Isolated compact elliptical galaxies: Stellar systems that ran away

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Science  24 Apr 2015:
Vol. 348, Issue 6233, pp. 418-421
DOI: 10.1126/science.aaa3344
  • Fig. 1 A sample of compact elliptical galaxies in all types of environment.

    Our current sample of compact elliptical galaxies (blue symbols) is compared with a data set compiled from the literature (red symbols). Dots and stars denote group/cluster and isolated compact elliptical galaxies, respectively. Square panels in the bottom part of the figure show representatives of the current sample, and top corner insets display Messier 32 and NGC 4486, prototypical cEs in the local universe, as they would look with the SDSS telescope at a 130 Mpc distance (z = 0.03). Each inset panel covers a 20 by 20 kpc region centered on a cE.

  • Fig. 2 Comparison of stellar population properties of isolated and nonisolated compact elliptical galaxies to a reference sample of elliptical galaxies from the SDSS.

    Mean stellar ages and metallicities of cE galaxies hosted by groups and clusters (round symbols) and isolated cEs (stars) were determined from the modeling of their SDSS spectra by using the NBursts full-spectrum fitting. The plotting symbols are color-coded according to the r-band absolute magnitudes derived from the SDSS photometry. We also computed ages and metallicities for a reference sample of 82,500 elliptical galaxies from SDSS DR7 in the same fashion, along with the median r-band absolute magnitude for every bin of age-metallicity parameter space. These magnitudes are shown as a background map, with the colors on the same scale as cE galaxies. This figure demonstrates that (i) stellar populations of isolated and group/cluster cEs do not differ statistically and (ii) cEs are on average much fainter than are normal elliptical galaxies of the same stellar age/metallicity.

  • Fig. 3 Positions of cE galaxies identified on caustic diagrams in galaxy clusters and rich groups with more than 20 members in the SDSS.

    (Top) An example of a caustic diagram showing projected distances and radial velocity differences of the members of the cluster MKW 03s that includes cE galaxies shown as stars. The derived caustics shown as solid blue lines roughly correspond to the escape velocity for a galaxy at a given distance from the cluster center. Shaded areas show statistical 1σ uncertainties of the caustic line computation. (Bottom) The caustic lines for an ensemble cluster of 2592 individual members are shown as gray shaded areas and light gray dots, respectively. The caustic lines normalized by corresponding velocity dispersions and half-mass radii (Reff) are shown for six individual clusters as colored solid lines. Small stars denote cEs located deeply inside the potential wells of these six clusters, and large stars (with same colors as caustic lines) indicate cEs that have projected radial velocities of at least 85% of the caustic amplitude. Vectors point to the location of identified host galaxies for corresponding cEs. Three of them are different than the cluster of central galaxies, indicating that these cEs belong to subgroups inside clusters and that the caustic diagnostic is irrelevant for them. However, the four circled cEs are examples of galaxies barely gravitationally bound to their host clusters.

Supplementary Materials

  • Isolated compact elliptical galaxies: Stellar systems that ran away

    Igor Chilingarian, Ivan Zolotukhin

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Supplementary Text
    • Figs. S1 to S4
    • Full Reference List
    Table S1
    Properties of 195 cEs and their host galaxies found in this study. Missing value in the host_objid column indicates that this row corresponds to an isolated cE galaxy without an identified host. The complete table along with column descriptions is available in Microsoft Excel format as a separate supplementary file.

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