Submm Galaxies Research Articles

A novel high-z submm galaxy efficient line survey in ALMA Bands 3 through 8 – an ANGELS pilot

ABSTRACT We use the Atacama Large sub/Millimetre Array (ALMA) to efficiently observe spectral lines across Bands 3, 4, 5, 6, 7, and 8 at high-resolution (0.5–0.1 arcsec) for 16 bright southern Herschel sources at $1.5 \lt z \lt 4.2$. With only six and a half hours of observations, we reveal 66 spectral lines in 17 galaxies. These observations detect emission from CO (3−2) to CO(18−17), as well as atomic ([C i](1−0), (2−1), [O i] 145 $\mu$m and [N ii] 205 $\mu$m) lines. Additional molecular lines are seen in emission (${\rm H_2O}$ and ${\rm H_2O^+}$) and absorption (OH$^+$ and CH$^+$). The morphologies based on dust continuum ranges from extended sources to strong lensed galaxies with magnifications between 2 and 30. CO line transitions indicate a diverse set of excitation conditions with a fraction of the sources ($\sim 35$ per cent) showcasing dense, warm gas. The resolved gas to star formation surface densities vary strongly per source, and suggest that the observed diversity of dusty star-forming galaxies could be a combination of lensed, compact dusty starbursts and extended, potentially merging galaxies. The predicted gas depletion time-scales are consistent with 100 Myr to 1 Gyr, but require efficient fuelling from the extended gas reservoirs onto the more central starbursts, in line with the Doppler-shifted absorption lines that indicate inflowing gas for two out of six sources. This pilot paper explores a successful new method of observing spectral lines in large samples of galaxies, supports future studies of larger samples, and finds that the efficiency of this new observational method will be further improved with the planned ALMA Wideband Sensitivity Upgrade.

A GMRT 610 MHz radio survey of the North Ecliptic Pole (NEP, ADF-N) / Euclid Deep Field North

Abstract This paper presents a 610 MHz radio survey covering 1.94 deg2 around the North Ecliptic Pole (NEP), which includes parts of the AKARI (ADF-N) and Euclid, Deep Fields North. The median 5σ sensitivity is 28 μJy beam−1, reaching as low as 19 μJy beam−1, with a synthesised beam of 3.6″× 4.1″. The catalogue contains 1675 radio components, with 339 grouped into multi-component sources and 284 ’isolated’ components likely part of double radio sources. Imaging, cataloguing, and source identification are presented, along with preliminary scientific results. From a non-statistical sub-set of 169 objects with multi-wavelength AKARI and other detections, luminous infrared galaxies (LIRGs) represent 66$\%$ of the sample, ultra-luminous infrared galaxies (ULIRGs) 4$\%$, and sources with LIR < 1011 L⊙ 30$\%$. In total, 56$\%$ of sources show some AGN presence, though only seven are AGN-dominated. ULIRGs require three times higher AGN contribution to produce high-quality SED fits compared to lower luminosity galaxies, and AGN presence increases with AGN fraction. The PAH mass fraction is not significant, although ULIRGs have about half the PAH strength of lower IR-luminosity galaxies. Higher luminosity galaxies show gas and stellar masses an order of magnitude larger, suggesting higher star formation rates. For LIRGs, AGN presence increases with redshift, indicating that part of the total luminosity could be contributed by AGN activity rather than star formation. Simple cross-matching revealed 13 ROSAT QSOs, 45 X-ray sources, and 61 sub-mm galaxies coincident with GMRT radio sources.

A dusty protocluster surrounding the binary galaxy HerBS-70 at z = 2.3

ABSTRACT We report on deep SCUBA-2 observations at 850 $\mu$m and NOrthern Extended Millimetre Array (NOEMA) spectroscopic measurements at 2 mm of the environment surrounding the luminous, massive (M* ≈ 2 × 1011 M⊙) Herschel-selected source HerBS-70. This source was revealed by previous NOEMA observations to be a binary system of dusty star-forming galaxies at z = 2.3, with the east component (HerBS-70E) hosting an active galactic nucleus. The SCUBA-2 observations detected, in addition to the binary system, 21 sources at >3.5σ over an area of ∼25 square comoving Mpc with a sensitivity of 1σ850 = 0.75 mJy. The surface density of continuum sources around HerBS-70 is three times higher than for field galaxies. The NOEMA spectroscopic measurements confirm the protocluster membership of three of the nine brightest sources through their CO(4–3) line emission, yielding a volume density 36 times higher than for field galaxies. All five confirmed sub-mm galaxies in the HerBS-70 system have relatively short gas depletion times (80−500 Myr), indicating the onset of quenching for this protocluster core due to the depletion of gas. The dark matter halo mass of the HerBS-70 system is estimated around 5 × 1013 M⊙, with a projected current-day mass of 1015 M⊙, similar to the local Virgo and Coma clusters. These observations support the claim that DSFGs, in particular the ones with observed multiplicity, can trace cosmic overdensities.

