Preben Grosbol

Pattern speed of main spiral arms in NGC 2997 ? Estimate based on very young stellar complexes

Context. Bright knots along arms are observed in K-band images of many grand-design spiral galaxies. They are identified as young starforming complexes using K-band spectra which show strong Br emission. Their alignment with spiral arms and ages <10 Myr suggest that they are associated with a starforming front linked to a density wave in the galaxies. Aims. Ages may be estimated for the youngest starforming complexes using NIR broad band colors and Br . A di erent angular speed of the density wave and material would lead to an azimuthal age gradient of newly formed objects. We aim to detect this gradient. Methods. Deep JHK-Br photometry of the southern arm of the grand-design spiral galaxy NGC 2997 was obtained by ISAAC/VLT. All sources in the field brighter than K=19 mag were located. Color-color diagrams were used to identify young stellar complexes among the extended sources. Ages can be estimated for the youngest complexes and correlated with azimuthal distances from the spiral arms defined by the K-band intensity variation. Results. The extended sources with MK< 12 mag display a di use appearance and are more concentrated inside the arm region than fainter ones, which are compact and uniformly distributed in the disk. The NIR colors of the bright di use objects are consistent with them being young starforming complexes with ages <10 Myr and reddened by up to 8 mag of visual extinction. They show a color gradient as a function of their azimuthal distance from the spiral arms. Interpreting this gradient as an age variation, the pattern speed p = 16 km sec 1 kpc 1 of the main spiral was derived assuming circular motion. Conclusions. The alignment and color gradient of the bright, di use complexes strongly support a density wave scenario for NGC 2997. Only the brightest complexes with MK < 12 mag show a well aligned structure along the arm, suggesting that a strong compression in the gas due to the spiral potential is required to form these most massive aggregates, while smaller starforming regions are formed more randomly in the disk. The sharp transition between the two groups at MK = 12 mag may be associated with expulsion of gas when the first supernovae explode in the complex.

Star-forming knots and density wave in NGC 2997

Context. Many grand design spiral galaxies show strings of bright knots along their arms on near-infrared K-band images. The alignment of such knots suggests a relation to the spiral pattern and possibly to a large-scale, star-forming front associated with a density wave. Aims. Bright knots in the southern.arm of NGC 2997 were studied to determine their nature and evolutionary state. Methods. Low resolution near-infrared K- and J-band spectra of the knots were observed with ISAAC on the VLT. Results. Most of the knots show strong Hi Br γ emission with some also having He I and H 2 emission. A few knots show indications of 12 CO absorption. This suggests that the knots are very young stellar clusters with masses up to 5 X 10 4 M ⊙ . Conclusions. The knots azimuthal distance from the K-band spiral correlates well with their Bry strength, indicating that they are located inside the co-rotation of the density wave, which triggered them through a large-scale, star-forming front. These relative azimuthal distances suggest an age spread of more than 1.6 Myr, which is incompatible with standard models for an instantaneous star burst. This indicates a more complex star-formation history, such as several bursts or continuous formation.

VLT Data Flow System: from concepts to operations

In order to realize the optimal scientific return from the VLT, ESO has undertaken to develop an end-to-end data flow system from proposal entry to science archive. The VLT Data Flow System (DFS) is being designed and implemented by the ESO Data Management and Operations Division in collaboration with VLT and Instrumentation Divisions. Tests of the DFS started in October 1996 on ESOs New Technology Telescope. Since then, prototypes of the Phase 2 Proposal Entry System, VLT Control System Interface, Data Pipelines, On-line Data Archive, Data Quality Control and Science Archive System have been tested. Several major DFS components have been run under operational conditions since February 1997. This paper describes the current status of the VLT DFS, the technological and operational challenges of such a system and the planing for VLT operations beginning in early 1999.

First detection of the field star overdensity in the Perseus arm

Aims. The main goal of this study is to detect the stellar overdensity associated with the Perseus arm in the anticenter direction. Methods. We used the physical parameters derived from Stromgren photometric data to compute the surface density distribution as a function of galactocentric distance for different samples of intermediate young stars. The radial distribution of the interstellar absorption has also been derived. Results. We detected the Perseus arm stellar overdensity at 1.6 ± 0.2 kpc from the Sun with a significance of 4–5σ and a surface density amplitude of around 10%, slightly depending on the sample used. Values for the radial scale length of the Galactic disk have been simultaneously fitted obtaining values in the range [2.9,3.5] kpc for the population of the B4–A1 stars. Moreover, the interstellar visual absorption distribution is congruent with a dust layer in front of the Perseus arm. Conclusions. This is the first time that the presence of the Perseus arm stellar overdensity has been detected through individual star counts, and its location matches a variation in the dust distribution. The offset between the dust lane and the overdensity indicates that the Perseus arm is placed inside the co-rotation radius of the Milky Way spiral pattern.

