Pilot Study and Early Results of the Cosmic Filaments and Magnetism Survey with Nenufar: The Coma Cluster Field

NenuFAR, the New Extension in Nancay Upgrading LOFAR, is currently in its early science phase. It is in this context that the Cosmic Filaments and Magnetism Pilot Survey is observing sources with the array as it is still under construction—with 57 (56 core, 1 distant) out of a total planned 102 (96 core, 6 distant) mini-arrays online at the time of observation—to get a first look at the low-frequency sky with NenuFAR. One of its targets is the Coma galaxy cluster: a well-known object, host of the prototype radio halo. It also hosts other features of scientific import, including a radio relic, along with a bridge of emission connecting it with the halo. It is thus a well-studied object.In this paper, we show the first confirmed NenuFAR detection of the radio halo and radio relic of the Coma cluster at 34.4 MHz, with associated intrinsic flux density estimates: we find an integrated flux value of 106.3 ± 3.5 Jy for the radio halo, and 102.0 ± 7.4 Jy for the radio relic. These are upper bound values, as they do not include point-source subtraction. We also give an explanation of the technical difficulties encountered in reducing the data, along with steps taken to resolve them. This will be helpful for other scientific projects which will aim to make use of standalone NenuFAR imaging observations in the future.

Extinction in the 11.2 µm PAH band and the low L11.2/LIR in ULIRGs

ABSTRACT We present a method for recovering the intrinsic (extinction-corrected) luminosity of the 11.2 μm PAH band in galaxy spectra. Using 105 high S/N Spitzer/IRS spectra of star-forming galaxies, we show that the equivalent width ratio of the 12.7 and 11.2 μm PAH bands is independent on the optical depth (τ), with small dispersion (∼5 per cent) indicative of a nearly constant intrinsic flux ratio Rint = (f12.7/f11.2)int = 0.377 ± 0.020. Conversely, the observed flux ratio, Robs = (f12.7/f11.2)obs, strongly correlates with the silicate strength (Ssil) confirming that differences in Robs reflect variation in τ. The relation between Robs and Ssil reproduces predictions for the Galactic Centre extinction law but disagrees with other laws. We calibrate the total extinction affecting the 11.2 μm PAH from Robs, which we apply to another sample of 215 galaxies with accurate measurements of the total infrared luminosity (LIR) to investigate the impact of extinction on L11.2/LIR. Correlation between L11.2/LIR and Robs independently on LIR suggests that increased extinction explains the well-known decrease in the average L11.2/LIR at high LIR. The extinction-corrected L11.2 is proportional to LIR in the range LIR = 109–1013 L⊙. These results consolidate L11.2 as a robust tracer of star formation in galaxies.

The ALMA Frontier Fields Survey

Context. Characterizing the number counts of faint (i.e., sub-mJy and especially sub-100 μJy), dusty star-forming galaxies is currently a challenge even for deep, high-resolution observations in the FIR-to-mm regime. They are predicted to account for approximately half of the total extragalactic background light at those wavelengths. Searching for dusty star-forming galaxies behind massive galaxy clusters benefits from strong lensing, enhancing their measured emission while increasing spatial resolution. Derived number counts depend, however, on mass reconstruction models that properly constrain these clusters. Aims. We aim to estimate the 1.1 mm number counts along the line of sight of three galaxy clusters, Abell 2744, MACS J0416.1–2403, and MACS J1149.5+2223, which are part of the ALMA Frontier Fields Survey. We have performed detailed simulations to correct these counts for lensing effects, probing down to the sub-mJy flux density level. Methods. We created a source catalog based on ALMA 1.1 mm continuum detections. We used several publicly available lensing models for the galaxy clusters to derive the intrinsic flux densities of these sources. We performed Monte Carlo simulations of the number counts for a detailed treatment of the uncertainties in the magnifications and adopted source redshifts. Results. We estimate lensing-corrected number counts at 1.1 mm using source detections down to S/N = 4.5. In each cluster field, we find an overall agreement among the number counts derived for the different lens models, despite their systematic variations regarding source magnifications and effective areas. Combining all cluster fields, our number counts span ∼2.5 dex in demagnified flux density, from several mJy down to tens of μJy. Both our differential and cumulative number counts are consistent with recent estimates from deep ALMA observations at a 3σ level. Below ≈0.1 mJy, however, our cumulative counts are lower by ≈1 dex, suggesting a flattening in the number counts. Conclusions. We derive 1.1 mm number counts around three well-studied galaxy clusters following a statistical approach. In our deepest ALMA mosaic, we estimate number counts for intrinsic flux densities ≈4 times fainter than the rms level. This highlights the potential of probing the sub-10 μJy population in larger samples of galaxy cluster fields with deeper ALMA observations.