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

Deep JHK-Brg 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. The extended sources with Mk <-12 mag display a diffuse 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 diffuse 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 Op = 16 km/s/kpc of the main spiral was derived assuming circular motion. The alignment and color gradient of the bright, diffuse 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.Comment: 4 pages, 5 figures. Accepted for publication as A&A Lette

NGC 1300 Dynamics: I. The gravitational potential as a tool for detailed stellar dynamics

In a series of papers we study the stellar dynamics of the grand design barred-spiral galaxy NGC~1300. In the first paper of this series we estimate the gravitational potential and we give it in a form suitable to be used in dynamical studies. The estimation is done directly from near-infrared observations. Since the 3D distribution of the luminous matter is unknown, we construct three different general models for the potential corresponding to three different assumptions for the geometry of the system, representing limiting cases. A pure 2D disc, a cylindrical geometry (thick disc) and a third case, where a spherical geometry is assumed to apply for the major part of the bar. For the potential of the disc component on the galactic plane a Fourier decomposition method is used, that allows us to express it as a sum of trigonometric terms. Both even and odd components are considered, so that the estimated potential accounts also for the observed asymmetries in the morphology. For the amplitudes of the trigonometric terms a smoothed cubic interpolation scheme is used. The total potential in each model may include two additional terms (Plummer spheres) representing a central mass concentration and a dark halo component, respectively. In all examined models, the relative force perturbation points to a strongly nonlinear gravitational field, which ranges from 0.45 to 0.8 of the axisymmetric background with the pure 2D being the most nonlinear one. We present the topological distributions of the stable and unstable Lagrangian points as a function of the pattern speed $(\Omega_p)$. In all three models there is a range of $\Omega_p$ values, where we find multiple stationary points whose stability affects the overall dynamics of the system.Comment: 14 pages, 11 figures, published in MNRA

NGC 1300 Dynamics: III. Orbital analysis

We present the orbital analysis of four response models, that succeed in reproducing morphological features of NGC 1300. Two of them assume a planar (2D) geometry with $\Omega_p$=22 and 16 \ksk respectively. The two others assume a cylindrical (thick) disc and rotate with the same pattern speeds as the 2D models. These response models reproduce most successfully main morphological features of NGC 1300 among a large number of models, as became evident in a previous study. Our main result is the discovery of three new dynamical mechanisms that can support structures in a barred-spiral grand design system. These mechanisms are presented in characteristic cases, where these dynamical phenomena take place. They refer firstly to the support of a strong bar, of ansae type, almost solely by chaotic orbits, then to the support of spirals by chaotic orbits that for a certain number of pat tern revolutions follow an n:1 (n=7,8) morphology, and finally to the support of spiral arms by a combination of orbits trapped around L$_{4,5}$ and sticky chaotic orbits with the same Jacobi constant. We have encountered these dynamical phenomena in a large fraction of the cases we studied as we varied the parameters of our general models, without forcing in some way their appearance. This suggests that they could be responsible for the observed morphologies of many barred-spiral galaxies. Comparing our response models among themselves we find that the NGC 130 0 morphology is best described by a thick disc model for the bar region and a 2D disc model for the spirals, with both components rotating with the same pattern speed $\Omega_p$=16 \ksk !. In such a case, the whole structure is included inside the corotation of the system. The bar is supported mainly by regular orbits, while the spirals are supported by chaotic orbits.Comment: 18 pages, 32 figures, accepted for publication in MNRA

Properties of young star cluster systems: the age signature from near-infrared integrated colours

A recent JHKs study of several grand-design spiral galaxies shows a bimodal distribution of their system of star clusters and star forming complexes in colour-magnitude and colour-colour diagrams. In a comparison with stellar population models including gas, the (J-H) vs (H-Ks) diagram reveals that embedded clusters, still immersed in their parental clouds of gas and dust, generally have a redder (H-Ks) colour than older clusters, whose gas and dust have already been ejected. This bimodal behaviour is also evident in the colour-magnitude diagram MK vs (J-Ks), where the brightest clusters split into two sequences separating younger from older clusters. In addition, the reddening-free index Qd = (H-Ks) - 0.884 (J-H) has been shown to correlate with age for the young clusters and thus provided an effective way to differentiate the embedded clusters from the older ones. We aim to study the behaviour of these photometric indices for star cluster systems in the Local Group. We investigate the effectiveness of the Qd index in sorting out clusters of different ages at their early evolutionary stages. Surface photometry was carried out for 2MASS images of populous clusters younger than ~100Myr whose ages were available. Some clusters, particularly the embedded ones, were studied for the first time using this method. The integrated magnitudes and colours extracted from the surface photometry of the most populous clusters/complexes in the Local Group shows the expected bimodal distribution in the colour-colour and colour-magnitude diagrams. In particular, we confirm the index Qd as a powerful tool for distinguishing clusters younger than about 7Myr from older clusters. (abridged)Comment: 10 pages, 11 figures; accepted for publication in A&

