arXiv subjects

J. H. Knapen

Publications and source records attributed to J. H. Knapen.

At least 145 records · Page 8Linked to original sources

arXiv2001

Penetrating the Mask: The Gravitational Torque of Bars

The Hubble classification scheme of galaxies is based on blue-light appearance. Atlases reveal the rich variety of responses of the Population I component (the "mask") of gas and dust to the underlying, older, stellar population. However, the Population I component may only constitute 5 percent of the dynamical mass of the galaxy; furthermore, dusty masks are highly effective in hiding bars. In the 1960s, Ken Freeman presented a meticulous study of the dynamics of bars at a time when nonbarred galaxies were called "normal" spirals and barred galaxies were regarded as curiosities. Now we know that it is more "normal" for a galaxy to be barred than to be nonbarred. What is the range for gravitational torques of bars? We describe here a recently developed method for deriving relative bar torques by using gravitational potentials inferred from near-infrared light distributions. We incorporate a bar torque class into the Block/Puerari dust-penetrated galaxy classification system. We find a huge overlap in relative bar torque between Hubble (Sa, Sb, ...) and (SBa, SBb, ...) classifications. Application of the method to the high redshift universe is briefly discussed.

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arXiv2001

The gravitational torque of bars in optically unbarred and barred galaxies

The relative bar torques for 45 galaxies observed at $K$- band with the 4.2m William Herschel Telescope are determined by transforming the light distributions into potentials and deriving the maximum ratios of the tangential forces relative to the radial forces. The results are combined with the bar torques for 30 other galaxies determined from our previous $K$-band survey (Buta & Block 2001). Relative bar torques determine the degree of spiral arm forcing, gas accretion, and bar evolution. They differ from other measures of bar strength, such as the relative amplitude of the bar determined photometrically, because they include the bulge and other disk light that contributes to the radial component of the total force. If the bulge is strong and the radial forcing large, then even a prominent bar can have a relatively weak influence on the azimuthal motions in the disk. Here we find that the relative bar torque correlates only weakly with the optical bar type listed in the Revised Shapley-Ames and de Vaucouleurs systems. In fact, some classically barred galaxies have weaker relative bar torques than classically unbarred galaxies. The optical class is a poor measure of azimuthal disk forcing for two reasons: some infrared bars are not seen optically, and some bars with strong bulges have their azimuthal forces so strongly diluted by the average radial force that they exert only small torques on their disks. The Hubble classification scheme poorly recognizes the gravitational influence of bars. Applications of our bar torque method to the high-redshift universe are briefly discussed.

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arXiv2001

Circumnuclear kinematics in NGC 5248: the origin of nuclear spiral arms

We present for the first time a two-dimensional velocity field of the central region of the grand-design spiral galaxy NGC 5248, at 0.9 arcsec spatial resolution. The H-alpha velocity field is dominated by circular rotation. While no systematic streaming motions are seen in the area of the nuclear grand-design spiral or the circumnuclear ring, the amplitude of residual velocities, after subtracting a model circular velocity field, reaches 20 km/s in projection. The rotation curve levels out at around 140 km/s, after a well-resolved and rather shallow rise. We have generated an analytical model for the nuclear spiral and fitted it to our observations to obtain estimates of the pattern speed of the spiral and the sound speed in the central region of NGC 5248. Our results are consistent with a low pattern speed, suggesting that the nuclear spiral rotates with the same rate as the main spiral structure in NGC 5248, and thus that the spiral structure is coupled from scales of a few hundred parsecs to several kiloparsecs. We have also compared the observed structure and kinematics between the nuclear regions of NGC 5248 and M100. Several similarities and differences are discussed, including the location of the peak emission regions on major and minor axes, and the spiral arm streaming motions. We find no kinematic evidence for a presence of a nuclear bar in NGC 5248.

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arXiv2000

Gas motions in the plane of the spiral galaxy NGC 3631

The velocity field of the nearly face-on galaxy NGC 3631, derived from observations in the H alpha line and the HI radio line, is analysed to study perturbations related to the spiral structure of the galaxy. We confirm our previous conclusion that the line-of-sight velocity field gives evidence of the wave nature of the observed two-armed spiral structure. Fourier analysis of the observed velocity field is used to determine the location of corotation of the spiral structure of this galaxy, and the radius of corotation R_c is found to be about 42 arcsec, or 3.2 kpc. The vector velocity field of the gas in the plane of the disc is restored, and taking into account that we previously investigated vertical motions, we now have a full 3D gaseous velocity field of the galaxy. We show clear evidence of the existence of two anticyclonic and four cyclonic vortices near corotation in a frame of reference rotating with the spiral pattern. The centres of the anticyclones lie between the observed spiral arms. The cyclones lie close to the observed spirals, but their centres are shifted from the maxima in brightness.

