Fourier photometric analysis of isolated galaxies in the context of the AMIGA project

Abstract

We present here the results of a Fourier photometric decomposition of a representative sample of ∼100 isolated CIG galaxies (Catalog of Isolated Galaxies) in the morphological range Sb–Sc. This study is an integral part of the AMIGA (Analysis of the Interstellar Medium of Isolated Galaxies) project. It complements the photometric analysis presented in our previous paper for the same sample of disc galaxies by allowing a description of the spiral structure morphology. We also estimate dynamical measures like torque strength for bar and spiral, and also the total non‐axisymmetric torque by assuming a constant mass‐to‐light ratio, and explore the interplay between the spiral and bar components of galaxies. Both the length (lbar) and the contrast (e.g. A2b) of the Fourier bars decrease along the morphological sequence Sb–Sbc–Sc, with bars in earlier types being longer and showing higher contrast. The bars of Sb galaxies are ∼three times longer than the bars in Sc types, consistent with our previous study. We find that the longer bars are not necessarily stronger (as quantified by the torque Qb measure), but longer bars show a higher contrast A2b, in very good agreement with theoretical predictions. Our data suggest that bar and spiral components are rather independent in the sense that the torque strengths of the two components are not correlated. The total strength Qg is a very reliable tracer of the bar strength measure Qb, the two quantities showing a very tight linear correlation. Comparison with a similar sample of disc galaxies (same morphological range) extracted from the OSUBGS (Ohio State University Bright Galaxy Survey) indicates that the isolated CIG/AMIGA galaxies host significantly longer Fourier bars and possibly show a different distribution of spiral torque Qs. The Fourier analysis also revealed a potential case of counterwinding spiral structure (KIG 652/NGC 5768), which deserves further kinematic study. We find that m= 2 (i.e. dominating two‐armed pattern) is the most common spiral arm multiplicity among the sample of Sb–Sc CIG/AMIGA galaxies (∼40 per cent), m= 2 and 3 and m= 1 and 2 are found in ∼28 and ∼13 per cent of isolated galaxies, respectively.

The AMIGA sample of isolated galaxies

“…If we consider a traditional feeding mechanism of black holes driven by major mergers (Kormendy et al 2011), the relatively high fraction of AGN in isolated galaxies, dominated by pseudo-bulges (Durbala et al 2009), might favour an alternative local feeding mechanism driven by the slow accretion of cold gas (Hopkins & Hernquist 2006). This mechanism is probably closer related to the activity found in galaxies with pseudobulges ).…”

“…Active galactic nuclei are hosted by earlier types and higher luminosity galaxies than TOs, while just the opposite happens for SFN. Because early types present a bigger fraction of the galaxy mass in the spheroidal component and AMIGA galaxies harbour pseudo-bulges instead of classic bulges (Durbala et al 2009), there might be a relation between the prevalence of AGN and the mass of the pseudo-bulge in AMIGA isolated galaxies.…”

The AMIGA sample of isolated galaxies

“…This has been obtained for a subsample of N = 96 CIG galaxies by Durbala et al (2009) using Fourier analysis over spiral and bar components separately (Buta et al 2003). Figure 16 shows the A flux ratio parameter with respect to the relative spiral strength (Q s ).…”

Angular momentum transport and evolution of lopsided galaxies

“…3). By carefully choosing the free parameters, the same functional form can roughly capture the typical radial variation of m = 2 perturbation such as bars/spirals (e.g., see Durbala et al 2009, for radial variation of A2) and we write this as:…”