Faint mm NIKA2 dusty star-forming galaxies: Finding the high-redshift population

Aims. High-redshift dusty star-forming galaxies (DSFGs) are proposed to be the progenitors of massive quiescent galaxies arising at cosmic noon, providing a crucial insight into the formation, assembly, and early quenching of massive galaxies in the early Universe. However, their high redshift combined with high dust obscuration adds significant difficulties to their redshift measurement, which is mandatory for detailed studies of their physical properties. Blind mm spectral scans are the most unbiased way in prinicple for obtaining accurate spectroscopic redshifts for these sources, but identifying faint molecular and atomic lines within limited telescope time for faint DSFGs is also difficult with these scans. Methods. We developed a new framework to constrain the source redshift. The method jointly accounts for the detection and/or nondetection of spectral lines and the prior information from the photometric redshift and total infrared luminosity from spectral energy distribution analysis. The method uses the estimated total infrared luminosity to predict the line fluxes at given redshifts and generates model spectra. The redshift-dependent spectral models were then compared with the observed spectra to determine the redshift. Results. We applied this joint redshift analysis method to four high-z dusty star-forming galaxy candidates selected from the NIKA2 observations of the HLSJ091828.6+514223 (HLS) field that were further observed by NOEMA with blind spectral scans. These sources only have Herschel SPIRE photometry as ancillary data. They were selected because SPIRE counterparts are faint or entirely lacking and thus favor to select the highest-redshift candidates. The method finds a spectroscopic redshift of 4 in the five NOEMA-counterpart detected sources, with z > 3. Based on these measurements, we derived the CO and [CI] lines and mm continuum fluxes from the NOEMA data and studied the properties of their interstellar medium and star formation. We find cold dust temperatures in some of the HLS sources compared to the general population of submm galaxies, which might be related to the bias introduced by the SPIRE-dropout selection. All sources except for one have a short gas-depletion time of a few hundred million years, which is typical of high-z submm galaxies. The only exception shows a longer gas-depletion time of up to a few billion years. This is comparable to the gas-depletion times of main-sequence galaxies at the same redshift. Furthermore, we identify a possible overdensity of dusty star-forming galaxies at z = 5.2 that is traced by two sources in our sample, as well as a lensed galaxy HLSJ091828.6+514223. Conclusions. We demonstrate that our method when applied to mm-selected DSFGs is able to determine the redshift accurately. This accuracy with only multiple emission lines with a low signal-to-noise ratio shows promising potential for the blind redshift search in large samples of high-z DSFGs, even in the absence of optical to near infrared photometric redshifts.

Brightest Cluster Galaxy Formation in the z = 4.3 Protocluster SPT 2349-56: Discovery of a Radio-loud Active Galactic Nucleus

We have observed the z = 4.3 protocluster SPT2349−56 with the Australia Telescope Compact Array (ATCA) with the aim of detecting radio-loud active galactic nuclei (AGNs) among the ∼30 submillimeter (submm) galaxies (SMGs) identified in the structure. We detect the central complex of submm sources at 2.2 GHz with a luminosity of L 2.2 = (4.42 ± 0.56) × 1025 W Hz−1. MeerKAT and the Australian Square Kilometre Array Pathfinder also detect the source at 816 MHz and 888 MHz, respectively, constraining the radio spectral index to α = −1.45 ± 0.16, implying L 1.4,rest = (2.2 ± 0.2) × 1026 W Hz−1. The radio observations do not have sufficient spatial resolution to uniquely identify one of the three Atacama Large Millimeter/submillimeter Array (ALMA) galaxies as the AGN, however the ALMA source properties themselves suggest a likely host. This radio luminosity is ∼100× higher than expected from star formation, assuming the usual far-infrared–radio correlation, indicating an AGN driven by a forming brightest cluster galaxy. None of the SMGs in SPT2349−56 show signs of AGNs in any other diagnostics available to us, highlighting the radio continuum as a powerful probe of obscured AGNs. We compare these results to field samples of radio sources and SMGs, along with the 22 gravitationally lensed SPT-SMGs also observed in the ATCA program, as well as powerful radio galaxies at high redshifts. The (3.3 ± 0.7) × 1038 W of power from the radio-loud AGN sustained over 100 Myr is comparable to the binding energy of the gas mass of the central halo, and similar to the instantaneous energy injection from supernova feedback from the SMGs in the core region. The SPT2349−56 radio-loud AGNs may be providing strong feedback on a nascent intracluster medium.

