arXiv subjects

Johan H. Knapen

Publications and source records attributed to Johan H. Knapen.

At least 109 records · Page 6Linked to original sources

arXiv2009

The H alpha Galaxy Survey. VIII. Close companions and interactions, and the definition of starbursts

(Shortened) We consider the massive star formation properties, radial profiles, and atomic gas masses of those galaxies in our H alpha Galaxy Survey, a representative sample of the local Universe of 327 disk galaxies, that have close companion galaxies, in comparison with a matched control sample of galaxies without companions. We find that the presence of a close companion raises the star formation rate by a factor of just under two, while increasing hardly at all the equivalent width of the H alpha emission. This means that although statistically galaxies with close companions form stars at a higher rate, they do this over extended periods of time, and not as bursts. We find no significant increase in the central concentration of the star formation as a result of the presence of a close companion. The fraction of truly interacting or merging galaxies is very small in the local Universe, at around 2%, and possibly 4% of bright galaxies. Most of these interacting galaxies currently have unremarkable star formation properties. We also study the properties of the Survey galaxies with the most extreme values for star formation indicators such as rate, equivalent width, star formation rate per area, and gas depletion timescale. We find that each of these indicators favors a different subset of galaxies, and use this information to discuss critically the possible definitions of the term starburst to describe galaxies with enhanced star formation activity. We conclude that no one starburst definition can be devised which is objective and generally discriminant. Unless one restricts the use of the term "starburst" to a very small number of galaxies, the term will continue to be used for a heterogeneous and wide-ranging collection of objects with no physical basis for their classification as starburst.

astro-ph.CO↗

arXiv2007

Spiral inflow feeding the nuclear starburst in M83, observed in H-alpha emission with the GHAFAS Fabry-Perot interferometer

We present observations of the nearby barred starburst galaxy, M83 (NGC5236), with the new Fabry-Perot interferometer GHAFAS mounted on the 4.2 meter William Herschel Telescope on La Palma. The unprecedented high resolution observations, of 16 pc/FWHM, of the H-alpha-emitting gas cover the central two kpc of the galaxy. The velocity field displays the dominant disk rotation with signatures of gas inflow from kpc scales down to the nuclear regions. At the inner Inner Lindblad Resonance radius of the main bar and centerd at the dynamical center of the main galaxy disk, a nuclear $5.5 (\pm 0.9) \times 10^8 M_\odot$ rapidly rotating disk with scale length of $60 \pm 20$ pc has formed. The nuclear starburst is found in the vicinity as well as inside this nuclear disk, and our observations confirm that gas spirals in from the outer parts to feed the nuclear starburst, giving rise to several star formation events at different epochs, within the central 100 pc radius of M83.

astro-ph↗

arXiv2007

Variation of Galactic Bar Length with Amplitude and Density as Evidence for Bar Growth over a Hubble Time

K_s-band images of 20 barred galaxies show an increase in the peak amplitude of the normalized m=2 Fourier component with the R_25-normalized radius at this peak. This implies that longer bars have higher $m=2$ amplitudes. The long bars also correlate with an increased density in the central parts of the disks, as measured by the luminosity inside 0.25R_25 divided by the cube of this radius in kpc. Because denser galaxies evolve faster, these correlations suggest that bars grow in length and amplitude over a Hubble time with the fastest evolution occurring in the densest galaxies. All but three of the sample have early-type flat bars; there is no clear correlation between the correlated quantities and the Hubble type.

astro-ph↗

arXiv2007

Panel discussion I: Star formation in galaxies: how do we continue?

This is the written account of the first of two panel discussions, on "Star formation in galaxies: how do we continue?" The chair of the panel was Phil James, and panel members were John Beckman, Torsten Boeker, Daniela Calzetti, Angeles Diaz, and Rob Kennicutt. The panel and audience discussed the four following questions: 1) What are the most critically needed techniques to give accurate measurements of total rates and efficiencies of star formation? 2) Do we understand the form of the initial mass function and its variation as a function of redshift and environment? 3) Are there multiple modes of star formation in galaxies (bulge vs. disk, burst vs. continuous) or does the Schmidt law explain everything? 4) How do we bring together our understanding of star formation in our Galaxy and in external systems?

