arXiv subjects

J. E. Beckman

Publications and source records attributed to J. E. Beckman.

64 records · Page 4Linked to original sources

arXiv1996

Global morphology and physical relations between the stars, gas and dust in the disc and arms of M100

We study star formation processes in the disc of the weakly barred grand design spiral galaxy M100 (NGC 4321) from a variety of images tracing recent massive star formation, old and young stars, dust, and neutral hydrogen. Differences between arm and interarm regions are specifically studied by decomposing the images into arm and non-arm zones. We find from a comparison of the morphology in H$α$, HI and dust that while the first two are coincident over most of the disc, they are offset from the dust lanes especially along the inner parts of the spiral arms: a picture which is indicative of a density wave shock moving through the arms. HI is formed near the young massive stars as a result of photo-dissociation. From radial profiles we find that in the region of the star-forming spiral arms the exponential scale lengths for H$α$, blue and near-infrared light, and 21 cm radio continuum are equal within the fitting errors. The scale lengths for the interarm region are also equal for all these tracers, but the arm scale lengths are significantly longer. This points to a common origin of the profiles in star formation, with little or no influence from radial population gradients or dust in the disc of this galaxy. The longer arm scale lengths are equivalent to an outwardly increasing arm-interarm contrast. We argue that the radial profiles of radio continuum and HI, as well as CO, are also directly regulated by star formation, and discuss the possible implications of this result for the interpretation of observed CO intensities in and outside spiral arms. We discuss the radial atomic hydrogen profile in some detail. Its almost perfectly flat shape in the region of the star-forming spiral arms may be explained by photodissociation and recombination processes in the presence of a limited

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arXiv1996

A new chemo-evolutionary population synthesis model for early-type galaxies. I: Theoretical basis

We have developed a new stellar population synthesis model designed to study early-type galaxies. It provides optical and near-infrared colors, and line indices for 25 absorption lines. It can synthesize single age, single metallicity stellar populations or follow the galaxy through its evolution from an initial gas cloud to the present time. The model incorporates the new isochrones of the Padova group and the latest stellar spectral libraries. The model has been extensively compared with previous ones in the literature to establish its accuracy as well as the accuracy of this kind of models in general. Using the evolutionary version of the model we find that we cannot fit the most metal-rich elliptical galaxies if we keep the IMF constant and do not allow infall of gas. We do however reproduce the results of Arimoto \& Yoshii (1986) for the evolution of the gas, and produce colors, and, for the first time with this type of models, absorption line-strengths. It is in fact possible to fit the data for the elliptical galaxies by varying the IMF with time. Our numerical model is in good broad agreement with the analytical 'simple model'. In the present paper we describe the model, and compare a few key observables with new data for three early-type {\em standard} galaxies. However the data, as well as our fits, will be discussed in much more detail in a second paper (Vazdekis {\it et al.} 1996), where some conclusions will be drawn about elliptical galaxies on the basis of this model.

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arXiv1996

Scale Lengths in Disk Surface Brightness as probes of Dust Extinction in 3 Spiral Galaxies: M 51, NGC 3631 and M 100

We have measured the radial brightness distributions in the disks of three nearby face-on spirals: M 51, NGC 3631, and NGC 4321 (M 100) in the photometric bands B through I, with the addition of the K band for M 51 only. The measurements were made by averaging azimuthally, in three modes, the two-dimensional surface brightness over the disks in photometric images of the objects in each band: (a) over each disk as a whole, (b) over the spiral arms alone, and (c) over the interarm zones alone. From these profiles scale-lengths were derived for comparison with schematic exponential disk models incorporating interstellar dust. These models include both absorption and scattering in their treatment of radiative transfer. The model fits show that the arms exhibit greater optical depth in dust than the interarm zones. The average fraction of emitted stellar light in V which is extinguished by dust within 3 scale-lengths of the center of each galaxy does not rise above 20% in any of them. We show that this conclusion is also valid for models with similar overall quantities of dust, but where this is concentrated in lanes. These can also account for the observed scale-lengths, and their variations.

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arXiv1995

NIR imaging and modeling of the core of M100

High-resolution NIR and optical images are used to constrain a dynamical model of the circumnuclear star forming (SF) region in the barred galaxy M100 (=NGC 4321). Subarcsecond resolution allowed us to distinguish important morphological details which are easily misinterpreted when using images at lower resolution. Small leading arms observed in our K-band image of the nuclear region are reproduced in the gas flow in our model, and lead us to believe that part of the K light comes from young stars, which trace the gas flow.

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arXiv1995

Molecular gas observations and enhanced massive star formation efficiencies in M100

We present new J=1-0 12-CO observations along the northern spiral arm of the grand-design spiral galaxy M100 (NGC 4321), and study the distribution of molecular hydrogen as derived from these observations, comparing the new data with a set of data points on the southern arm published previously. We compare these measurements on both spiral arms and on the interarm regions with observations of the atomic and ionized hydrogen components. We determine massive star formation efficiency parameters, defined as the ratio of H alpha luminosity to total gas mass, along the arms and compare the values to those in the interarm regions adjacent to the arms. We find that these parameters in the spiral arms are on average a factor of 3 higher than outside the arms, a clear indication of triggering of the star formation in the spiral arms. We discuss possible mechanisms for this triggering, and conclude that a density wave system is probably responsible for it. We discuss several possible systematical effects in some detail, and infer that the conclusions on triggering are sound. We specifically discuss the possible effects of extinction in H alpha, or a non-standard CO to H_2 conversion factor (X), and find that our conclusions on the enhancement of the efficiencies in the arms are reinforced rather than weakened by these considerations. A simple star forming scheme involving threshold densities for gravitational collapse is discussed for NGC 4321, and comparison is made with M51. We find that the gas between the arms is generally stable against gravitational collapse whereas the gas in the arms is not, possibly leading to the observed arm-interarm differences in efficiency, but also note that these results, unlike the others obtained in this paper, do depend critically on the assumed value for the conversion factor.

