arXiv subjects

M. Balcells

Publications and source records attributed to M. Balcells.

52 records · Page 3Linked to original sources

arXiv2004

Thick Disks of Lenticular Galaxies

Thick disks are faint and extended stellar components found around several disk galaxies including our Milky Way. The Milky Way thick disk, the only one studied in detail, contains mostly old disk stars (~10 Gyr), so that thick disks are likely to trace the early stages of disk evolution. Previous detections of thick disk stellar light in external galaxies have been originally made for early-type, edge-on galaxies but detailed 2D thick/thin disk decompositions have been reported for only a scant handful of mostly late-type disk galaxies. We present in this paper for the first time explicit 3D thick/thin disk decompositions characterising the presence and properties (eg scalelength and scaleheight) for a sample of eight lenticular galaxies by fitting 3D disk models to the data. For six out of the eight galaxies we were able to derive a consistent thin/thick disk model. The mean scaleheight of the thick disk is 3.6 times larger than that of the thin disk. The scalelength of the thick disk is about twice, and its central luminosity density between 3-10% of, the thin disk value. Both thin and thick disk are truncated at similar radii. This implies that thick disks extend over fewer scalelengths than thin disks, and turning a thin disk into a thick one requires therefore vertical but little radial heating. All these structural parameters are similar to thick disk parameters for later Hubble-type galaxies previously studied. We discuss our data in respect to present models for the origin of thick disks, either as pre- or post-thin-disk structures, providing new observational constraints.

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arXiv2003

Tidal Streams around External Galaxies

We have the unique opportunity to observe and model nearby streams around the two large Local Group spirals Milky Way and M31 in great detail. However, the detection of streams around other external galaxies is required to verify the general application of the derived results. We give a short summary of streams around other galaxies known in the literature, measuring for the first time the surface brightness of Malin's M83 stream with modern CCD imaging. In addition, we present four new detections of possible stellar streams around disk galaxies.

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arXiv2003

Evidence for a Large Stellar Bar in the LSB Galaxy UGC 7321

Late-type spiral galaxies are thought to be the dynamically simplest type of disk galaxies and our understanding of their properties plays a key role in the galaxy formation and evolution scenarios. The low surface brightness (LSB) galaxy UGC 7321, a nearby, isolated, ``superthin'' edge-on galaxy, is an ideal object to study those purely disk dominated bulge-less galaxies. Although late type spirals are believed to exhibit the simplest possible structure, even prior observations showed deviations from a pure single component exponential disk in the case of UGC 7321. We present for the first time photometric evidence for peanut-shaped outer isophotes from a deep optical (R-band) image of UGC 7321. Observations and dynamical modeling suggest that boxy/peanut-shaped (b/p) bulges in general form through the buckling instability in bars of the parent galaxy disks. Together with recent HI observations supporting the presence of a stellar bar in UGC 7321 this could be the earliest known case of the buckling process during the evolutionary life of a LSB galaxy, whereby material in the disk-bar has started to be pumped up above the disk, but a genuine bulge has not yet formed.

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arXiv2002

Galactic bulges from Hubble Space Telescope NICMOS observations: the lack of r^{1/4} bulges

We use HST near-infrared imaging to explore the shapes of the surface brightness profiles of bulges of S0-Sbc galaxies at high resolution. Modeling extends to the outer bulge via bulge-disk decompositions of combined HST - ground based profiles. Compact, central unresolved components similar to those reported by others are found in ~84% of the sample. We also detect a moderate frequency (~34%) of nuclear components with exponential profiles which may be disks or bars. Adopting the Sérsic r^{1/n} functional form for the bulge, none of the bulges have an r^{1/4} behaviour; derived Sérsic shape-indices are = 1.7 \pm 0.7. For the same sample, fits to NIR ground-based profiles yield Sérsic indices up to n = 4-6. The high-$n$ of ground-based profiles are a result of nuclear point sources blending with the bulge extended light due to seeing. The low Sérsic indices are not expected from merger violent relaxation, and argue against significant merger growth for most bulges.

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arXiv2002

The Central Mass Distribution in Dwarf and Low Surface Brightness Galaxies

We present Halpha rotation curves for a sample of 15 dwarf and LSB galaxies. From these, we derive limits on the slopes of the central mass distributions. Assuming the density distributions of dark matter halos follow a power-law at small radii, rho(r)~r^(-alpha), we find inner slopes in the range 0<alpha<1 for most galaxies. In general, halos with constant density cores (α=0) provide somewhat better fits, but the majority of our galaxies (~75%) are also consistent with alpha=1, provided that the R-band mass-to-light ratios are smaller than about 2. Halos with alpha=1.5, however, are ruled out in virtually every case. To investigate the robustness of these results we discuss and model several possible causes of systematic errors including non-circular motions, slit width, seeing, and slit alignment errors. Taking the associated uncertainties into account, we conclude that even for the 25% of the cases where alpha=1 seems inconsistent with the rotation curves, we cannot rule out cusp slopes this steep. Inclusion of literature samples similar to the one presented here leads to the same conclusion when possible systematic errors are taken into account. In the ongoing debate on whether the rotation curves of dwarf and LSB galaxies are consistent with predictions for a CDM universe, we argue that our sample and the literature samples discussed in this paper provide insufficient evidence to rule out halos with alpha=1. At the same time, we note that none of the galaxies in these samples require halos with steep cusps, as most are equally well or better explained by constant density cores. (abridged)

