arXiv subjects

H. Dejonghe

Publications and source records attributed to H. Dejonghe.

At least 37 records · Page 2Linked to original sources

arXiv2004

Local kinematics of K and M giants from Coravel/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters

The availability of the Hipparcos Catalogue triggered many kinematic and dynamical studies of the solar neighbourhood. Nevertheless, those studies generally lacked the third component of the space velocities, i.e., the radial velocities. This work presents the kinematic analysis of 5952 K and 739 M giants in the solar neighbourhood which includes for the first time radial velocity data from a large survey performed with the CORAVEL spectrovelocimeter. It also uses proper motions from the Tycho-2 catalogue, which are expected to be more accurate than the Hipparcos ones. The UV-plane constructed from these data for the stars with precise parallaxes reveals a rich small-scale structure, with several clumps corresponding to the Hercules stream, the Sirius moving group, and the Hyades and Pleiades superclusters. A maximum-likelihood method, based on a bayesian approach, has been applied to the data, in order to make full use of all the available stars and to derive the kinematic properties of these subgroups. Isochrones in the Hertzsprung-Russell diagram reveal a very wide range of ages for stars belonging to these groups. These groups are most probably related to the dynamical perturbation by transient spiral waves rather than to cluster remnants. A possible explanation for the presence of young clusters in the same area of the UV-plane is that they have been put there by the spiral wave associated with their formation, while the kinematics of the older stars of our sample has also been disturbed by the same wave. The term "dynamical stream" for the kinematic groups is thus more appropriate than the traditional term "supercluster" since it involves stars of different ages, not born at the same place nor at the same time.

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arXiv2004

The PNLF distance to the Sculptor Group galaxy NGC 55

We identified 21 new Planetary Nebula (PN) candidates in the Sculptor Group galaxy NGC 55. We determined a most likely distance of 2.00 +/- 0.2 Mpc using the Planetary Nebulae Luminosity Function (PNLF) method. The distance to NGC 55 is larger than previously determined distances, which means that the Sculptor Group is a bit further away from the Local Group than previously thought. The distance to NGC 55 is again similar to the distance of NGC 300, adding support to the suggestion that these galaxies form a bound pair.

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arXiv2004

Evidence for a warm ISM in Fornax dEs - II. FCC032, FCC206 and FCCB729

We present R-band and H-alpha+[N II] narrow-band imaging of FCC032, FCC206 and FCCB729, three dwarf elliptical galaxies (dEs) in the Fornax Cluster. These dEs contain significant amounts of ionized gas. FCC032 harbours a large ionized gas complex, consisting of several individual clouds, a superbubble and a filament that extends away from the galaxy centre. The ionized gas structures observed in FCC032 bear a strong resemblance to those observed in more gas-rich and more fiercely star-forming dwarf galaxies. FCC206, a very low surface brightness dE, contains one faint extended emission region, and two compact clouds. In FCCB729, the only nucleated galaxy in this sample, one of the ionized gas clouds coincides with the stellar nucleus. We derive ionized gas masses of a few 100 to 1000 solar masses for these galaxies. This brings our sample of dEs with ionized gas with H-alpha+[N II] narrow-band imaging to five objects. The ionized gas morphologies in these galaxies range from pure nuclear emission peaks to extended emission complexes. This morphological diversity could also indicate a diversity in ionizing processes in dEs with a warm interstellar medium (active galactic nuclei, starbursts, post-AGB stars,...). Inside FCC206, four star clusters appear to be on the verge of merging to form a nucleus in this presently non-nucleated dE. Understanding the formation of nuclei in dEs could give us important clues to the formation of super-massive black holes.

