arXiv subjects

Marc Balcells

Publications and source records attributed to Marc Balcells.

46 records · Page 3Linked to original sources

arXiv2008

Bulges of disk galaxies at intermediate redshifts.II. Nuclear, disk and global colours in the Groth Strip

We analyse colours of the nuclear regions of intermediate redshift disk galaxies, with the aim of obtaining empirical information of relative ages of bulges and disks at 0.1 < z < 1.3. We work with an apparent-diameter limited parent sample of 248 galaxies from the HST Groth Strip Survey. We apply a conservative criterion to identify bulges and potential precursors of present-day bulges based on nuclear surface brightness excess above the exponential profile of the outer parts. We measure bulge colours on wedge profiles opening on the semi-minor axis, and compare them to disk, and global galaxy colours. For 60% of galaxies with bulges, the rest-frame nuclear colour distribution shows a red sequence that is well fit by passive evolution models of various ages, while the remainder 40% scatters towards bluer colours. In contrast, galaxies without central brightness excess show typical colours of star forming population and lack a red sequence. We also see that, as in the local Universe, most of the minor axis colour profiles are negative (bluer outward), and fairly gentle, indicating that nuclear colours are not distinctly different from disk colours. This is corroborated when comparing nuclear, global and disk colours: these show strong correlations, for any value of the central brightness prominence of the bulge. Comparison with synthetic models of red sequence bulge colours suggests that such red bulges have stopped forming stars at an epoch earlier than ~ 1 Gyr before the observation. The correlation between nuclear and disk colours and the small colour gradients hints at an intertwined star formation history for bulges and disks: probably, most of our red bulges formed in a process in which truncation of star formation in the bulge did not destroy the disk.

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arXiv2008

The HST/ACS Coma Cluster Survey: I - Survey Objectives and Design

We describe the HST ACS Coma cluster Treasury survey, a deep two-passband imaging survey of one of the nearest rich clusters of galaxies, the Coma cluster (Abell 1656). The survey was designed to cover an area of 740 square arcmin in regions of different density of both galaxies and intergalactic medium within the cluster. The ACS failure of January 27th 2007 leaves the survey 28% complete, with 21 ACS pointings (230 square arcmin) complete, and partial data for a further 4 pointings (44 square arcmin). Predicted survey depth for 10 sigma detections for optimal photometry of point sources is g' = 27.6 in the F475W filter, and IC=26.8 mag in F814 (AB magnitudes). Initial simulations with artificially injected point sources show 90% recovered at magnitude limits of g' = 27.55 and IC = 26.65. For extended sources, the predicted 10 sigma limits for a 1 square arcsecond region are g' = 25.8 mag/sq. arcsec and IC = 25.0 mag/sq. arcsec. We highlight several motivating science goals of the survey, including study of the faint end of the cluster galaxy luminosity function, structural parameters of dwarf galaxies, stellar populations and their effect on colors and color gradients, evolution of morphological components in a dense environment, the nature of ultra compact dwarf galaxies, and globular cluster populations of cluster galaxies of a range of luminosities and types. This survey will also provide a local rich cluster benchmark for various well known global scaling relations and explore new relations pertaining to the nuclear properties of galaxies.

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arXiv2007

The z=0.8 precursors of today's bulges

We study the color structure of disk galaxies in the Groth strip at redshifts 0.1<z<1.2. Our aim is to test formation models in which bulges form before/after the disk. We find smooth color distributions with gentle outward blueing across the galaxy image: bulges are not distinctly redder than their disks; and bulge colors strongly correlate with global colors. The results suggest a roughly coeval evolution of bulges and disks. About 50% of the nuclei of galaxies with central light excesses above the outer exponential profile hold passively evolving red populations. The remainder 50% are galaxies with central blue colors similar to their disks. They may be bulges in formation, or the central parts of disks with non-exponential surface brightness profiles.

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arXiv2005

The Formation of Galactic Bulges

We summarise some recent results about nearby galactic bulges that are relevant to their formation. We highlight a number of significant advances in our understanding of the surface brightness profiles, stellar populations, and especially the very centers of spiral galaxies. We also view our own Milky Way as if it were an external galaxy. Our main conclusions are that bulges of early-type spirals (S0 -- Sb) contain central nuclear components, just like late-type spirals and most other types of galaxies. The luminosities of these central components correlate best with total bulge luminosity, and not as well with morphological type. Bulges of early-type spiral galaxies follow the fundamental plane and the colour/line strength vs. luminosity relations of elliptical galaxies. Although we have a reasonable idea about bulges of early-type spirals we know much less about late-type bulges. However, the close resemblance of our Milky Way Bulge to bulges in external disk galaxies makes us suspect that bulges of late-type spirals might be very similar as well.

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arXiv2003

Minor axis kinematics of 19 S0-Sbc bulges

We present minor axis kinematic profiles for a well-studied sample of 19 early- to intermediate-type disk galaxies. We introduce, for the first time, the use of single-burst stellar population (SSP) models to obtain stellar velocities, velocity dispersions and higher order Gauss-Hermite moments (h3,h4) from galaxy spectra in the near-infrared Ca II triplet region. SSP models, which employs the synthetic spectra of Vazdekis et al. 2003, provide a means to address the template-mismatch problem, and are shown to provide as good or better fits as traditional stellar templates. We anticipate the technique to be of particular use for high-redshift galaxy kinematics. We give the measurement of a recently defined CaT* index Cenarro et al. 2001a, and describe the global properties of the bulge kinematics as derived from the kinematic profiles. We detect small-amplitude minor-axis rotation, generally due to inner isophotal twists as a result of slightly triaxial bulges or misaligned inner disks; such inner features do not show peculiar colors or distinct CaT* index values. Velocity dispersion profiles, which extend well into the disk region, show a wide range of slopes. Flattened bulges tend to have shallower velocity dispersion profiles. The inferred similarity of bulge and disk radial velocity dispersions supports the interpretation of these bulges as thickened disks.