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arXiv2002

The Westerbork HI Survey of Spiral and Irregular Galaxies II. R-Band Surface Photometry of Late-type Dwarf Galaxies

R-band surface photometry is presented for 171 late-type dwarf and irregular galaxies. For a subsample of 46 galaxies B-band photometry is presented as well. We present surface brightness profiles as well as isophotal and photometric parameters including magnitudes, diameters and central surface brightnesses. Absolute photometry is accurate to 0.1 mag or better for 77% of the sample. For over 85% of the galaxies the radial surface brightness profiles are consistent with published data within the measured photometric uncertainty. For most of the galaxies in the sample HI data have been obtained with the Westerbork Synthesis Radio Telescope. The galaxies in our sample are part of the WHISP project (Westerbork HI Survey of Spiral and Irregular Galaxies), which aims at mapping about 500 nearby spiral and irregular galaxies in HI. The availability of HI data makes this data set useful for a wide range of studies of the structure, dark matter content and kinematics of late-type dwarf galaxies.

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arXiv2001

HI in the shell elliptical NGC 3656

VLA neutral hydrogen observations of the shell elliptical NGC 3656 reveal an edge-on, warped minor axis gaseous disk (M_HI ~ 2.10^9 Msun) extending 7 kpc. HI is also found outside the optical image, on two complexes to the North-East and North-West that seem to trace an outer broken HI disk or ring, or possibly one or two tidal tails. Integral-field optical fiber spectroscopy at the region of the bright southern shell of NGC 3656 has provided a determination of the stellar velocities of the shell. The shell has traces of HI with velocities bracketing the stellar velocities, providing evidence for a dynamical association of HI and stars at the shell. Within the errors the stars have systemic velocity, suggesting a possible phase wrapping origin for the shell. We detect five dwarf galaxies with HI masses ranging from 2.10^8 Msun to 2.10^9 Msun all within 180 kpc from NGC 3656 and all within the velocity range (450 \kms) of the HI of NGC 3656. For the NGC 3656 group to be bound requires a total mass of 3-7.4x10^{12} Msun, yielding a mass to light ratio from 125 to 300. The overall HI picture presented by NGC 3656 supports the hypothesis of a disk-disk merger origin, or possibly an ongoing process of multiple merger with nearby dwarfs.

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arXiv2001

The Central kpc of Galaxy Bulges

We study the innermost regions of bulges with surface brightness data derived from combined HST/NICMOS and ground-based NIR profiles. Bulge profiles to 1-2 kpc may be fit with Sersic laws, and show a trend with bulge-to-disk ratio: low-B/D bulges are roughly exponential, whereas higher-B/D bulges show increasing Sersic shape index $n$, indicating higher peak central densities and more extended brightness tails. N-body models of accretion of satellites onto disk-bulge-halo galaxies show that satellite accretion contributes to the increase of the shape index $n$ as the bulge grows by accretion. The N-body results demonstrate that exponential profiles are fragile to merging, hence bulges with exponential surface brightness profiles cannot have experienced significant growth by accretion of dense satellites.

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arXiv2001

Nuclear Components in the Bulges of Disk Galaxies

By combining surface brightness profiles from images taken in the HST/NICMOS F160W and ground-based (GB) $K$ bands, we have obtained NIR profiles for a well studied sample of inclined disk galaxies, spanning radial ranges from 20 pc to a few kpc. We fit PSF-convolved Sersic-plus-exponential laws to the profiles, and compare the results with the fits to the ground-based data alone. HST profiles show light excesses over the best-fit Sersic law in the inner ~1 arcsec. This is often as a result of inner power-law cusps similar to the inner profiles of intermediate-luminosity elliptical galaxies.

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arXiv2000

Growth of Galactic Bulges by Mergers: I. Dense Satellites

Andredakis, Peletier & Balcells (1995) fit Sersic's law $μ(r) \sim r^{1/n}$ to the bulges of the Balcells & Peletier (1994) galaxy sample, and infer that $n$ drops with morphological type T from $n \approx$ 4--6 for S0 to $n=1$ (exponential) for Sc's. We use collisionless N body simulations to test the assumption that initially the surface brightness profiles of all bulges were exponential, and that the steepening of the profiles toward the early-types is due to satellite accretion. The results are positive. After the accretion of a satellite, bulge-disk fits show that the bulge grows and that the bulge profile index $n$ increases proportional to the satellite mass. For a satellite as massive as the bulge, $n$ rises from 1 to 4. We present kinematic diagnostics on the remnants and disk thickening. The latter suggests that the bulge growth must have occurred before the last formation of a thin disk in the galaxy. The thick disks created by the merger are reminiscent of thick disks seen in early-type edge-on galaxies. The efficiency of the process suggests that present day bulges of late-type spirals showing exponential profiles cannot have grown significantly by collisionless mergers.

