Quantifying the (X/peanut)-shaped structure in edge-on disc galaxies: length, strength, and nested peanuts

Abstract

X-shaped or peanut-shaped (X/P) bulges are observed in more than 40% of (nearly) edge-on disc galaxies, though to date a robust method to quantify them is lacking. Using Fourier harmonics to describe the deviation of galaxy isophotes from ellipses, we demonstrate with a sample of 11 such galaxies (including NGC 128) that the sixth Fourier component (B 6 ) carries physical meaning by tracing this X/P structure. We introduce five quantitative diagnostics based on the radial B 6 profile, namely: its 'peak' amplitude (Π max ); the (projected major-axis) 'length' where this peak occurs (R Π ,max ); its vertical 'height' above the disc plane (z Π ,max ); a measure of the B 6 profile's integrated 'strength' (S Π ); and the B 6 peak 'width' (W Π ). We also introduce different 'classes' of B 6 profile shape. Furthermore, we convincingly detect and measure the properties of multiple (nested) X/P structures in individual galaxies which additionally display the signatures of multiple bars in their surface brightness profiles, thus consolidating further the scenario in which peanuts are associated with bars. We reveal that the peanut parameter space ('length', 'strength' and 'height') for real galaxies is not randomly populated, but the three metrics are inter-correlated (both in kpc and disc scale-length h). Additionally, the X/P 'length' and 'strength' appear to correlate with (v rot /σ ), lending further support to the notion that peanuts 'know' about the galactic disc in which they reside. Such constraints are important for numerical simulations, as they provide a direct link between peanuts and their host disc. Our diagnostics reveal a spectrum of X/P properties and could provide a means of distinguishing between different peanut formation scenarios discussed in the literature. Moreover, nested peanuts, as remnants of bar buckling events, can provide insights into the disc and bar instability history.

Inner bars also buckle. The MUSE TIMER view of the double-barred galaxy NGC 1291

“…This kinematic criterion to detect B/P in face-on galaxies has been previously confirmed in main bars (Méndez-Abreu et al 2008b, 2014, but this is the first time a B/P is detected in a face-on inner bar. Hints of multiple B/P in edge-on galaxies have also been discussed in Ciambur & Graham (2016). Remarkably, these results strongly suggest that inner and outer bars in double-barred galaxies are analogous systems, governed by the same physical processes.…”

The haloes and environments of nearby galaxies (HERON) – II. The outer structure of edge-on galaxies

“…This lenticular galaxy is famous for its prominent peanut-shaped bulge, which is thought to be associated with a buckling instability of a stellar bar (Raha et al 1991). Ciambur & Graham (2016) discovered that there are two such nested structures, possibly associated with two stellar bars. Also, it has a strongly warped stellar disc.…”

“…The same applies to five galaxies with prominent B/PS or thick boxy bulges (labelled in Fig. 2 by 'X'), as noted in Savchenko et al (2017), Ciambur & Graham (2016) and Lütticke et al (2004), and galaxies with a central peculiar component (NGC 2683 and M 82). The vertical extent of these central components may contribute to the measured outer structure to some degree.…”

Structural and photometric properties of barred galaxies from the Auriga cosmological simulations

“…Several methods to measure the B/P strength (or B/P scaleheight) and size have been proposed, not necessarily from edge-on unsharp-masked images (e.g. Erwin & Debattista 2013, Fragkoudi et al 2015, Ciambur & Graham 2016, Savchenko et al 2017). The method followed in this paper is based on Laurikainen & Salo (2017).…”