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Register a new userEscape, Accretion or Star Formation? The Competing Depleters of Gas in the Quasar Markarian 231
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Katherine Alatalo
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We report on high resolution CO(1-0), CS(2-1) and 3mm continuum Combined Array for Research in Millimeter Astronomy (CARMA) observations of the molecular outflow host and nearest quasar Markarian 231. We use the CS(2-1) measurements to derive a dense gas mass within Mrk231 of $1.8pm0.3times10^{10}~M_odot$, consistent with previous measurements. The CS(2-1) data also seem to indicate that the molecular disk of Mrk231 is forming stars at about normal efficiency. The high resolution CARMA observations were able to resolve the CO(1-0) outflow into two distinct lobes, allowing for a size estimate to be made and further constraining the molecular outflow dynamical time, further constraining the molecular gas escape rate. We find that 15% of the molecular gas within the Mrk231 outflow actually exceeds the escape velocity in the central kiloparsec. Assuming that molecular gas is not constantly being accelerated, we find the depletion timescale of molecular gas in Mrk231 to be 49Myr, rather than 32Myr, more consistent with the poststarburst stellar population observed in the system.
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The oxygen-bearing molecular ions OH+, H2O+, and H3O+ are key species that probe the ionization rate of (partially) molecular gas that is ionized by X-rays and cosmic rays permeating the interstellar medium. We report Herschel far-infrared and submillimeter spectroscopic observations of OH+ in Mrk 231, showing both ground-state P-Cygni profiles, and excited line profiles with blueshifted absorption wings extending up to ~1000 km s^{-1}. In addition, OH+ probes an excited component peaking at central velocities, likely arising from the torus probed by the OH centimeter-wave megamaser. Four lines of H2O+ are also detected at systemic velocities, but H3O+ is undetected. Based on our earlier OH studies, we estimate an abundance ratio of OH/OH+~5-10 for the outflowing components and ~20 for the torus, and an OH+ abundance relative to H nuclei of ~>10^{-7}. We also find high OH+/H2O+ and OH+/H3O+ ratios, both are ~>4 in the torus and ~>10-20 in the outflowing gas components. Chemical models indicate that these high OH+ abundances relative to OH, H2O+, and H3O+ are characteristic of gas with a high ionization rate per unit density, zeta/n_H~(1-5)x10^{-17} cm^3 s^{-1} and ~(1-2)x10^{-16} cm^3 s^{-1} for the above components, respectively, and an ionization rate of zeta~(0.5-2)x10^{-12} s^{-1}. X-rays appear to be unable to explain the inferred ionization rate, and thus we suggest that low-energy (10-400 MeV) cosmic-rays are primarily responsible for the ionization with dot{M}_{CR}~0.01 M_{sun} yr^{-1} and dot{E}_{CR}~10^{44} erg s^{-1}, the latter corresponding to 1% of the AGN luminosity and similar to the energetics of the molecular outflow. We suggest that cosmic-rays accelerated in the forward shock associated with the molecular outflow are responsible for the ionization, as they diffuse through the outflowing molecular phase downstream.
This paper discusses how cosmic gas accretion controls star formation, and summarizes the physical properties expected for the cosmic gas accreted by galaxies. The paper also collects observational evidence for gas accretion sustaining star formation. It reviews evidence inferred from neutral and ionized hydrogen, as well as from stars. A number of properties characterizing large samples of star-forming galaxies can be explained by metal-poor gas accretion, in particular, the relationship between stellar mass, metallicity, and star formation rate (the so-called fundamental metallicity relationship). They are put forward and analyzed. Theory predicts gas accretion to be particularly important at high redshift, so indications based on distant objects are reviewed, including the global star formation history of the universe, and the gas around galaxies as inferred from absorption features in the spectra of background sources.
Cosmological numerical simulations of galaxy evolution show that accretion of metal-poor gas from the cosmic web drives the star formation in galaxy disks. Unfortunately, the observational support for this theoretical prediction is still indirect, and modeling and analysis are required to identify hints as actual signs of star-formation feeding from metal-poor gas accretion. Thus, a meticulous interpretation of the observations is crucial, and this observational review begins with a simple theoretical description of the physical process and the key ingredients it involves, including the properties of the accreted gas and of the star-formation that it induces. A number of observations pointing out the connection between metal-poor gas accretion and star-formation are analyzed, specifically, the short gas consumption time-scale compared to the age of the stellar populations, the fundamental metallicity relationship, the relationship between disk morphology and gas metallicity, the existence of metallicity drops in starbursts of star-forming galaxies, the so-called G dwarf problem, the existence of a minimum metallicity for the star-forming gas in the local universe, the origin of the alpha-enhanced gas forming stars in the local universe, the metallicity of the quiescent BCDs, and the direct measurements of gas accretion onto galaxies. A final section discusses intrinsic difficulties to obtain direct observational evidence, and points out alternative observational pathways to further consolidate the current ideas.
In order to quantify the relationship between gas accretion and star formation, we analyse a sample of 29 nearby galaxies from the WHISP survey which contains galaxies with and without evidence for recent gas accretion. We compare combined radial profiles of FUV (GALEX) and IR 24 {mu}m (Spitzer) characterizing distributions of recent star formation with radial profiles of CO (IRAM, BIMA, or CARMA) and HI (WSRT) tracing molecular and atomic gas contents to examine star formation efficiencies in symmetric (quiescent), asymmetric (accreting), and interacting (tidally disturbed) galaxies. In addition, we investigate the relationship between star formation rate and HI in the outer discs for the three groups of galaxies. We confirm the general relationship between gas surface density and star formation surface density, but do not find a significant difference between the three groups of galaxies.
A major uncertainty in models for photoionised outflows in AGN is the distance of the gas to the central black hole. We present the results of a massive multiwavelength monitoring campaign on the bright Seyfert 1 galaxy Mrk 509 to constrain the location of the outflow components dominating the soft X-ray band. Mrk 509 was monitored by XMM-Newton, Integral, Chandra, HST/COS and Swift in 2009. We have studied the response of the photoionised gas to the changes in the ionising flux produced by the central regions. We were able to put tight constraints on the variability of the absorbers from day to year time scales. This allowed us to develop a model for the time-dependent photoionisation in this source. We find that the more highly ionised gas producing most X-ray line opacity is at least 5 pc away from the core; upper limits to the distance of various absorbing components range between 20 pc up to a few kpc. The more lowly ionised gas producing most UV line opacity is at least 100 pc away from the nucleus. These results point to an origin of the dominant, slow (v<1000 km/s) outflow components in the NLR or torus-region of Mrk 509. We find that while the kinetic luminosity of the outflow is small, the mass carried away is likely larger than the 0.5 Solar mass per year accreting onto the black hole. We also determined the chemical composition of the outflow as well as valuable constraints on the different emission regions. We find for instance that the resolved component of the Fe-K line originates from a region 40-1000 gravitational radii from the black hole, and that the soft excess is produced by Comptonisation in a warm (0.2-1 keV), optically thick (tau~10-20) corona near the inner part of the disk.
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