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arXiv1999

The shell elliptical NGC2865: evolutionary population synthesis of a kinematically distinct core

We report on the discovery of a rapidly co-rotating stellar and gas component in the nucleus of the shell elliptical NGC2865. The stellar component extends ~ 0.51/h100 kpc along the major axis, and shows depressed velocity dispersion and absorption line profiles skewed in the opposite sense to the mean velocity. Associated with it is a young stellar population with enhanced \hbeta, lowered Mg and same Fe indices relative to the underlying elliptical. Its recent star formation history is constrained by considering ``bulge+burst'' models under 4 physically motivated scenarios, using evolutionary population synthesis. Scenarios in which the nuclear component is formed over a Hubble time or recently from continuous gas inflow are ruled out. Our results argue for a gas-rich accretion or merger origin for the shells and kinematic subcomponent in NGC2865. Arguments based on stellar populations and gas dynamics suggest that one of the progenitors is likely a Sb or Sc spiral. We demonstrate that despite the age and metallicity degeneracy of the underlying elliptical, the age and metallicity of the kinematic subcomponent can be constrained. This work strengthens the link between KDCs and shells, and demonstrates that a KDC can be formed from a late merger.

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arXiv1997

Near-Infrared Surface Photometry of Bulges and Disks of Spiral Galaxies. The Data

We present optical and near-infrared (NIR) surface brightness and colour profiles,in bands ranging from U to K, for the disk and bulge components of a complete sample of 30 nearby S0 to Sbc galaxies with inclinations larger than 50 degrees. We describe in detail the observations and the determination of colour parameters. Calibrated monochromatic and real-colour images are presented, as well as colour index maps. This data set, tailored for the study of the population characteristics of galaxy bulges, provides useful information on the colours of inner disks as well. In related papers, we have used them to quantify colour gradients in bulges, and age differentials between bulge and inner disk.

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arXiv1996

Ages of galaxy bulges and disks from optical and near-infrared colors

We compare optical and near-infrared colors of disks and bulges in a diameter-limited sample of inclined, bright, nearby, early-type spirals. Color profiles along wedge apertures at 15 degree from the major axis and on the minor axis on the side of the galaxy opposite to the dust lane are used to assign nominal colors for the inner disks (at 2 scale length) and for the bulges (~ 0.5 r_eff), respectively. We estimate that the effects of dust reddening and the cross-talk between the colors of the two components is negligible. We find that color differences (bulge -- disk) are very small: Delta(U-R)=0.126 +/- 0.165, Delta(R-K)=0.078 +/- 0.165. Disks tend to be bluer by an amount three times smaller than that reported by Bothun & Gregg (1990) for S0's. Color variations from galaxy to galaxy are much larger than color differences between disk and bulge in each galaxy. Probably, the underlying old population of disks and bulges is much more similar than the population paradigm would lead us to believe. Implied age differences, assuming identical metallicities, are less than 30%.

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arXiv1995

Stellar Populations of Bulges and Disks: New Insights from Near-IR Colours

Colours of spiral galaxies generally are affected by the underlying old stellar population, younger stars, and extinction by dust. Old and young populations generally can be disentangled using a combination of blue and near-infrared colours. Extinction effects are very hard to take into account, except for galaxies with special orientations. In this paper we give some results of one of the first studies of galactic bulges and disks in various optical and near-infrared bands, and its implications for the stellar populations of spirals.

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arXiv1995

Ages of galaxy bulges and disks from optical and near-infrared colours

For a sample of bright nearby early-type galaxies we have obtained surface photometry in bands ranging from U to K. Since the galaxies have inclinations larger than 50 deg. it is easy to separate bulges and disks. By measuring the colours in special regions, we minimize the effects of extinction, and by looking at B-K colour gradients we can show that for these type of spirals the colours mainly give information about stellar populations, and not extinction. We find that the differences between bulges and disks in all colours is very small, and using simple population models we can show that on the average the age difference between the bulge and the disk at 2 scale length is smaller than 30%, and much smaller if part of the difference is caused by a gradient in metallicity.

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arXiv1993

Colors and color gradients in bulges of galaxies

We have obtained surface photometry in U, B, R and I for a complete optically selected sample of 45 early-type spiral galaxies, to investigate the colors and color gradients of spiral bulges. Color profiles in U-R, B-R, U-B and R-I have been determined in wedges opening on the minor axes. Based on several criteria, we have defined a subsample of 18 objects whose colors are largely unaffected by dust. We find that the colors of bulges are predominantly bluer than those of ellipticals. This result holds even when bulges are compared to ellipticals of the same luminosity, and indicates that bulges are younger and/or more metal-poor than elliptical galaxies. Most bulges do not reach solar metallicities. Bulges show predominantly negative color gradients (bluer outward) that increases with bulge luminosity. The similarity with ellipticals suggests that the formation of the disk did not affect the stellar populations of the bulge in a major way.

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