ALMACAL

It may be unsurprising that the most common approach to finding proto-clusters is to search for over-densities of galaxies. Upgrades to submillimetre (submm) interferometers and the advent of the James Webb Space Telescope will soon offer the opportunity to find more distant candidate proto-clusters in deep sky surveys without any spectroscopic confirmation. In this Letter, we report the serendipitous discovery of an extremely dense region centred on the blazar, J0217−0820, at z = 0.6 in the ALMACAL sky survey. Its density is eight times higher than that predicted by blind submm surveys. Among the seven submm-bright galaxies, three are as bright as conventional single-dish submm galaxies, with S870 μm > 3 mJy. The over-density is thus comparable to the densest known and confirmed proto-cluster cores. However, their spectra betray a wide range of redshifts. We investigate the likelihood of line-of-sight projection effects using light cones from cosmological simulations, finding that the deeper we search, the higher the chance that we will suffer from such projection effects. The extreme over-density around J0217−0820 demonstrates the strong cosmic variance we may encounter in the deep submm surveys. Thus, we should also question the fidelity of galaxy proto-cluster candidates selected via over-densities of galaxies, where the negative K correction eases the detection of dusty galaxies along an extraordinarily extended line of sight.

ALMA confirmation of an obscured hyperluminous radio-loud AGN at z = 6.853 associated with a dusty starburst in the 1.5 deg2 COSMOS field

ABSTRACT We present band 6 ALMA observations of a heavily obscured radio-loud (L1.4 GHz = 1025.4 W Hz−1) active galactic nucleus (AGN) candidate at zphot = 6.83 ± 0.06 found in the 1.5 deg2 COSMOS field. The ALMA data reveal detections of exceptionally strong [C ii]158 $\mu$m (z[C ii] = 6.8532) and underlying dust continuum emission from this object (COS-87259), where the [C ii] line luminosity, line width, and 158 $\mu$m continuum luminosity are comparable to those seen from z ∼ 7 sub-mm galaxies and quasar hosts. The 158 $\mu$m continuum detection suggests a total infrared luminosity of $9\times 10^{12}\, \mathrm{ L}_\odot$ with corresponding very large obscured star formation rate (1300 M⊙ yr−1) and dust mass ($2\times 10^9\, \mathrm{ M}_\odot$). The strong break seen between the VIRCam and IRAC photometry perhaps suggests that COS-87259 is an extremely massive reionization-era galaxy with $M_\ast \approx 1.7\times 10^{11}\, \mathrm{ M}_\odot$. Moreover, the MIPS, PACS, and SPIRE detections imply that this object harbours an AGN that is heavily obscured ($\tau _{_{\mathrm{9.7\,\mu m}}}=2.3$) with a bolometric luminosity of approximately $5\times 10^{13}\, \mathrm{ L}_\odot$. Such a very high AGN luminosity suggests that this object is powered by an ≈1.6 × 10$^9\, \mathrm{ M}_\odot$ black hole if accreting near the Eddington limit, and is effectively a highly obscured version of an extremely ultraviolet (UV)-luminous (M1450 ≈ −27.3) z ∼ 7 quasar. Notably, these z ∼ 7 quasars are an exceedingly rare population (∼0.001 deg−2), while COS-87259 was identified over a relatively small field. Future very wide area surveys with e.g. Roman and Euclid have the potential to identify many more extremely red yet UV-bright z ≳ 7 objects similar to COS-87259, providing richer insight into the occurrence of intense obscured star formation and supermassive black hole growth among this population.

The nature of 500 micron risers – II. Multiplicities and environments of sub-mm faint dusty star-forming galaxies

