Origins of Scatter in the Relationship Between HCN 1-0 and Dense Gas Mass in the Galactic Center

Abstract

We investigate the correlation of HCN 1-0 with dense gas mass in the Galactic center. We find that in general (on the ~10 pc size scale of individual molecular cloud cores) HCN 1-0 is well correlated with the dense molecular gas mass using a standard log-log relationship. However individual clouds in this environment show systematic deviations from this relationship that contribute to around 0.75 dex of scatter. Most notably, Sgr B2, the most massive cloud in the Galactic center, has an integrated HCN 1-0 intensity of cloud less than half its mass, and several other clouds including GCM-0.02-0.07 show an enhancement of HCN 1-0 by a factor of 2-3 relative to clouds of comparable mass. We also find that HCN 1-0 is more intense for a given mass in the large-scale diffuse emission in the central 300 parsecs compared to the compact emission from individual cloud cores. We identify the two primary sources of scatter in the relationship between HCN 1-0 and dense gas mass to be self-absorption (which reduces the observed HCN 1-0 intensity of Sgr B2) and variations in HCN abundance, which appear to increase the brightness of HCN 1-0 in GCM-0.02-0.07 and other clouds. We find that these sources of scatter would only contribute to an ~10% error in the dense gas mass inferred from the HCN 1-0 intensity in the Galactic center. However, the implied order of magnitude HCN abundance variations between Galactic center clouds, and the systematic nature of these variations warn of potential biases in the use of HCN as dense gas mass tracer in more extreme environments, such as AGN and shock-dominated regions. Finally, we also investigate other tracers having transitions near 3 mm, finding that HNCO, HNC and HCO+ largely behave like HCN, while HC3N, and CH3CN are higher fidelity tracers of the amount of gas in clouds like Sgr B2 that suffer from strong self-absorption of the HCN 1-0 line. [abridged]

19 pages, 6 figures, ApJ accepted

References in corpus (19)

• Cosmic Star Formation History
• Combined CO & Dust Scaling Relations of Depletion Time and Molecular Gas Fractions with Cosmic Time, Specific Star Formation Rate and Stellar Mass
• MAMBO Mapping of Spitzer c2d Small Clouds and Cores
• COLD GASS, an IRAM Legacy Survey of Molecular Gas in Massive Galaxies: II. The non-universality of the Molecular Gas Depletion Timescale
• Diagnostics of irradiated dense gas in galaxy nuclei. II. A grid of XDR and PDR models
• An Improved Distance and Mass Estimate for Sgr A* from a Multistar Orbit Analysis
• The dynamical evolution of molecular clouds near the Galactic Centre - I. Orbital structure and evolutionary timeline
• ALMA Reveals the Molecular Medium Fueling the Nearest Nuclear Starburst
• Evidence of enhanced star formation efficiency in luminous and ultraluminous infrared galaxies
• Is HCN a True Tracer of Dense Molecular Gas in LIRGs and ULIRGs?
• Vibrationally Excited HCN in the Luminous Infrared Galaxy NGC 4418
• High resolution observations of HCN and HCO+ J=3-2 in the disk and outflow of Mrk231 -Detection of vibrationally excited HCN in the warped nucleus
• HCN Observations of Dense Star-Forming Gas in High Redshift Galaxies
• Overluminous HNC Line Emission in Arp220, NGC4418 and Mrk231 - Global IR Pumping or XDRs?
• High-J CO SLEDs in nearby infrared bright galaxies observed by Herschel-PACS
• Nobeyama Millimeter Interferometric HCN(1-0) and HCO+(1-0) Observations of Further Luminous Infrared Galaxies
• Luminous HC3N line emission in NGC4418 - buried AGN or nascent starburst?
• Abundant Methanol Masers but no New Evidence for Star Formation in GCM0.253+0.016
• The Comparison of Physical Properties Derived from Gas and Dust in a Massive Star-Forming Region

Cited by in corpus (15)

• Ubiquitous velocity fluctuations throughout the molecular interstellar medium
• CMZoom: Survey Overview and First Data Release
• Star formation in 'the Brick': ALMA reveals an active proto-cluster in the Galactic centre cloud G0.253+0.016
• CMZoom II: Catalog of Compact Submillimeter Dust Continuum Sources in the Milky Way's Central Molecular Zone
• The centres of M83 and the Milky Way: opposite extremes of a common star formation cycle
• SOFIA/HAWC+ Far-InfraRed Polarimetric Large Area CMZ Exploration (FIREPLACE) Survey I: General Results from the Pilot Program
• The Molecular ISM in the Super Star Clusters of the Starburst NGC 253
• Fiery Cores: Bursty and Smooth Star Formation Distributions across Galaxy Centers in Cosmological Zoom-in Simulations
• Virial Clumps in Central Molecular Zone Clouds
• Central molecular zones in galaxies: multitransition survey of dense gas tracers HCN, HNC, and HCO+
• Cold Molecular Gas in Merger Remnants. II. The properties of dense molecular gas
• CMZoom III: Spectral Line Data Release
• Evidence for an interaction between the Galactic Center clouds M0.10-0.08 and M0.11-0.11
• HCN $J$=4-3, HNC $J$=1-0, ${\mathrm{H}}^{13}\mathrm{CN}$ $J$=1-0, and $\mathrm{H}{\mathrm{C}}_3\mathrm{N}$ $J$=10-9 Maps of Galactic Center Region II.: Physical Properties of Dense Gas Clumps and Probability of Star Formation
• Clustered Star Formation in the center of NGC 253 Contributes to Driving the Ionized Nuclear Wind