A TIDALLY DISRUPTING DWARF SPHEROIDAL AROUND THE GALAXY NGC 253

Image: Spiral galaxy NGC 253 Credit: Robert Gendler/Jim Mistin

The modern paradigm of cold dark matter with a cosmological constant (ΛCDM) predicts that galaxies form hierarchically - growing through the gradual merging of many smaller galaxies.

A giant spiral like our Milky Way is expected to undergo a succession of dwarf galaxy accretion events which have different observational signatures, depending on their occurrence in the past, present, or future. Ancient accretion events can be detected through careful sifting of the chemodynamical phase-space of halo stars. Ongoing accretion is implied by the presence of satellite galaxies within the halo, and imminent accretion is marked by the existence of field dwarfs near to their future hosts.

Left: Amateur images of NGC 253 and its satellite NGC 253-dw2.
The field of view is of ~ 67 x 60 kpc. North is up and East is left.
The zoom-in on NGC 253-dw2 covers ~ 8 x 7 kpc.
Right: Subaru/Suprime-Cam image.
Credit: Romanowsky, Martinez-Delgago et. al 2016

Some researchers (Romanowsky, Martinez-Delgago et. al 2016) report the discovery of NGC 253-dw2, a dwarf spheroidal (dSph) galaxy candidate undergoing tidal disruption around a nearby spiral galaxy NGC 253 in the Sculptor group: the first such event identified beyond the Local Group. it is noteworthy that the dwarf was found using small-aperture amateur telescopes, and followed up with Suprime-Cam on the 8 m Subaru Telescope in order to resolve its brightest stars.

The morphological properties of NGC 253-dw2 mark it as distinct from normal dSphs and imply ongoing disruption at a projected distance of ~ 50 kpc from the main galaxy. Their observations support the hierarchical paradigm wherein massive galaxies continously accrete less massive ones, and provide a new case study for dSph infall and dissolution dynamics.

The paper (Romanowsky, Martinez-Delgago et. al 2016) is available online and is published in the MNRAS. >>
http://mnrasl.oxfordjournals.org/content/457/1/L103.full.pdf
http://arxiv.org/pdf/1512.03815v2.pdf

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