Bulges Of Spiral Galaxies Research Articles

Realistic triaxial density-potential-force profiles for stellar systems and dark matter haloes

Popular models for describing the luminosity-density profiles of dynamically hot stellar systems (e.g. Jaffe, Hernquist, Dehnen) were constructed to match the deprojected form of de Vaucouleurs' R 1/4 light-profile. However, we now know that elliptical galaxies and bulges display a mass-dependent range of structural profiles. To compensate this, the model in Terzic & Graham was designed to closely match the deprojected form of Sersic R 1/n light-profiles, including deprojected exponential light-profiles and galaxies with partially depleted cores. It is thus applicable for describing bulges in spiral galaxies, dwarf elliptical galaxies, both 'power-law' and 'core' elliptical galaxies, also dark matter haloes formed from A cold dark matter cosmological simulations. In this paper, we present a new family of triaxial density-potential-force triplets, which generalizes the spherical model reported in Terzic & Graham to three dimensions. If the (optional) power-law core is present, it is a five-parameter family, while in the absence of the core it reduces to three parameters. The isodensity contours in the new family are stratified on confocal ellipsoids and the potential and forces are expressed in terms of integrals which are easy to evaluate numerically. We provide the community with a suite of numerical routines for orbit integration, which feature: optimized computations of potential and forces for this family; the ability to run simulations on parallel platforms; and modular and easily editable design.

Completely analytical families of anisotropic -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.

News from bulges hosted by low surface brightness galaxies

We present new results on stellar populations of bulges in spiral galaxies, from optical and near-IR data (images and spectra). In particular, we analyze a sample of bulges hosted by low surface brightness galaxies (LSBs). Correlations between metallicity, scale lengths of disk and bulges, and HI content are reported, supporting the bulge secular evolution hypothesis, as well as systematics between the derived evolutionary parameters of the embedded stellar populations. Using spectra of moderate resolution, we also investigate the stellar histories of bulges using Lick/IDS indices and recent α-enhancement models. Results indicate that small bulges tend to be smaller and metal poor, compared to larger ones. However, all bulges appear to have been formed in the same time-scale. We present some ongoing studies and some prospects of our research.

Bulge Evolution in Face-On Spiral and Low Surface Brightness Galaxies

It is an observational fact that bulges of spiral galaxies contain a high fraction of old and metal-rich stars. Following this observational fact, we have investigated the colors of 21 bulges hosted by a selected sample of high surface brightness spiral galaxies and low surface brightness galaxies observed in the B and R optical bands and the J and Ks near-IR bands. Using structural parameters derived from these observations, we obtain evidence that bulges could be formed by pure disk evolution (secular evolution), in agreement with suggestions by some authors. The color profiles, especially the near-IR ones, show null or almost null color gradients, supporting the hypothesis that the disk stellar populations are similar to those present in the bulge and/or that some bulges can be understood as disks with enhanced stellar density (or pseudobulges). In the optical, half of the galaxies present an inverse color gradient, giving additional evidence in favor of secular evolution for the sample investigated. The comparison of the observed colors with those obtained from spectrophotometric models of galaxy evolution suggests that bulges of the selected sample have solar and subsolar metallicities and are independent of the current stellar formation rate. Also, we obtain evidence suggesting that galaxies hosting small bulges tend to be systematically metal-poor compared to those with larger bulges. These results are being checked more carefully with high signal-to-noise ratio spectroscopy.

Stellar populations in bulges of spiral galaxies

We present line strengths in the bulges and inner discs of 38 galaxies in the local Universe, including several galaxies whose bulges were previously identified as being disc like in their colours or kinematics, to see if their spectral properties reveal evidence for secular evolution. We find that red bulges of all Hubble types are similar to luminous ellipticals in their central stellar populations. They have large luminosity-weighted ages, metallicities, and α/Fe ratios. Blue bulges can be separated into a metal-poor class that is restricted to late types with small velocity dispersion and a young, metal-rich class that includes all Hubble types and velocity dispersions. Luminosity-weighted metallicities and α/Fe ratios are sensitive to central velocity dispersion and maximum disc rotational velocity. Red bulges and ellipticals follow the same scaling relations. We see differences in some scaling relations between blue and red bulges and between bulges of barred and unbarred galaxies. Most bulges have decreasing metallicity with increasing radius; galaxies with larger central metallicities have steeper gradients. Where positive age gradients (with the central regions being younger) are present, they are invariably in barred galaxies. The metallicities of bulges are correlated with those of their discs. While this and the differences between barred and unbarred galaxies suggest that secular evolution cannot be ignored, our results are generally consistent with the hypothesis that mergers have been the dominant mechanism responsible for bulge formation.