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arXiv2000

Near-infrared spectroscopy of the circumnuclear star formation regions in M100: Evidence for sequential triggering

We present low resolution (R~450) K-band spectroscopy for 16 of the 43 circumnuclear star-forming knots in M100 identified by Ryder & Knapen (1999). We compare our measurements of equivalent widths for the Br-gamma emission line and CO 2.29 micron absorption band in each knot with the predictions of starburst models from the literature, and derive ages and burst parameters for the knots. The majority of these knots are best explained by the result of short, localised bursts of star formation between 8 and 10 Myr ago. By examining both radial and azimuthal trends in the age distribution, we present a case for sequential triggering of star formation, most likely due to the action of a large-scale shock. In an appendix, we draw attention to the fact that the growth in the CO spectroscopic index with decreasing temperature in supergiant stars is not as regular as is commonly assumed.

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arXiv2000

Nuclear star formation in the hot-spot galaxy NGC2903

We present high-resolution near-infrared imaging obtained using adaptive optics and HST/NICMOS, and ground-based spectroscopy of the hot-spot galaxy NGC2903. Our near-infrared resolution imaging enables us to resolve the infrared hot spots into individual young stellar clusters or groups of these. The spatial distribution of the stellar clusters is not coincident with that of the bright HII regions, as revealed by the HST/NICMOS Pa_alpha image. Overall, the circumnuclear star formation in NGC2903 shows a ring-like morphology with an approximate diameter of 625pc. The SF properties of the stellar clusters and HII regions have been studied using the photometric and spectroscopic information in conjunction with evolutionary synthesis models. The population of bright stellar clusters shows a very narrow range of ages, 4-7 X 10^6 yr after the peak of star formation, or absolute ages 6.5-9.5 X 10^6yr (for the assumed short-duration Gaussian bursts), and luminosities similar to the clusters found in the Antennae interacting galaxy. This population of young stellar clusters accounts for some 7-12% of the total stellar mass in the central 625pc of NGC2903. The HII regions in the ring of star formation have luminosities close to that of the super-giant HII region 30 Doradus, they are younger than the stellar clusters, and will probably evolve into bright infrared stellar clusters similar to those observed today. We find that the star formation efficiency in the central regions of NGC2903 is higher than in normal galaxies, approaching the lower end of infrared luminous galaxies.

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arXiv2000

Circumnuclear regions in barred spiral galaxies. I. Near-infrared imaging

We present sub-arcsecond resolution ground-based near-infrared images of the central regions of a sample of twelve barred galaxies with circumnuclear star formation activity, which is organized in ring-like regions typically one kiloparsec in diameter. We also present Hubble Space Telescope near-infrared images of ten of our sample galaxies, and compare them with our ground-based data. Although our sample galaxies were selected for the presence of circumnuclear star formation activity, our broad-band near-infrared images are heterogeneous, showing a substantial amount of small-scale structure in some galaxies, and practically none in others. We argue that, where it exists, this structure is caused by young stars, which also cause the characteristic bumps or changes in slope in the radial profiles of ellipticity, major axis position angle, surface brightness and colour at the radius of the circumnuclear ring in most of our sample galaxies. In 7 out of 10 HST images, star formation in the nuclear ring is clearly visible as a large number of small emitting regions, organised into spiral arm fragments, which are accompanied by dust lanes. NIR colour index maps show much more clearly the location of dust lanes and, in certain cases, regions of star formation than single broad-band images. Circumnuclear spiral structure thus outlined appears to be common in barred spiral galaxies with circumnuclear star formation.

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arXiv2000

Populations of high-luminosity density-bounded H II regions in spiral galaxies? Evidence and implications