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arXiv2004

Dwarf elliptical galaxies with kinematically decoupled cores

{We present, for the first time, photometric and kinematical evidence, obtained with FORS2 on the VLT, for the existence of kinematically decoupled cores (KDCs) in two dwarf elliptical galaxies; FS76 in the NGC5044 group and FS373 in the NGC3258 group. Both kinematically peculiar subcomponents rotate in the same sense as the main body of their host galaxy but betray their presence by a pronounced bump in the rotation velocity profiles at a radius of about 1". The KDC in FS76 rotates at 10+/-3km/s, with the host galaxy rotating at 15+/-6km/s; the KDC in FS373 has a rotation velocity of 6+/-2km/s while the galaxy itself rotates at 20+/-5km/s. FS373 has a very complex rotation velocity profile with the velocity changing sign at 1.5 R_e. The velocity and velocity dispersion profiles of FS76 are asymmetric at larger radii. This could be caused by a past gravitational interaction with the giant elliptical NGC5044, which is at a projected distance of 50kpc. We argue that these decoupled cores are most likely not produced by mergers in a group or cluster environment because of the prohibitively large relative velocities. A plausible alternative is offered by flyby interactions between a dwarf elliptical or its disky progenitor and a massive galaxy. The tidal forces during an interaction at the relative velocities and impact parameters typical for a group environment exert a torque on the dwarf galaxy that, according to analytical estimates, transfers enough angular momentum to its stellar envelope to explain the observed peculiar kinematics.

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arXiv2003

Modelling galactic spectra: I - A dynamical model for NGC3258

In this paper we present a method to analyse absorption line spectra of a galaxy designed to determine the stellar dynamics and the stellar populations by a direct fit to the spectra. This paper is the first one to report on the application of the method to data. The modelling results in the knowledge of distribution functions that are sums of basis functions. The practical implementation of the method is discussed and a new type of basis functions is introduced. With this method, a dynamical model for NGC 3258 is constructed. This galaxy can be successfully modelled with a potential containing 30% dark matter within 1r_e with a mass of 1.6x10^11 M_o. The total mass within 2r_e is estimated as 5x10^11 M_o, containing 63% dark matter. The model is isotropic in the centre, is radially anisotropic between 0.2 and 2 kpc (0.88 r_e) and becomes tangentially anisotropic further on. The photometry reveals the presence of a dust disk near the centre.

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arXiv2003

Modelling galactic spectra: II - Simultaneous study of stellar dynamics and stellar mix in NGC3258

In this paper we adopt a method to analyse absorption line spectra from elliptical galaxies that determines the dynamics of the galaxy and at the same time offers a way to study the stellar populations in that galaxy by a direct fit to the spectra. The result of the modelling is a distribution function for each stellar population that is considered in the fit. The method is described in detail in an accompanying paper. This paper reports on a dynamical stellar population study in NGC3258, based on spectra in two different wavelength regions, the near-IR Ca II triplet around 8600A and the Ca H and K lines around 3900A. These absorption lines have discriminating power toward various stellar types. The dynamical model shows an increase in dwarfs (represented by G2V stars) toward the centre. Most of the rotation in the model is delivered by the giants (represented by M1III stars). Moreover, the different models that were considered indicate that establishing a potential for a galaxy is dependent on the wavelength range used for the modelling.

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arXiv2003

The puzzlingly large Ca II triplet absorption in dwarf elliptical galaxies

We present central CaT, PaT, and CaT* indices for a sample of fifteen dwarf elliptical galaxies (dEs). Twelve of these have CaT* ~ 7 A and extend the negative correlation between the CaT* index and central velocity dispersion sigma, which was derived for bright ellipticals (Es), down to 20 < sigma < 55 km/s. For five dEs we have independent age and metallicity estimates. Four of these have CaT* ~ 7 A, much higher than expected from their low metallicities (-1.5 < [Z/H] < -0.5). The observed anti-correlation of CaT* as a function of sigma or Z is in flagrant disagreement with theory. We discuss some of the amendments that have been proposed to bring the theoretical predictions into agreement with the observed CaT*-values of bright Es and how they can be extended to incorporate also the observed CaT*-values of dEs. Moreover, 3 dEs in our sample have CaT* ~ 5 A, as would be expected for metal-poor stellar systems. Any theory for dE evolution will have to be able to explain the co-existence of low-CaT* and high-CaT* dEs at a given mean metallicity. This could be the first direct evidence that the dE population is not homogeneous, and that different evolutionary paths led to morphologically and kinematically similar but chemically distinct objects.

