Star Formation in Nuclear Rings of Barred Galaxies

Abstract

Nuclear rings in barred galaxies are sites of active star formation. We use hydrodynamic simulations to study temporal and spatial behavior of star formation occurring in nuclear rings of barred galaxies where radial gas inflows are triggered solely by a bar potential. The star formation recipes include a density threshold, an efficiency, conversion of gas to star particles, and delayed momentum feedback via supernova explosions. We find that star formation rate (SFR) in a nuclear ring is roughly equal to the mass inflow rate to the ring, while it has a weak dependence on the total gas mass in the ring. The SFR typically exhibits a strong primary burst followed by weak secondary bursts before declining to very small values. The primary burst is associated with the rapid gas infall to the ring due to the bar growth, while the secondary bursts are caused by re-infall of the ejected gas from the primary burst. While star formation in observed rings persists episodically over a few Gyr, the duration of active star formation in our models lasts for only about a half of the bar growth time, suggesting that the bar potential alone is unlikely responsible for gas supply to the rings. When the SFR is low, most star formation occurs at the contact points between the ring and the dust lanes, leading to an azimuthal age gradient of young star clusters. When the SFR is large, on the other hand, star formation is randomly distributed over the whole circumference of the ring, resulting in no apparent azimuthal age gradient. Since the ring shrinks in size with time, star clusters also exhibit a radial age gradient, with younger clusters found closer to the ring. The cluster mass function is well described by a power law, with a slope depending on the SFR. Giant gas clouds in the rings have supersonic internal velocity dispersions and are gravitationally bound.

Star Clusters in a Nuclear Star Forming Ring: The Disappearing String of Pearls

“…Though it is difficult to exactly define the contact points in our case the mere presence of an age gradient is consistent with a dynamically (e.g. bar) induced cluster formation in the ring, a "pearls-on-a-string" scenario (Böker et al 2008;Seo & Kim 2013) rather than localised gravitational instabilities resulting in a more random pattern (a "popcorn model"). Also, any systematics in the age distribution indicate that multiple rotations, the rotational period of material at the ring radius is ∼30 Myr, have not washed out the signal confirming the generally young ages of clusters in NGC 2328.…”

Equilibrium Sequences and Gravitational Instability of Rotating Isothermal Rings

“…Wheṅ> M M c ; on the other hand, the inflowing gas overflows the contact points and is transferred into other parts of the ring, resulting in popcorn-style star formation when it becomes gravitationally unstable. Seo & Kim (2013) found numericallẏ…”

“…Because star clusters age as they orbit about the galaxy center, the pearls-on-a-string model naturally explains the presence of an azimuthal age gradient, while clusters with different ages are well mixed in the popcorn model (see also, e.g., Ryder et al 2001Ryder et al , 2010Allard et al 2006;Sandstrom et al 2010;van der Laan et al 2013). The most important factor that determines the dominating type of star formation appears to be the mass inflow rateṀ to the ring along the dust lanes (Seo & Kim 2013. WhenṀ is less than a critical valueṀ c , all inflowing gas can be consumed at the contact points, and star formation occurs in the pearls-on-a-string fashion.…”

THE MASS PROFILE AND SHAPE OF BARS IN THE SPITZER SURVEY OF STELLAR STRUCTURE IN GALAXIES (S4G): SEARCH FOR AN AGE INDICATOR FOR BARS

“…Being dissipative, the gas loses angular momentum and energy and flows inward toward the galactic center (Knapen et al 1995;Regan et al 1999;Sheth et al 2000Sheth et al , 2002, accumulating in the central ∼1 kpc of galaxies (e.g., Sakamoto et al 1999;Sheth et al 2005). The accumulation of gas in the central parts leads to high levels of circumnuclear star formation activity (Sérsic & Pastoriza 1965;Hawarden et al 1986;Devereux 1987;Martin 1995;Ho et al 1997;Sheth et al 2000Sheth et al , 2005Gadotti & dos Anjos 2001;Ellison et al 2011;Coelho & Gadotti 2011;Wang et al 2012); this circumnuclear star formation is often in the shape of nuclear rings Comerón et al 2010; Kim et al 2012;Seo & Kim 2013) and nuclear star clusters (Böker et al 2002(Böker et al , 2004(Böker et al , 2011. Such star formation activities may help to create disky pseudobulges (Kormendy & Kennicutt 2004;Sheth et al 2005;Athanassoula 2005; Debattista et al 2006).…”