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J. E. Beckman

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

At least 55 records · Page 3Linked to original sources
arXiv2003

Ionized gas kinematics and massive star formation in NGC 1530

We present emission line mapping of the strongly barred galaxy NGC 1530 obtained from Fabry-Perot interferometry in Halpha, at significantly enhanced angular resolution compared with published studies. The main point of the work is to examine in detail the non-circular components of the velocity field of the gas, presumably induced by the strongly non-axisymmetric gravitational potential of the bar. These reveal quasi-elliptical motions with speeds of order 100 km/s aligned with the bar. It also shows how these flows swing in towards and around the nucleus as they cross a circumnuclear resonance, from the dominant "x1 orbits" outside the resonance to "x2 orbits" within it. Comparing cross-sections of the residual velocity map along and across the bar with the Halpha intensity map indicates a systematic offset between regions of high non-circular velocity and massive star formation. To investigate further we produce velocity gradient maps along and across the bar. These illustrate very nicely the shear compression of the gas, revealed by the location of the dust lanes along loci of maximum velocity gradient perpendicular to the bar. They also show clearly how shear acts to inhibit massive star formation, whereas shocks act to enhance it. Although the inhibiting effect of gas shear flow on star formation has long been predicted, this is the clearest observational illustration so far of the effect. It is also the clearest evidence that dust picks out shock-induced inflow along bars. These observations should be of considerable interest to those modelling massive star formation in general.

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arXiv2002

The origin of the ionization of the diffuse interstellar medium in spiral galaxies. II. Modelling the distribution of ionizing radiation in NGC 157

In this paper we make a quantitative study of the hypothesis that the diffuse H-alpha emitted from the discs of spiral galaxies owes its origin to the ionizing photons escaping from HII regions. We use the H-alpha measurements of the complete set of HII regions in the spiral NGC 157, for which an HI density map was available, to derive a family of models which predict the ionizing photon distribution in the disc of this galaxy. The predicted diffuse H-alpha surface brightness distributions from our models were compared with the observed distributions showing that, in general terms, the hypothesis of density bounding for the HII regions allows us to predict well the spatial distribution of the diffuse ionized gas. In the model yielding the best fit to the data, the regions of lower luminosity lose a constant fraction of their ionizing flux to their surroundings, while for HII region luminosities above a specific transition value the ionizing escape fraction is a rising function of the Ha luminosity.

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arXiv2002

Generation of galactic disc warps due to intergalactic accretion flows onto the disc

A new method is developed to calculate the amplitude of the galactic warps generated by a torque due to external forces. This takes into account that the warp is produced as a reorientation of the different rings which constitute the disc in order to compensate the differential precession generated by the external force, yielding a uniform asymptotic precession for all rings. Application of this method to gravitational tidal forces in the Milky Way due to the Magellanic Clouds leads to a very low amplitude of the warp. If the force were due to an extragalactic magnetic field, its intensity would have to be very high, to generate the observed warps. An alternative hypothesis is explored: the accretion of the intergalactic medium over the disk. A cup-shaped distortion is expected, due to the transmission of the linear momentum; but, this effect is small and the predominant effect turns out to be the transmission of angular momentum, i.e. a torque giving an integral-sign shape warp. The torque produced by a flow of velocity ~100 km/s and baryon density \~10^{-25} kg/m^3 is enough to generate the observed warps and this mechanism offers quite a plausible explanation. First, because this order of accretion rate is inferred from other processes observed in the Galaxy, notably its chemical evolution. The inferred rate of infall of matter, ~1 solar-mass/yr, to the Galactic disc that this theory predicts agrees with the quantitative predictions of this chemical evolution resolving key issues, notably the G-dwarf problem. Second, because the required density of the intergalactic medium is within the range of values compatible with observation. By this mechanism, we can explain the warp phenomenon in terms of intergalactic accretion flows onto the disk of the galaxy.

