Radial Color Profiles Research Articles

Detection of a Spatially Extended Stellar Population in M33: A Shallow Stellar Halo?

We analyze the outer regions of M33, beyond 15 kpc in projected distance from its center, using Subaru/Hyper Suprime-Cam multicolor imaging. We identify red giant branch (RGB) stars and red clump (RC) stars using the surface-gravity-sensitive NB515 filter for the RGB sample and a multicolor selection for both samples. We construct the radial surface density profiles of these RGB and RC stars and find that M33 has an extended stellar population with a shallow power-law index of α > −3, depending on the intensity of the contamination. This result represents a flatter profile than the stellar halo that was detected by the previous study focusing on the central region, suggesting that M33 may have a double-structured halo component, i.e., inner/outer halos or a very extended disk. Also, the slope of this extended component is shallower than those typically found for halos in large galaxies, implying intermediate-mass galaxies may have different formation mechanisms (e.g., tidal interaction) from large spirals. We also analyze the radial color profiles of RC/RGB stars and detect a radial gradient, consistent with the presence of an old and/or metal-poor population in the outer region of M33, thereby supporting our proposal that the stellar halo extends beyond 15 kpc. Finally, we estimate that the surface brightness of this extended component is μ V = 35.72 ± 0.08 mag arcsec−2. If our detected component is the stellar halo, this estimated value is consistent with the detection limit of previous observations.

Structure and Color Gradients of Ultradiffuse Galaxies in Distant Massive Galaxy Clusters

We have measured structural parameters and radial color profiles of 108 ultradiffuse galaxies (UDGs), carefully selected from six distant massive galaxy clusters in the Hubble Frontier Fields (HFF) in a redshift range from 0.308 to 0.545. Our best-fitting GALFIT models show that the HFF UDGs have a median Sérsic index of 1.09, which is close to 0.86 for local UDGs in the Coma cluster. The median axis-ratio value is 0.68 for HFF UDGs and 0.74 for Coma UDGs, respectively. The structural similarity between HFF and Coma UDGs suggests that they are the same kind of galaxies seen at different times, and that the structures of UDGs do not change at least for several billion years. By checking the distribution of HFF UDGs in the rest-frame UVJ and UVI diagrams, we find a large fraction of them are star forming. Furthermore, a majority of HFF UDGs show small U − V color gradients within the 1 ∗ R e,SMA region, and the fluctuation of the median radial color profile of HFF UDGs is smaller than 0.1 mag, which is compatible to Coma UDGs. Our results indicate that cluster UDGs may fade or quench in a self-similar way, irrespective of the radial distance, in less than ∼4 Gyr.

The Lyman Alpha Reference Sample

Context. The Lyman α Reference Sample (LARS) of 14 star-forming galaxies offers a wealth of insight into the workings of these local analogs to high-redshift star-forming galaxies. The sample has been well-studied in terms of Lyα and other emission line properties, such as H I mass, gas kinematics, and morphology. Aims. We analyze deep surface photometry of the LARS sample in UBIK broadband imaging obtained at the Nordic Optical Telescope and the Canada-France-Hawaii Telescope, and juxtaposition their derived properties with a sample of local high-redshift galaxy analogs, namely, with blue compact galaxies (BCGs). Methods. We construct radial surface brightness and color profiles with both elliptical and isophotal integration, as well as RGB images, deep contours, color maps, a burst fraction estimate, and a radial mass-to-light ratio profile for each LARS galaxy. Standard morphological parameters like asymmetry, clumpiness, the Gini and M20 coefficients are also obtained and analyzed, as well as isophotal asymmetry profiles for each galaxy. In this context, we compare the LARS sample to the properties of the BCG sample and highlight the differences. Results. Several of these diagnostics indicate that the LARS galaxies have highly disturbed morphologies even at the level of the faintest outer isophotes, with no hint at a regular underlying population, as found in many BCG sample galaxies. The ground-based photometry reaches isophotes down to ~28 mag arcsec−2, while the space-based data reach only ~26 mag arcsec−2. The ground-based observations therefore reveal previously unexplored isophotes of the LARS galaxies. The burst fraction estimate suggests a spatially more extended burst region in LARS than in the BCGs. Comparison to stellar evolutionary models in color–color diagrams reveals complex behavior of the radial color profiles, often inconsistent with a single stellar population of any age and metallicity, but instead suggesting a mixture of at least two stellar populations with a typical young mass fraction in the range 0.1–1%. Conclusions. The galaxies in the LARS sample appear to be in earlier stages of a merger event compared to the BCGs. Standard morphological diagnostics like asymmetry, clumpiness, Gini and M20 coefficients cannot separate the two samples, although an isophotal asymmetry profile successfully captures the average difference in morphology. These morphological diagnostics do not show any correlation with the equivalent width or the escape fraction of Lyman Alpha.