Density-potential pairs for spherical stellar systems with Sérsic light profiles and (optional) power-law cores

Popular models for describing the luminosity-density profiles of dynamically hot stellar systems (e.g. Jaffe, Hernquist, Dehnen) were constructed with the desire to match the deprojected form of an R1/4 light profile. Real galaxies, however, are now known to have a range of different light-profile shapes that scale with mass. Consequently, although highly useful, the above models have implicit limitations, and this is illustrated here through their application to a number of real galaxy density profiles. On the other hand, the analytical density profile given by Prugniel & Simien closely matches the deprojected form of Sérsic R1/n light profiles – including deprojected exponential light profiles. It is thus applicable for describing bulges in spiral galaxies, dwarf elliptical galaxies, and both ordinary and giant elliptical galaxies. Moreover, the observed Sérsic quantities define the parameters of the density model. Here we provide simple equations, in terms of elementary and special functions, for the gravitational potential and force associated with this density profile. Furthermore, to match galaxies with partially depleted cores, and better explore the supermassive black hole/galaxy connection, we have added a power-law core to this density profile and derived similar expressions for the potential and force of this hybrid profile. Expressions for the mass and velocity dispersion, assuming isotropy, are also given. These spherical models may also prove appropriate for describing the dark matter distribution in haloes formed from ΛCDM cosmological simulations.

Integral Field Spectroscopy of 23 Spiral Bulges

We have obtained integral-field spectroscopy for 23 spiral bulges using INTEGRAL on the William Herschel Telescope and SPIRAL on the Anglo-Australian Telescope. This is the first two-dimensional survey directed solely at the bulges of spiral galaxies. Eleven galaxies of the sample do not have previous measurements of the stellar velocity dispersion (σ*). These data are designed to complement our Space Telescope Imaging Spectrograph program for estimating black hole masses in the range 106-108 M☉ using gas kinematics from nucleated disks. These observations will serve to derive the stellar dynamical bulge properties using the traditional Mg b and Ca II triplets. We use both cross-correlation and maximum penalized likelihood to determine projected σ* in these systems and present radial velocity fields, major axis rotation curves, curves of growth, and σ* fields. Using cross-correlation to extract the low-order two-dimensional stellar dynamics we generally see coherent radial rotation and irregular velocity dispersion fields suggesting that σ* is a nontrivial parameter to estimate.

The expected abundance of Lyman-αemitting primeval galaxies

We present model calculations for the expected surface density of Ly-α emitting primeval galaxies (PGs) at high redshifts. We assume that elliptical galaxies and bulges of spiral galaxies (=spheroids) formed early in the universe and that the Ly-α emitting PGs are these spheroids during their first burst of star formation at high redshift. One of the main assumptions of the models is that the Ly-α bright phase of this first starburst in the spheroids is confined to a short period after its onset due to rapid formation of dust. The models do not only explain the failure of early surveys for Ly-α emitting PGs but are also consistent with the limits of new surveys (e.g. the Calar Alto Deep Imaging Survey - CADIS). At faint detection limits S lim ≤ 10 −20 Wm −2 the surface density of Ly-α emitters is expected to vary only weakly in the redshift range between z = 3a nd z = 6 with values >10 3 /deg 2 /∆z = 0.1 reaching its maximum at z0 ≈ 4. At shallower detection limits, S lim ≥ 3 × 10 −20 Wm −2 the surface density of high-z Ly-α emitters is expected to be a steep function of redshift and detection limit. This explains the low success in finding bright Ly-α galaxies at z > 5. We demonstrate how the observed surface densities of Ly-α emitting PGs derived from recent surveys constrain the parameters of our models. Finally, we discuss the possibility that two Ly-α bright phases occur in the formation process of galaxies: An initial - primeval - phase in which dust is virtually non-existant, and a later secondary phase in which strong galactic winds as observed in some Lyman break galaxies facilitate the escape of Ly-α photons after dust has already been formed.