ABSTRACT We present Submillimeter Common User Bolometer Array 2 (SCUBA-2) and Submillimeter Array (SMA) follow-up observations of four candidate high-redshift dusty star-forming galaxies, selected as sources with rising spectral energy distributions in the 250, 350, and 500 μm Herschel SPIRE bands. Previous SMA observations showed no counterparts to these sources, but in our deeper sub-mm observations we detect counterparts to all four 500 μm risers, with three resolving into multiple systems. For these three multiple systems, the SMA 345 GHz (≈870 μm) observations recover $123 \pm 73{{\ \rm per\ cent}}$, $60 \pm 15{{\ \rm per\ cent}}$, and $19 \pm 4{{\ \rm per\ cent}}$ of the integrated 850 μm flux density from SCUBA-2, indicating that there may be additional sources below our SMA detection limit making up a dense, protocluster core. The fourth 500 μm riser was observed at a lower frequency and so we cannot make a similar comparison. We estimate photometric redshifts based on FIR/sub-mm colours, finding that three fourths likely lie at z ≥ 2. This fits with the interpretation that the 500 μm riser selection criterion selects both intrinsically red, individual galaxies at z > 4, and multiple systems at more moderate redshifts, artificially reddened by the effects of blending. We use the SCUBA-2 850 μm maps to investigate the environments of these 500 μm risers. By constructing cumulative number counts and estimating photometric redshifts for surrounding SCUBA-2 detections, we find that one of our 500 μm risers could plausibly reside in a z ≥ 2 protocluster. We infer that bright 500 μm risers with faint 850 μm flux densities are typically multiple systems at z ≥ 2 that may reside in overdensities of bright sub-mm galaxies.

Compact molecular gas emission in local LIRGs among low- and high-z galaxies

We present new CO(2–1) observations of a representative sample of 24 local (z < 0.02) luminous infrared galaxies (LIRGs) at high spatial resolution (< 100 pc) from the Atacama Large Millimeter/submillimeter Array (ALMA). Our LIRGs lie above the main sequence (MS), with typical stellar masses in the range 1010–1011 M⊙ and SFR ∼ 30 M⊙ yr−1. We derive the effective radii of the CO(2–1) and the 1.3 mm continuum emissions using the curve-of-growth method. LIRGs show an extremely compact cold molecular gas distribution (median RCO ∼ 0.7 kpc), which is a factor 2 smaller than the ionized gas (median RHα ∼ 1.4 kpc), and 3.5 times smaller than the stellar size (median Rstar ∼ 2.4 kpc). The molecular size of LIRGs is similar to that of early-type galaxies (ETGs; RCO ∼ 1 kpc) and about a factor of 6 more compact than local spiral galaxies of similar stellar mass. Only the CO emission in low-z ULIRGs is more compact than these local LIRGs by a factor of 2. Compared to high-z (1 < z < 6) systems, the stellar sizes and masses of local LIRGs are similar to those of high-z MS star-forming galaxies (SFGs) and about a factor of 2–3 lower than submillimeter (submm) galaxies (SMGs). The molecular sizes of high-z MS SFGs and SMGs are larger than those derived for LIRGs by a factor of ∼3 and ∼8, respectively. Contrary to high-z SFGs and SMGs, which have comparable molecular and stellar sizes (median Rstar/RCO = 1.8 and 1.2, respectively), local LIRGs show more centrally concentrated molecular gas distribution (median Rstar/RCO = 3.3). A fraction of the low-z LIRGs and high-z galaxies share a similar range in the size of the ionized gas distribution, from 1 to 4 kpc. However, no LIRGs with a very extended (above 4 kpc) radius are identified, while for high-z galaxies no compact (less than 1 kpc) emission is detected. These results indicate that while low-z LIRGs and high-z MS SFGs have similar stellar masses and sizes, the regions of current star formation (traced by the ionized gas) and of potential star formation (traced by the molecular gas) are substantially smaller in LIRGs, and constrained to the central kiloparsec (kpc) region. High-z galaxies represent a wider population but their star-forming regions are more extended, even covering the entire extent of the galaxy. High-z galaxies have larger fractions of gas than low-z LIRGs, and therefore the formation of stars could be induced by interactions and mergers in extended disks or filaments with sufficiently large molecular gas surface density involving physical mechanisms similar to those identified in the central kpc of LIRGs.

