Pieter Buyle

Observational evidence for a connection between supermassive black holes and dark matter haloes

We present new velocity dispersion measurements of a sample of 12 spiral galaxies for which extended rotation curves are available. These data are used to refine a recently discovered correlation between the circular velocity and the central velocity dispersion of spiral galaxies. We find a slightly steeper slope for our larger sample, confirm the negligible intrinsic scatter on this correlation and find a striking agreement with the corresponding relation for elliptical galaxies. We combine this correlation with the well-known M B H -σ relation to obtain a tight correlation between the circular velocities of galaxies and the masses of the supermassive black holes they host. This correlation is the observational evidence for an intimate link between dark matter haloes and supermassive black holes. Apart from being an important ingredient for theoretical models of galaxy formation and evolution, the relation between M B H and circular velocity can serve as a practical tool to estimate black hole masses in spiral galaxies.

The H I Content of E+A Galaxies

We present deep single-dish H I observations of a sample of six nearby E+A galaxies (0.05 < z < 0.1). A nonnegligible fraction of a local sample of E+As are detected in H I. In four galaxies, we have detected up to a few times 109 M☉ of neutral gas. These E+A galaxies are almost as gas-rich as spiral galaxies with comparable luminosities. There appears to exist no direct correlation between the amount of H I present in an E+A galaxy and its star formation rate as traced by radio continuum emission. Moreover, the end of the starburst does not necessarily require the complete exhaustion of the neutral gas reservoir. Most likely, an intense burst of star formation consumed the dense molecular clouds, which are the sites of massive star formation. This effectively stops star formation, even though copious amounts of diffuse neutral gas remain. The remaining H I reservoir may eventually lead to further episodes of star formation. This may indicate that some E+As are observed in the inactive phase of the star formation duty cycle.

Upper limits on the central black hole masses of 47 Tuc and NGC 6397 from radio continuum emission

We present upper-limits on the masses of the putative central intermediate-mass black holes in two nearby Galactic globular clusters: 47Tuc (NGC104), the second brightest Galactic globular cluster, and NGC6397, a core-collapse globular cluster and, with a distance of 2.7 kpc, quite possibly the nearest globular cluster, using a technique suggested by T. Maccarone. These mass estimates have been derived from 3sigma upper limits on the radio continuum flux at 1.4 GHz, assuming that the putative central black hole accretes the surrounding matter at a rate between 0.1% and 1% of the Bondi accretion rate. For 47Tuc, we find a 3sigma upper limit of 2060 - 670 solar masses, depending on the actual accretion rate of the black hole and the distance to 47Tuc. For NGC6397, which is closer to us, we derive a 3sigma upper limit of 1290 - 390 solar masses. While estimating mass upper-limits based on radio continuum observations requires making assumptions about the gas density and the accretion rate of the black hole, their derivation does not require complex and time consuming dynamical modeling. Thus, this method offers an independent way of estimating black hole masses in nearby globular clusters. If, generally, central black holes in stellar systems accrete matter faster than 0.1% of the Bondi accretion rate, then these results indicate the absence of black holes in these globular clusters with masses as predicted by the extrapolated M-sigma relation.

The dynamical structure of isotropic spherical galaxies with a central black hole

We discuss the kinematical structure of a two-parameter family of isotropic models with a central black hole. The family contains the slope of the central density cusp and the relative black hole mass as parameters. Most of the basic kine- matical quantities of these models can be expressed analytically. This family contains three distinct models where also the distribution function, differential energy distribution and spatial LOSVDs can be expressed completely analytically. Each of these models shows a drastically different behaviour of the distribution function. Although the effect of a black hole on the distribution function is very strong, in particular for models with a shallow density cusp, the differential energy distribution is only marginally affected. We discuss in detail the effects of a central black hole on the LOSVDs. The projected velocity dispersion increases with black hole mass at small projected radii, but the effect of a black hole on the shape of the LOSVDs (characterized by the h4 parameter) is less straightforward to interpret. Too much reliance on the wings of the LOSVDs and the value of the h4 parameter to determine black hole masses might hence be dangerous.

The CO content of the Local Group dwarf irregular galaxies IC 5152, UGCA 438 and the Phoenix dwarf

We present a search for CO(1 → 0) emission in three Local Group dwarf irregular galaxies: IC 5152, the Phoenix dwarf and UGCA 438, using the ATNF Mopra radio telescope. Our scans largely cover the optical extent of the galaxies and the stripped H I cloud west of the Phoenix dwarf. Apart from a tentative but non-significant emission peak at one position in the Phoenix dwarf, no significant emission was detected in the CO spectra of these galaxies. For a velocity width of 6km s -1 , we derive 4σ upper limits of 0.03, 0.04 and 0.06 K km s -1 for IC 5152, the Phoenix dwarf and UGCA 438, respectively. This is an improvement of over a factor of 10 compared with previous observations of IC 5152; the other two galaxies had not yet been observed at millimetre wavelengths. Assuming a Galactic CO-to-H 2 conversion factor, we derive upper limits on the molecular gas mass of 6.2 x 10 4 , 3.7 x 10 3 and 1.4 × 10 5 M ⊙ for IC 5152, the Phoenix dwarf and UGCA 438, respectively. We investigate two possible causes for the lack of CO emission in these galaxies. On the one hand, there may be a genuine lack of molecular gas in these systems, in spite of the presence of large amounts of neutral gas. However, in the case of IC 5152 which is actively forming stars, molecular gas is at least expected to be present in the star-forming regions. On the other hand, there may be a large increase in the CO-to-H 2 conversion factor in very low-metallicity dwarfs (-2 ≤ [Fe/H] < - 1), making CO a poor tracer of the molecular gas content in dwarf galaxies.