astro-ph↗

arXiv2007

Panel discussion II: Reconciling observations and modeling of star formation at high redshifts

This is the written account of the second of two panel discussions, on "Reconciling observations and modeling of star formation at high redshifts". The chair of the panel was Pavel Kroupa, and panel members were Marc Balcells, John Beckman, Christopher Conselice, and Joseph Silk. After a short introduction by each of the panelists, panel and audience entered into a lively discussion, centered around the following six themes: the mass function of pre-stellar gas clouds; a possible top-heavy initial mass function at high redshifts versus mini-quasars as the first sources of ionization; the integrated galactic initial mass function; possible differences in specific star formation rates in disks and in massive galaxies; whether merging rates yield a wrong prediction for massive galaxies, and what is the physics behind the onset of the red sequence of galaxies; and the case of dark matter-dominated dwarf galaxies versus tidal dwarf galaxies.

astro-ph↗

arXiv2007

A Connection between Star Formation in Nuclear Rings and their Host Galaxies

We present results from a photometric H-alpha survey of 22 nuclear rings, aiming to provide insight into their star formation properties, including age distribution, dynamical timescales, star formation rates, and galactic bar influence. We find a clear relationship between the position angles and ellipticities of the rings and those of their host galaxies, which indicates the rings are in the same plane as the disk and circular. We use population synthesis models to estimate ages of each H-alpha emitting HII region, which range from 1 Myr to 10 Myrs throughout the rings. We find that approximately half of the rings contain azimuthal age gradients that encompass at least 25% of the ring, although there is no apparent relationship between the presence or absence of age gradients and the morphology of the rings or their host galaxies. NGC1343, NGC1530, and NGC4321 show clear bipolar age gradients, where the youngest HII regions are located near the two contact points of the bar and ring. We speculate in these cases that the gradients are related to an increased mass inflow rate and/or an overall higher gas density in the ring, which would allow for massive star formation to occur on short timescales, after which the galactic rotation would transport the HII regions around the ring as they age. Two-thirds of the barred galaxies show correlation between the locations of the youngest HII region(s) in the ring and the location of the contact points, which is consistent with predictions from numerical modeling.

astro-ph↗

arXiv2007

Evolution of Structure in Late-type Spiral Galaxies I: Ionized Gas Kinematics in NGC 628

We study two dimensional Fabry-Perot interferometric observations of the nearby face-on late-type spiral galaxy, NGC 628, in order to analyse the ionized gas component of the interstellar medium. Covering the galaxy out to a radius larger than 12 kpc, and with a spatial sampling of 1.6 arcsec, we investigate the large-scale dynamics as well as feedback from individual HII regions into their surrounding medium. We study the role of gravitational perturbations along with that of external triggers which can disturb the kinematics and morphology of NGC 628. We verify the presence of an inner rapidly rotating disc-like component in NGC 628, which we interpret as caused by slow secular evolution of the large-scale spiral arms and oval structure. In combination with auxiliary data, we find indication for that gas is falling in from the outer parts towards the central regions, where a nuclear ring has formed at the location of the inner Lindblad resonance radius of an m=2 perturbation which could help build a pseudo-bulge in NGC 628. Moreover, we calculate radial profiles of the emission-line velocity dispersion which we use to study the role of feedback from individual HII regions. The mean velocity dispersion for the ionized gas (even when excluding pixels belonging to individual HII regions) is almost constant out to 12 kpc, although it varies from 14 to 20 km/s, with a steady decline in the outer parts. The current paper demonstrates a number of tools that we have developed for building a solid frame work for studying the evolution of structure in spiral galaxies using two dimensional kinematic observations.

astro-ph↗

arXiv2007

Structure and evolution of star-forming gas in late-type spiral galaxies

We study two dimensional Fabry-Perot interferometric observations of the nearby face-on late-type spiral galaxy, NGC 628. We investigate the role of the individual HII regions together with the large-scale gravitational mechanisms which govern star formation and overall evolution in spiral galaxies. Our kinematical analysis (reinforced by literature maps in HI and CO at lower angular resolution) enables us to verify the presence of an inner rapidly rotating inner disk-like component which we attribute to long term secular evolution of the large-scale spiral arms and oval structure. We find that gas is falling in from the outer parts towards the bluer central regions. This could be an early phase in the formation of a pseudo-bulge. We find signatures of radial motions caused by an m=2 perturbation, which are likely to be responsible for the inflow of material forming the circumnuclear ring and the rapidly rotating inner structure.

