Toward a Unification of Star Formation Rate Determinations in the Milky Way and Other Galaxies

Abstract

The star formation rate (SFR) of the Milky Way remains poorly known, with often-quoted values ranging from 1 to 10 M yr −1 . This situation persists despite the potential for the Milky Way to serve as the ultimate SFR calibrator for external galaxies. We show that various estimates for the Galactic SFR are consistent with one another once they have been normalized to the same initial mass function (IMF) and massive star models, converging to 1.9 ± 0.4 M yr −1 . However, standard SFR diagnostics are vulnerable to systematics founded in the use of indirect observational tracers sensitive only to high-mass stars. We find that absolute SFRs measured using resolved low/intermediate-mass stellar populations in Galactic H ii regions are systematically higher by factors of ∼2-3 compared with calibrations for SFRs measured from mid-IR and radio emission. We discuss some potential explanations for this discrepancy and conclude that it could be allayed if (1) the power-law slope of the IMF for intermediate-mass (1.5 M < m < 5M ) stars were steeper than the Salpeter slope or (2) a correction factor was applied to the extragalactic 24 μm SFR calibrations to account for the duration of star formation in individual mid-IR-bright H ii regions relative to the lifetimes of O stars. Finally, we present some approaches for testing whether a Galactic SFR of ∼2 M yr −1 is consistent with what we would measure if we could view the Milky Way as external observers. Using luminous radio supernova remnants and X-ray point sources, we find that the Milky Way deviates from expectations at the 1σ -3σ level, hinting that perhaps the Galactic SFR is overestimated or extragalactic SFRs need to be revised upward.

RAPID CIRCUMSTELLAR DISK EVOLUTION AND AN ACCELERATING STAR FORMATION RATE IN THE INFRARED DARK CLOUD M17 SWex

“…The SFR measured within the ACIS FOV is  M 0.0036 M ☉ yr −1 , at least four times the SFR of the Orion Nebula Cluster (ONC), and the extrapolated SFR for the entire M17 SWex complex (recalling that only half of the YSOs in M17 SWex are concentrated in our ACIS FOV) rivals or exceed the SFR of = M 0.005 M ☉ yr −1 in NGC 6618, the cluster ionizing the bright M17 H II region itself (Chomiuk & Povich 2011). This is not an artifact of time-averaging, since the durations of star formation in M17 SWex, NGC 6618, and the ONC are approximately the same (Povich et al 2009;Chomiuk & Povich 2011;Getman et al 2014).…”

“…If such clouds are common, they could represent a significant mode of "dark" star formation, invisible to widely used SFR diagnostics in external galaxies (Chomiuk & Povich 2011;Kennicutt & Evans 2012). …”

The cosmic dust rate across the Universe

“…The spiral galaxy is characterized by a continuous SFR which is higher than the one of irregulars. The models are able to reproduce the present day SFR observed in irregulars (Chomiuk & Povich 2011) and spirals (Harris & Zaritsky 2009). In the other panels of the Figure, the evolution of various processes related to dust evolution (stellar production, accretion, and destruction) are shown.…”

26Al kinematics: superbubbles following the spiral arms?

“…We therefore used the star cluster population up to 48 Myr for our model. Following Chomiuk & Povich (2011), we took 1.9 M yr −1 for the star formation rate of the Milky Way. This sets the constant in Eq.…”