Molecular gas properties of Planck-selected protocluster candidates at z ≃ 1.3–3

We report on IRAM 30-m/EMIR observations of 38 Herschel sources chosen as the brightest red submillimeter (submm) sources in 18 Planck-selected fields drawn from the Planck high-z (PHz) sample of protocluster candidates. These fields host overdensities of red Herschel sources, with high star formation rates (∼10 000 M⊙ yr−1), as obtained from the Planck measurements. The goals of these observations are to measure the spectroscopic redshifts of the Herschel sources in the PHz fields, investigate the origin of their bright submm emission, and find evidence of their association with high-z protoclusters. We detected 40 CO lines on a total of 24 bright (S​350 μm > 40 mJy) Herschel sources in 14 of the 18 PHz fields. The measured average redshift is ⟨zCO⟩ = 2.25 ± 0.09, spanning a range from 1.32 to 2.75. We measured the redshifts for multiple Herschel sources in projected proximity in eight PHz fields. In half of those fields, we detected between two and three objects at similar redshifts, supporting the idea that the PHz fields contain high-z protoclusters. The detection of sources at different redshifts in the same field demonstrates that foreground and background sources also contribute to the total submm emission. We compared the properties of the molecular gas and of the star formation activity of our sources with samples of normal star-forming galaxies (SFGs), submm galaxies (SMGs), and CO-detected cluster and protocluster galaxies drawn from the literature at similar redshifts. We find that the PHz-IRAM sources are mainly normal SFGs, with only ∼20% undergoing a starburst phase. The PHz-IRAM sources are characterized by star formation rates (⟨SFR⟩ = 1043 ± 157 M⊙ yr−1) and gas masses (⟨Mgas⟩ = (4.0 ± 0.7) × 1011 M⊙) that are, on average, eight and five times higher than those typical of normal SFGs at similar redshifts. Their dust temperatures (⟨Tdust⟩ = 29.2 ± 0.9 K) and depletion timescales (⟨τdep⟩ = 0.47 ± 0.07 Gyr) are instead consistent with those of normal SFGs. The analysis of the CO spectral line energy distribution, available for ten PHz-IRAM sources, peaks at a low quantum rotation number (Jup = 3) in most of the cases, implying low gas excitation. These properties imply that a significant number of PHz-IRAM sources contain extended and cold molecular gas reservoirs at low excitation and that their star formation is driven by secular processes. Multiplicity and moderate gravitational lensing might also play a role in producing the observed properties. Some of these properties are also observed in other CO-detected infrared-luminous protocluster galaxies at z ≃ 1.3 − 3. We find that the protoclusters with the highest level of star formation, drawn from the largest volume simulations available to date, exhibit similar SFRs as compared to the PHz protoclusters; however, they separate out into a higher number of star-forming galaxies. Millimeter and CO observations at higher spatial resolution than those presented here would be necessary to further elucidate the properties of our PHz-IRAM sources and determine which mechanisms drive star formation in infrared-luminous protocluster galaxies.

The nature of sub-millimetre galaxies II: an ALMA comparison of SMG dust heating mechanisms

ABSTRACT We compare the contribution of active galactic nuclei (AGNs) and star formation towards dust heating in sub-mm galaxies (SMGs). We have used ALMA at 0.1-arcsec resolution to image a complete flux-limited sample of seven sub-mm sources previously shown to have spectral energy distributions that were as well-fitted by obscured AGN as star-forming galaxy templates. Indeed, two sub-mm sources were known to be quasars from their absorbed X-ray emission. We find the sub-mm sizes of all SMGs to be small (≈1−2 kpc) and generally ∼3 times smaller than any host detected in the near-infrared (NIR). In all cases, the five SMGs are comparable in sub-mm size to the two known quasars and four z ≈ 6 quasars, also observed with ALMA. We detect no evidence of diffuse spiral arms in this complete sample. We then convert the far-infrared (FIR) luminosities to star formation rate (SFR) surface densities and find that the SMGs occupy the same range as the known quasars in our sample. We conclude that in terms of sub-mm size, extent relative to host and SFR density as well as luminosity and mid-IR (MIR) colour, there is little distinction between the SMGs and sub-mm bright quasars. Finally, we present preliminary evidence that SMGs with higher MIR luminosities and sub-mm loud quasars tend to have dust components that range to hotter temperatures than their less luminous SMG counterparts. In light of these results, we continue to suggest that luminous SMGs may host dust-absorbed quasars that may simultaneously dominate the FIR and hard X-ray backgrounds.

ALMA survey of Lupus class III stars: Early planetesimal belt formation and rapid disc dispersal