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arXiv2003

Realistic error estimates on kinematic parameters

Current error estimates on kinematic parameters are based on the assumption that the data points in the spectra follow a Poisson distribution. For realistic data that have undergone several steps in a reduction process, this is generally not the case. Neither is the noise distribution independent in adjacent pixels. Hence, the error estimates on the derived kinematic parameters will (in most cases) be smaller than the real errors. In this paper we propose a method that makes a diagnosis of the characteristics of the observed noise The method also offers the possibility to calculate more realistic error estimates on kinematic parameters. The method was tested on spectroscopic observations of NGC3258. In this particular case, the realistic errors are almost a factor of 2 larger than the errors based on least squares statistics.

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arXiv2003

Embedded disks in Fornax dwarf ellipticals

We present photometric and kinematic evidence for the presence of stellar disks, seen practically edge-on, in two Fornax dwarf galaxies, FCC204 (dS0(6)) and FCC288 (dS0(7)). This is the first time such structures have been identified in Fornax dwarfs. FCC2088 has only a small bulge and a bright flaring and slightly warped disk that can be traced out to 23" from the center (2.05 kpc for H_0=75 km/s/Mpc). FCC204's disk can be traced out to 20" (1.78 kpc). This galaxy possesses a large bulge. These results can be compared to the findings of Jerjen et al. (2000) and Barazza et al. (2002) who discovered nucleated dEs with spiral and bar features in the Virgo Cluster.

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arXiv2002

Three-component Stackel potentials satisfying recent estimates of Milky Way parameters

We present a set of three-component Stackel potentials defined by five parameters and designed to model the Milky Way. We review the fundamental constraints that any model of the Milky Way must satisfy, including the most recent ones derived from Hipparcos data, and we study how the parameters of the presented potentials can vary in order to match these constraints. Five different valid potentials are presented and analyzed in detail: they are designed to be confronted with kinematical surveys in the future, by the construction of three-integral analytic distribution functions.

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arXiv2002

Evidence for a warm ISM in the Fornax dEs FCC046 and FCC207

We present Halpha+[NII] narrow-band imaging of FCC046 and FCC207, two dES in the Fornax Cluster. B-R and B-I color-maps clearly show signs of dust-absorption in FCC207. FCC046 has a very bright blue nucleus, offset by about 1.1'' with respect to the outer isophotes. Moreover, FCC046 shows a pronounced lopsided shape. The emitting regions differ considerably between the two galaxies. Whereas FCC207 has only one central emission region, FCC046 also contains fainter emission regions. Based on broad-band colours, its disturbed shape and its very bright nucleus, FCC046 is akin to the class of amorphous dwarfs. The central emission regions of both galaxies are resolved and we estimate their diameters at about 60 pc. Their Halpha luminosities can be explained as photo-ionisation by post-AGB stars in an old population. Some of the emission regions in FCC046 are resolved and have diameters of the order of 50-150 pc and H$α$ luminosities of the order of 10^30 W, comparable to supernova remnants or nebulae around Wolf-Rayet stars. Hence, FCC046 is clearly undergoing star-formation while for FCC207 the case is not as clearcut. We estimate the mass of the HII gas in FCC046 at M(HII) = 40-150 Mo. The ionised-gas content of FCC207 is somewhat higher : M(HII) = 60-190 Mo.

