Accretion-Inhibited Star Formation in the Warm Molecular Disk of the Green-valley Elliptical Galaxy NGC 3226

TL;DR

This study investigates the complex interplay of accretion, star formation, and molecular gas in the elliptical galaxy NGC 3226, revealing that accretion may inhibit rather than promote star formation in its molecular disk.

Contribution

The paper provides detailed multi-wavelength observations of NGC 3226, showing that accretion from tidal streams inhibits star formation in its molecular disk, challenging previous assumptions about galaxy rejuvenation.

Findings

Presence of a warm molecular hydrogen disk with low cold gas detection.

Low star formation rate (~0.04 M_sun/yr) over 100 Myrs.

Accretion from tidal streams appears to inhibit star formation.

Abstract

We present archival Spitzer photometry and spectroscopy, and Herschel photometry, of the peculiar "Green Valley" elliptical galaxy NGC~3226. The galaxy, which contains a low-luminosity AGN, forms a pair with NGC~3227, and is shown to lie in a complex web of stellar and HI filaments. Imaging at 8 and 16$\mu$m reveals a curved plume structure 3 kpc in extent, embedded within the core of the galaxy, and coincident with the termination of a 30 kpc-long HI tail. In-situ star formation associated with the IR plume is identified from narrow-band HST imaging. The end of the IR-plume coincides with a warm molecular hydrogen disk and dusty ring, containing 0.7-1.1 $\times$ 10$^7$ M$_{\odot}$ detected within the central kpc. Sensitive upper limits to the detection of cold molecular gas may indicate that a large fraction of the H$_2$ is in a warm state. Photometry, derived from the UV to the far-IR, shows evidence for a low star formation rate of $\sim$0.04 M$_{\odot}$ yr$^{-1}$ averaged over the last 100 Myrs. A mid-IR component to the Spectral Energy Distribution (SED) contributes $\sim$20$\%$ of the IR luminosity of the galaxy, and is consistent with emission associated with the AGN. The current measured star formation rate is insufficient to explain NGC3226's global UV-optical "green" colors via the resurgence of star formation in a "red and dead" galaxy. This form of "cold accretion" from a tidal stream would appear to be an inefficient way to rejuvenate early-type galaxies, and may actually inhibit star formation.