Characterizing Intracluster Light in the Hubble Frontier Fields

Abstract We investigate the intracluster light (ICL) in the six Hubble Frontier Field clusters at 0.3 < z < 0.6 . We employ a new method, which is free from any functional form of the ICL profile, and exploit the unprecedented depth of this Hubble Space Telescope imaging to map the ICL’s diffuse light out to clustrocentric radii R ∼ 300 kpc ( μ ICL ∼ 27 mag arcsec−2). From these maps, we construct radial color and stellar mass profiles via SED fitting and find clear negative color gradients in all systems with increasing distance from the Brightest Cluster Galaxy (BCG). While this implies older/more metal-rich stellar components in the inner part of the ICL, we find that the ICL mostly consists of a ≲ 2 Gyr population, and plausibly originated with log M * / M ⊙ ≲ 10 cluster galaxies. Furthermore, we find that 10%–15% of the ICL’s mass at large radii ( ≳ 150 kpc) lies in a younger/bluer stellar population (∼1 Gyr), a phenomenon not seen in local samples. We attribute this light to the higher fraction of star-forming/(post-)starburst galaxies in clusters at z ∼ 0.5 . Ultimately, we find the ICL’s total mass to be log M * ICL / M ⊙ ∼ 11 –12, constituting 5%–20% of the clusters’ total stellar mass, or about half of the value at z ∼ 0 . The above implies distinct formation histories for the ICL and BCGs/other massive cluster galaxies; i.e., the ICL at this epoch is still being constructed rapidly ( ∼ 40 M ⊙ yr−1), while the BCGs have mostly completed their evolution. To be consistent with the ICL measurements of local massive clusters, such as Virgo, our data suggest mass acquisition mainly from quiescent cluster galaxies is the principal source of ICL material in the subsequent ∼5 Gyr of cosmic time.

UVI colour gradients of 0.4 < z < 1.4 star-forming main-sequence galaxies in CANDELS: dust extinction and star formation profiles

This paper uses radial colour profiles to infer the distributions of dust, gas and star formation in z=0.4-1.4 star-forming main sequence galaxies. We start with the standard UVJ-based method to estimate dust extinction and specific star formation rate (sSFR). By replacing J with I band, a new calibration method suitable for use with ACS+WFC3 data is created (i.e. UVI diagram). Using a multi-wavelength multi-aperture photometry catalogue based on CANDELS, UVI colour profiles of 1328 galaxies are stacked in stellar mass and redshift bins. The resulting colour gradients, covering a radial range of 0.2--2.0 effective radii, increase strongly with galaxy mass and with global $A_V$. Colour gradient directions are nearly parallel to the Calzetti extinction vector, indicating that dust plays a more important role than stellar population variations. With our calibration, the resulting $A_V$ profiles fall much more slowly than stellar mass profiles over the measured radial range. sSFR gradients are nearly flat without central quenching signatures, except for $M_*>10^{10.5} M_{\odot}$, where central declines of 20--25 per cent are observed. Both sets of profiles agree well with previous radial sSFR and (continuum) $A_V$ measurements. They are also consistent with the sSFR profiles and, if assuming a radially constant gas-to-dust ratio, gas profiles in recent hydrodynamic models. We finally discuss the striking findings that SFR scales with stellar mass density in the inner parts of galaxies, and that dust content is high in the outer parts despite low stellar-mass surface densities there.