The lensing properties of the Sersic model

It is well known that the surface brightness of elliptical galaxies and of bulges of spiral galaxies is best fitted by the Sersic $r^{1/n}$ profile. It is thus interesting to explore the lensing properties of the Sersic model because of its wide range of applicability. To this aim, we evaluate the lensing potential, the deflection angle, the time delay between the images and the amplification for a circularly symmetric lens whose surface density is described by the Sersic profile. We estimate the same quantities also adding a shear term to the deflection potential in order to study (at the lowest order) the impact of deviations from radial symmetry or the contribution from other nearby lenses. Moreover, we investigate the systematic errors due to the use of a de Vaucouleurs profile instead of the correct Sersic model also taking into account the presence of a dark halo.

Morphology and kinematics in thick box‐peanut bulge galaxies

AbstractBoth, observational and theoretical results support the close connection between bars and box/peanut‐shaped bulges in spiral galaxies. However, for the sub‐class of thick box/peanut‐shaped bulges, an environmental process instead of an internal mechanism is suggested to account for the prominence of the box/peanut component. In this paper a multiwavelength study of the morphology and the kinematics of a sample of thick box/peanut‐shaped bulges is presented. The results suggest a scenario in which interaction events play a significant role in the history of thick box/peanut‐shaped bulge galaxies. Most likely these sources have experienced a mass infall. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hypernovae/Gamma-Ray Bursts in the Galactic Center as Possible Sources of Galactic Positrons

The observation of a strong and extended positron-electron line annihilation emission in the central regions of the Galaxy by the SPectrometer on the International Gamma-Ray Astrophysical Laboratory (INTEGRAL/SPI), consistent with the Galactic bulge geometry and without any counterpart in the gamma-ray range, neither at high energy nor in the 1809 keV 26Al decay line, is challenging. Leaving aside the geometrical question, we address the problem of the adequate positron sources, showing the potentiality of a new category of Type Ic supernovae (SNe Ic), exemplified by SN 2003dh, that is associated with a gamma-ray burst (GRB). This kind of supernova/hypernova/GRB event is interpreted as the result of a bipolar Wolf-Rayet explosion, which produces a large amount of 56Ni and ejects it at high velocity along the rotation axis. The bulk of positrons resulting from 56Co decay escapes in the surrounding medium as a result of the rapid thinning of the ejecta in the polar direction. We show that a rate of about 0.02 SN 2003dh-like events per century in the central region of the Galaxy is sufficient to explain the positron flux detected by INTEGRAL/SPI. In order to explain this flux by SN Ia events alone, a rate of 0.5 per century is necessary, much higher than indicated by Galactic evolutionary models applied to the bulge. Further observations of late light curves of SNe Ia and SNe Ic in the bulge of spiral galaxies, together with three-dimensional hydrodynamic calculations of anisotropic ejections of 56Ni in SN Ic/GRB events, will allow us to estimate the separate contributions of SNe Ia and SNe Ic to positron injection.

Stellar populations in bulges of spiral galaxies

We present a few results of a work in progress tackling the radial spectroscopic properties of bulges of spiral galaxies.