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arXiv2003

The near-IR Ca II triplet - sigma relation for bulges of spiral galaxies

We present measurements of the near-infrared Calcium II triplet (CaT, CaT*), Paschen (PaT) and Magnesium (MgI) indices for a well-studied sample of 19 bulges of early to intermediate spiral galaxies. We find that both the CaT* and CaT indices decrease with central velocity dispersion (sigma) with small scatter. This dependence is similar to that recently found by Cenarro (2002) for elliptical galaxies, implying an uniform CaT* -- sigma relation that applies to galaxies from ellipticals to intermediate-type spirals. The decrease of CaT and CaT* with sigma contrasts with the well-known increase of another alpha-element index, Mg_2, with sigma. We discuss the role of Ca underabundance ([Ca/Fe]<0) and IMF variations in the onset of the observed relations.

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arXiv2003

Spheroid and disk signatures in galaxy bulges

Recent progress on the structure and dynamics of bulges is reviewed. Those aspects that link galaxy bulges either to oblate spheroids akin to elliptical galaxies or to rapidly-rotating, flattened systems more nearly resembling the products of disk internal transformations, are highlighted. The analysis of surface brightness profiles derived from HST data is reviewed to show that unresolved nuclear components detected by HST have biased the determination of surface brightness profiles obtained from the ground; r^1/4 profiles are virtually nonexistent in galaxy bulges. Predictions from accretion N-body models on the shape of surface brightness profiles are discussed. The position of bulges on the Fundamental Plane of elliptical galaxies is examined to infer clues on bulge population ages and bulge dynamical structure. Kinematic diagnostics on the internal dynamics of bulges are examined. Finally, a new approach to the kinematic analysis of galaxies, based on the use of synthetic spectra of single stellar population models instead of the standard stellar templates, is presented.

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arXiv2002

Bulges on the Fundamental Plane of Early-Type Galaxies

In an ongoing effort to study the formation and evolution of galactic bulges we have investigated the position of 19 bulges of type S0-Sbc on the fundamental plane (FP). We find that bulges, both in B and K band, lie close to but slightly below the FP defined by ellipticals and S0s, i.e. are slightly brighter. There are hints that bulges of later morphological type are situated below the other bulges in our sample. The FP results are consistent with the picture, obtained from our recent analysis of HST-colours, that bulges are old, except for the Sbc galaxies. The fact that bulges lie so close to the FP of ellipticals and S0s implies that their formation epoch must have been similar, at most about 2.5 Gyr earlier, than cluster Es and S0s, and and that the surrounding disk does not significantly affect their structure.

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arXiv1999

Galactic Bulges from HST-NICMOS Observations: Ages and Dust

We present optical and near-infrared colour maps of the central regions of bulges of S0 and spiral galaxies obtained with WFPC2 and NICMOS on the HST. By combined use of HST and ground-based data the colour information spans from a few tens of pc to a few kpc. In almost all galaxies the colour profiles in the central 100-200 pc become more rapidly redder. We attribute the high central colour indices to a central concentration of dust. We infer an average extinction at the center of A_V = 0.6 - 1.0 mag. Several objects show central dust rings or disks at sub-kpc scales similar to those found by others in giant ellipticals. For galactic bulges of types S0 to Sb, the tightness of the B-I vs I-H relation suggests that the age spread among bulges of early type galaxies is small, of at most 2 Gyr. Colours at 1 R_eff, where we expect extinction to be negligible are similar to than those of elliptical galaxies in the Coma cluster, suggesting that these bulges formed at the same time as the bright galaxies in Coma. Furthermore the galaxy ages are found to be independent of their environment. Since it is likely that Coma was formed at redshift z > 3, our bulges, which are in groups and in the field, also must have been formed at this epoch. Bulges of early-type spirals cannot be formed by secular evolution of bars at recent epochs, since such bulges would be much younger. There are three galaxies of type Sbc and later, their bulges are younger and could perhaps arise from secular evolution of transient bars. Our results are in good agreement with semi-analytic predictions (Baugh et al. 1996, 1998, Kauffmann 1996), who also predict that bulges, in clusters and in the field, are as old as giant ellipticals in clusters.

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arXiv1997

Two tails in NGC 3656, and the major merger origin of shell and minor axis dust lane ellipticals

I report on the discovery of two faint (~ 26.8 Rmag/arcsec^2) tidal tails around the shell elliptical NGC 3656 (Arp 155). This galaxy had previously been interpreted as a case of accretion, or minor merger. The two tidal tails are inconsistent with a minor merger, and point instead to a disk-disk major merger origin. NGC 3656 extends Toomre's merger sequence toward normal elliptical galaxies, and hints at a major merger origin for shells and minor-axis dust lanes. A dwarf galaxy lies at the tip of one of the tidal tails. A prominent shell, which shows sharp azymuthal color discontinuities, belongs to a rotating dynamical component of young stars which includes the inner dust lane.

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