Automated bar detection in local disk galaxies from the SDSS

This paper describes an automatic isophotal fitting procedure that succeeds, without the support of any visual inspection of neither the images nor the ellipticity/P.A. radial profiles, at extracting a fairly pure sample of barred LTGs among thousands of optical images from the SDSS. The procedure relies on the methods described in Consolandi et al. (2016) to robustly extract the photometrical properties of a large sample of local SDSS galaxies and is tailored to extract bars on the basis of their well-known peculiarities in their P.A. and ellipticity profiles. It has been run on a sample of 5853 galaxies in the Coma and Local supercluster. The procedure extracted for each galaxy a color, an ellipticity and a position angle radial profile of the ellipses fitted to the isophotes. Examining automatically the profiles of 922 face-on late-type galaxies (B/A >0.7) the procedure found that ~ 36 % are barred. The local bar fraction strongly increases with stellar mass. The sample of barred galaxies is used to construct a set of template radial color profiles in order to test the impact of the barred galaxy population on the average color profiles shown by Consolandi et al. (2016) and to test the bar-quenching scenario proposed in Gavazzi et al. (2015). The radial color profile of barred galaxy shows that bars are on average redder than their surrounding disk producing an outside-in gradient toward red in correspondence of their corotation radius. The distribution of the extension of the deprojected length of the bar suggests that bars have strong impacts on the gradients of averaged color profiles. The dependence of the profiles on the mass is consistent with the bar-quenching scenario, i.e. more massive barred galaxies have redder colors (hence older stellar population and suppressed star formation) inside their corotation radius with respect to their lower mass counterparts.

Hubbleimaging of V1331 Cygni: proper motion study of its circumstellar structures

The young star V1331 Cyg received previous attention because it is surrounded by an optical, arc-like reflection nebula. V1331 Cyg is commonly considered to be a candidate for an object that has undergone an FU-Ori (FUOR) the outbreak in the past. This in turn could lead to a time-varying appearance of the dusty arcs that may be revealed by multi-epoch imaging. In particular, a radial colour analysis of the dust arcs can then be attempted to check whether radial grain size distribution was modified by a previous FUOR wind. Second-epoch imaging of V1331 Cyg was obtained by us in 2009 using the Hubble Space Telescope (HST). By comparing this to archival HST data from 2000, we studied the time evolution of the circumstellar nebulae. After a point spread function subtraction using model point spread functions, we used customised routines to perform a proper motion analysis. The nebula expansion was first derived by deconvolving and correlating the two-epoch radial brightness profiles. Additional data from other facilities TLS, UKIDSS, SPITZER, and HERSCHEL were also incorporated to improve our understanding of the star in terms of environment, viewing angle, bipolar outflow length, and the FUOR phenomenon. The derived radial colour profiles do not indicate a spatial separation of the dust grain sizes. The HERSCHEL 160 micron images show for the time thermal emission from dust probably residing in the outer arc. By viewing V1331 Cyg almost pole-on, the length of the bipolar outflow exceeds previous estimates by far. The outer arc expansion timescale is consistent with the implantation time of the CO torus, which supports the hypothesis of an outburst that occurred a few thousand years ago. The azimuthal colour variation of the outer arc is probably due to changes of the scattering angle, imposed by a tilt or helical geometry of the dust configuration.