astro-ph↗

arXiv2007

Resolving the Stellar Populations in the Circumnuclear Ring of NGC 7469

We investigate the stellar populations in the star forming ring of the luminous infrared galaxy NGC 7469. We use Hubble Space Telescope multi-wavelength (UV through NIR) imaging complemented with new K-band ground-based long-slit spectroscopy, and mid-IR and radio maps. Spectral energy distributions (SEDs) and evolutionary synthesis models have been used to characterize the star formation at different scales from those of individual star clusters (tens of pc) to that of the entire star-forming ring (kpc scale). At the smallest scales two different populations of massive (1-10 x 10^6 Msun) clusters are identified. About 25% of the clusters are young (1-3 Myr) and extincted (A_V ~ 3 mag), whereas the vast majority are of intermediate age (~9 to 20 Myr) and less obscured (A_V ~ 1 mag). At larger (hundreds of pc) scale, an analysis of the integrated SED and spectroscopic data of the ring indicates the presence of two stellar populations. The young (5-6 Myr) and obscured stellar population accounts for the Br_gamma emission and most of the IR luminosity, and about one-third of the stellar mass of the ring. The much less obscured intermediate-age population has properties similar to those of the majority of the (older) 1.1um-selected star clusters. These two populations are spatially anti-correlated. The UV-optical-NIR continuum (including the majority of the clusters) of the ring traces mostly the mildly obscured intermediate-age population, while the MIR and radio peaks mark the location of the youngest and obscured star-forming regions. This study emphasizes the need for multi-wavelength, high-angular resolution observations to characterize the star formation in the dust-obscured regions commonly present in LIRGs.

astro-ph↗

arXiv2006

The Star Formation History and Evolution of the Circumnuclear Region of M100

(Abridged) Star-forming nuclear rings in barred galaxies are common in nearby spirals, and their detailed study can lead to important insights into the evolution of galaxies, their bars, and their central regions. We present integral field spectroscopic observations obtained with SAURON of the bar and circumnuclear region of the barred spiral galaxy M100, complemented by new {\it Spitzer Space Telescope} imaging of the region. We use these data to enhance our understanding of the formation, evolution, and current properties of the bar and ring. We derive the kinematics of the gas and the stars and quantify circular and non-circular motions using kinemetry. We analyse this in conjunction with the optical and infrared morphology, and our previously published dynamical modelling. By comparing line indices to simple stellar population models we estimate the ages and metallicities of the stellar populations present within the region, especially in and around the ring. Our kinematic and morphological results all confirm the picture in which the nuclear ring in M100, considered typical, is fed by gas flowing in from the disc under the action of the bar, is slowed down near a pair of resonances, and forms significant amounts of massive stars. Detailed stellar population modelling shows how the underlying bulge and disc were put in place a number of Gyr ago, and that the nuclear ring has been forming stars since about 500~Myr ago in a stable succession of bursts. This confirms that nuclear rings of this kind can form under the influence of a resonant structure set up by a bar, and proves that they are stable features of a galaxy rather than one-off starburst events.

astro-ph↗

arXiv2006

Pseudobulges in the Disk Galaxies NGC 7690 and NGC 4593

We present Ks-band surface photometry of NGC 7690 (Hubble type Sab) and NGC 4593 (SBb). We find that, in both galaxies, a major part of the "bulge" is as flat as the disk and has approximately the same color as the inner disk. In other words, the "bulges" of these galaxies have disk-like properties. We conclude that these are examples of "pseudobulges" -- that is, products of secular dynamical evolution. Nonaxisymmetries such as bars and oval disks transport disk gas toward the center. There, star formation builds dense stellar components that look like -- and often are mistaken for -- merger-built bulges but that were constructed slowly out of disk material. These pseudobulges can most easily be recognized when, as in the present galaxies, they retain disk-like properties. NGC 7690 and NGC 4593 therefore contribute to the growing evidence that secular processes help to shape galaxies. NGC 4593 contains a nuclear ring of dust that is morphologically similar to nuclear rings of star formation that are seen in many barred and oval galaxies. The nuclear dust ring is connected to nearly radial dust lanes in the galaxy's bar. Such dust lanes are a signature of gas inflow. We suggest that gas is currently accumulating in the dust ring and hypothesize that the gas ring will starburst in the future. The observations of NGC 4593 therefore suggest that major starburst events that contribute to pseudobulge growth can be episodic.

