A SOFIA Survey of [C ii] in the Galaxy M51. I. [C ii] as a Tracer of Star Formation

Abstract

We present a [C ii] 158 µm map of the entire M51 (including M51b) grand-design spiral galaxy observed with the FIFI-LS instrument on SOFIA. We compare the [C ii] emission with the total far-infrared (TIR) intensity and star formation rate (SFR) surface density maps (derived using Hα and 24µm emission) to study the relationship between [C ii] and the star formation activity in a variety of environments within M51 on scales of 16 corresponding to ∼660 pc. We find that [C ii] and the SFR surface density are well correlated in the central, spiral arm, and inter-arm regions. The correlation is in good agreement with that found for a larger sample of nearby galaxies at kpc scales. We find that the SFR, and [C ii] and TIR luminosities in M51 are dominated by the Pineda, J.L. et al.extended emission in M51's disk. The companion galaxy M51b, however, shows a deficit of [C ii] emission compared with the TIR emission and SFR surface density, with [C ii] emission detected only in the S-W part of this galaxy. The [C ii] deficit is associated with an enhanced dust temperature in this galaxy. We interpret the faint [C ii] emission in M51b to be a result of suppressed star formation in this galaxy, while the bright midand far-infrared emission, which drive the TIR and SFR values, are powered by other mechanisms. A similar but less pronounced effect is seen at the location of the black hole in M51's center. The observed [C ii] deficit in M51b suggests that this galaxy is a valuable laboratory to study the origin of the apparent [C ii] deficit observed in ultra-luminous galaxies.

SOFIA/FIFI-LS Full-disk [C ii] Mapping and CO-dark Molecular Gas across the Nearby Spiral Galaxy NGC 6946

“…Our new SOFIA/FIFI-LS [C II] observations of the nearby (D∼7.7 Mpc, see references in Leroy et al 2019), molecular gas rich (M H2 ∼10 9.6 M ☉ , Leroy et al 2013), actively starforming ( Leroy et al 2019) and relatively face-on (i≈33°) spiral galaxy NGC 6946 provides a testbed to address these topics across a full galaxy disk. Our high resolution (15″, corresponding to ∼560 pc) sets these observations apart from prior low-resolution work in this galaxy (Madden et al 1993;Contursi et al 2002) or observations covering only part of the disk (e.g., from Herschel KINGFISH, de Blok et al 2016;Smith et al 2017) and is in line with a recent SOFIA study of M51 (Pineda et al 2018). NGC 6946 has a moderate metallicity gradient of~r 0.3 dex 25 (central oxygen abundance ( ) + = 12 log O H 9.13 and 9.05 at r 0.4 25 , whereas the galaxy average is ∼8.99, Moustakas et al 2010), using the calibration by Kobulnicky & Kewley (2004), where the optical radius = ¢ r 5.74…”

“…The scatter in each bin (see error bars) is between 0.2 and 0.4 dex and is largest at higher luminosities. The data distribution covers roughly the same range of [C II] and SFR surface densities than in other resolved, kiloparsecscale studies of nearby galaxies (e.g., Herrera-Camus et al 2015;Pineda et al 2018). Because [C II]-SFR trends are commonly parameterized by power-law fits in the literature, we repeat this exercise for the purpose of comparing to prior work (though not for an exhaustive statistical characterization of these data).…”

“…Extragalactic observations, however, reveal significant variations in the [C II]/TIR ratio among galaxies. In particular, in infrared luminous environments, [C II] emission appears less luminous relative to that expected from TIR emission, tracing the heating rate by massive star formation in the ISM ("cooling line deficit," e.g., Malhotra et al 1997;Graciá-Carpio et al 2011;Díaz-Santos et al 2017;Smith et al 2017;Pineda et al 2018).…”

Simulations of the star-forming molecular gas in an interacting M51-like galaxy

“…At t = 418 Myr, the time at which our simulation is at an equivalent evolutionary phase to M51, we find an SFR of 4 M yr −1 which is comparable to the observed value of 4.6 M yr −1 (Pineda et al 2018). However, if we consider that we started with a galaxy that had only half of the gas mass of M51a, we conclude that our depletion times are most likely too short by a factor of about two.…”

Diagnosing the interstellar medium of galaxies with far-infrared emission lines

“…or gas density. However, the impact from other physical parameters such as dust temperatures (Pineda et al 2018) or the AGN (Herrera-Camus et al 2018b) are still not clear. In Fig.…”