[Paper Review] Dwarfs walking in a row. The filamentary nature of the NGC3109 association

M. Bellazzini, Tom Oosterloo+2 more|arXiv (Cornell University)|Oct 23, 2013

Galaxies: Formation, Evolution, Phenomena2 references29 citations

TL;DR

This paper proposes that the NGC 3109 dwarf galaxy association is a rare, highly elongated filamentary structure in space, with all members—including the newly discovered Leo P—aligned along a 1.07 Mpc-long line and exhibiting a coherent velocity gradient. The tight spatial and kinematic correlation, with only 16.8 km s⁻¹ rms scatter, suggests the group formed via tidal interaction or accretion as a cold filament, challenging standard Λ-CDM expectations.

ABSTRACT

We re-consider the association of dwarf galaxies around NGC3109, whose known members were NGC3109, Antlia, Sextans A and Sextans B, based on a new updated list of nearby galaxies and the most recent data. We find that the original members of the NGC3109 association, together with the recently discovered and adjacent dwarf irregular Leo P, form a very tight and elongated configuration in space. All these galaxies lie within ~100 kpc of a line that is ~1070 kpc long, from one extreme (NGC3109) to the other (Leo P), and they show a gradient in the Local Group standard of rest velocity with a total amplitude of 43 km/s Mpc and a r.m.s. scatter of just 16.8 km/s. It is shown that the reported configuration is exceptional given the known dwarf galaxies in the Local Group and its surroundings. We conclude that (a) Leo P is likely an additional member of the NGC3109 association, and (b) the association is highly ordered in space and velocity, and it is very elongated, suggesting that it has been originated by a tidal interaction or it was accreted as a filamentary sub-structure.

Motivation & Objective

• To reassess the spatial and kinematic structure of the NGC 3109 dwarf galaxy association using updated data.
• To determine whether the recently discovered faint dwarf Leo P is a physical member of the association.
• To evaluate the statistical significance of the observed alignment and velocity gradient in the context of random groupings.
• To investigate the origin of the structure, considering tidal interactions or filamentary accretion within the Λ-CDM framework.

Proposed method

• Used the M12 catalogue of Local Group and nearby galaxies, supplemented with updated distances and radial velocities from Dalcanton et al. (2009) and Giovanelli et al. (2013).
• Defined a NGC 3109-centered Cartesian coordinate system (Xa, Ya, Za) and introduced a radial coordinate R to parametrize distance along the line of best fit.
• Performed independent linear fits in the (Xa, Ya) and (Xa, Za) planes to determine the best-fit line in 3D space, using the squared sum of residuals (rmsT) as a measure of scatter.
• Quantified velocity coherence via the rms scatter of radial velocities (rmsV) along the best-fit line in the Local Group standard of rest frame.
• Conducted a statistical test by randomly sampling groups of four galaxies from the M12 catalogue to assess the rarity of such alignments.
• Compared the observed structure to known associations (e.g., NGC 55 subgroup) and tested for physical association with IC 3104 via spatial and kinematic consistency.

Experimental results

Research questions

• RQ1 Is Leo P a physical member of the NGC 3109 association based on spatial and kinematic consistency?
• RQ2 How rare is the observed alignment and velocity gradient among four-galaxy configurations in the Local Group?
• RQ3 Can the observed linear structure be explained by chance alignment or random clustering?
• RQ4 What is the origin of the filamentary structure—tidal interaction or accretion along a cosmological filament?

Key findings

• Leo P is very likely a physical member of the NGC 3109 association, extending its spatial configuration to a total length of 1070 kpc.
• The five members (NGC 3109, Antlia, Sextans A, Sextans B, and Leo P) are aligned along a line with a global scatter of only 95.8 kpc, yielding a length-to-thickness ratio of 11.2.
• The radial velocities in the Local Group standard of rest show a coherent gradient with a total amplitude of 43 km s⁻¹ and an rms scatter of just 16.8 km s⁻¹.
• The probability of such a tight alignment and velocity coherence occurring by chance is less than 0.5%, as confirmed by random sampling of galaxy groups.
• The structure's length (1070 kpc) and mass (≈3.2 × 10¹¹ M⊙) are consistent with a gravitationally bound system, with a turnaround diameter of ~980 kpc.
• The alignment and kinematic coherence strongly support a common origin, most plausibly via tidal disruption or accretion along a cold cosmological filament.