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arXiv2002

Three-integral models for axisymmetric galactic discs

We present new equilibrium component distribution functions that depend on three analytic integrals in a Stackel potential, and that can be used to model stellar discs of galaxies. These components are generalizations of two-integral ones and can thus provide thin discs in the two-integral approximation. Their most important properties are the partly analytical expression for their moments, the disc-like features of their configuration space densities (exponential decline in the galactic plane and finite extent in the vertical direction) and the anisotropy of their velocity dispersions. We further show that a linear combination of such components can fit a van der Kruit disc.

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arXiv2001

Potentials and distribution functions to be used for dynamical modeling with GAIA-like data

We present new tools to establish axisymmetric equilibrium models of the Milky Way. The models we wish to establish are pairs (V,F) where V is the gravitational potential generated by the whole mass distribution including the dark matter, and F is the distribution function in phase space for late-type tracer stars. We present a set of Stackel potentials that fit some fundamental parameters of the Milky Way (mass density in the solar neighbourhood and Oort constants). Then we define new component distribution functions that can be combined with these potentials in order to reproduce kinematical data like those that will be provided by GAIA.

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arXiv2001

Spiral Galaxies with HST/NICMOS. I. Nuclear Morphologies, Color Maps and Distinct Nuclei

This is the first of two papers where we present the analysis of an HST NICMOS-Cam2 near-infared (NIR) snapshot survey in the F160W (H) filter for a sample of 78 spiral galaxies selected from the UGC and ESOLV catalogs. For 69 of these objects we provide nuclear color information derived by combining the H data either with additional NICMOS F110W (J) images or with V WFPC2/HST data. Here we present the NIR images and the optical-NIR color maps. We focus our attention on the properties of the photometrically-distinct `nuclei' which are found embedded in most of the galaxies, and provide measurements of their half-light radii and magnitudes in the H (and when available, in the J) band. We find that: (i) In the NIR, the nuclei embedded in the bright early- to intermediate-type galaxies span a much larger range in brightness than the nuclei which are typically found embedded in bulgeless late-type disks; (ii) Nuclei are found in both non-barred and barred hosts, in large-scale ($\gta$ 1 kpc) as well as in nuclear (up to a few 100pc) bars; (iii) There is a significant increase in half-light radius with increasing luminosity of the nucleus in the early/intermediate types; (iv) The nuclei of early/intermediate-type spirals cover a large range of optical-NIR colors, from $V-H\approx$ -0.5 to 3. Some nuclei are bluer and others redder than the surrounding galaxy, indicating the presence of activity or reddening by dust in many of these systems; (v) Some early/intermediate nuclei are elongated and/or slightly offset from the isophotal center of the host galaxy. On average, however, these nuclei appear as centered, star-cluster-like structures similar to those which are found in the late-type disks. [abridged]

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arXiv2001

Spiral Galaxies with HST/NICMOS. II. Isophotal Fits and Nuclear Cusp Slopes

We present surface brightness profiles for 56 of the 78 spiral galaxies observed in the HST/{\tt NICMOS/Camera-2} F160W snapshot survey introduced in Carollo et al. (2001; paper I), as well as surface brightness profiles for 23 objects out of the 41 that were also observed in the F110W filter. We fit these surface brightness profiles with the Nuker law of Lauer et al. (1995), and use the smooth analytical descriptions of the data to compute the average nuclear stellar cusp slopes $<γ>$ in the 0.1$''-0.5''$ radial range. Our main result is the startling similarity between the nuclear stellar cusp slopes $<γ>$ in the near-infared compared to those derived in the visual passband. This similarity has several implications: (1.) There are typically little or no optical-NIR {\it global} color gradients, and thus no global stellar population variations, inside $\sim 50$-100 pc from the nucleus in nearby spirals. (2.) The large observed range of the strength of the nuclear stellar cusps seen in the HST optical study of spiral galaxies reflects a physical difference between galaxies, and is not an artifact caused by nuclear dust and/or recent star formation. (3.) The dichotomy between $R^{1/4}$ bulges, with steep nuclear stellar cusps $<γ> \sim 1$, and exponential bulges, with shallow nuclear stellar cusps $<γ> < 0.3$, is also not an artifact of the effects of dust or recent star formation. (4) A surrounding massive disk appears to have no effect on the rise of the stellar density distribution within the innermost hundred pc of the

