Wednesday 31st July 2013

The Densest Galaxy

Jay Strader (Michigan St), Anil Seth (Utah), Duncan Forbes (Swinburne), Giuseppina Fabbiano (CfA), Aaron Romanowsky (San Jose St, Santa Cruz), Jean Brodie (Santa Cruz), Charlie Conroy (Santa Cruz), Nelson Caldwell (CfA), Vincenzo Pota (Swinburne), Christopher Usher (Swinburne), Jacob Arnold (Santa Cruz)

We report the discovery of a remarkable ultra-compact dwarf galaxy around the massive Virgo elliptical galaxy NGC 4649 (M60), which we term M60-UCD1. With a dynamical mass of 2.0 x 10^8 M_sun but a half-light radius of only ~ 24 pc, M60-UCD1 is more massive than any ultra-compact dwarfs of comparable size, and is arguably the densest galaxy known in the local universe. It has a two-component structure well-fit by a sum of Sersic functions, with an elliptical, compact (r_h=14 pc; n ~ 3.3) inner component and a round, exponential, extended (r_h=49 pc) outer component. Chandra data reveal a variable central X-ray source with L_X ~ 10^38 erg/s that could be an active galactic nucleus associated with a massive black hole or a low-mass X-ray binary. Analysis of optical spectroscopy shows the object to be old (~> 10 Gyr) and of solar metallicity, with elevated [Mg/Fe] and strongly enhanced [N/Fe] that indicates light element self-enrichment; such self-enrichment may be generically present in dense stellar systems. The velocity dispersion (~ 70 km/s) and resulting dynamical mass-to-light ratio (M/L_V=4.9 +/- 0.7) are consistent with—but slightly higher than—expectations for an old, metal-rich stellar population with a Kroupa initial mass function. The presence of a massive black hole or a mild increase in low-mass stars or stellar remnants is therefore also consistent with this M/L_V. The stellar density of the galaxy is so high that no dynamical signature of dark matter is expected. However, the properties of M60-UCD1 suggest an origin in the tidal stripping of a nucleated galaxy with M_B ~ -18 to -19.


"Direct" Gas-phase Metallicities, Stellar Properties, and Local Environments of Emission-line Galaxies at Redshift below 0.90

Chun Ly (1,6), Matthew A. Malkan (2), Tohru Nagao (3), Nobunari Kashikawa (4,5), Kazuhiro Shimasaku (4), Masao Hayashi (4) ((1) STScI, (2) UCLA, (3) Kyoto U., (4) U. Tokyo, (5) NAOJ, (6) Giacconi Fellow)
(Submitted on 29 Jul 2013)

Using deep narrow-band imaging and optical spectroscopy from Keck and MMT, we identify a sample of 20 emission-line galaxies at z=0.065-0.90 where the weak auroral emission line, [O III]4363, is detected at >3\sigma. These detections allow us to determine the gas-phase metal abundances using the "direct" method. After correcting for dust attenuation using Balmer decrements, we find that 10 of these low-mass galaxies are extremely metal-poor with 12+log(O/H) <= 7.65 dex or one-tenth solar. Considering measurement uncertainties, we argue that 8 and 4 of them are extremely metal-poor at the 85% and 95% confidence levels, respectively. Our three most metal-deficient galaxies have 12+log(O/H)=7.05^{+0.43}_{-0.12}, 7.09^{+0.53}_{-0.25}, and 7.12^{+0.33}_{-0.24} dex (95% confidence), similar to some of the lowest metallicity galaxies identified in the local universe. We also find that our galaxies are all undergoing significant star formation with average specific star formation rate (SFR) of (100 Myr)^{-1}, and that they have high central SFR surface densities (average of 0.5 Msun/yr/kpc^2). In addition, more than two-thirds of our galaxies have between one and four nearby companions within a projected radius of 100 kpc, which we find is an excess among star-forming galaxies at z=0.4-0.8. We also examine how the gas-phase metallicities compare with the stellar mass and the SFR of the galaxies, and find that while roughly half of our galaxies lie along the M-Z-(SFR) relation, the other half have metallicities that are 0.1-1 dex lower than predicted. Our analysis suggests that this discrepancy is real at the 95% confidence. This indicates that the local M-Z-(SFR) relation may not hold for all low-mass galaxies (<10^9 Msun) at z=0.4-0.8. The high ionization parameter and high electron density seen in our galaxies suggest that they are lower redshift analogs to typical z > 1 galaxies.

Deep Chandra Observations of Abell 2199: the Interplay between Merger-Induced Gas Motions and Nuclear Outbursts in a Cool Core Cluster

Paul E. J. Nulsen (1), Zhiyuan Li (1, 2), William R. Forman (1), Ralph P. Kraft (1), Dharam V. Lal (1, 3), Christine Jones (1), Irina Zhuravleva (4), Eugene Churazov (5), Jeremy S. Sanders (6), Andrew C. Fabian (7), Ryan E. Johnson (8), Stephen S. Murray (9) ((1) CfA, (2) UCLA, (3) NCRA-TIFR, Pune, India, (4) KIPAC, (5) MPA, Germany, (6) MPIfE, Germany, (7) IoA, Cambridge, (8) Denison Univsersity, (9) Johns Hopkins)

We present new Chandra observations of Abell 2199 that show evidence of gas sloshing due to a minor merger, as well as impacts of the radio source, 3C 338, hosted by the central galaxy, NGC 6166, on the intracluster gas. The new data are consistent with previous evidence of a Mach 1.46 shock 100" from the cluster center, although there is still no convincing evidence for the expected temperature jump. Other interpretations of this feature are possible, but none is fully satisfactory. Large scale asymmetries, including enhanced X-ray emission 200" southwest of the cluster center and a plume of low entropy, enriched gas reaching 50" to the north of the center, are signatures of gas sloshing induced by core passage of a merging subcluster about 400 Myr ago. An association between the unusual radio ridge and low entropy gas are consistent with this feature being the remnant of a former radio jet that was swept away from the AGN by gas sloshing. A large discrepancy between the energy required to produce the 100" shock and the enthalpy of the outer radio lobes of 3C 338 suggests that the lobes were formed by a more recent, less powerful radio outburst. Lack of evidence for shocks in the central 10" indicates that the power of the jet now is some two orders of magnitude smaller than when the 100" shock was formed.

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