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

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
e-mail: simondiazgar@gmail.com
² Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
³ Department for Physics, Engineering Physics and Astrophysics, Queen’s University, Kingston, ON K7L 3N6, Canada
⁴ Departamento de Física Teórica y del Cosmos, Campus de Fuentenueva, Universidad de Granada, 18071 Granada, Spain
⁵ Instituto Carlos I de Física Teórica y Computacional, Facultad de Ciencias, 18071 Granada, Spain
⁶ Observatoire de Paris, LERMA, Collège de France, CNRS, PSL University, Sorbonne University, 75014 Paris, France
⁷ SKA Organization, Lower Withington, Macclesfield, Cheshire SK11 9DL, UK
⁸ Joint ALMA Observatory, Alonso de Cordova 3107, Vitacura, Santiago 763-0355, Chile
⁹ Universidad Internacional de Valencia (VIU), C. del Pintor Sorolla 21, 46002 Valencia, Spain
¹⁰ SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹¹ UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹² Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía, 18008 Granada, Spain

Received: 26 February 2021
Accepted: 22 June 2021

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 log₁₀(M_⋆/M_⊙)∈[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 μm images within the beam-pointings, covering the full bar extent (SFRs were also derived from far-UV and 22 μm).

Results. We detect molecular gas along the bars of all probed galaxies. Molecular gas and SFR surface densities span the ranges log₁₀(Σ_mol/[M_⊙ pc⁻²]) ∈ [0.4,2.4] and log₁₀(Σ_SFR/[M_⊙ pc⁻¹ kpc⁻²]]) ∈ [−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 ∈[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 (∼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 Σ_SFR (NGC 4548 and NGC 5850, SFE ≲ 0.25 Gyr⁻¹, Σ_SFR ≲ 10^−2.25 M_⊙ yr⁻¹ kpc⁻², M_⋆ ≳ 10^10.7 M_⊙) 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.