Molecular gas and star formation within 12 strong galactic bars observed with IRAM-30m

Abstract

Context. While some galactic bars show recent massive star formation (SF) along them, some others do not. Whether bars with low level of SF are a consequence of low star formation efficiency, low gas inflow rate, or dynamical effects remains a matter of debate. Aims. In order to study the physical conditions that enable or prevent SF, we perform a multi-wavelength analysis of 12 strongly barred galaxies with total stellar masses log(10)(M-*/M-circle dot)is an element of[10.2,11], chosen to host different degrees of SF along the bar major axis without any prior condition on gas content. We observe the CO(1-0) and CO(2-1) emission within bars with the IRAM-30 m telescope (beam sizes of 1.7-3.9 kpc and 0.9-2.0 kpc, respectively; 7-8 pointings per galaxy on average). Methods. We estimated molecular gas masses (M-mol) from the CO(1-0) and CO(2-1) emissions. SF rates (SFRs) were calculated from GALEX near-ultraviolet (UV) and WISE 12 mu m images within the beam-pointings, covering the full bar extent (SFRs were also derived from far-UV and 22 mu m). Results. We detect molecular gas along the bars of all probed galaxies. Molecular gas and SFR surface densities span the ranges log(10)(sigma(mol)/[M-circle dot pc(-2)]) is an element of [0.4,2.4] and log(10)(sigma(SFR)/[M-circle dot pc(-1) kpc(-2)]]) is an element of [-3.25, -0.75], respectively. The star formation efficiency (SFE; i.e., SFR/M-mol) in bars varies between galaxies by up to an order of magnitude (SFE is an element of[0.1,1.8] Gyr(-1)). On average, SFEs are roughly constant along bars. SFEs are not significantly different from the mean value in spiral galaxies reported in the literature (similar to 0.43 Gyr(-1)), regardless of whether we estimate M-mol from CO(1-0) or CO(2-1). Interestingly, the higher the total stellar mass of the host galaxy, the lower the SFE within their bars. In particular, the two galaxies in our sample with the lowest SFE and sigma(SFR) (NGC 4548 and NGC 5850, SFE less than or similar to 0.25 Gyr(-1), sigma(SFR)less than or similar to 10(-2.25)M(circle dot) yr(-1) kpc(-2), M greater than or similar to 10(10.7)M(circle dot)) are also those hosting massive bulges and signs of past interactions with nearby companions. Conclusions. We present a statistical analysis of the SFE in bars for a sample of 12 galaxies. The SFE in strong bars is not systematically inhibited (either in the central, middle, or end parts of the bar). Both environmental and internal quenching are likely responsible for the lowest SFEs reported in this work.