arXiv subjects

Georges Meylan

Publications and source records attributed to Georges Meylan.

25 records · Page 2Linked to original sources

arXiv2001

Mass Determinations of Star Clusters

Mass determinations are difficult to obtain and still frequently characterised by deceptively large uncertainties. We review below the various mass estimators used for star clusters of all ages and luminosities. We highlight a few recent results related to (i) very massive old star clusters, (ii) the differences and similarities between star clusters and cores of dwarf elliptical galaxies, and (iii) the possible strong biases on mass determination induced by tidal effects.

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arXiv2000

Kinematics, ages, and metallicities of star clusters in NGC 1316: A 3 Gyr old merger remnant

We report on multi-object spectroscopy of 37 candidate star clusters around the giant early-type radio galaxy NGC 1316 (Fornax A), the brightest galaxy in the Fornax cluster. For the star clusters in this sample, we measure a mean radial velocity of 1698 +/- 46 km/s, a rotation velocity of 175 +/- 70 km/s, and a velocity dispersion of 227 +/- 33 km/s within a galactocentric radius of 24 kpc. Using the projected mass estimator and assuming isotropic orbits, the estimated total mass is (6.6 +/- 1.7) E+11 solar masses within a radius of 24 kpc. The mass is uncertain by about a factor of two, depending on the orbital assumptions. The implied M/L_B ratio is in the range 3-6. Four star clusters in our sample are exceptionally luminous (M_V < -12.3). This means that (1) at least this many clusters in NGC 1316 are up to an order of magnitude more luminous than the most luminous star cluster in our Galaxy or M31, and (2) that the S/N ratio of their spectra allows us to measure line strengths with good accuracy. By comparing the colours and equivalent widths of H-alpha and the Ca II triplet absorption lines for those bright star clusters in our sample with those of single-burst population models, we find that they are coeval with an age of 3.0 +/- 0.5 Gyr. Their metallicities are found to be solar to within 0.15 dex. These results show once again that globular clusters with near-solar metallicity do form during galactic mergers, and, moreover, that they can survive disruption processes taking place during the merger (e.g., dynamical friction, tidal disruption), as well as evaporation. In this respect, NGC 1316 provides a hitherto "missing" evolutionary link between young merger remnants of age ~0.5 Gyr such as NGC 3597, NGC 3921 and NGC 7252 on one side, and older giant ellipticals featuring bimodal colour distributions on the other side.

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arXiv2000

Mass Segregation in Star Clusters

Star clusters - open and globulars - experience dynamical evolution on time scales shorter than their age. Consequently, open and globular clusters provide us with unique dynamical laboratories for learning about two-body relaxation, mass segregation from equipartition of energy, and core collapse. We review briefly, in the framework of star clusters, some elements related to the theoretical expectation of mass segregation, the results from N-body and other computer simulations, as well as the now substantial clear observational evidence.

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arXiv1999

Tidal Tails around 20 Galactic Globular Clusters

We present our study of the 2-D structures of the tidal tails associated with 20 galactic globular clusters, structures obtained by using the wavelet transform to detect weak structures at large scale and filter the strong background noise for the low galactic latitude clusters (Leon, Meylan & Combes 2000). We also present N-body simulations of globular clusters, in orbits around the Galaxy, in order to study quantitatively and geometrically the tidal effects they encounter (Combes, Leon & Meylan 2000).

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arXiv1998

Towards an Understanding of the Globular Cluster Over--abundance around the Central Giant Elliptical NGC 1399

We investigate the kinematics of a combined sample of 74 globular clusters around NGC 1399. Their high velocity dispersion, increasing with radius, supports their association with the gravitational potential of the galaxy cluster rather than with that of NGC 1399 itself. We find no evidence for rotation in the full sample, although some indication for rotation in the outer regions. The data do not allow us to detect differences between the kinematics of the blue and red sub-populations of globular clusters. A comparison between the globular cluster systems of NGC 1399 and those of NGC 1404 and NGC 1380 indicates that the globular clusters in all three galaxies are likely to have formed via similar mechanisms and at similar epochs. The only property which distinguishes the NGC 1399 globular cluster system from these others is that it is ten times more abundant. We summarize the evidence for associating these excess globulars with the galaxy cluster rather than with NGC 1399 itself, and suggest that the over-abundance can be explained by tidal stripping, at an early epoch, of neighboring galaxies and subsequent accumulation of globulars in the gravitational potential of the galaxy cluster.

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arXiv1997

Radial Velocities of Globular Clusters in the Giant Elliptical Galaxy NGC 1399

We report on the radial velocity measurements of 18 globular clusters and one dE galaxy in the field of the giant elliptical galaxy NGC 1399, the dominant galaxy of the Fornax cluster. We present also the radial velocity measurements of some candidates for young and super metal-rich globular clusters, which turn out to be foreground or background objects, viz. 28 stars and 7 galaxies. The bonafide globular clusters are selected, on the basis of their magnitudes and colors, to be metal-rich. For this cluster sample we measure a mean radial velocity of v_{rad} = 1353 +/- 79 km/s, and a velocity dispersion of 338 +/- 56 km/s. Using a few different mass estimators, this implies M/L values in the range 50-130 within a radius of 28 kpc, consistent with M/L rising with radius. Our velocity dispersion estimate is intermediate between the value computed from the integrated stellar light at smaller radii, and that computed at large radii from recent X-ray observations.

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arXiv1997

Discovery of Extended Blue Horizontal Branches in Two Metal-Rich Globular Clusters

We have used WFPC2 to construct B, V color-magnitude diagrams of four metal-rich globular clusters, NGC 104 (47 Tuc), NGC 5927, NGC 6388, and NGC 6441. All four clusters have well populated red horizontal branches (RHB), as expected for their metallicity. However, NGC 6388 and 6441 also exhibit a prominent blue HB (BHB) extension, including stars reaching as faint in V as the turnoff luminosity. This discovery demonstrates directly for the first time that a major population of hot HB stars can exist in old, metal-rich systems. This may have important implications for the interpretation of the integrated spectra of elliptical galaxies. The cause of the phenomenon remains uncertain. We examine the possibility that NGC 6388 and 6441 are older than the other clusters, but a simple difference in age may not be sufficient to produce the observed distributions along the HB. The high central densities in NGC 6388 and 6441 suggest that the existence of the blue HB (BHB) tails might be caused by stellar interactions in the dense cores of these clusters, which we calculate to have two of the highest collision rates among globular clusters in the Galaxy. Tidal collisions might act in various ways to enhance loss of envelope mass, and therefore populate the blue side of the HB. However, the relative frequency of tidal collisions does not seem large enough (compared to that of the clusters with pure RHBs) to account for such a drastic difference in HB morphology. While a combination of an age difference and dynamical interactions may help, prima facie the lack of a radial gradient in the BHB/RHB star ratio seems to argue against dynamical effects playing a role.

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