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arXiv2001

Photoionization Models of NGC 346

We present spherically symmetric and plane parallel photoionization models of NGC 346, an HII region in the Small Magellanic Cloud. The models are based on CLOUDY and on the observations of Peimbert, Peimbert, & Ruiz (2000). We find that approximately 45% of the H ionization photons escape from the HII region providing an important ionizing source for the low density interstellar medium of the SMC. The predicted I(4363)/I(5007) value is smaller than that observed, probably implying that there is an additional source of energy not taken into account by the models. From the ionization structure of the best model and the observed line intensities we determine the abundances of N, Ne, S, Ar, and Fe relative to O.

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arXiv2001

Kinematic properties of gas and stars in 20 disc galaxies

Ionized gas and stellar kinematical parameters have been measured along the major axis of 20 nearby disc galaxies. We discuss the properties of each sample galaxy distinguishing between those characterized by regular or peculiar kinematics. In early-type disc galaxies ionized gas tends to rotate faster than stars and to have a lower velocity dispersion, whereas in late-type spirals gas and stars show almost the same rotation velocities and velocity dispersions. Incorporating the early-type disc galaxies studied by Bertola et al. (1995), Fisher (1997) and Corsini et al. (1999), we have compiled a sample of some 40 galaxies for which the major-axis radial profiles of both the stellar and gaseous components have been measured. The value of the stellar velocity dispersion measured at Re/4 turns out to be strongly correlated with the galaxy morphological type, while the gaseous velocity dispersion is not and sometimes takes values above the range expected from thermal motions or small-scale turbulence.

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arXiv2000

Modelling gaseous and stellar kinematics in the disc galaxies NGC 772, NGC 3898, and NGC 7782

We present V-band surface photometry and major-axis kinematics of stars and ionized gas of three early-type spiral galaxies, namely NGC 772, NGC 3898 and NGC 7782. For each galaxy we present a self-consistent Jeans model for the stellar kinematics, adopting the light distribution of bulge and disc derived by means of a two-dimensional parametric photometric decomposition. This allowed us to investigate the presence of non-circular gas motions, and derive the mass distribution of luminous and dark matter in these objects. NGC 772 and NGC 7782 have apparently normal kinematics with the ionized gas tracing the gravitational equilibrium circular speed. This is not true in the innermost region (r < 8'') of NGC 3898 where the ionized gas is rotating more slowly than the circular velocity predicted by dynamical modelling. This phenomenon is common in the bulge-dominated galaxies for which dynamical modelling enables us to make the direct comparison between the gas velocity and the circular speed, and it poses questions about the reliability of galaxy mass distributions derived by the direct decomposition of the observed ionized-gas rotation curve into the contributions of luminous and dark matter.

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arXiv2000

NGC 4340: Double Bar + Fossil Nuclear Ring

NGC 4340 is a double-barred SB0 galaxy in the Virgo cluster (Wozniak et al. 1995). Here, we present evidence that this galaxy also posseses a luminous stellar nuclear ring of relatively old stars with little or no gas. The ring lies just outside the inner bar, at the probable inner inner Lindblad resonance (IILR) of the outer bar. Careful inspection of the isophotes and unsharp masks shows that the two bars are slightly misaligned, which suggests they may be independently rotating.

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arXiv2000

Modeling gas and stellar kinematics in disc galaxies

We present V-band surface photometry and major-axis kinematics of stars and ionized gas of three early-type spiral galaxies, namely NGC 772, NGC 3898 and NGC 7782. For each galaxy we built a self-consistent Jeans model for the stellar kinematics, adopting the light distribution of bulge and disc derived by means of a two-dimensional parametric photometric decomposition. This allowed us to investigate the presence of non-circular gas motions, and derive the mass distribution of luminous and dark matter in these objects. We found that the observed gas rotation corresponds to the circular velocity except for the innermost region (|r|<8") of NGC 3898. This behaviour is quite common, although not ubiquitous, in the few bulge-dominated galaxies, for which dynamical modeling allows the comparison between the gas velocity and the circular speed.

