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Register a new userMetallicity gradients in the globular cluster systems of early-type galaxies: In-situ and accreted components?
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Massive early-type galaxies typically have two subpopulations of globular clusters (GCs) which often reveal radial colour (metallicity) gradients. Collating gradients from the literature, we show that the gradients in the metal-rich and metal-poor GC subpopulations are the same, within measurement uncertainties, in a given galaxy. Furthermore, these GC gradients are similar in strength to the {it stellar} metallicity gradient of the host galaxy. At the very largest radii (e.g. greater than 8 galaxy effective radii) there is some evidence that the GC gradients become flat with near constant mean metallicity. Using stellar metallicity gradients as a proxy, we probe the assembly histories of massive early-type galaxies with hydrodynamical simulations from the Magneticum suite of models. In particular, we measure the stellar metallicity gradient for the in-situ and accreted components over a similar radial range as those observed for GC subpopulations. We find that the in-situ and accreted stellar metallicity gradients are similar but have a larger scatter than the metal-rich and metal-poor GC subpopulations gradients in a given galaxy. We conclude that although metal-rich GCs are predominately formed during the in-situ phase and metal-poor GCs during the accretion phase of massive galaxy formation, they do not have a strict one-to-one connection.
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Here we examine the Milky Ways GC system to estimate the fraction of accreted versus in situ formed GCs. We first assemble a high quality database of ages and metallicities for 93 Milky Way GCs from literature deep colour-magnitude data. The age-metallicity relation for the Milky Ways GCs reveals two distinct tracks -- one with near constant old age of ~12.8 Gyr and the other branches to younger ages. We find that the latter young track is dominated by globular clusters associated with the Sagittarius and Canis Major dwarf galaxies. Despite being overly simplistic, its age-metallicity relation can be well represented by a simple closed box model with continuous star formation. The inferred chemical enrichment history is similar to that of the Large Magellanic Cloud, but is more enriched, at a given age, compared to the Small Magellanic Cloud. After excluding Sagittarius and Canis Major GCs, several young track GCs remain. Their horizontal branch morphologies are often red and hence classified as Young Halo objects, however they do not tend to reveal extended horizontal branches (a possible signature of an accreted remnant nucleus). Retrograde orbit GCs (a key signature of accretion) are commonly found in the young track. We also examine GCs that lie close to the Fornax-Leo-Sculptor great circle defined by several satellite galaxies. We find that several GCs are consistent with the young track and we speculate that they may have been accreted along with their host dwarf galaxy, whose nucleus may survive as a GC. Finally, we suggest that 27-47 GCs (about 1/4 of the entire system), from 6-8 dwarf galaxies, were accreted to build the Milky Way GC system we seen today.
We study the azimuthal distribution of globular clusters (GCs) in early-type galaxies and compare them to their host galaxies using data from the ACS Virgo Cluster Survey. We find that in host galaxies with visible elongation (epsilon > 0.2) and intermediate to high luminosities (M_z<-19), the GCs are preferentially aligned along the major axis of the stellar light. The red (metal-rich) GC subpopulations show strong alignment with the major axis of the host galaxy, which supports the notion that these GCs are associated with metal-rich field stars. The metal-rich GCs in lenticular galaxies show signs of being more strongly associated with disks rather than bulges. Surprisingly, we find that the blue (metal-poor) GCs can also show the same correlation. If the metal-poor GCs are part of the early formation of the halo and built up through mergers, then our results support a picture where halo formation and merging occur anisotropically, and where the present day major axis is an indicator of the preferred merging axis.
We present a new model for the evolution of gas phase metallicity gradients in galaxies from first principles. We show that metallicity gradients depend on four ratios that collectively describe the metal equilibration timescale, production, transport, consumption, and loss. Our model finds that most galaxy metallicity gradients are in equilibrium at all redshifts. When normalized by metal diffusion, metallicity gradients are governed by the competition between radial advection, metal production, and accretion of metal-poor gas from the cosmic web. The model naturally explains the varying gradients measured in local spirals, local dwarfs, and high-redshift star-forming galaxies. We use the model to study the cosmic evolution of gradients across redshift, showing that the gradient in Milky Way-like galaxies has steepened over time, in good agreement with both observations and simulations. We also predict the evolution of metallicity gradients with redshift in galaxy samples constructed using both matched stellar masses and matched abundances. Our model shows that massive galaxies transition from the advection-dominated to the accretion-dominated regime from high to low redshifts, which mirrors the transition from gravity-driven to star formation feedback-driven turbulence. Lastly, we show that gradients in local ultraluminous infrared galaxies (major mergers) and inverted gradients seen both in the local and high-redshift galaxies may not be in equilibrium. In subsequent papers in this series, we show that the model also explains the observed relationship between galaxy mass and metallicity gradients, and between metallicity gradients and galaxy kinematics.
We present a study of globular clusters (GCs) in 17 relatively nearby early-type galaxies, based on deep HST/WFPC2 F555W and F814W images. We compare color distributions, cluster sizes and luminosity functions with those of GCs in the Milky Way. In nearly all cases, a KMM test returns a high confidence level for the hypothesis that a sum of two Gaussians provides a better fit to the observed color distribution than a single Gaussian, although histograms of the V-I distribution are not always obviously bimodal. The blue and red peak colors both correlate with absolute host galaxy B band magnitude and central velocity dispersion (at about the 2-3 sigma level), but we see no clear correlation with host galaxy V-I or J-K color. Red GCs are generally smaller than blue GCs by about 20%. The size difference is seen at all radii and exists also in the Milky Way and Sombrero (M104) spiral galaxies. Fitting t5 functions to the luminosity functions of blue and red GC populations separately, we find that the V-band turn-over of the blue GCs is generally brighter than that of the red ones by about 0.3 mag, as expected if the two GC populations have similar ages and mass distributions but different metallicities. Brighter than M_V ~ -7.5, the luminosity functions (LFs) are well approximated by power-laws with an exponent of about -1.75. This is similar to the LF for young star clusters, suggesting that young and old globular clusters form by the same basic mechanism. We discuss scenarios for GC formation and conclude that our data appear to favor ``in-situ models in which all GCs in a galaxy formed after the main body of the proto-galaxy had assembled into a single potential well.
We search for ongoing major dry-mergers in a well selected sample of local Brightest Cluster Galaxies (BCGs) from the C4 cluster catalogue. 18 out of 515 early-type BCGs with redshift between 0.03 and 0.12 are found to be in major dry-mergers, which are selected as pairs (or triples) with $r$-band magnitude difference $dm<1.5$ and projected separation $rp<30$ kpc, and showing signatures of interaction in the form of significant asymmetry in residual images. We find that the fraction of BCGs in major dry-mergers increases with the richness of the clusters, consistent with the fact that richer clusters usually have more massive (or luminous) BCGs. We estimate that present-day early-type BCGs may have experienced on average $sim 0.6 (tmerge/0.3Gyr)^{-1}$ major dry-mergers and through this process increases their luminosity (mass) by $15% (tmerge/0.3Gyr)^{-1} (fmass/0.5)$ on average since $z=0.7$, where $tmerge$ is the merging timescale and $fmass$ is the mean mass fraction of companion galaxies added to the central ones. We also find that major dry-mergers do not seem to elevate radio activities in BCGs. Our study shows that major dry-mergers involving BCGs in clusters of galaxies are not rare in the local Universe, and they are an important channel for the formation and evolution of BCGs.
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