Abstract
We report new detections and limits from a NOEMA and ALMA CO(1-0) search for molecular outflows in 13 local galaxies with high far-infrared surface brightness, and combine these with local universe CO outflow results from the literature. The CO line ratios and spatial outflow structure of our targets provide some constraints on the conversion steps from observables to physical quantities such as molecular mass outflow rates. Where available, ratios between outflow emission in higher J CO transitions and in CO(1-0) are typically consistent with excitationRi1 ≲ 1. However, for IRAS 13120−5453,R31 = 2.10 ± 0.29 indicates optically thin CO in the outflow. Like much of the outflow literature, we useαCO(1 − 0)= 0.8, and we present arguments for usingC = 1 in deriving molecular mass outflow ratesṀout=CMoutvout/Rout. We compare the two main methods for molecular outflow detection: CO millimeter interferometry andHerschelOH-based spectroscopic outflow searches. For 26 sources studied with both methods, we find an 80% agreement in detectingvout≳ 150 km s−1outflows, and non-matches can be plausibly ascribed to outflow geometry and signal-to-noise ratio. For a published sample of 12 bright ultraluminous infrared galaxies with detailed OH-based outflow modeling, CO outflows are detected in all but one. Outflow masses, velocities, and sizes for these 11 sources agree well between the two methods, and modest remaining differences may relate to the different but overlapping regions sampled by CO emission and OH absorption. Outflow properties correlate better with active galactic nucleus (AGN) luminosity and with bolometric luminosity than with far-infrared surface brightness. The most massive outflows are found for systems with current AGN activity, but significant outflows in nonAGN systems must relate to star formation or to AGN activity in the recent past. We report scaling relations for the increase of outflow mass, rate, momentum rate, and kinetic power with bolometric luminosity. Short flow times of ∼106yr and some sources with resolved multiple outflow episodes support a role of intermittent driving, likely by AGNs.
Highlights
We report new detections and limits from a NOrthern Extended Millimeter Array (NOEMA) and Atacama Large Millimeter/submillimeter Array (ALMA) CO(1-0) search for molecular outflows in 13 local galaxies with high far-infrared surface brightness, and combine these with local universe CO outflow results from the literature
Gas outflows triggered by intense star formation and/or active galactic nuclei (AGNs) are important agents in the evolution of galaxies, in regulating the fraction of baryons inside a dark matter halo that are converted to stars, and for the metal enrichment of the circumgalactic and intergalactic medium
In extreme cases and if velocities are high enough to escape the galaxy, extrapolation of the current outflow rate would clear out most of the gas content of a galaxy within 107 yr. While both star formation and active galactic nucleus (AGN) may contribute to driving outflows, the OH outflow velocity in ULIRGs as well as the CO-based outflow rates seem to be best correlated with AGN luminosity (Veilleux et al 2013; Cicone et al 2014; Fiore et al 2017; Fluetsch et al 2019)
Summary
Gas outflows triggered by intense star formation and/or active galactic nuclei (AGNs) are important agents in the evolution of galaxies, in regulating the fraction of baryons inside a dark matter halo that are converted to stars, and for the metal enrichment of the circumgalactic and intergalactic medium (see, e.g., reviews by Veilleux et al 2005; Fabian 2012). Three major sources of uncertainty currently affect the derivation of molecular outflow rates from interferometric CO data: (i) the emission from outflowing gas has to be separated from that of the host; (ii) a CO conversion factor has to be adopted to convert from luminosity to gas mass, but may not necessarily be the same as for typical Galactic molecular clouds; and (iii) if only an outflow velocity and overall size are known, the conversion from outflow mass to outflow rate is uncertain by at least a factor of three due to different possible geometrical assumptions
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