WFPC2 Images of the Central Regions of Early-Type Galaxies. I. The Data

We present high-resolution R-band images of the central regions of 67 early-type galaxies obtained with the Wide Field and Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope (HST). This homogeneously selected sample roughly doubles the number of early-type galaxies that have now been imaged at HST resolution and complements similar data on the central regions of radio galaxies and the bulges of spiral galaxies. Our sample strikingly confirms the complex morphologies of the central regions of early-type galaxies which have become apparent from previous studies with HST. In particular, we detect dust, either in the form of nuclear disks or with a filamentary distribution, in 43% of all galaxies, in good agreement with previous estimates. In addition, we find evidence for embedded stellar disks in a remarkably large fraction of 51%. In 14 of those galaxies the disklike structures are misaligned with the main galaxy, suggesting that they correspond to stellar bars in S0 galaxies. We analyze the luminosity profiles of the galaxies in our sample and classify galaxies according to their central cusp slope. To a large extent we confirm the results from previous HST surveys in that early-type galaxies reveal a clear dichotomy: the bright ellipticals (MB ≲ -20.5) are generally boxy and have luminosity profiles that break from steep outer power laws to shallow inner cusps (referred to as "core" galaxies). The fainter ellipticals, on the other hand, typically have disky isophotes and luminosity profiles that lack a clear break and have a steep central cusp (referred to as "power-law" galaxies). The advantages and shortcomings of classification schemes utilizing the extrapolated central cusp slope γ are discussed, and it is shown that γ might be an inadequate representation for galaxies whose luminosity profile slope changes smoothly with radius rather than resembling a broken power law. Thus, we introduce a new, alternative parameter and show how this affects the classification. In fact, we find evidence for an "intermediate" class of galaxies that cannot unambiguously be classified as either core or power-law galaxies and that have central cusp slopes and absolute magnitudes intermediate between those of core and power-law galaxies. It is unclear at present, however, whether these galaxies make up a physically distinct class or whether distance and/or resolution effects cause them to lose their distinct core or power-law characteristics.

The Nuclear Cusp Slopes of Dwarf Elliptical Galaxies

We derive the light profiles for a sample of 25 dwarf elliptical galaxies observed by us with Hubble Space Telescope's Wide Field and Planetary Camera 2 in F555W and F814W. These profiles are fitted with Nuker, R1/4, exponential, and Sersic laws and are also used to derive the nuclear cusp slopes γ. We discuss the correlation of nuclear cusp slope with galactic luminosity, the presence of a nucleus, and the type of light profile. The results are compared with those found in the literature for elliptical galaxies and the bulges of spiral galaxies. We find that, as a class, the nuclear regions of dwarf ellipticals are very similar to those of the exponential bulges of spiral galaxies and have nuclear cusp slopes shallower than those of bulges with the same luminosity that were well fitted by a de Vaucouleurs R1/4 profile. For the 14 nucleated galaxies in our sample, this conclusion is less certain than for the 11 nonnucleated objects, since it relies on an extrapolation of galaxy light under the nucleus. In terms of their light profiles and nuclear properties, most spheroidal stellar systems can be broadly divided into two subclasses: the exponential shallow cusp objects and the R1/4 steep cusp objects. Membership of a class does not appear to correlate with the presence of a massive stellar disk.

An Investigation into the Prominence of Spiral Galaxy Bulges

From a diameter-limited sample of 86 `face-on' spiral galaxies, the bulge-to-disk size and luminosity ratios, and other quantitative measurements for the prominence of the bulge are derived. The bulge and disk parameters have been estimated using a seeing convolved Sersic r^(1/n) bulge and a seeing convolved exponential disk. In general, early-type spiral galaxy bulges have Sersic values of n>1, and late-type spiral galaxy bulges have values of n<1. In the B-band, only 8 galaxies have a bulge shape parameter n consistent with the exponential value of 1, and only 5 galaxies do in the K-band. Application of the r^(1/n) bulge models results in a larger mean r_e/h ratio for the early-type spiral galaxies than the late-type spiral galaxies. Although, this result is shown not to be statistically significant. The mean B/D luminosity ratio is, however, significantly larger (> 3-sigma) for the early-type spirals than the late-type spirals. This apparent contradiction with the r_e/h values can be explained with an iceberg-like scenario, in which the bulges in late-type spiral galaxies are relatively submerged in their disk. This can be achieved by varying the relative bulge/disk stellar density while maintaining the same effective bulge-to-disk size ratio. The absolute bulge magnitude - log(n) diagram is used as a diagnostic tool for comparative studies with dwarf elliptical and ordinary elliptical galaxies. At least in the B-band, these objects occupy distinctly different regions of this parameter space. While the dwarf ellipticals appear to be the faint extension to the brighter elliptical galaxies, the bulges of spiral galaxies are not.