(Abridged) We present evidence that the HII regions of high luminosity in disk galaxies may be density bounded, so that a significant fraction of the ionizing photons emitted by their exciting OB stars escape from the regions. The key piece of evidence is the presence, in the Hαluminosity functions (LFs) of the populations of HII regions, of glitches, local sharp peaks at an apparently invariant luminosity, defined as the Stromgren luminosity (L_ Str), L_Hα= L_Str = 10^38.6 (\pm 10^0.1) erg/s (no other peaks are found in any of the LFs) accompanying a steepening of slope for L_Hα> L_Str. This behavior is readily explicable via a physical model whose basic premises are: (a) the transition at L_Hα= L_Str marks a change from essentially ionization bounding at low luminosities to density bounding at higher values, (b) for this to occur the law relating stellar mass in massive star-forming clouds to the mass of the placental cloud must be such that the ionizing photon flux produced within the cloud is a function which rises more steeply than the mass of the cloud. Supporting evidence for the hypothesis of this transition is presented. If confirmed, the density-bounding hypothesis would imply that the density-bounded regions were the main sources of the photons which ionize the diffuse gas in disk galaxies. We estimate that these regions emit sufficient Lyman continuum not only to ionize the diffuse medium, but to cause a typical spiral to emit significant ionizing flux into the intergalactic medium. The low scatter observed in L_Str, less than 0.1 mag rms in the still quite small sample measured to date, is an invitation to widen the data base, and to calibrate against primary standards, with the aim of obtaining a precise standard candle.

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arXiv1999

Kinematics of ionized and molecular hydrogen in the core of M100

We present high angular and velocity resolution two-dimensional kinematic observations in the spectral lines of H alpha and CO J=1-0 of the circumnuclear starburst region in the barred spiral galaxy M100, and compare them with kinematics derived from our previously published numerical modeling. The H alpha data, fully sampled and at sub-arcsecond resolution, show a rotation curve that is rapidly rising in the central ~140 pc, and stays roughly constant, at the main disk value, further out. Non-circular motions are studied from the H alpha and CO data by detailed consideration of the velocity fields, residual velocity fields after subtraction of the rotation curve, and sets of position-velocity diagrams. These motions are interpreted as the kinematic signatures of gas streaming along the inner part of the bar, and of density wave streaming motions across a two-armed mini-spiral. Comparison with a two-dimensional velocity field and rotation curve derived from our 1995 dynamical model shows good qualitative and quantitative agreement for the circular and non-circular kinematic components. Both morphology and kinematics of this region require the presence of a double inner Lindblad Resonance in order to explain the observed twisting of the near-infrared isophotes and the gas velocity field. These are compatible with the presence of a global density wave driven by the moderately strong stellar bar in this galaxy. We review recent observational and modeling results on the circumnuclear region in M100, and discuss the implications for bar structure and gas dynamics in the core of M100 and other disk galaxies.

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arXiv1999

A subarcsecond resolution near-infrared study of Seyfert and `normal' galaxies: I. Imaging data

We present new high-resolution near-infrared observations in the J,H and K bands, obtained to study the properties of Seyfert host galaxies. The dataset consists of images in the three bands of practically the entire CfA sample of Seyfert galaxies, and K-band images of a control sample of non-active, `normal', galaxies, matched to the Seyfert sample in the distribution of type and inclination. The spatial resolution and sampling of the new images is a factor 2 better than previously published K-band data. In this paper, we present the data in the form of profiles of surface brightness and color, ellipticity and major axis position angle, as well as greyscale maps of surface brightness in H or K and both J-H and H-K colors. We compare our surface brightness and color profiles with the literature, and find good agreement. Our data are discussed in detail in three subsequent publications, where we analyze the morphologies of Seyfert and normal hosts, quantify the strength of nonaxisymmetric features in disks and their relationship to nuclear activity, address the question of bar fraction in Seyferts and normal galaxies, and analyze the color information in the framework of emission mechanisms in Seyfert 1s and 2s, and in non-active galaxies.

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arXiv1998

A nuclear grand-design spiral within the normal disc spiral of NGC 5248

We report the discovery of trailing grand-design nuclear spiral structure in the spiral galaxy NGC 5248. Two tightly-wound red spiral arms, with widths less than 1 arcsec (~75 pc), emanate at 1 arcsec outside the quiescent nucleus of the galaxy, and can be followed for 3 arcsec, appearing to end before the radius of the circumnuclear starburst "ring", which is at 5--6 arcsec or around 400 parsec. Combining our near-infrared Canada-France-Hawaii Telescope adaptive optics results with traditional near-infrared and optical imaging, we show that spiral structure is present in this galaxy at spatial scales covering two orders of magnitude, from a hundred parsecs to 15 kpc. Comparison with a Hubble Space Telescope ultraviolet image shows how the location of the circumnuclear star formation relates to the nuclear spiral structure. We briefly discuss possible mechanisms which could be responsible for the observed nuclear grand-design spiral structure.