ABSTRACT Class III stars are those in star forming regions without large non-photospheric infrared emission, suggesting recent dispersal of their protoplanetary discs. We observed 30 class III stars in the 1–3 Myr Lupus region with ALMA at ∼856μm, resulting in four detections that we attribute to circumstellar dust. Inferred dust masses are 0.036–0.093M⊕, ∼1 order of magnitude lower than any previous measurements; one disc is resolved with radius ∼80 au. Two class II sources in the field of view were also detected, and 11 other sources, consistent with sub-mm galaxy number counts. Stacking non-detections yields a marginal detection with mean dust mass ∼0.0048M⊕. We searched for gas emission from the CO J = 3–2 line, and present its detection to NO Lup inferring a gas mass (4.9 ± 1.1) × 10−5 M⊕ and gas-to-dust ratio 1.0 ± 0.4. Combining our survey with class II sources shows a gap in the disc mass distribution from 0.09–2M⊕ for ${\gt}0.7\, \mathrm{M}_\odot$ Lupus stars, evidence of rapid dispersal of mm-sized dust from protoplanetary discs. The class III disc mass distribution is consistent with a population model of planetesimal belts that go on to replenish the debris discs seen around main sequence stars. This suggests that planetesimal belt formation does not require long-lived protoplanetary discs, i.e. planetesimals form within ∼2 Myr. While all four class III discs are consistent with collisional replenishment, for two the gas and/or mid-IR emission could indicate primordial circumstellar material in the final stages of protoplanetary disc dispersal. Two class III stars without sub-mm detections exhibit hot emission that could arise from ongoing planet formation processes inside ∼1 au.

The star formation histories of z ∼ 1 post-starburst galaxies

ABSTRACT We present the star formation histories of 39 galaxies with high-quality rest-frame optical spectra at 0.5 < z < 1.3 selected to have strong Balmer absorption lines and/or Balmer break, and compare to a sample of spectroscopically selected quiescent galaxies at the same redshift. Photometric selection identifies a majority of objects that have clear evidence for a recent short-lived burst of star formation within the last 1.5 Gyr, i.e. ‘post-starburst’ galaxies, however we show that good quality continuum spectra are required to obtain physical parameters such as burst mass fraction and burst age. Dust attenuation appears to be the primary cause for misidentification of post-starburst galaxies, leading to contamination in spectroscopic samples where only the [O ii] emission line is available, as well as a small fraction of objects lost from photometric samples. The 31 confirmed post-starburst galaxies have formed 40–90 per cent of their stellar mass in the last 1–1.5 Gyr. We use the derived star formation histories to find that the post-starburst galaxies are visible photometrically for 0.5–1 Gyr. This allows us to update a previous analysis to suggest that 25–50 per cent of the growth of the red sequence at z ∼ 1 could be caused by a starburst followed by rapid quenching. We use the inferred maximum historical star formation rates of several 100–1000 M⊙ yr−1 and updated visibility times to confirm that sub-mm galaxies are likely progenitors of post-starburst galaxies. The short quenching time-scales of 100–200 Myr are consistent with cosmological hydrodynamic models in which rapid quenching is caused by the mechanical expulsion of gas due to an acive galactic neucleus.

ALMA band 8 observations of DLA 2233+131 at z = 3.150

Abstract We present our ALMA Band 8 observations of a damped Ly$\alpha$ absorption (DLA) system at $z = 3.150$ observed in the spectrum of the quasar Q2233+131 at $z = 3.295$. The optical counterpart of this DLA has been identified and it shows a double-peaked Ly$\alpha$ emission line. Since one possible origin of DLAs at high redshift is an outflowing gas from star-forming galaxies, DLA 2233+131 provides a good laboratory to investigate the nature of high-z DLAs. Motivated by this, we have carried out ALMA band 8 observations to study the [C ii] line in this system. However, we do not detect any significant emission line in the observed pass bands. Instead, we have serendipitously found three submm continuum sources in the observed sky area. One appears to be the quasar Q2233+131 itself while the other two sources are newly identified submm galaxies (SMGs), called SMG1 and SMG2 in this paper. They are located at a separation of ${4{^{\prime \prime }_{.}}7}$ and ${8{^{\prime \prime }_{.}}1}$ from Q2233+131, respectively. Their 646 μm fluxes are $6.35\:$mJy and $6.43\:$mJy, respectively, being higher than that of Q2233+131, $3.62\:$mJy. Since these two SMGs are not detected in the optical images obtained with the Hubble Space Telescope and the Subaru Telescope, they have a very red spectral energy distribution. It is, therefore, suggested that they are high-redshift galaxies or very dusty galaxies at intermediate redshift, although we cannot rule out the possibility that they are optically very faint SMG analogs at low redshift. Follow-up observations will be necessary to explore the nature of this interesting region.

