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Post Info TOPIC: NGC 3923
Blobrana


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RE: NGC 3923
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Title: Deep imaging of the shell elliptical galaxy NGC3923 with MegaCam
Author: Michal Bílek, Jean-Charles Cuillandre, Stephen Gwyn, Ivana Ebrová, Katerina Bartoková, Bruno Jungwiert, Lucie Jílková

Context. The elliptical galaxy NGC 3923 is known to be surrounded by a number of stellar shells, a probable remnant of an accreted galaxy. Despite its uniqueness, the deepest images of its outskirts come from the 80s. Bilek et al. (2014) predicted a new shell to lie in this region on the basis of the MOND theory of modified dynamics.
Aims. To obtain the deepest image ever of the galaxy and to map the tidal features in it.
Methods. The image of the galaxy was taken by the MegaCam camera at the Canada-France-Hawaii Telescope in the g' band. It reached the surface-brightness limit of 29 mag arcsec-2. Moreover, we reanalyse an archival HST image of the galaxy.
Results. We detected up to 42 shells in NGC 3923. This is by far most of all galaxies. We present the description of the shells and other tidal features in the galaxy. A probable progenitor of some of these features was discovered. The shell system likely originates from two or more progenitors. The predicted shell was not detected, but we found that the prediction was based on incorrect assumptions and poor data.

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Blobrana


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PGC 37061
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NGC 3923 (also known as ESO 440-17, MCG -5-28-12 and PGC 37061) is a magnitude +9.8 elliptical galaxy located 80 million light-years away in the constellation Hydra.

The galaxy was discovered by German-British astronomer William Herschel using a 47.5 cm (18.7 inch) f/13 speculum reflector at Windsor Road in Slough, Berkshire on the 7th March 1791

Position (2000): Right Ascension 11h 51m 01.783s, Declination -28° 48' 22.36"



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Blobrana


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Posts: 131433
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RE: NGC 3923
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Hubble Spots the Layers of NGC 3923

NGC 3923 is an example of a shell galaxy where the stars in its halo are arranged in layers.
Finding concentric shells of stars enclosing a galaxy is quite common and is observed in many elliptical galaxies. In fact, every tenth elliptical galaxy exhibits this onion-like structure, which has never been observed in spiral galaxies. The shell-like structures are thought to develop as a consequence of galactic cannibalism, when a larger galaxy ingests a smaller companion. As the two centers approach, they initially oscillate about a common center, and this oscillation ripples outwards forming the shells of stars just as ripples on a pond spread when the surface is disturbed.
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Blobrana


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Title: Testing MOND gravity in the shell galaxy NGC 3923
Authors: Michal Bilek, Bruno Jungwiert, Lucie Jilkova, Ivana Ebrova, Katerina Bartoskova, Miroslav Krizek

Context. The elliptical galaxy NGC 3923 is surrounded by numerous stellar shells that are concentric arcs centred on the galactic core. They are very likely a result of a minor merger and they consist of stars in nearly radial orbits. For a given potential, the shell radii at a given time after the merger can be calculated and compared to observations. The Modified Newtonian Dynamics (MOND) is a theory that aims to solve the missing mass problem by modifying the laws of classical dynamics in the limit of small accelerations. Hernquist & Quinn(1987) claimed that the shell distribution of NGC 3923 contradicted MOND, but Milgrom(1988) found several substantial insufficiencies in their work.
Aims. We test whether the observed shell distribution in NGC 3923 is consistent with MOND using the current observational knowledge of the shell number and positions and of the host galaxy surface brightness profile, which supersede the data available in the 1980s when the last (and negative) tests of MOND viability were performed on NGC 3923.
Methods. Using the 3.6 um bandpass image of NGC 3923 from the Spitzer space telescope we construct the mass profile of the galaxy. The evolution of shell radii in MOND is then computed using analytical formulae. We use 27 currently observed shells and allow for their multi-generation formation, unlike the Hernquist & Quinn one-generation model that used the 18 shells known at the time.
Results. Our model reproduces the observed shell radii with a maximum deviation of 5% for 25 out of 27 known shells while keeping a reasonable formation scenario. A multi-generation nature of the shell system, resulting from successive passages of the surviving core of the tidally disrupted dwarf galaxy, is one of key ingredients of our scenario supported by the extreme shell radial range. The 25 reproduced shells are interpreted as belonging to three generations.

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Blobrana


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Title: The Globular Cluster Kinematics and Galaxy Dark Matter Content of NGC 3923
Authors: Mark A. Norris, Karl Gebhardt, Ray M. Sharples, Favio Raul Faifer, Terry Bridges, Duncan A. Forbes, Juan C. Forte, Stephen E. Zepf, Michael A. Beasley, David A. Hanes, Robert Proctor, Sheila J. Kannappan

This paper presents further results from our spectroscopic study of the globular cluster (GC) system of the group elliptical NGC 3923. From observations made with the GMOS instrument on the Gemini South telescope, an additional 50 GC and Ultra Compact Dwarf (UCD) candidates have been spectroscopically confirmed as members of the NGC 3923 system. When the recessional velocities of these GCs are combined with the 29 GC velocities reported previously, a total sample of 79 GC/UCD velocities is produced. This sample extends to over 6 arcmin (>6 Re \sim30 kpc) from the centre of NGC 3923, and is used to study the dynamics of the GC system and the dark matter content of NGC 3923. It is found that the GC system of NGC 3923 displays no appreciable rotation, and that the projected velocity dispersion is constant with radius within the uncertainties. The velocity dispersion profiles of the integrated light and GC system of NGC 3923 are indistinguishable over the region in which they overlap. We find some evidence that the diffuse light and GCs of NGC 3923 have radially biased orbits within \sim130". The application of axisymmetric orbit-based models to the GC and integrated light velocity dispersion profiles demonstrates that a significant increase in the mass-to-light ratio (from M/Lv = 8 to 26) at large galactocentric radii is required to explain these observations. We therefore confirm the presence of a dark matter halo in NGC 3923. We find that dark matter comprises 17.5% of the mass within 1 Re, 41.2% within 2 Re, and 75.6% within the radius of our last kinematic tracer at 6.9 Re. The total dynamical mass within this radius is found to be 1.5 x 10^12 solar masses. In common with other studies of large ellipticals, we find that our derived dynamical mass profile is consistently higher than that derived by X-ray observations, by a factor of around 2.

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