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arXiv2000

Production of Lithium in the Galactic Disk

We examine the evolution of Li in the Galactic disk,basing our model on the compilation of Li v.Fe observations in stars of iron abundance ranging between halo values,which give a baseline for the disk Li abundance,and 0.2 dex above solar.The upper envelope of these observations is taken to represent the locus of minimum stellar Li depletion,and thus to follow the Galactic IS Li abundance.The most striking feature of this envelope is its steep rise between Fe/H ~ -0.4 and -0.2,corresponding to a relatively late epoch.The Li abundance increases by an order of magnitude,from its halo value of logLi ~2.2 to the "current" value of ~3.2 within this narrow range of Fe abundances.It is well known that spallation reactions between CNO and 4He in the ISM produce Li,but models give a nearly tenfold shortfall for this process.The alpha+alpha fusion reaction appeared to yield too much early Li or too little current Li.These failures led to the exploration of stellar sources:supernovae,novae,and the He flash phase in AGB stars.The Li-Fe envelope is a strong constraint on any process.Our models can account for this in the context of disk chemical evolution with a near constant or growing rate of low metallicity gas infall,and a purely interstellar Li source: alpha-alpha fusion.The Li production rate is found to be proportional to the gas expulsion rate from intermediate and low mass stars.Low-energy alphas,emitted by these stars,but accelerated in more energetic processes associated with SNe,or the bow-shocks of stellar winds,can yield the observed abundance and its variation with Fe or O.Our model is consistent with the cosmic ray spectrum in the few MeV range,corrected for solar modulation and with new 7Li/6Li ratios found by Knauth et al.(2000) in the local ISM.

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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

High-velocity clouds as dark matter in the Local Group

The High-Velocity Clouds (HVCs) observed in the Galactic neighbourhood, have been proposed to be remnants of the formation of the galaxies in the Local Group, having distances, and thus masses, predominantly of dark matter, considerably larger than hitherto hypothesized. This hypothesis is plausibly supported by observational evidence that their kinematical centre is the Local Group barycentre. Evolutionary models to account for the evolution of the light elements in the Galaxy demand infall of metal poor gas to the plane, which could well be supplied by these HVCs. Modelling the time dependence of this infall, taking into account that an accreting galaxy shows an increasing cross-section to the infalling clouds, and produces increasing mean infall velocities, we deduce that the HVCs must currently represent at least around one half of the total mass of the Local Group, given that the accretion rate, as inferred from chemical evolution, has not decreased significantly during the disc lifetime. This fraction is consistent with dynamical estimates of the relative masses of the Local Group as a whole and its constituent galaxies. The HVCs may thus form a significant constituent of baryonic, and of non-baryonic, dark matter.

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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

Correlation between kinematics and Hubble Sequence in disk galaxies?

We present a comparison between ionized gas and stellar kinematics for a sample of 5 early-to-intermediate disc galaxies. We measured the major axis V and sigma radial profiles for both gas and stars, and the h3 and h4 radial profiles of the stars. We also derived from the R-band surface photometry of each galaxy the light contribution of their bulges and discs. In order to investigate the differences between the velocity fields of the sample galaxies we adopted the self-consistent dynamical model by Pignatelli & Galletta (1999), which takes into account the asymmetric drift effects, the projection effects along the line-of-sight and the non-Gaussian shape of the line profiles due to the presence of different components with distinct dynamical behavior. We find for the stellar component a sizeable asymmetric drift effect in the inner regions of all the sample galaxies, as it results by comparing their stellar rotation curves with the circular velocity predicted by the models. The galaxy sample is not wide enough to draw general conclusions. However, we have found a possible correlation between the presence of slowly-rising gas rotation curves and the ratio of the bulge/disc half luminosity radii, while there is no obvious correlation with the key parameter represented by the morphological classification, namely the bulge/disc luminosity ratio. Systems with a diffuse dynamically hot component (bulge or lens) with a scale length comparable to that of the disc are characterized by slowly-rising gas rotation curves. On the other hand, in systems with a small bulge the gas follows almost circular motions, regardless of the luminosity of the bulge itself.