THE DIFFERENTIAL SIZE GROWTH OF FIELD AND CLUSTER GALAXIES ATz= 2.1 USING THE ZFOURGE SURVEY

There is ongoing debate regarding the extent that environment affects galaxy size growth beyond z>1. To investigate the differences in star-forming and quiescent galaxy properties as a function of environment at z=2.1, we create a mass-complete sample of 59 cluster galaxies Spitler et al. (2012) and 478 field galaxies with log(M)>9 using photometric redshifts from the ZFOURGE survey. We compare the mass-size relation of field and cluster galaxies using measured galaxy semi-major axis half-light radii ($r_{1/2,maj}$) from CANDELS HST/F160W imaging. We find consistent mass normalized (log(M)=10.7) sizes for quiescent field galaxies ($r_{1/2,maj}=1.81\pm0.29$ kpc) and quiescent cluster galaxies ($r_{1/2,maj}=2.17\pm0.63$ kpc). The mass normalized size of star-forming cluster galaxies ($r_{1/2,maj}=4.00\pm0.26$ kpc ) is 12% larger (KS test $2.1\sigma$) than star-forming field galaxies ($r_{1/2,maj}=3.57\pm0.10$ kpc). From the mass-color relation we find that quiescent field galaxies with 9.7<log(M)<10.4 are slightly redder (KS test $3.6\sigma$) than quiescent cluster galaxies, while cluster and field quiescent galaxies with log(M)>10.4 have consistent colors. We find that star-forming cluster galaxies are on average 20% redder than star-forming field galaxies at all masses. Furthermore, we stack galaxy images to measure average radial color profiles as a function of mass. Negative color gradients are only present for massive star-forming field and cluster galaxies with log(M)>10.4, the remaining galaxy masses and types have flat profiles. Our results suggest given the observed differences in size and color of star-forming field and cluster galaxies, that the environment has begun to influence/accelerate their evolution. However, the lack of differences between field and cluster quiescent galaxies indicates that the environment has not begun to significantly influence their evolution at z~2.

QUENCHING DEPENDS ON MORPHOLOGIES: IMPLICATIONS FROM THE ULTRAVIOLET-OPTICAL RADIAL COLOR DISTRIBUTIONS IN GREEN VALLEY GALAXIES

In this Letter, we analyse the radial UV-optical color distributions in a sample of low redshift green valley (GV) galaxies, with the Galaxy Evolution Explorer (GALEX)+Sloan Digital Sky Survey (SDSS) images, to investigate how the residual recent star formation distribute in these galaxies. We find that the dust-corrected $u-r$ colors of early-type galaxies (ETGs) are flat out to $R_{90}$, while the colors turn blue monotonously when $r>0.5R_{50}$ for late-type galaxies (LTGs). More than a half of the ETGs are blue-cored and have remarkable positive NUV$-r$ color gradients, suggesting that their star formation are centrally concentrated; the rest have flat color distributions out to $R_{90}$. The centrally concentrated star formation activity in a large portion of ETGs is confirmed by the SDSS spectroscopy, showing that $\sim$50 % ETGs have EW(H$\rm \alpha$)$>6.0$ \AA. For the LTGs, 95% of them show uniform radial color profiles, which can be interpreted as a red bulge plus an extended blue disk. The links between the two kinds of ETGs, e.g., those objects having remarkable "blue-cored" and those having flat color gradients, are less known and require future investigations. It is suggested that the LTGs follow a general picture that quenching first occur in the core regions, and then finally extend to the rest of the galaxy. Our results can be re-examined and have important implications for the IFU surveys, such as MaNGA and SAMI.

The SAURON project - XXI. The spatially resolved UV-line strength relations of early-type galaxies