A SCUBA-2 850 μm survey of heavily reddened quasars at z ∼ 2

ABSTRACT We present new 850 μm SCUBA-2 observations for a sample of 19 heavily reddened Type-I quasars at redshifts z ∼ 2 with dust extinctions of AV ≃ 2–6 mag. Three of the 19 quasars are detected at >3σ significance corresponding to an 850 μm flux-limit of ≳4.8 mJy. Assuming the 850 μm flux is dominated by dust heating due to star formation, very high star formation rates (SFR) of ∼2500–4500 M⊙ yr−1 in the quasar host galaxies are inferred. Even when considering a large contribution to the 850 μm flux from dust heated by the quasar itself, significant SFRs of ∼600–1500 M⊙ yr−1 are nevertheless inferred for two of the three detected quasars. We stack the remaining 16 heavily reddened quasars and derive an average 3σ upper limit on the SFRs in these quasar host galaxies of <880 M⊙ yr−1. The number counts of sub-mm galaxies in the total survey area (134.3 arcmin2) are consistent with predictions from blank-field surveys. There are, however, individual quasars where we find evidence for an excess of associated sub-mm galaxies. For two quasars, higher spatial resolution and spectroscopic ALMA observations confirm the presence of an excess of sub-mm sources. We compare the 850 μm detection rate of our quasars to both unobscured, ultraviolet luminous quasars as well as the much more obscured population of mid-infrared luminous Hot Dust Obscured Galaxies (HotDOGs). When matched by luminosity and redshift, we find no significant differences in the 850 μm flux densities of these various quasar populations given the current small sample sizes.

A machine-learning approach for identifying the counterparts of submillimetre galaxies and applications to the GOODS-North field

ABSTRACT Identifying the counterparts of submillimetre (submm) galaxies (SMGs) in multiwavelength images is a critical step towards building accurate models of the evolution of strongly star-forming galaxies in the early Universe. However, obtaining a statistically significant sample of robust associations is very challenging due to the poor angular resolution of single-dish submm facilities. Recently, a large sample of single-dish-detected SMGs in the UKIDSS UDS field, a subset of the SCUBA-2 Cosmology Legacy Survey (S2CLS), was followed up with the Atacama Large Millimeter/submillimeter Array (ALMA), which has provided the resolution necessary for identification in optical and near-infrared images. We use this ALMA sample to develop a training set suitable for machine-learning (ML) algorithms to determine how to identify SMG counterparts in multiwavelength images, using a combination of magnitudes and other derived features. We test several ML algorithms and find that a deep neural network performs the best, accurately identifying 85 per cent of the ALMA-detected optical SMG counterparts in our cross-validation tests. When we carefully tune traditional colour-cut methods, we find that the improvement in using machine learning is modest (about 5 per cent), but importantly it comes at little additional computational cost. We apply our trained neural network to the GOODS-North field, which also has single-dish submm observations from the S2CLS and deep multiwavelength data but little high-resolution interferometric submm imaging, and we find that we are able to classify SMG counterparts for 36/67 of the single-dish submm sources. We discuss future improvements to our ML approach, including combining ML with spectral energy distribution fitting techniques and using longer wavelength data as additional features.

The nature of submillimetre and highly star-forming galaxies in the EAGLE simulation

ABSTRACT We exploit EAGLE, a cosmological hydrodynamical simulation, to reproduce the selection of the observed submillimetre (submm) galaxy population by selecting the model galaxies at z ≥ 1 with mock submm fluxes $S_{850\, \mu \mathrm{m}}$ ≥ 1 mJy. We find a reasonable agreement between the model galaxies within this sample and the properties of the observed submm population, such as their star formation rates (SFRs) at z < 3, redshift distribution, and many integrated galaxy properties. We find that the median redshift of the $S_{850\, \mu \mathrm{m}}$ ≥ 1 mJy model population is z ≈ 2.5, and that they are massive galaxies (M* ∼ 1011 M⊙) with high dust masses (Mdust ∼ 108 M⊙), gas fractions (fgas ≈ 50 per cent), and SFRs ($\dot{M}_* \approx 100$ M⊙ yr−1). In addition, we find that they have major and minor merger fractions similar to the general population, suggesting that mergers are not the sole driver of the high SFRs in the model submm galaxies. Instead, the $S_{850\, \mu \mathrm{m}}$ ≥ 1 mJy model galaxies yield high SFRs primarily because they maintain a significant gas reservoir as a result of hosting an undermassive black hole relative to comparably massive galaxies. Not all ‘highly star-forming’ ($\dot{M}_* \ge 80$ M⊙ yr−1) eagle galaxies have submm fluxes $S_{850\, \mu \mathrm{m}}$ ≥ 1 mJy. We investigate the nature of these highly star-forming ‘Submm-Faint’ galaxies (i.e. $\dot{M}_* \ge 80$ M⊙ yr−1 but $S_{850\, \mu \mathrm{m}}$ < 1 mJy) and find that they are similar to the model submm galaxies, being gas rich and hosting undermassive black holes. However, they are also typically at higher redshifts (z > 4) and are lower mass (M* ∼ 1010 M⊙). These typically higher redshift galaxies show stronger evidence for having been triggered by major mergers, and critically, they are likely missed by most current submm surveys due to their higher dust temperatures and lower dust masses.