astro-ph↗

arXiv2005

Cool Gas and Massive Stars - the Nuclear Ring in M100

The SAURON integral field spectrograph was used to observe the central area of the barred spiral galaxy M100 (NGC 4321). M100 contains a nuclear ring of star formation, fueled by gas channeled inward by the galaxy's bar. We present maps of emission line strengths, absorption line strength indices, and the gas velocity dispersion across the field. The H beta emission is strongest in the ring, along two curved bar dustlanes and at the ends of the bar. The Mg b absorption line strength shows a younger population of stars within the ring as compared to the surrounding area. The gas velocity dispersion is notably smaller than elsewhere in the field both in the ring and along the leading edge of the dustlanes. Low gas dispersion is correlated spatially with the H beta emission. We thus see stars being formed from cold (low dispersion) gas which is being channeled inward along the dustlanes under the influence of a large bar, and accumulated into a ring near the location of the inner Lindblad resonances. This lends further strong support to the interpretation of nuclear rings in barred galaxies as resonance phenomena.

astro-ph↗

arXiv2005

Fueling and morphology of central starbursts

Of the mechanisms proposed to bring gaseous fuel into the central starburst regions of a galaxy, non-axisymmetries in the gravitational potential set up by interactions or by bars are among the most promising. Nevertheless, direct observational evidence for a connection between interactions and bars on the one hand, and starburst (as well as AGN) activity on the other, remains patchy. These general issues are reviewed before proceeding to discuss, within this context, massive star formation in the circumnuclear regions of spiral galaxies, in particular star-forming nuclear rings and pseudo-rings. Such rings are common, and are closely linked to the dynamics of their host galaxy, which in almost all cases is barred. The possible existence of a population of nuclear rings on scales which are too small for detection with standard ground- or even space-based techniques will be discussed.

astro-ph↗

arXiv2004

Kinematics in the starbursting circumnuclear region of M100

We have obtained integral-field spectroscopic data, using the SAURON instrument, of the bar and starbursting circumnuclear region in the barred spiral galaxy M100. From our data we have derived kinematic maps of the mean velocity and velocity dispersion of the stars and the gas, which we present here. We have also produced maps of the total, [OIII], and Hbeta intensity. The gas velocity field shows significant kinematic signatures of gas streaming along the inner part of the bar, and of density wave streaming motions across the miniature spiral arms in the nuclear pseudo-ring. The stellar velocity field shows similar non-circular motions. The gas velocity dispersion is notably smaller where the star formation occurs in the nuclear zone and HII regions.

astro-ph↗

arXiv2004

Fuelling starbursts and AGN

There is considerable evidence that the circumnuclear regions of galaxies are intimately related to their host galaxies, most directly through their bars. There is also convincing evidence for relations between the properties of supermassive black holes in the nuclei of galaxies and those of their host galaxy. It is much less clear, however, how stellar (starburst) and non-stellar (AGN) activity in the nuclear regions can be initiated and fuelled. I review gas transport from the disk to the nuclear and circumnuclear regions of galaxies, as well as the statistical relationships between the occurrence of nuclear activity and mechanisms which can cause central gas concentration, in particular bars and interactions. There are strong indications from theory and modelling for bar-induced central gas concentration, accompanied by limited observational evidence. Bars are related to activity, but this is only a weak statistical effect in the case of Seyferts, whereas the relation is limited to specific cases in starbursts. There is no observational evidence for a statistical connection between interactions and activity in Seyferts, and some evidence for this in starbursts, but probably limited to the extremes, e.g., ULIRGs. Some interesting hints at relations between rings, including nuclear rings, and the presence of nuclear activity are emerging. It is likely that the connection between the inflow of gaseous fuel from the disk of a galaxy on the one hand and the activity in its nuclear region on the other is not as straightforward as sometimes suggested, because the spatial- or time-scales concerned may be significantly different.

