Early science with the Large Millimeter Telescope: a 1.1 mm AzTEC survey of red-Herschel dusty star-forming galaxies

Abstract

We present LMT/AzTEC 1.1mm observations of ∼100 luminous high-redshift dusty star-forming galaxy candidates from the ∼600 sq.deg Herschel-ATLAS survey, selected on the basis of their SPIRE red far-infrared colours and with $S_{500\, \mu \rm m}=35-80$ mJy. With an effective θFWHM ≈ 9.5 arcsec angular resolution, our observations reveal that at least 9 per cent of the targets break into multiple systems with SNR ≥4 members. The fraction of multiple systems increases to ∼23 per cent (or more) if some non-detected targets are considered multiples, as suggested by the data. Combining the new AzTEC and deblended Herschel photometry we derive photometric redshifts, IR luminosities, and star formation rates. While the median redshifts of the multiple and single systems are similar (zmed ≈ 3.6), the redshift distribution of the latter is skewed towards higher redshifts. Of the AzTEC sources ∼85 per cent lie at zphot > 3 while ∼33 per cent are at zphot > 4. This corresponds to a lower limit on the space density of ultra-red sources at 4 < z < 6 of ∼3 × 10−7 Mpc−3 with a contribution to the obscured star-formation of ≳ 8 × 10−4 M⊙ yr−1 Mpc−3. Some of the multiple systems have members with photometric redshifts consistent among them suggesting possible physical associations. Given their angular separations, these systems are most likely galaxy over-densities and/or early-stage pre-coalescence mergers. Finally, we present 3mm LMT/RSR spectroscopic redshifts of six red-Herschel galaxies at zspec = 3.85 − 6.03, two of them (at z ∼ 4.7) representing new redshift confirmations. Here we release the AzTEC and deblended Herschel photometry as well as catalogues of the most promising interacting systems and z > 4 galaxies.

The Nature of 500 Micron Risers II: Sub-mm Faint Dusty Star Forming Galaxies

“…This result supports the conclusions of G20 and, by extension, is in good agreement with the multiplicity fractions for 500 𝜇m risers quoted by Oteo et al (2017), despite their ALMA observations achieving significantly better sensitivities and angular resolutions. These results are also in reasonably good agreement with the more extreme scenario in Montaña et al (2021), and well within the range of potential multiplicity fractions of the brightest (𝑆 500 > 60 mJy) sources in their sample. These multiplicity fractions indicate that 500 𝜇m risers are a much more diverse population than previously predicted.…”

“…As well as being effective at selecting intrinsically red, high redshift DSFGs, the 500 𝜇m riser selection criterion will also naturally include some fraction of intermediate redshift systems that are artificially reddened by the blending of multiple sources at moderately large separations. For example, Montaña et al (2021) find in their sample of ∼ 100 500 𝜇m risers that the photometric redshift distribution of those that resolve into multiple components is skewed towards lower redshifts, with a slightly lower median redshift (𝑧 med = 3.5 vs. 𝑧 med = 3.8) and a higher fraction residing at lower redshifts (27% at 𝑧 phot < 3 vs. 10%) compared to single systems. We find that our 500 𝜇m risers typically fall into the category of multiple systems at moderate redshifts and, based on their FIR/sub-mm flux densities and intermediate photometric redshifts, we would expect them to be examples of Hyper-Luminous Infrared Galaxies (HLIRGS: 𝐿 FIR > 10 13 L ), similar to those discussed in G20.…”

“…Bussmann et al 2015;Duivenvoorden et al 2018). For example, Ma et al (2019) and Montaña et al (2021) estimate that ∼ 20% and ∼ 25% of their samples respectively would not pass the 500 𝜇m riser selection criterion after accounting for the effects of blending. We additionally find that our SCUBA-2 observations may miss a significant fraction of the integrated flux density at 850 𝜇m (particularly for Bootes15 and Bootes27) and that there may be additional faint sources contributing to the integrated FIR/sub-mm flux density that remain undetected in our SMA observations (particularly for Bootes27).…”

“…This multiplicity fraction rises to ∼ 39% when sources showing evidence of strong gravitational lensing are removed. Montaña et al (2021) carry out 1.1 mm LMT/AzTEC observations of ∼ 100 500 𝜇m risers with 𝑆 500 ≈ 35 − 80 mJy, achieving an average beam FWHM of 9.6 ± 0.5 arcseconds and 1𝜎 RMS sensitivities of ≈ 0.7 − 2.8 mJy. They find that ∼ 9% show direct evidence of multiplicity, rising to ∼ 23% if 500 𝜇m risers which display evidence of multiplicity but are slightly below their detection threshold are included, and up to ∼ 50% if 500 𝜇m risers without counterparts in their higher resolution observations are assumed to be multiple sources.…”

“…Based on this assumption, G20 estimated that ∼ 60% of faint (S 500 < 60 mJy) 500 𝜇m risers and ∼ 35% of bright (S 500 > 60 mJy) 500 𝜇m risers should be blends, indicating that this population is likely much more diverse than predicted. Similarly, based on LMT/AzTEC 1.1 mm follow-up observations of their sample of bright Herschel 500 𝜇m risers, Montaña et al (2021) find that 45/93 have no counterpart in the higher resolution imaging, with some of these non detections expected to be multiple systems and others expected to be individual, resolved sources that are faint at 1.1 mm due to a large dust spectral emissivity index 𝛽. The multiplicity of their sample could therefore be ∼ 9% in the most conservative scenario, or ∼ 50 − 60% in the most extreme scenario.…”