Sub-millimetre non-contaminated detection of the disc around TWA 7 by ALMA

Debris disks can be seen as the left-overs of giant planet formation and the possible nurseries of rocky planets. While M-type stars out-number more massive stars we know very little about the time evolution of their circumstellar disks at ages older than $\sim 10$\,Myr. Sub-millimeter observations are best to provide first order estimates of the available mass reservoir and thus better constrain the evolution of such disks. Here, we present ALMA Cycle\,3 Band\,7 observations of the debris disk around the M2 star TWA\,7, which had been postulated to harbor two spatially separated dust belts, based on unresolved far-infrared and sub-millimeter data. We show that most of the emission at wavelengths longer than $\sim 300$\,$\mu$m is in fact arising from a contaminant source, most likely a sub-mm galaxy, located at about 6.6" East of TWA\,7 (in 2016). Fortunately, the high resolution of our ALMA data allows us to disentangle the contaminant emission from that of the disc and report a significant detection of the disk in the sub-millimeter for the first time with a flux density of 2.1$\pm$0.4 mJy at 870 $\mu$m. With this detection, we show that the SED can be reproduced with a single dust belt.

Discovery of a Lyα-emitting Dark Cloud within the z ∼ 2.8 SMM J02399-0136 System

We present Keck/KCWI integral field spectrograph observations of the complex system surrounding SMM J02399$-$0136 (a lensed $z=2.8$ sub-mm galaxy), including an associated Ly$\rm \alpha$ nebula, a dust-obscured, broad-absorption-line quasar, and neighboring galaxies. At a 3$\sigma$ surface brightness contour of 1.6$\times$ 10$^{-17}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$, the Ly$\rm \alpha$ nebula extends over 17 arcsec ($\gtrsim 140$ physical kpc) and has a total Ly$\rm \alpha$ luminosity of $2.5 \times 10^{44} \, \rm erg \, s^{-1}$ (uncorrected for lensing). The nebula exhibits a kinematic shear of $\sim$ 1000 km s$^{-1}$ over 100 pkpc with lowest velocities east of SMM J02399$-$0136 and increasing to the southwest. We also discover a bright, Ly$\rm \alpha$ emitter, separated spatially and kinematically from the nebula, at a projected separation of $\approx$60 kpc from the quasar. This source has no clear central counterpart in deep $Hubble \, Space \, Telescope$ imaging, giving an intrinsic Ly$\rm \alpha$ rest-frame equivalent width greater than 312 $ \rm \mathring{A}$ (5-$\sigma$). We argue that this 'dark cloud' is illuminated by the quasar with a UV flux that is orders of magnitude brighter than the emission along our sightline. This result confirms statistical inferences that luminous quasars at $z>2$ emit UV radiation anisotropically. Future KCWI observations of other lines, e.g. Ly$\rm \beta$, He$\rm _{II}$, C$\rm _{IV}$, etc, and with polarimetry will further reveal the origin of the Ly$\rm \alpha$ nebula and nature of the dark cloud.

Herschel-ATLAS : the spatial clustering of low- and high-redshift submillimetre galaxies

We present measurements of the angular correlation function of sub-millimeter (sub-mm) galaxies (SMGs) identified in four out of the five fields of the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) - GAMA-9h, GAMA-12h, GAMA-15h and NGP - with flux densities $S_{250\mu m}$>30 mJy at 250 {\mu}m. We show that galaxies selected at this wavelength trace the underlying matter distribution differently at low and high redshifts. We study the evolution of the clustering finding that at low redshifts sub-mm galaxies exhibit clustering strengths of $r_0$ $\sim$ 2 - 3 $h^{-1}$ Mpc, below z < 0.3. At high redshifts, on the other hand, we find that sub-mm galaxies are more strongly clustered with correlation lengths $r_0$ = 8.1 $\pm$ 0.5, 8.8 $\pm$ 0.8 and 13.9 $\pm$ 3.9 $h^{-1}$Mpc at z = 1 - 2, 2 - 3 and 3 - 5, respectively. We show that sub-mm galaxies across the redshift range 1 < z < 5, typically reside in dark-matter halos of mass of the order of ~ $10^{12.5}$ - $10^{13.0}$ $h^{-1} \, M_{\odot}$ and are consistent with being the progenitors of local massive elliptical galaxies that we see in the local Universe.