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arXiv1995

Statistics and properties of HII regions in a sample of grand design galaxies I. Luminosity functions

We present new high quality continuum-subtracted H alpha images of the grand-design galaxies NGC 157, NGC 3631, NGC 6764 and NGC 6951. We have determined the positions, angular sizes, and fluxes of their individual HII regions, and describe statistical properties of the HII region samples. We construct luminosity functions for all the HII regions in the disc and separately for arm and interarm zones for each galaxy. The slopes of the luminosity functions for the complete sample agree well with values published for other spiral galaxies of comparable morphological type. For three galaxies we determined the slopes of the luminosity functions for the spiral arm and interarm zones separately. We find that for NGC 157, NGC 3631, and NGC 6951 these slopes are equal within the errors of determination. We compare our results to those found from earlier work, specifically for M51 and NGC 6814, and discuss implications for massive star forming processes.

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arXiv1995

The Kinematics of the Ionized and Molecular Hydrogen in the Starburst Galaxy NGC 253

Near-infrared H_2 1-0 S(1) and Br_gamma velocity curves along the major axis of NGC 253 have revealed a central velocity gradient that is seven times steeper than that shown by the optical velocity curve. This is interpreted as an optical depth effect due to dust. Approximately 35 mag of visual extinction in the center is required to match the SW side of the optical velocity curve. The spatial variation of the ratio of these lines to the CO (J=1-0) line is compared among starburst galaxies NGC 253, M82, and NGC 4945 to investigate the excitation mechanism responsible for the H_2 1-0 S(1) line.

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arXiv1995

The extinction by dust in the outer parts of spiral galaxies

To investigate the distribution of dust in Sb and Sc galaxies we have analyzed near-infrared and optical surface photometry for an unbiased sample of 37 galaxies. Since light in the $K$-band is very little affected by extinction by dust, the $B-K$ colour is a good indicator of the amount of extinction, and using the colour-inclination relation we can statistically determine the extinction for an average Sb/Sc galaxy. We find in general a considerable amount of extinction in spiral galaxies in the central regions, all the way out to their effective radii. In the outer parts, at D$_{K,21}$, or at 3 times the typical exponential scale lengths of the stellar distribution , we find a maximum optical depth of 0.5 in $B$ for a face-on galaxy. If we impose the condition that the dust is distributed in the same way as the stars, this upper limit would go down to 0.1.

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arXiv1995

The Central Region in M100: Observations and Modeling

We present new high-resolution observations of the center of the late-type spiral M100 (NGC 4321) supplemented by 3D numerical modeling of stellar and gas dynamics, including star formation (SF). NIR imaging has revealed a stellar bar, previously inferred from optical and 21 cm observations, and an ovally-shaped ring-like structure in the plane of the disk. The K isophotes become progressively elongated and skewed to the position angle of the bar (outside and inside the `ring') forming an inner bar-like region. The galaxy exhibits a circumnuclear starburst in the inner part of the K `ring'. Two maxima of the K emission have been observed to lie symmetrically with respect to the nucleus and equidistant from it slightly leading the stellar bar. We interpret the twists in the K isophotes as being indicative of the presence of a double inner Lindblad resonance (ILR) and test this hypothesis by modeling the gas flow in a self-consistent gas + stars disk embedded in a halo, with an overall NGC4321-like mass distribution. We have reproduced the basic morphology of the region (the bar, the large scale trailing shocks, two symmetric K peaks corresponding to gas compression maxima which lie at the caustic formed by the interaction of a pair of trailing and leading shocks in the vicinity of the inner ILR, both peaks being sites of SF, and two additional zones of SF corresponding to the gas compression maxima, referred usually as `twin peaks').

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arXiv1995

THE STRIKING NEAR-INFRARED MORPHOLOGY OF THE INNER REGION IN M100

New optical and NIR K-band images of the inner 3 kpc region of the nearby Virgo spiral M100 (NGC 4321) display remarkable morphological changes with wavelength. While in the optical the light is dominated by a circumnuclear zone of enhanced star formation, the features in the 2.2\mum image correspond to a newly discovered kpc-size stellar bar, and a pair of leading arms situated inside an ovally-shaped region. Only 5% of the K flux is emitted in antisymmetric structures. This indicates that the morphology seen in the NIR is dominated by a global density wave. Making a first-order correction for the presence of localized dust extinction in K using the I-K image, we find that the observed leading arm morphology is slightly hidden by dust. Possible mechanisms responsible for the optical and NIR morphology are discussed, and tests are proposed to discriminate between them. Conclusions are supported with an evolutionary stellar population model reproducing the optical and NIR colors in a number of star forming zones. We argue that this morphology is compatible with the presence of a pair of inner Lindblad resonances in the region, and show this explicitly in an accompanying paper by detailed numerical modeling. The observed phenomena may provide insight into physical processes leading to central activity in galaxies.

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