Spiral arms in near{infrared bands ? Broad- and narrow-band NIR photometry

We investigate the contribution of Br and H2 emission due to young objects in the arms of spiral galaxies observed in the K 0 lter. Out of a sample of disk galaxies for which we obtained deep surface photometry in broad- and narrow-band near{infrared lters, we selected two grand design spirals (NGC 5861, NGC 7412), which clearly have sharp knots along their arms both in optical and NIR images. For these galaxies we estimate the amount of light coming from Br and H2 emission and we conclude that it represents only a few percent of the observed K 0 light. For comparison we used the spiral galaxy NGC 4603, which has high recessional velocity. In this case the emission lines we study are practically shifted out of the narrow{band lter. Comparing its flux with what we found in the two former cases, we conclude that a major contribution from young objects in K 0 comes from continuum radiation which in the arm regions can amount to 20%.

Quality control systems for the Very Large Telescope

The operational applications needed to quantitatively assess VLT calibration and science data are provided by the VLT Quality Control system (QC). In the Data Flow observation life-cycle, QC relates data pipeline processing and observation preparation. It allows the ESO Quality Control Scientists of the Data Flow Operations group to populate and maintain the pipeline calibration database, to measure and verify the quality of observations, and to follow instrument trends. The QC system also includes models allowing users to predict instrument performance, and the Exposure Time Calculators are probably the QC applications most visible to the astronomical community. The Quality Control system is designed to cope with the large data volumes of the VLT, the geographical distribution of data handling, and the parallelism of observations executed on the different unit telescopes and instruments.

Mortality and dust expulsion in early phases of stellar clusters - Evidence from NIR photometry of nearby, spiral galaxies

Context. It is often argued that young stellar clusters suffer a significant infant mortality that is partly related to the expulsion of dust and gas in their early phases caused by radiation pressure from hot stars and supernovae. Near-infrared (J − K)− MK diagrams of young stellar clusters in nearby spiral galaxies show a bi-modal distribution that is consistent with a fast decline of their intrinsic extinction at an early epoch. Aims. The distinct features in the color-magnitude diagrams (CMD) and the fast change of colors for the youngest clusters allow us to place constraints on their early evolutionary phases, including the time scale for the decreasing extinction caused for instance by gas and dust expulsion. Methods. Monte Carlo simulations of cluster populations were performed using the power-law distribution function g(M ,τ ) ∝ M α τ γ . Integrated colors were computed from Starburst99 models. The simulated near-infrared CMD were compared with those observed for six grand-design, spiral galaxies using statistical goodness-of-fit tests. Results. The CMDs indicate a significant mortality of young, massive clusters with γ = −1.4±0.5. High initial extinction AV = 8−11 m and strong nebular emission are required to reproduce the bi-modal color distributions of the clusters. An extended star formation phase of longer than 5 Myr is suggested. The reduction of the internal extinction of the clusters starts during their active star formation and lasts for a period of 5−10 Myr.

Design of the VLT data flow model

The basic objective of modern observatories is to globally maximize their efficiency and ensure a high, constant and predictable data quality. These challenges can only be met if the scientific operation of such facilities, from the submission of observing programs to the archiving of all information, is carried out in a consistent and well controlled manner. The size, complexity and long operational lifetime of such systems make it difficult to predict and control their behavior with the necessary accuracy. Moreover they are subject to changes and are cumbersome to maintain. We present in this paper an object-oriented end-to-end operations model which describes the flow of science data associated with the operation of the VLT. The analysis model helped us to get a clear understanding of the problem domain. We were able in the design phase to partition the system into subsystems, each of them being allocated to a team for detailed design and implementation. Each of these subsystems is addressed in this paper. Prototypes will be implemented in the near future and tested on the new technology telescope (NTT). They will allow us to clarify the astronomical requirements and check the new operational concepts introduced to meet the ambitious goals of the VLT.

Stellar physical parameters from Strömgren photometry - Application to the young stars in the Galactic anticenter survey

M. Monguio, F. Figueras, and P. Grosbol, “Stellar physical parameters from Stromgren photometry. Application to the young stars in the Galactic anticenter survey”, Astronomy & Astrophysics, Vol. 568, September 2014. Thhis version of record is available online at: https://doi.org/10.1051/0004-6361/201423703 Reproduced with Permission from Astronomy and Astrophysics,

The FITS Data Format

The first proposal of a Flexible Image Transport System for astronomy was made by Wells and Greisen (1979). It described a general way to encode both a definition of the data and the data itself in a machine independent way using magnetic tape as the standard transport medium. The advantage of using a standard format for transport of astronomical images was soon realized and most major observatories implemented it as the prime format for data exchange. Subsequently, the FITS tape format (Wells et al., 1981) was recommended as the standard format for interchange of image data between all observatories by Commission 5 at the 1982 General Assembly of International Astronomical Union (IAU) in Patras (IAU Inf. Bull. 49, 1983). The first extension of the FITS format was also recommended during that meeting. This “random-groups” extension (Greisen and Harten, 1981) defined the way to transport a large number of data matrices with irregular spacing.