NGC 1300 Dynamics: II. The response models

We study the stellar response in a spectrum of potentials describing the barred spiral galaxy NGC 1300. These potentials have been presented in a previous paper and correspond to three different assumptions as regards the geometry of the galaxy. For each potential we consider a wide range of $\Omega_p$ pattern speed values. Our goal is to discover the geometries and the $\Omega_p$ supporting specific morphological features of NGC 1300. For this purpose we use the method of response models. In order to compare the images of NGC 1300 with the density maps of our models, we define a new index which is a generalization of the Hausdorff distance. This index helps us to find out quantitatively which cases reproduce specific features of NGC 1300 in an objective way. Furthermore, we construct alternative models following a Schwarzschild type technique. By this method we vary the weights of the various energy levels, and thus the orbital contribution of each energy, in order to minimize the differences between the response density and that deduced from the surface density of the galaxy, under certain assumptions. We find that the models corresponding to $\Omega_p\approx16$\ksk and $\Omega_p\approx22$\ksk are able to reproduce efficiently certain morphological features of NGC 1300, with each one having its advantages and drawbacks.Comment: 13 pages, 10 figures, accepted for publication in MNRA

Software for producing trichromatic images in astronomy

We present a software package for combining three monochromatic images of an astronomical object into a trichromatic color image. We first discuss the meaning of "true" colors in astronomical images. We then describe the different steps of our method, choosing the relevant dynamic intensity range in each filter, inventorying the different colors, optimizing the color map, modifying the balance of colors, and enhancing contrasts at low intensity levels. While the first steps are automatic, the last two are interactive.Comment: 12 pages (including 2 figures) uuencoded compressed Postscript file. To appear in Experimental Astronom

Stellar physical parameters from Str ömgren photometry. Application to the young stars in the Galactic anticenter survey

M. Monguió, F. Figueras, and P. Grosbøl, “Stellar physical parameters from Strömgren 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, © ESO 2016.Aims. The aim is to derive accurate stellar distances and extinctions for young stars of our survey in the Galactic anticenter direction using the Str\"omgren photometric system. This will allow a detailed mapping of the stellar density and absorption toward the Perseus arm. Methods. We developed a new method for deriving physical parameters from Str\"omgren photometry and also implemented and tested it. This is a model-based method that uses the most recent available stellar atmospheric models and evolutionary tracks to interpolate in a 3D grid of the unreddened indexes [m1], [c1] and Hbeta. Distances derived from both this method and the classical pre-Hipparcos calibrations were tested against Hipparcos parallaxes and found to be accurate. Results. Systematic trends in stellar photometric distances derived from empirical calibrations were detected and quantified. Furthermore, a shift in the atmospheric grids in the range Teff=[7000,9000]K was detected and a correction is proposed. The two methods were used to compute distances and reddening for about 12000 OBA-type stars in our Str\"omgren anticenter survey. Data from the IPHAS and 2MASS catalogs were used to complement the detection of emission line stars and to break the degeneracy between early and late photometric regions. We note that photometric distances can differ by more than 20%, those derived from the empirical calibrations being smaller than those derived with the new method, which agree better with the Hipparcos data.Peer reviewe

First detection of the field star overdensity in the Perseus arm

M. Monguió, P. Grosbøl, and F. Figueras, “First detection of the field star overdensity in the Perseus arm”, Astronomy & Astrophysics, Vol. 577, May 2015. This version of record is available online at: https://doi.org/10.1051/0004-6361/201424896 Reproduced with Permission from Astronomy and Astrophysics, © ESO 2016.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 Str\"omgren 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 4-5{\sigma} 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.Peer reviewe

1.65 micron (H-band) surface photometry of galaxies. VIII: the near-IR k-space at z=0

We present the distribution of a statistical sample of nearby galaxies in the k-space (k1 ~ log M, k2 ~ log Ie, k3 ~ log M/L). Our study is based on near-IR (H-band: lambda = 1.65 micron) observations, for the first time comprising early- and late-type systems. Our data confirm that the mean effective dynamical mass-to-light ratio M/L of the E+S0+S0a galaxies increases with increasing effective dynamical mass M, as expected from the existence of the Fundamental Plane relation. Conversely, spiral and Im/BCD galaxies show a broad distribution in M/L with no detected trend of M/L with M, the former galaxies having M/L values about twice larger than the latter, on average. For all the late-type galaxies, the M/L increases with decreasing effective surface intensity Ie, consistent with the existence of the Tully--Fisher relation. These results are discussed on the basis of the assumptions behind the construction of the k-space and their limitations. Our study is complementary to a previous investigation in the optical (B-band: lambda = 0.44 micron) and allows us to study wavelength-dependences of the galaxy distribution in the k-space. As a first result, we find that the galaxy distribution in the k1--k2 plane reproduces the transition from bulge-less to bulge-dominated systems in galaxies of increasing dynamical mass. Conversely, it appears that the M/L of late-types is higher (lower) than that of early-types with the same M in the near-IR (optical). The origins of this behaviour are discussed in terms of dust attenuation and star formation history.Comment: 11 pages, 1 Postscript table, 3 Postscript figures, accepted for publication in MNRA