Testing MOND gravity in the shell galaxy NGC 3923

Cuillandre

Gwyn³

et al. 2016

A&A

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Context. The elliptical galaxy NGC 3923 is known to be surrounded by a number of stellar shells, probable remnants of an accreted galaxy. Despite its uniqueness, the deepest images of its outskirts come from the 1980s. On the basis of the modified Newtonian dynamics (MOND), it has recently been predicted that a new shell lies in this region. Aims. We obtain the deepest image ever of the galaxy, map the tidal features in it, and search for the predicted shell. 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 . In addition, we reanalyzed an archival HST image of the galaxy. Results. We detected up to 42 shells in NGC 3923. This is by far the highest number among all shell 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 the new image revealed that the prediction was based on incorrect assumptions and poor data.

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Deep imaging of the shell elliptical galaxy NGC 3923 with MegaCam

Cuillandre

Gwyn³

et al. 2016

A&A

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“…The first solution of these problems appeared in Bílek et al (2013). Our method allows testing the compatibility of an observed radial distribution of shells with a given gravitational potential.…”

Section: Shell Identificationmentioning

confidence: 99%

“…1), which is the main subject of this paper, is decorated by the biggest known shell in the Universe. Its radius is more than 100 kpc (Table 1) and, according to the MONDian model of the galaxy NGC 3923 from Bílek et al (2013), it is exposed to a gravitational acceleration of a 0 /5 from the host galaxy. In this work, we theoretically predict a new -currently not yet observed -shell at a distance of about 220 kpc, which would extend to the acceleration of a 0 /10.…”

Section: Introductionmentioning

confidence: 99%

MOND prediction of a new giant shell in the elliptical galaxy NGC 3923

Bartošková

Ebrová

et al. 2014

A&A

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Context. Stellar shells, which form axially symmetric systems of arcs in some elliptical galaxies, are most likely remnants of radial minor mergers. They are observed up a radius of ∼100 kpc. The stars in them oscillate in radial orbits. The radius of a shell depends on the free-fall time at the position of the shell and on the time since the merger. We previously verified the consistency of shell radii in the elliptical galaxy NGC 3923 with its most probable MOND potential. Our results implied that an as yet undiscovered shell exists at the outskirts of the galaxy. Aims. We here extend our study by assuming more general models for the gravitational potential to verify the prediction of the new shell and to estimate its position. Methods. We tested the consistency of the shell radial distribution observed in NGC 3923 with a wide variety of MOND potentials of the galaxy. The potentials differed in the mass-to-light ratio and in distance to the galaxy. We considered different MOND interpolation functions, values of the acceleration constant a 0 , and density profiles of the galaxy. We verified the functionality of our code on a Newtonian self-consistent simulation of the formation of a shell galaxy. Results. Our method reliably predicts that exactly one new outermost shell exists at a galactocentric radius of about 1900 (∼210 kpc) on the southwestern side of the galaxy. Its estimated surface brightness is about 28 mag arcsec −2 in B -a value accessible by current instruments. This prediction enables a rare test of MOND in an elliptical down to an acceleration of a 0 /10. The predictive power of our method is verified by reconstructing the position of the largest known shell from the distribution of the remaining shells.

MOND implications for spectral line profiles of shell galaxies: shell formation history and mass-velocity scaling relations

Jungwiert

Ebrová

et al. 2015

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Context. Many ellipticals are surrounded by round stellar shells probably stemming from minor mergers. A new method for constraining gravitational potential in elliptical galaxies has recently been suggested. It uses the spectral line profiles of these shells to measure the circular velocity at the edge of the shell and the expansion velocity of the shell itself. MOND is an alternative to the dark matter framework aiming to solve the missing mass problem. Aims. We study how the circular and expansion velocities behave in MOND for large shells. Methods. The asymptotic behavior for infinitely large shells is derived analytically. The applicability of the asymptotic results for finitely sized shells is studied numerically on a grid of galaxies modeled with Sérsic spheres. Results. Circular velocity settles asymptotically at a value determined by the baryonic mass of the galaxy forming the baryonic TullyFisher relation known for disk galaxies. Shell expansion velocity also becomes asymptotically constant. The expansion velocities of large shells form a multibranched analogy to the baryonic Tully-Fisher relation, together with the galactic baryonic masses. For many -but not all -shell galaxies, the asymptotic values of these two types of velocities are reached under the effective radius. If MOND is assumed to work in ellipticals, then the shell spectra allow many details of the history to be revealed about the formation of the shell system, including its age. The results pertaining to circular velocities apply to all elliptical galaxies, not only those with shells.