The unexpected rising flux of early-type galaxies at decreasing ultraviolet (UV) wavelengths is a long-standing mystery. One important observational constraint is the correlation between UV-optical colours and Mg2 line strengths found by Burstein et al. (1988). The simplest interpretation of this phenomenon is that the UV strength is related to the Mg line strength. Under this assumption, we expect galaxies with larger Mg gradients to have larger UV colour gradients. By combining UV imaging from GALEX, optical imaging from MDM and SAURON integral-field spectroscopy, we investigate the spatially-resolved relationships between UV colours and stellar population properties of 34 early-type galaxies from the SAURON survey sample. We find that galaxies with old stellar populations show tight correlations between the FUV colours (FUV-V and FUV-NUV) and the Mgb index, H{\beta} index and metallicity [Z/H]. The equivalent correlations for the Fe5015 index, {\alpha}-enhancement [{\alpha}/Fe] and age are present but weaker. We have also derived logarithmic internal radial colour, measured line strength and derived stellar population gradients for each galaxy and again found a strong dependence of the FUV-V and FUV-NUV colour gradients on both the Mg b line strength and the metallicity gradients for galaxies with old stellar populations. In particular, global gradients of Mg b and [Z/H] with respect to the UV colour across galaxies are consistent with their local gradients within galaxies, suggesting that the global correlations also hold locally. From a simple model based on multi-band colour fits of UV upturn and UV-weak galaxies, we have identified a plausible range of parameters that reproduces the observed radial colour profiles. In these models, the centers of elliptical galaxies, where the UV flux is strong, are enhanced in metals by roughly 60% compared to UV-weak regions.

Mass segregation and fractal substructure in young massive clusters - I. The McLuster code and method calibration

By analysing models of the young massive cluster R136 in 30 Doradus, set-up using the herewith introduced and publicly made available code McLuster, we investigate and compare different methods for detecting and quantifying mass segregation and substructure in non-seeing limited N-body data. For this purpose we generate star cluster models with different degrees of mass segregation and fractal substructure and analyse them. We quantify mass segregation by measuring, from the projected 2d model data, the mass function slope in radial annuli, by looking for colour gradients in radial colour profiles, by measuring Allison's Lambda parameter, and by determining the local stellar surface density around each star. We find that these methods for quantifying mass segregation often produce ambiguous results. Most reliable for detecting mass segregation is the mass function slope method, whereas the colour gradient method is the least practical in an R136-like configuration. The other two methods are more sensitive to low degrees of mass segregation but are computationally much more demanding. We also discuss the effect of binaries on these measures. Moreover, we quantify substructure by looking at the projected radial stellar density profile, by comparing projected azimuthal stellar density profiles, and by determining Cartwright & Whitworth's Q parameter. We find that only high degrees of substructure affect the projected radial density profile, whereas the projected azimuthal density profile is very sensitive to substructure. The Q parameter is also sensitive to substructure but its absolute value shows a dependence on the radial density gradient of the cluster and is strongly influenced by binaries. (abridged)

Nearby early-type galaxies with ionized gas: the UV emission from GALEX observations★

We present GALEX far-ultraviolet (FUV, $\lambda_{eff}$=1538 \AA) and near-ultraviolet (NUV, $\lambda_{eff}$=2316 \AA) surface photometry of 40 early-type galaxies (ETGs) selected from a wider sample of 65 nearby ETGs showing emission lines in their optical spectra. We derive FUV and NUV surface brightness profiles, (FUV-NUV) colour profiles and D$_{25}$ integrated magnitudes. We extend the photometric study to the optical {\it r} band from SDSS imaging for 14 of these ETGs. In general, the (FUV-NUV) radial colour profiles become redder with galactocentric distance in both rejuvenated ($\leq 4$ Gyr) and old ETGs. Colour profiles of NGC 1533, NGC 2962, NGC 2974, NGC 3489, and IC 5063 show rings and/or arm-like structures, bluer than the body of the galaxy, suggesting the presence of recent star formation. Although seven of our ETGs show shell systems in their optical image, only NGC 7135 displays shells in the UV bands. We characterize the UV and optical surface brightness profiles, along the major axis, using a Sersic law. The Sersic law exponent, $n$, varies from 1 to 16 in the UV bands. S0 galaxies tend to have lower values of $n$ ($\leq5$). The Sersic law exponent $n=4$ seems to be a watershed: ETGs with $n>4$ tend to have [$\alpha$/Fe] greater than 0.15, implying a short star-formation time scale. We find a significant correlation between the FUV$-$NUV colour and central velocity dispersions $\sigma$, with the UV colours getting bluer at larger $\sigma$. This trend is likely driven by a combined effect of `downsizing' and of the mass-metallicity relation.