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arXiv1999

A major star formation region in the receding tip of the stellar Galactic bar. II. Supplementary information and evidence that the bar is not the same structure as the triaxial bulge previouly reported

This paper is the second part of Garzon et al. (1997: ApJ 491, L31) in which we presented an outline of the analysis of 60 spectra from a follow-up program to the Two Micron Galactic Survey (TMGS) project in the l=27 deg., b=0 deg. area. In this second part, we present a more detailed explanation of the analysis as well a library of the spectra for more complete information for each of the 60 stars, and further discussions on the implications for the structure of the Galaxy. This region contains a prominent excess in the flux distribution and star counts previously observed in several spectral ranges, notably in the TMGS. More than 50% of the spectra of the stars detected with m_K<5.0 mag, within a very high confidence level, correspond to stars of luminosity class I, and a significant proportion of the remainder are very late giants which must also be rapidly evolving. We make the case, using all the available evidence, that we are observing a region at the nearer end of the Galactic bar, where the Scutum spiral arm breaks away, and that this is powerful evidence for the presence of the bar. Alternative explanations do not give nearly such a satisfactory account of the observations. The space localization of one and, a fortiori, of both ends of the bar allows us to infer a position angle for the bar of around 75 deg. with respect to the Sun-Galactic centre line. The angle is different from that given by other authors for the bar and this, we think, is because they refer to the triaxial bulge and not to the bar as detected here.

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arXiv1998

Kinematics of Gas and Stars in Early-type Disk Galaxies

We present a comparison between ionized gas and stellar kinematics for a sample of 5 early-to-intermediate disk galaxies. We measured the major axis V and velocity dispersion radial profiles for both gas and stars, and the h_3 and h_4 radial profiles of the stars. We also derived from the R-band surface photometry of each galaxy the light contribution of their bulges and disks. In order to investigate the differences in the velocity fields of our sample galaxies we adopted the self-consistent dynamical model by Pignatelli & Galletta (1998), which takes into account the asymmetric drift effects, the projection effects along the line-of-sight and the non-Gaussian shape of the line profiles due to the presence of different components with distinct dynamical behavior.

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arXiv1997

A major star formation region in the receding tip of the stellar Galactic bar

We present an analysis of the optical spectroscopy of 58 stars in the Galactic plane at $l=27$\arcdeg, where a prominent excess in the flux distribution and star counts have been observed in several spectral regions, in particular in the Two Micron Galactic Survey (TMGS) catalog. The sources were selected from the TMGS, to have a $K$ magnitude brighter than +5 mag and be within 2 degrees of the Galactic plane. More than 60% of the spectra correspond to stars of luminosity class I, and a significant proportion of the remainder are very late giants which would also be fast evolving. This very high concentration of young sources points to the existence of a major star formation region in the Galactic plane, located just inside the assumed origin of the Scutum spiral arm. Such regions can form due to the concentrations of shocked gas where a galactic bar meets a spiral arm, as is observed at the ends of the bars of face-on external galaxies. Thus, the presence of a massive star formation region is very strong supporting evidence for the presence of a bar in our Galaxy.

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arXiv1997

A new chemo-evolutionary population synthesis model for early-type galaxies. II: Observations and Results

We present here the results of applying a new chemo-evolutionary stellar population model developed by ourselves in a previous paper (Vazdekis et al. 1996) to new high quality observational data of the nuclear regions of two representative elliptical galaxies and the bulge of the Sombrero galaxy. Here we fit in detail about 20 absorption lines and 6 optical and near-infrared colors following two approaches: fitting a single-age single-metallicity model and fitting our full chemical evolutionary model. We find that all of the iron lines are weaker than the best fitting models predict, indicating that the iron-abundance is anomalous and deficient. We also find that the Ca_I index at 4227 A is much lower than predicted by the models. We can obtain good fits for all the other lines and observed colors with models of old and metal-rich stellar populations, and can show that the observed radial gradients are due to metallicity decreasing outward. We find that good fits are obtained both with fully evolutionary models and with single-age single-metallicity models. This is due to the fact that in the evolutionary model more than 80% of stars form with in 1.5 Gyr after the formation of the galaxies. The fact that slightly better fits are obtained with evolutionary models indicates these galaxies contain a small spread in metallicity.

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