A Fundamental Plane of Spiral Structure in Disk Galaxies

Abstract

Spiral structure is the most distinctive feature of disk galaxies and yet debate persists about which theory of spiral structure is correct. Many versions of the density wave theory demand that the pitch angle be uniquely determined by the distribution of mass in the bulge and disk of the galaxy. We present evidence that the tangent of the pitch angle of logarithmic spiral arms in disk galaxies correlates strongly with the density of neutral atomic hydrogen in the disk and with the central stellar bulge mass of the galaxy. These three quantities, when plotted against each other, form a planar relationship that we argue should be fundamental to our understanding of spiral structure in disk galaxies. We further argue that any successful theory of spiral structure must be able to explain this relationship.

Catalogue of the morphological features in theSpitzerSurvey of Stellar Structure in Galaxies (S4G)

“…The pitch angle is not necessarily constant within a galaxy. It can stay nearly constant in certain regions of the galaxies (Davis et al 2012), or can change smoothly along the galaxy radius even in grand design galaxies (Davis et al 2015). Possible reasons for these changes are discussed by Davis et al (2015).…”

“…It can stay nearly constant in certain regions of the galaxies (Davis et al 2012), or can change smoothly along the galaxy radius even in grand design galaxies (Davis et al 2015). Possible reasons for these changes are discussed by Davis et al (2015). Furthermore, the spiral arms often appear asymmetrically at the two sides of the galaxies (see Elmegreen et al 2011).…”

“…Morphology of the spiral arms is connected to the physical properties of the galaxies. For example, pitch angle is found to correlate with the central mass concentration and the density of the HI gas in galaxies (Davis et al 2015). It also depends on the total galaxy mass so that the spiral arms are more open in galaxies with rising rotation curves, whereas those with falling rotation curves are generally tightly wound (Seigar et al 2005(Seigar et al , 2006(Seigar et al , 2014.…”

The Physical Origin of Long Gas Depletion Times in Galaxies

“…A given gas mass encounters a spiral arm on a timescale of t arm ∼ 2π/(m[Ω(R) − Ω p ]), where Ω(R) = V rot /R is the angular frequency of gas rotation, Ω p is the pattern speed of spiral arms, and m is the number of spiral arms. This timescale is t arm ∼ 50 − 200 Myr, if we assume Ω p ∼ 20 km s −1 kpc −1 (e.g., Bissantz et al 2003), m ∼ 2 − 4 (e.g., Davis et al 2015) and V rot ∼ 220 km s −1 typically derived for MW-like galaxies. Numerical simulations of gaseous galactic disks show that star-forming molecular clouds may form on even shorter timescales of a few tens of Myrs (Dobbs et al 2012(Dobbs et al , 2015.…”

The Most Ancient Spiral Galaxy: A 2.6-Gyr-old Disk with a Tranquil Velocity Field

“…Spiral arms play an active role in driving the radial and azimuthal mixing of the metals, redistributing angular momentum, and smoothing out small-scale mass distributions (e.g., Sellwood & Binney 2002;Sellwood 2014;Grand et al 2015Grand et al , 2016. The number and pitch angle of spiral arms are strongly correlated with the mass distribution of the disk and can be a powerful tool to constrain the bulge and black hole masses (Athanassoula et al 1987;Kennicutt 1981;Elmegreen & Elmegreen 1990;Berrier et al 2013;Dobbs & Baba 2014;Seigar et al 2014;Davis et al 2015Davis et al , 2017. The onset of spiral structures offers crucial insights into the origin of the Hubble sequence (Driver et al 1998; Email: tiantianyuan@swin.edu.au * ASTRO 3D Fellow Cen 2014; Genel et al 2015).…”