The red haloes of SDSS low surface brightness disc galaxies

The faint stellar halos of galaxies contain key information about the oldest stars and the process of galaxy formation. A previous study of stacked SDSS images of disk galaxies has revealed a halo with an abnormally red r-i colour, seemingly inconsistent with our current understanding of stellar halos. Here, we investigate the statistical properties of the faint envelopes of low surface brightness disk galaxies to look for further support for a red excess. 1510 edge-on low surface brightness galaxies were selected from the SDSS Data Release 5, rescaled to the same apparent size, aligned and stacked. This procedure allows us to reach a surface brightness of mu_g ~ 31 mag arcsec^-2. After a careful assessment of instrumental light scattering effects, we derive median and average radial surface brightness and colour profiles in g,r and i. The sample is then divided into 3 subsamples according to g-r colour. All three samples exhibit a red colour excess in r-i in the thick disk/halo region. The halo colours of the full sample, g-r = 0.60+-0.15 and r-i = 0.80+-0.15, are found to be incompatible with the colours of any normal type of stellar population. The fact that no similar colour anomaly is seen at comparable surface brightness levels along the disk rules out a sky subtraction residual as the source of the extreme colours. A number of possible explanations for these abnormally red halos are discussed. We find that two different scenarios -- dust extinction of extragalactic background light and a stellar population with a very bottom-heavy initial mass function -- appear to be broadly consistent with our observations and with similar red excesses reported in the halos of other types of galaxies.

The SAURON project - XIII. SAURON-GALEX study of early-type galaxies: the ultraviolet colour-magnitude relations and Fundamental Planes

We present GALEX far (FUV) and near (NUV) ultraviolet imaging of 34 nearby early-type galaxies from the SAURON representative sample of 48 E/S0 galaxies, all of which have ground-based optical imaging from the MDM Observatory. The surface brightness profiles of nine galaxies show regions with blue UV-optical colours suggesting recent star formation. Five of these show blue integrated UV-optical colours that set them aside in the NUV integrated colour-magnitude relation. They also have other properties confirming they have had recent star formation, in particular H_beta absorption higher than expected for a quiescent population and a higher CO detection rate. NUV-blue galaxies are generally drawn from the lower stellar velocity dispersion and thus lower dynamical mass part of the sample. We have also constructed the first UV Fundamental Planes and show that NUV blue galaxies bias the slopes and increase the scatters. If they are eliminated the fits get closer to expectations from the virial theorem. Although our analysis is based on a limited sample, it seems that a dominant fraction of the tilt and scatter of the UV Fundamental Planes is due to the presence of young stars in preferentially low-mass early-type galaxies. Interestingly, the UV-optical radial colour profiles reveal a variety of behaviours, with many galaxies showing signs of recent star formation, a central UV-upturn phenomenon, smooth but large-scale age and metallicity gradients, and in many cases a combination of these. In addition, FUV-NUV and FUV-V colours even bluer than those normally associated with UV-upturn galaxies are observed at the centre of some quiescent galaxies. Four out of the five UV-upturn galaxies are slow rotators. These objects should thus pose interesting challenges to stellar evolutionary models of the UV-upturn.

Color gradients of spiral disks in the Sloan Digital Sky Survey

We investigate the radial color gradients of galactic disks using a sample of ∼ 20 000 face-on spiral galaxies selected from the fourth data release of the Sloan Digital Sky Survey (SDSS-DR4). We combine galaxies with similar concentrations, sizes and luminosities to construct composite galaxies, and then measure their color profiles by stacking the azimuthally averaged radial color profiles of all the member galaxies. Except for the smallest galaxies (R50 < 3 kpc), almost all galaxies show negative disk color gradients with mean g – r gradient Ggr = −0.006 mag kpc−1 and r – z gradient Grz = −0.018 mag kpc−1. The disk color gradients are independent of the morphological types of galaxies and strongly dependent on the disk surface brightness μd, with lower surface brightness galactic disks having steeper color gradients. We quantify the intrinsic correlation between color gradients and surface brightness as Ggr = −0.011 μd + 0.233 and Grz = −0.015 μd + 0.324. These quantified correlations provide tight observational constraints on the formation and evolution models of spiral galaxies.