The H i content of Fornax dwarf elliptical galaxies: FCC032 and FCC336

We present H I21-cm line observations, obtained with the Australia Telescope Compact Array, of two dwarf elliptical galaxies (dEs) in the Fomax cluster: FCC032 and FCC336. The optical positions and velocities of these galaxies place them well within the Fornax cluster. FCC032 was detected at the 3σ significance level with a total HI flux density of 0.66 ± 0.22 Jy km s -1 or an HI mass of 5.0 ± 1.7 x 10 7 h -2 75 M ○. . Based on our deep Ha + [N II] narrow-band images, obtained with FORS2 mounted on the Very Large Telescope, this dE was already known to contain 600 -1800 h -2 75 M ○. of ionized hydrogen (depending on the relative strengths of the Hα and [N II] emission lines). Hence, this is the first study of the complex, multiphase interstellar medium of a dE outside the Local Group. FCC336 was detected at the same significance level: 0.37 ± 0.10 Jy km s -1 or a total H I mass of 2.8 ± 0.7 x 10 7 h -2 75 M ○. . Using a compilation of H I data of dwarf galaxies, we find that the observed high HI mass boundary of the distribution of dwarf irregulars, blue compact dwarfs and dwarf ellipticals in a log L B versus log M HI diagram is in good agreement with a simple chemical evolution model with continuous star formation. The existence of many gas-poor dEs (undetected at 21 cm) suggests that the environment (or, more particularly, a galaxys orbit within a cluster) also plays a crucial role in determining the amount of gas in present-day dEs; for example, FCC032 and FCC336 are located in the sparsely populated outskirts of the Fomax cluster. This is in agreement with H I surveys of dEs in the Virgo cluster, and an Ha survey of the Fornax cluster, which also tend to place gas-rich dwarf galaxies in the cluster periphery.

The conversion of late-type into early-type dwarf galaxies by ram-pressure stripping in the Fornax cluster

We put to the test the hypothesis that the Fornax cluster dwarf galaxies are mostly a relatively recently acquired population, of which the star-forming, late-type members are converted into quiescent, early-type ones by ram-pressure stripping while being on orbits that plunge inside the inner few hundred kiloparsecs of the cluster. We construct dynamical models with different anisotropy profiles for the dwarf galaxy population and show that only extremely radially anisotropic orbital distributions are in agreement with the available morphological, positional, and kinematical data, especially with the radially increasing late-to-early-type ratio. This corroborates the idea that the Fornax cluster dwarfs are an infall population and that environmental factors, in this case ram-pressure stripping, play a prominent role in converting late-type dwarfs into early-type ones.

Completely analytical families of anisotropic γ-models

We present new analytical distribution functions for anisotropic spherical galaxies. They have the density profiles of the γ-models, which allow a wide range of central density slopes, and are widely used to fit elliptical galaxies and the bulges of spiral galaxies. Most of our models belong to two two-parameter families. One of these parameters is the slope y of the central density cusp. The other allows a wide range of varying radial and tangential anisotropies, at either small or large radii. We give analytical formulas for their distribution functions, velocity dispersions, and the manner in which energy and transverse velocity are distributed between orbits. We also give some of their observable properties, including line-of-sight velocity profiles which have been computed numerically. Our models can be used to provide a useful tool for creating initial conditions for N-body and Monte Carlo simulations.

Internal Kinematics and Stellar Populations of the Poststarburst Galaxy SDSS J230743.41+152558.4

We present the first 3D-spectroscopic observations of a nearby Hi detected poststarburst, or E+A, galaxy, SDSS J230743.41+152558.4, obtained with the VIMOS IFU spectrograph at ESO VLT. Using the NBURSTS full spectral fitting technique, we derive maps of stellar kinematics, age, and metallicity out to 2-3 half-light radii. Our analysis reveals a large-scale rapidly rotating disk (nu(circ) = 300 km s(-1)) with a positive age gradient (0.6-1.5 Gyr), and a very metal-rich central region ([Fe/H] = +0.25 dex). If a merger or interaction is responsible for triggering the starburst, the presence of this undisturbed disk suggests a minor merger with a gas-rich satellite as the most plausible option, rather than a disruptive major merger. We find spectroscopic evidence for the presence of an active galactic nucleus. This is an important clue to the feedback mechanism that truncated the starburst. The presently observed quiescent phase may well be a temporary episode in the galaxys life. SDSS J230743.41+152558.4 is gas-rich and may restart forming stars, again becoming blue before finally settling at the red sequence.

The vc–σc relation in low‐mass and low surface brightness galaxies

We present an updated investigation of the relation between large-scale disc circular velocity, v c , and bulge velocity dispersion, σ c . New bulge velocity dispersions are measured for a sample of 11 low surface brightness (LSB) and seven high surface brightness (HSB) spiral galaxies for which v c is known from published optical or H I rotation curves. We find that, while LSB galaxies appear to define the upper envelope of the region occupied by HSB galaxies (having relatively larger v c for any given σ c ), the distinction between LSB and HSB galaxies in the v c -σ c plane becomes less pronounced for σ c ≤ 80 km s -1 . We conclude that either the scatter of the v c -σ c relation is a function of v c (and hence galaxy mass) or the character of the v c -σ c relation changes at v c ∼ 80 km s -1 . Some implications of our findings are discussed.