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Register a new userA Novel Approach to Constrain the Mass Ratio of Minor Mergers in Elliptical Galaxies: Application to NGC 4889, the Brightest Cluster Galaxy in Coma
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Authors
Meng Gu
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Minor mergers are thought to be important for the build-up and structural evolution of massive elliptical galaxies. In this work, we report the discovery of a system of four shell features in NGC 4889, one of the brightest members of the Coma cluster, using optical images taken with the Hubble Space Telescope and the Sloan Digital Sky Survey. The shells are well aligned with the major axis of the host and are likely to have been formed by the accretion of a small satellite galaxy. We have performed a detailed two-dimensional photometric decomposition of NGC 4889 and of the many overlapping nearby galaxies in its vicinity. This comprehensive model allows us not only to firmly detect the low-surface brightness shells, but, crucially, also to accurately measure their luminosities and colors. The shells are bluer than the underlying stars at the same radius in the main galaxy. We make use of the colors of the shells and the color-magnitude relation of the Coma cluster to infer the luminosity (or mass) of the progenitor galaxy. The shells in NGC 4889 appear to have been produced by the minor merger of a moderate-luminosity (M_I ~ -18.7 mag) disk (S0 or spiral) galaxy with a luminosity (mass) ratio of ~ 90:1 with respect to the primary galaxy. The novel methodology presented in this work can be exploited to decode the fossil record imprinted in the photometric substructure of other nearby early-type galaxies.
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Many recent observations and numerical simulations suggest that nearby massive, early-type galaxies were formed through a two-phase process. In the proposed second phase, the extended stellar envelope was accumulated through many dry mergers. However, details of the past merger history of present-day ellipticals, such as the typical merger mass ratio, are difficult to constrain observationally. Within the context and assumptions of the two-phase formation scenario, we propose a straightforward method, using photometric data alone, to estimate the average mass ratio of mergers that contributed to the build-up of massive elliptical galaxies. We study a sample of nearby massive elliptical galaxies selected from the Carnegie-Irvine Galaxy Survey, using two-dimensional analysis to decompose their light distribution into an inner, denser component plus an extended, outer envelope, each having a different optical color. The combination of these two substructures accurately recovers the negative color gradient exhibited by the galaxy as whole. The color difference between the two components (<Delta(B-V)> ~ 0.10 mag; <Delta(B-R)> ~ 0.14 mag), based on the slope of the M_stellar-color relation for nearby early-type galaxies, can be translated into an estimate of the average mass ratio of the mergers. The rough estimate, 1:5 to 1:10, is consistent with the expectation of the two-phase formation scenario, suggesting that minor mergers were largely responsible for building up to the outer stellar envelope of present-day massive ellipticals. With the help of accurate photometry, large sample size, and more choices of colors promised by ongoing and future surveys, the approach proposed here can reveal more insights into the growth of massive galaxies during the last few Gyr.
We propose a phylogenetic approach (PA) as a novel and robust tool to detect galaxy populations (GPs) based on their chemical composition. The branches of the tree are interpreted as different GPs and the length between nodes as the internal chemical variation along a branch. We apply the PA using 30 abundance indices from the Sloan Digital Sky Survey to 475 galaxies in the Coma Cluster and 438 galaxies in the field. We find that a dense environment, such as Coma, shows several GPs, which indicates that the environment is promoting galaxy evolution. Each population shares common properties that can be identified in colour magnitude space, in addition to minor structures inside the red sequence. The field is more homogeneous, presenting one main GP. We also apply a principal component analysis (PCA) to both samples, and find that the PCA does not have the same power in identifying GPs.
Using the Oxford Short Wavelength Integral Field specTrograph (SWIFT), we trace radial variations of initial mass function (IMF) sensitive absorption features of three galaxies in the Coma cluster. We obtain resolved spectroscopy of the central 5kpc for the two central brightest-cluster galaxies (BCGs) NGC4889, NGC4874, and the BCG in the south-west group NGC4839, as well as unresolved data for NGC4873 as a low-$sigma_*$ control. We present radial measurements of the IMF-sensitive features sodium NaI$_{rm{SDSS}}$, calcium triplet CaT and iron-hydride FeH0.99, along with the magnesium MgI0.88 and titanium oxide TiO0.89 features. We employ two separate methods for both telluric correction and sky-subtraction around the faint FeH feature to verify our analysis. Within NGC4889 we find strong gradients of NaI$_{rm{SDSS}}$ and CaT but a flat FeH profile, which from comparing to stellar population synthesis models, suggests an old, $alpha$-enhanced population with a Chabrier, or even bottom-light IMF. The age and abundance is in line with previous studies but the normal IMF is in contrast to recent results suggesting an increased IMF slope with increased velocity dispersion. We measure flat NaI$_{rm{SDSS}}$ and FeH profiles within NGC4874 and determine an old, possibly slightly $alpha$-enhanced and Chabrier IMF population. We find an $alpha$-enhanced, Chabrier IMF population in NGC4873. Within NGC4839 we measure both strong NaI$_{rm{SDSS}}$ and strong FeH, although with a large systematic uncertainty, suggesting a possible heavier IMF. The IMFs we infer for these galaxies are supported by published dynamical modelling. We stress that IMF constraints should be corroborated by further spectral coverage and independent methods on a galaxy-by-galaxy basis.
We use ACS data from the HST Treasury survey of the Coma cluster (z~0.02) to study the properties of barred galaxies in the Coma core, the densest environment in the nearby Universe. This study provides a complementary data point for studies of barred galaxies as a function of redshift and environment. From ~470 cluster members brighter than M_I = -11 mag, we select a sample of 46 disk galaxies (S0--Im) based on visual classification. The sample is dominated by S0s for which we find an optical bar fraction of 47+/-11% through ellipse fitting and visual inspection. Among the bars in the core of the Coma cluster, we do not find any very large (a_bar > 2 kpc) bars. Comparison to other studies reveals that while the optical bar fraction for S0s shows only a modest variation across low-to-intermediate density environments (field to intermediate-density clusters), it can be higher by up to a factor of ~2 in the very high-density environment of the rich Coma cluster core.
We study the stellar population far into the halo of one of the two brightest galaxies in the Coma cluster, NGC 4889, based on deep medium resolution spectroscopy with FOCAS at the Subaru 8.2m telescope. We fit single stellar population models to the measured line-strength (Lick) indices (Hbeta, Mgb, [MgFe] and <Fe>). Combining with literature data, we construct radial profiles of metallicity, [alpha/Fe] element abundance ratio and age for NGC 4889, from the center out to ~60 kpc (~4Re). We find evidence for different chemical and star formation histories for stars inside and outside 1.2Re = 18 kpc radius. The inner regions are characterized by a steep [Z/H] gradient and high [alpha/Fe] at ~2.5 times solar value. In the halo, between 18 and 60 kpc, the [Z/H] is near-solar with a shallow gradient, while [alpha/Fe] shows a strong negative gradient, reaching solar values at ~60 kpc. We interpret these data in terms of different formation histories for both components. The data for the inner galaxy are consistent with a rapid, quasi-monolithic, dissipative merger origin at early redshifts, followed by one or at most a few dry mergers. Those for the halo argue for later accretion of stars from old systems with more extended star formation histories. The half-light radius of the inner component alone is estimated as ~6 kpc, suggesting a significantly smaller size of this galaxy in the past. This may be the local stellar population signature of the size evolution found for early-type galaxies from high-redshift observations.
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