Microwave Continuum Emission and Dense Gas Tracers in NGC 3627: Combining Jansky Vla and Alma Observations

Abstract

We present Karl G. Jansky Very Large Array (VLA) Ka band (33 GHz) and Atacama Large Millimeter Array (ALMA) Band 3 (94.5 GHz) continuum images covering the nucleus and two extranuclear star-forming regions within the nearby galaxy NGC 3627 (M 66), observed as part of the Star Formation in Radio Survey (SFRS). Both images achieve an angular resolution of 2 ′′ , allowing us to map the radio spectral indices and estimate thermal radio fractions at a linear resolution of 90 pc at the distance of NGC 3627. The thermal fraction at 33 GHz reaches unity at and around the peaks of each Hii region; we additionally observed the spectral index between 33 and 94.5 GHz to become both increasingly negative and positive away from the peaks of the Hii regions, indicating an increase of non-thermal extended emission from diffusing cosmic-ray electrons and the possible presence of cold dust, respectively. While the ALMA observations were optimized for collecting continuum data, they also detected line emission from the J = 1 → 0 transitions of HCN and HCO + . The peaks of dense molecular gas traced by these two spectral lines are spatially offset from the peaks of the 33 and 94.5 GHz continuum emission for the case of the extranuclear star-forming regions, indicating that our data reach an angular resolution at which one can spatially distinguish sites of recent star formation from the sites of future star formation. Finally, we find trends of decreasing dense gas fraction and velocity dispersion with increasing star formation efficiency among the three regions observed, indicating that the dynamical state of the dense gas, rather than its abundance, plays a more significant role in the star formation process.

Dense gas is not enough: environmental variations in the star formation efficiency of dense molecular gas at 100 pc scales in M 51

“…This is true for our new measurements in M51 (ρ SFE dense −σ HCN = −0.62), and it remains true when we include the values for other galaxies from the literature (ρ SFE dense −σ HCN = −0.53). The trend goes in the same direction as that already pointed out by Murphy et al (2015) based on HCN emission in NGC 3627 (with three datapoints at ∼300 pc resolution), although there is a vertical offset between the trend in NGC 3627 and that in M51. Our measurements extend to the dense gas phase the result from Leroy et al (2017a), who already identified in M51 a correlation between the velocity dispersion and star formation efficiency of the bulk molecular gas traced by CO (using the full PAWS resolution, 40 pc), in the sense that larger linewidths are associated with lower star formation efficiencies.…”

“…Given that systematic offsets exist among the different SFR tracers used by the studies from the literature, we rescale those SFRs by an empirically derived factor to enforce consistency (see Appendix A for details). In NGC 3627, for the nuclear pointing in Murphy et al (2015) the HCN line was not fully covered by their spectral set-up, so we use the information from HCO + (1-0) instead in that particular case.…”

“…High-resolution maps of dense gas are still rare, although there are some exceptions (e.g. Murphy et al 2015;Bigiel et al 2015;Chen et al 2017;Kepley et al 2018;Viaene et al 2018). A second issue affecting these studies is the difficulty to trace star formation locally in a way that is robust to extinction by dust.…”

A 3 mm Spectral Line Survey toward the Barred Spiral Galaxy NGC 3627

“…Before deriving the column densities, the integrated intensities are divided by the beam filling factor θ 2 source /(θ 2 source +θ 2 beam ), where θ source and θ beam are the source size and the FWHM of the telescope beam, respectively, to compensate for frequency dependence of the beam size. Here, the source size (θ source ) is assumed to be 10 ′′ , because distributions of HCO + (J = 1−0) are resolved to be ∼ 10 ′′ with ALMA in BE and NR (Murphy et al 2015). We assume the same source size in SA for simplicity.…”

The GOODS-N Jansky VLA 10 GHz Pilot Survey: Sizes of Star-forming μJY Radio Sources

“…Constructing the star-formation history of the universe requires converting the synchrotron luminosities measured by low-frequency surveys to star formation rates via the tight, but empirical and local, far-infrared/radio correlation (de Jong et al 1985;Helou et al 1985;Yun et al 2001). Surveys at observing frequencies 10 GHz measure flux densities closer to the rest-frame frequencies n  30 GHz, where the total radio emission is dominated by the free-free radiation that is more directly proportional to the rate of massive star formation (e.g., Mezger & Henderson 1967;Turner & Ho 1983Klein & Graeve 1986;Kobulnicky & Johnson 1999;Murphy et al 2012aMurphy et al , 2015bNikolic & Bolton 2012), are still unbiased by dust emission or absorption, and yield higher angular resolution for a given array size.…”