astro-ph↗

arXiv2003

From gas to starburst and AGN

Although it is clear that the circumnuclear regions of galaxies are intimately related to their host galaxies, most directly through their bars, it remains unclear what exactly initiates and fuels nuclear stellar (starburst) and non-stellar (AGN) activity. Deviations from axisymmetry in the gravitational potential of a galaxy, set up by a bar or an interaction, are known to cause gas inflow, and must at some scale and level be related to the fueling of AGN and starbursts. We review the observed relations between bars and interactions on the one, and nuclear activity of the Seyfert and starburst variety on the other hand, and conclude that none of these relations is particularly significant in a statistical sense, except in extreme and rare cases, such as ultra-luminous infrared galaxies. Nuclear rings of star formation, however, are not only related directly to non-axisymmetries in the potential, but also, it seems, to the occurrence of nuclear activity. Their role as potential tracers of the fueling process must be further explored.

astro-ph↗

arXiv2002

Discovery and Implications of a New Large-Scale Stellar Bar in NGC 5248

For decades, the grand-design SAB spiral galaxy NGC 5248 has been postulated to host a short bar of semi-major axis 22" (1.6 kpc). From dynamical and morphological arguments, we argue, however, that its spiral structure is being driven by a large-scale bar whose corotation radius lies at around 115" (8.6 kpc).Our estimate is based partially on a deep R-band image which reveals that the feature previously thought to be an inclined disk is in fact an extended stellar bar. The bar is embedded within a fainter outer disk visible out to a radius of 230" (17.2 kpc). The bar has a deprojected ellipticity of 0.44 and a semi-major axis of 95" (7.1 kpc). The classical grand-design spirals of NGC 5248, prominent in B, R, and K light, lie on the leading edge of the large-scale stellar bar and are accompanied by concave dust lanes out to at least 70". The offset between the dust and young stars is consistent with our understanding of gas flows in barred galaxies, where shocks along the leading edges of a moderately strong bar compress the gas to form massive young stars. While in many strongly barred galaxies, optical spiral arms are prominent outside the bar but not within it, NGC 5248 illustrates how intense star formation along a moderately strong bar can lead to conspicuous open spiral arms within the bar itself. NGC 5248 also provides a clear example of how a large-scale stellar bar embedded within a faint outer optical disk can be misidentified as an inclined disk when imaging studies lack the sensitivity to detect the actual outer disk. We discuss the implications for the estimated bar fraction at higher redshifts.

astro-ph↗

arXiv2002

Gas Dynamics in NGC 5248: Fueling a Circumnuclear Starburst Ring of Super Star Clusters

Through observations and modeling, we demonstrate how the recently discovered large-scale bar in NGC 5248 generates spiral structure which extends from 10 kpc down to 100 pc, fuels star formation on progressively smaller scales, and drives disk evolution. Deep inside the bar, two massive molecular spirals cover nearly 180 degrees in azimuth, show streaming motions of 20--40 km/s and feed a starburst ring of super star clusters at 375 pc. They also connect to two narrow K-band spirals which delineate the UV-bright star clusters in the ring. The data suggest that the K-band spirals are young, and the starburst has been triggered by a bar-driven spiral density wave (SDW). The latter may even have propagated closer to the center where a second Halpha ring and a dust spiral are found. The molecular and HST data support a scenario where stellar winds and supernovae efficiently clear out gas from dense star-forming regions on timescales less than a few Myrs. We have investigated the properties of massive CO spirals within the framework of bar-driven SDWs, incorporating the effect of gas self-gravity. We find good agreement between the model predictions and the observed morphology, kinematics, and pitch angle of the spirals. This combination of observations and modeling provides the best evidence to date for a strong dynamical coupling between the nuclear region and the surrounding disk. It also confirms that a low central mass concentration, which may be common in late-type galaxies, is particularly favorable to the propagation of a bar-driven gaseous SDW deep into the central region of the galaxy, whereas a large central mass concentration favors other processes, such as the formation and decoupling of nuclear bars.

astro-ph↗