Chemical Evolution of Bulges

We show our results from the application of the Multiphase chemical evolution model to a set of Bulges of spiral galaxies

On the bulge-to-disk size ratio for spiral galaxies

From a detailed structural analysis of the sample of spiral galaxies from de Jong and van der Kruit (1994), we present new results on the bulge-to-disk size ratio as a function of morphological type. A Sersic r 1/n bulge and an exponential disk model, both convolved with an appropriate Gaussian PSF, were simultaneously applied to the galaxy light profiles to obtain the best-fitting bulge effective radius, r e , and disk scale-length, h. The Hubble sequence of spiral galaxies is scale-free if the ratio of these two quantities is uncorrelated with morphological type. This was in fact claimed to be the case by de Jong (1996) and also Courteau et al. (1996) using R-band data; although, applying exponential bulge models to their K-band data resulted in their mean r e / h ratio for the early-type spirals being smaller than the mean r e / h ratio for the late-type spirals at the 98% significance level. However, early-type spiral galaxy bulges are known to be better described with models having Sersic shape parameters n>1(Andredakis, Peletier and Balcells, 1995; Graham and Prieto, 1999). It was therefore of interest to re-evaluate the above claim allowing for the full range of bulge profile shapes that exist and can be described by the Sersic r 1/n model. In so doing, the mean r e / h ratio for the early-type spirals (Sa,Sab,Sb) was found to be significantly (3σ) larger than the mean r e / h ratio obtained when using the exponential bulge models. Intriguingly, the mean r e / h ratio for the early-type galaxies was discovered to be larger than the mean r e / h ratio for the late-type (Scd-Sm) spirals at only the 1.5–2σ significance level in the optical bands and at the lσ significance level in the K-band. These results suggest that the Hubble sequence for spiral galaxies is indeed scale-free. A fuller discussion is presented in Graham (2001).

Formation of Galactic Bulges

We use cosmological hydrodynamic simulations to investigate the formation of galactic bulges within the framework of hierarchical clustering in a representative cold dark matter (CDM) cosmological model. We show that the largest objects forming at cosmological redshifts z ~ 4 resemble observed bulges of spiral galaxies or moderate-sized ellipticals in their general properties, such as size, shape, and density profile. This is consistent with observational data indicating the existence of old bulges and ellipticals at more moderate redshifts. These bulges are gas dominated at redshift z = 3, with high rates of star formation, and would appear to be good candidates for small blue galaxies seen in the Hubble Deep Field.

A Near-Infrared Photometric Plane for Elliptical Galaxies and Bulges of Spiral Galaxies.

We report the existence of a single plane in the space of global photometric parameters describing elliptical galaxies and the bulges of early-type spiral galaxies. The three parameters that define the plane are obtained by fitting the Sersic form to the brightness distribution obtained from near-infrared K-band images. We find, from the range covered by their shape parameters, that the elliptical galaxies form a more homogeneous population than the bulges. Known correlations like the Kormendy relation are projections of the photometric plane. The existence of the plane has interesting implications for bulge formation models.

The Nature of Boxy/Peanut-Shaped Bulges in Spiral Galaxies

We present a systematic observational study of the relationship between bars and boxy/peanut-shaped (B/PS) bulges. We first review and discuss proposed mechanisms for their formation, focusing on accretion and bar-buckling scenarios. Using new methods relying on the kinematics of edge-on disks, we then look for bars in a large sample of edge-on spiral galaxies with a B/PS bulge and in a smaller control sample of edge-on spirals with more spheroidal bulges. We present position-velocity diagrams of the ionized gas obtained from optical long-slit spectroscopy. We show that almost all B/PS bulges are due to a thick bar viewed edge-on, while only a few extreme cases may be due to the accretion of external material. This strongly supports the bar-buckling mechanism for the formation of B/PS bulges. None of the galaxies in the control sample show evidence for a bar, which suggests conversely that bars are generally B/PS.We consider the effects of dust in the disk of the galaxies but conclude that it does not significantly affect our results. Unusual emission-line ratios correlating with kinematical structures are observed in many objects, and we argue that this is consistent with the presence of strong bars in the disk of the galaxies. As expected from N-body simulations, the boxy-peanut transition appears to be related to the viewing angle, but more work is required to derive the precise orientation of the bars in the bulges. The reliable identification of bars in edge-on spiral galaxies opens up for the first time the possibility of studying observationally the vertical structure of bars.