THE ORIGIN OF COLOR GRADIENTS IN EARLY-TYPE SYSTEMS AND THEIR COMPACTNESS AT HIGH- z

In this Letter, we present mean optical+NIR color gradient estimates for 5080 early-type galaxies (ETGs) in the grizYJHK wavebands of the Sloan Digital Sky Survey (SDSS) plus UKIRT Infrared Deep Sky Survey (UKIDSS). The color gradient is estimated as the logarithmic slope of the radial color profile in ETGs. With such a large sample size, we study the variation of the mean color gradient as a function of waveband with unprecedented accuracy. We find that (i) color gradients are mainly due, on average, to a metallicity variation of about -0.4dex per decade in galaxy radius; and (ii) a small, but significant, positive age gradient is present, on average, in ETGs, with the inner stellar population being slightly younger, by ~0.1dex per radial decade, than the outer one. Also, we show that the presence of a positive mean age gradient in ETGs, as found in the present study, implies their effective radius to be smaller at high z, consistent with observations.

The nature of late-type spiral galaxies: structural parameters, optical and near-infrared colour profiles and dust extinction

We analyse V- and H-band surface photometry of a sample of 18 Sb–Sd galaxies. Combining high-resolution Hubble Space Telescope (HST) images with ground-based near-infrared observations, we extract photometric profiles, which cover the whole disc and provide the highest possible resolution. This is the first photometric study of late-type spirals for which the stellar kinematics have been measured. For 10 out of the 18 galaxies, HST data in both F160W (H) and F606W (V) are available, and, for those, we present colour maps and radial colour profiles at the resolution of the HST. Colours vary significantly from galaxy to galaxy, but tend to be highly homogeneous within each galaxy, with smooth and flat colour profiles. Some of the colour maps show jumps in the inner regions, likely due to dust. We determine extinction maps in an almost model-independent way using the V−H colour map and the SAURON Mgb absorption line map of Ganda et al. The maps show that AV ranges from 0 to 2 mag, in the centre from 0 to 1.5 mag, in agreement with the models of Tuffs et al. We describe the surface brightness profiles as the superposition of an exponential disc and a Sérsic bulge. The bulges are small (0.1–2.5 kpc), and show a shape parameter n ranging from ≈0.7 to 3, with a mean value smaller than two: well below the value for the classical de Vaucouleurs bulges. Most galaxies (16 out of 18) show a central light excess above the Sérsic fit to the bulge, which can be interpreted as a nuclear cluster, as shown by previous studies. We provide zero-order estimates for the magnitude of these components. We discuss the correlations among the structural galaxy parameters and with other relevant quantities, such as Hubble type and stellar velocity dispersion. We compare these results with a recent paper by Graham & Worley, who present a summary of most of the NIR surface photometry of spirals in the literature. For both early- and late-type spirals, bulge luminosity strongly correlates with central velocity dispersion; at constant velocity dispersion, later type bulges are larger and less dense, and have lower Sérsic n-values.