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arXiv1998

Statistical properties of HII regions in the disc of M100

(Abridged) From a new H alpha mosaic image of the complete disc of the spiral galaxy M100, a catalogue is composed listing 1948 individual HII regions. I give details of the data collection and reduction procedure, and of the production of the HII region catalogue. For each HII region, the catalogue gives its position relative to the centre of the galaxy, its deprojected distance to the centre, its radius, and its calibrated luminosity. I present the results of a statistical study of properties of the HII regions. The luminosity function of the complete ensemble of HII regions is well fitted by a power-law slope in the higher luminosity range, confirming results for galaxies like M100. Luminosity function slopes for arm and interarm HII regions separately are equal within the errors of the fits, indicating that whereas the density wave accumulates material into the arm regions, and may trigger star formation there, it does not in fact change the mass distribution of the star-forming clouds, nor the statistical properties of the HII region population. Diameter and radial number density distributions are discussed. The huge number of HII regions allowed the construction of a number of independent luminosity functions at different distances to the nucleus. The slope of the luminosity function shows a marginal decrease with increasing distance from the centre, which could indicate a gradual change toward shallower IMF slopes with increasing galactocentric distance, or an evolutionary effect.

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arXiv1997

Resolved structure in the nuclear region of the ultraluminous infrared galaxy Mrk 273

We have studied the core morphology of the ultraluminous infrared galaxy Mrk 273 by combining a high-resolution adaptive optics near-infrared image with an optical image from the Hubble Space Telescope and interferometric radio continuum data, all at spatial resolutions of 150 mas or better. The near-infrared image reveals that the nucleus has two main components, both of which have radio counterparts. The strongest component (N) shows very similar extended structure in the radio and near-infrared. It has a flat radio spectrum and is resolved into a double-lobed structure (Ne; Nw), with a separation of 90\pm5 mas (70 parsec). A similar structure is detected in the near-infrared. We identify this component as the location of the active nucleus. The second component (SW), strong in the near-infrared but relatively weak in the radio, is located $\sim1$ arcsecond to the southwest. We interpret this as an obscured starburst region associated with the merger. The radio continuum images show a third, strong, component (SE) which has previously been interpreted as a second nucleus. However, it shows no associated optical or near-infrared emission, suggesting that it is in fact a background source.

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arXiv1997

Evidence for the Large-Scale Dissociation of Molecular Gas in the Inner Spiral Arms of M81

We compare the detailed distributions of HI, H alpha, and 150 nm far-UV continuum emission in the spiral arms of M81 at a resolution of 9" (linear resolution 150 pc at 3.7 Mpc distance). The bright H alpha emission peaks are always associated with peaks in the far-UV emission. The converse is not always true; there are many regions of far-UV emission with little corresponding H alpha. The HI and the far-UV are always closely associated, in the sense that the HI is often brightest around the edges of the far-UV emission. The effects of extinction on the morphology are small, even in the far-UV. Extensive far-UV emission, often with little corresponding H alpha, indicates the presence of many ``B-stars'', which produce mostly non-ionizing UV photons. These far-UV photons dissociate a small fraction of an extensive layer of H_2 into HI. The observed morphology can be understood if ``chimneys'' are common in the spiral arms of M81, where holes are blown out of the galactic disk, exposing the bright HII regions and the corresponding far-UV associated with vigorous star formation. These ``naked'' star-forming regions show little obscuration. H_2 is turned into HI by UV photons impinging on the interior surfaces of these chimneys. The intensity of the far-UV radiation measured by UIT can dissociate the underlying H_2 with a typical density of ~10 H nucleii cm**-3 to produce the observed amount of HI in the spiral arms of M81. Except for thin surface layers locally heated in these photo-dissociation regions close to the far-UV sources, the bulk of the molecular gas in the inner disk of M81 is apparently too cold to produce much 12CO(1-0) emission.

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arXiv1996

Atomic hydrogen in the spiral galaxy NGC 3631

New high resolution, high sensitivity WSRT HI synthesis observations of the spiral galaxy NGC 3631 are presented. In the total atomic hydrogen map, the spiral arms are well distinguished from the interarm regions, while the sensitivity allows detection of HI in all but a few isolated regions of the areas between the spiral arms. Most of the atomic hydrogen is located within the optical disc, but the HI extends to some 1.5R_opt. The HI follows the spiral arms, and streaming motions of up to ~15 kms (projected) can be identified from the velocity field. Assuming a constant inclination angle of 17 deg, a rotation curve is derived which is slightly falling in the outer parts of the disc. Analysis of a residual velocity field, obtained after subtraction of an axisymmetric model based on the rotation curve, confirms the existence of streaming motions near the spiral arms in an otherwise undisturbed disc.

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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

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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