Morphological and Kinematical Analysis of the Double-barred Galaxy NGC 3504 Using ALMA CO (2-1) Data

TL;DR

This study uses high-resolution ALMA CO (2-1) data to analyze the gas dynamics and structures within the double-barred galaxy NGC 3504, revealing an inner gas bar, nuclear ring, and detailed kinematic properties.

Contribution

It provides the first detailed kinematic analysis of an inner gas bar in a double-barred galaxy using high-resolution ALMA data, including pattern speed and mass inflow rate estimates.

Findings

Inner gas bar and nuclear ring detected within 1 kpc.

Dominance of circular motion at certain radii with radial motion inside.

Estimated pattern speed of outer bar as 18±5 km/s/kpc.

Abstract

We present results obtained from ALMA CO (2-1) data of the double-barred galaxy NGC 3504. With three times higher angular resolution (~ 0."8) than previous studies, our observations reveal an inner molecular gas bar, a nuclear ring, and four inner spiral arm-like structures in the central 1 kpc region. Furthermore, the CO emission is clearly aligned with the two dust lanes in the outer bar region, with differences in shape and intensity between them. The total molecular gas mass in the observed region (50"x57") is estimated to be $\sim 3.1\times 10^9 \, {\rm M}_{\odot}$, which is 17 per cent of the stellar mass. We used the Kinemetry package to fit the velocity field and found that circular motion strongly dominates at $R= 0.3-0.8$ kpc, but radial motion becomes important at $R<0.3$ kpc and $R=1.0-2.5$ kpc, which is expected due to the presence of the inner and outer bars. Finally, assuming that the gas moves along the dust lanes in the bar rotating frame, we derived the pattern speed of the outer bar to be $ 18\pm5$ km s$^{-1}$ kpc$^{-1}$, the average streaming velocities on each of the two dust lanes to be 165 and 221 km s$^{-1}$, and the total mass inflow rate along the dust lanes to be 12 M$_{\odot}$ yr$^{-1}$. Our results give a new example of an inner gas bar within a gas-rich double-barred galaxy and suggest that the formation of double-barred galaxies could be associated with the existence of such gas structures.