepancies are seen in the Teff-(B-V) plane below Teff~3800K at [M/H]=0.0, due to difficulties in reproducing the 'turn-off' to the bluer colors which is seen in the observed data at Teff~3600K. Note that at [M/H]=-2.0 effective temperatures given by the scale of Alonso et al. (1999) are generally lower than those resulting from other Teff-color relations based both on observed and synthetic colors.Comment: 2 pages, 1 figure. Proceedings of the IAU Symposium 232 "The Scientific Requirements for Extremely Large Telescopes", eds. P. Whitelock, B. Leibundgut, and M. Dennefel

Deep near-IR observations of the Globular Cluster M4: Hunting for Brown Dwarfs

We present an analysis of deep HST/WFC3 near-IR (NIR) imaging data of the globular cluster M4. The best-photometry NIR colour-magnitude diagram (CMD) clearly shows the main sequence extending towards the expected end of the Hydrogen-burning limit and going beyond this point towards fainter sources. The white dwarf sequence can be identified. As such, this is the deepest NIR CMD of a globular cluster to date. Archival HST optical data were used for proper-motion cleaning of the CMD and for distinguishing the white dwarfs (WDs) from brown dwarf (BD) candidates. Detection limits in the NIR are around F110W approx 26.5 mag and F160W approx27 mag, and in the optical around F775W approx 28 mag. Comparing our observed CMDs with theoretical models, we conclude that we have reached beyond the H-burning limit in our NIR CMD and are probably just above or around this limit in our optical-NIR CMDs. Thus, any faint NIR sources that have no optical counterpart are potential BD candidates, since the optical data are not deep enough to detect them. We visually inspected the positions of NIR sources which are fainter than the H-burning limit in F110W and for which the optical photometry did not return a counterpart. We found in total five sources for which we did not get an optical measurement. For four of these five sources, a faint optical counterpart could be visually identified, and an upper optical magnitude was estimated. Based on these upper optical magnitude limits, we conclude that one source is likely a WD, one source could either be a WD or BD candidate, and the remaining two sources agree with being BD candidates. For only one source no optical counterpart could be detected, which makes this source a good BD candidate. We conclude that we found in total four good BD candidates.Comment: ApJ accepted, 28 pages including 16 figure

MEMO: mass spectrometry-based sample vectorization to explore chemodiverse datasets

In natural products research, chemodiverse extracts coming from multiple organisms are explored for novel bioactive molecules, sometimes over extended periods. Samples are usually analyzed by liquid chromatography coupled with fragmentation mass spectrometry to acquire informative mass spectral ensembles. Such data is then exploited to establish relationships among analytes or samples (e.g., via molecular networking) and annotate metabolites. However, the comparison of samples profiled in different batches is challenging with current metabolomics methods since the experimental variation-changes in chromatographical or mass spectrometric conditions - hinders the direct comparison of the profiled samples. Here we introduce MEMO-MS2 BasEd SaMple VectOrization-a method allowing to cluster large amounts of chemodiverse samples based on their LC-MS/MS profiles in a retention time agnostic manner. This method is particularly suited for heterogeneous and chemodiverse sample sets. MEMO demonstrated similar clustering performance as state-of-the-art metrics considering fragmentation spectra. More importantly, such performance was achieved without the requirement of a prior feature alignment step and in a significantly shorter computational time. MEMO thus allows the comparison of vast ensembles of samples, even when analyzed over long periods of time, and on different chromatographic or mass spectrometry platforms. This new addition to the computational metabolomics toolbox should drastically expand the scope of large-scale comparative analysis