Chemical evolution with radial mixing

Models of the chemical evolution of our Galaxy are extended to include radial migration of stars and flow of gas through the disc. The models track the production of both iron and alpha elements. A model is chosen that provides an excellent fit to the metallicity distribution of stars in the Geneva-Copenhagen survey (GCS) of the solar neighbourhood, and a good fit to the local Hess diagram. The model provides a good fit to the distribution of GCS stars in the age-metallicity plane although this plane was not used in the fitting process. Although this model's star-formation rate is monotonic declining, its disc naturally splits into an alpha-enhanced thick disc and a normal thin disc. In particular the model's distribution of stars in the ([O/Fe],[Fe/H]) plane resembles that of Galactic stars in displaying a ridge line for each disc. The thin-disc's ridge line is entirely due to stellar migration and there is the characteristic variation of stellar angular momentum along it that has been noted by Haywood in survey data. Radial mixing of stellar populations with high sigma_z from inner regions of the disc to the solar neighbourhood provides a natural explanation of why measurements yield a steeper increase of sigma_z with age than predicted by theory. The metallicity gradient in the ISM is predicted to be steeper than in earlier models, but appears to be in good agreement with data for both our Galaxy and external galaxies. The models are inconsistent with a cutoff in the star-formation rate at low gas surface densities. The absolute magnitude of the disc is given as a function of time in several photometric bands, and radial colour profiles are plotted for representative times.

Galaxy Zoo: a sample of blue early-type galaxies at low redshift

We report the discovery of a population of nearby, blue early-type galaxies with high star formation rates (0.5 < SFR < 50 Msun/yr). They are identified by their visual morphology as provided by Galaxy Zoo for SDSS DR6 and their u-r colour. We select a volume-limited sample in the redshift range 0.02 < z < 0.05, corresponding to luminosities of approximately L* and above, and with u-r colours significantly bluer than the red sequence. We confirm the early-type morphology of the objects in this sample and investigate their environmental dependence and star formation properties. Blue early-type galaxies tend to live in lower-density environments than `normal' red sequence early-types and make up 5.7 +/-0.4% of the low-redshift early-type galaxy population. We find that such blue early-type galaxies are virtually absent at high velocity dispersions above 200 km/s. Our analysis uses emission line diganostic diagrams and we find that ~25% of them are actively starforming, while another ~25% host both star formation and an AGN. Another ~12% are AGN. The remaining 38% show no strong emission lines. When present and uncontaminated by an AGN contribution, the star formation is generally intense. We consider star formation rates derived from Halpha, u-band and infrared luminosities, and radial colour profiles, and conclude that the star formation is spatially extended. Of those objects that are not currently undergoing star formation must have ceased doing so recently in order to account for their blue optical colours. The gas phase metallicity of the actively starforming blue early-types galaxies is supersolar in all cases. We discuss the place of these objects in the context of galaxy formation. A catalogue of all 204 blue early-type galaxies in our sample, including star formation rates and emission line classification, is provided.

STELLAR DISKS OF COLLISIONAL RING GALAXIES. I. NEW MULTIBAND IMAGES, RADIAL INTENSITY AND COLOR PROFILES, AND CONFRONTATION WITHN-BODY SIMULATIONS

We present new multi-band imaging data in the optical (BV RI and Hα) and near infrared bands (JHK) of 15 candidate ring galaxies from the sample of Appleton & Struck-Marcell (1987). We use these data to obtain color composite images, global magnitudes and colors of both the ring galaxy and its companion(s), and radial profiles of intensity and colors. We find that only nine of the observed galaxies have multi-band morphologies expected for the classical collisional scenario of ring formation, indicating the high degree of contamination of the ring galaxy sample by galaxies without a clear ring morphology. The radial intensity profiles, obtained by masking the off-centered nucleus, peak at the position of the ring, with the profiles in the continuum bands broader than that in the Hα line. The images as well as the radial intensity and color profiles clearly demonstrate the existence of the pre-collisional stellar disk outside the star-forming ring, which is in general bluer than the disk internal to the ring. The stellar disk seems to have retained its size, with the disk outside the ring having a shorter exponential scale length as compared to the values expected in normal spiral galaxies of comparable masses. The rings in our sample of galaxies are found to be located preferentially at around half-way through the stellar disk. The most likely reason for this preference is bias against detecting rings when they are close to the center (they would be confused with the resonant rings), and at the edge of the disk the gas surface density may be below the critical density required for star formation. Most of the observed characteristics point to relatively recent collisions (< 80 Myr ago) according to the N-body simulations of Gerber et al. (1996). Subject headings: galaxies: photometry — galaxies: interactions