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arXiv2001

The dynamics of the dE galaxy FS76 : bridging the kinematic dichotomy between Es and dEs

We present major and minor axis kinematics out to 2 half-light radii for the bright (M_B=-16.7) dwarf elliptical FS76, a member of the NGC5044 group. Its velocity dispersion is 46 \pm 2 km/s in the center and rises to 70 \pm 10 km/s at half-light radius. Beyond 1R_e the dispersion starts to fall again. The maximum rotation velocity is 15 \pm 6 km/s, about the value expected for an oblate isotropic rotator with the same flattening as FS76 (dE1). Hence, FS76 is the first dE discovered so far that is not flattened predominantly by anisotropy. Using dynamical models, we estimate the mass-to-light ratio to be between 3.2 and 9.1, consistent with predictions based on CDM cosmological scenarios for galaxy formation.

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arXiv2000

HST Optical-NIR Colors of Nearby $R^{1/4}$ and Exponential Bulges

We have analysed $V$, $H$ and $J$ HST images for a sample of early- to late-type spiral galaxies, and reported elsewhere the statistical frequency of $R^{1/4}$-law and exponential bulges in our sample as a function of Hubble type, and the frequency of occurrence and structural properties of the resolved central nuclei hosted by intermediate- to late-type bulges and disks (see references in the text). Here we use these data to show that: {\it (i)} The $V-H$ color distribution of the $R^{1/4}$ bulges peaks around $ \sim 1.3$, with a sigma $Δ(V-H) \sim 0.1$ magnitudes. Assuming a solar metallicity, these values correspond to stellar ages of $\approx 6 \pm 3$ Gyrs. In contrast, the $V-H$ color distribution of the exponential bulges peaks at $ \sim 0.9$ and has a sigma $Δ(V-H) \sim 0.4$ mags. This likely implies significantly smaller ages and/or lower metallicities for (a significant fraction of the stars in) the exponential bulges compared to the $R^{1/4}$-law spheroids. {\it (ii)} Most of the central nuclei hosted by the exponential bulges have $V-H$ and $J-H$ colors which are compatible with relatively unobscured stellar populations. Assuming no or little dust effects, ages $\gta 1$ Gyrs are suggested for these nuclei, which in turn imply masses of about a few 10$^6$ to a few 10$^7$ M$_\odot$, i.e., sufficient to dissolve progenitor bars with masses consistent with those inferred for the exponential bulges by their luminosities. {\it (iii)} While different bulge-nucleus pairs cover a large range of $V-H$ colors, each bulge-nucleus pair has quite similar $V-H$ colors, and thus possibly similar stellar populations. (abridged)

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arXiv2000

Distribution functions for evolved stars in the inner galactic Plane

We present dynamical distribution functions for evolved stars in the inner galactic plane. We use an axisymmetric, two-component Stackel potential that satisfies recent constraints on the galactic potential, amongst others a slightly declining local rotation curve. We show that this potential is adequate to model stellar-kinematic samples with radial extent ranging from \~100 pc to ~5 kpc in the Galaxy. The two-integral model that gives the best fit to the first three projected moments provides a very good global representation of the data but fails to reproduce the central dispersion, the central apparent scaleheight and the cylindrical rotation at intermediate longitudes. All these features are fitted well by a three--integral model. We discuss various properties of the 2I- and 3I models and the implications for galactic structure. A somewhat thicker disk component is needed to explain the distribution of older AGB stars in the plane; this component also fits the kinematics at higher latitudes better. We find that the Disk and the Bulge, as traced by AGB stars, are very similar dynamically and could well be one and the same component. There is a dynamically distinct component in the inner 100 pc of the Bulge, however.

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