The Cold Dust Content of the Nearby Galaxies IC 5325, NGC 7496, NGC 7590, and NGC 7599

Star-forming galaxies are rich reservoirs of dust, both warm and cold. But the cold dust emission is faint alongside the relatively bright and ubiquitous warm dust emission. Recently, evidence for a very cold dust component has also been revealed via millimeter/submillimeter photometry of some galaxies. This component, despite being the most massive of the three dust components in star-forming galaxies, is by virtue of its very low temperature, faint and hard to detect together with the relatively bright emission from warmer dust. Here we analyze the dust content of a carefully selected sample of four galaxies detected by IRAS, WISE, and SPT, whose spectral energy distributions (SEDs) were modeled to constrain their potential cold dust content. Low-frequency radio observations using the GMRT were carried out to segregate cold dust emission from non-thermal emission in millimeter/submillimeter wavebands. We also carried out AstroSat/UVIT observations for some galaxies to constrain their SED at shorter wavelengths so as to enforce energy balance for the SED modeling. We constructed their SEDs across a vast wavelength range (extending from ultraviolet to radio frequencies) by assembling global photometry from GALEX FUV+NUV, UVIT, Johnson BRI, 2MASS, WISE, IRAC, IRAS, AKARI, ISOPHOT, Planck HFI, SPT, and GMRT. The SEDs were modeled with CIGALE to estimate their basic properties, in particular to constrain the masses of their total and very cold dust components. Although the galaxies’ dust masses are dominated by warmer dust, there are hints of very cold dust in two of the targets, NGC 7496 and NGC 7590.

The substructure of the Perseus star-forming region: a survey with Gaia DR2

ABSTRACT We use photometric and kinematic data from Gaia DR2 to explore the structure of the star-forming region associated with the molecular cloud of Perseus. Apart from the two well-known clusters, IC 348 and NGC 1333, we present five new clustered groups of young stars, which contain between 30 and 300 members, named Autochthe, Alcaeus, Heleus, Electryon, and Mestor. We demonstrate that these are co-moving groups of young stars, based on how the candidate members are distributed in position, proper motion, parallax, and colour–magnitude space. By comparing their colour–magnitude diagrams to isochrones, we show that they have ages between 1 and 5 Myr. Using 2MASS and WISE colours, we find that the fraction of stars with discs in each group ranges from 10 to ∼50 per cent. The youngest of the new groups is also associated with a reservoir of cold dust, according to the Planck map at 353 GHz. We compare the ages and proper motions of the five new groups to those of IC 348 and NGC 1333. Autochthe is clearly linked with NGC 1333 and may have formed in the same star formation event. The seven groups separate roughly into two sets that share proper motion, parallax, and age: Heleus, Electryon, and Mestor as the older set, and NGC 1333 and Autochthe as the younger set. Alcaeus is kinematically related to the younger set, but at a more advanced age, while the properties of IC 348 overlap with both sets. All older groups in this star-forming region are located at higher galactic latitude.

Exhausted gas reservoirs drive massive galaxy quenching in the early universe

When the Universe was merely three billion years old, about half of massive galaxies had already formed the bulk of their stars and new star formation plummeted [1]. How galaxies quench at such early times remains a puzzle, as their dark matter halos contain large gas reservoirs [2-4]. This gas should cool efficiently, sustaining star formation over long periods [5,6]. Here we present sensitive 1.3mm wavelength observations of cold dust in six quenched galaxies in the redshift range z=1.6 to z=3.2 with stellar masses ranging from 2.5x1010M⊙ to 5x1011M⊙, which are magnified by foreground galaxy clusters. Even with factors of up to 30 in magnification, four of the six galaxies are undetected at this wavelength. We show that these quenched galaxies have extremely little dust at early times, and by proxy very little cold molecular gas. The median dust mass is <0.01% of the stellar mass (molecular gas mass <1%), more than two orders of magnitude less than star-forming galaxies at this epoch [4]. The implication is that most early galaxies shut off star formation because their reservoir of molecular gas was rapidly depleted or removed, and is not being replenished.

Dynamical evidence from the sub-parsec counter-rotating disc for a close binary of supermassive black holes in NGC 1068

ABSTRACT A puzzle in NGC 1068 is how to secularly maintain the counter-rotating disc (CRD) from 0.2 to $7\,$ pc unambiguously detected by recent ALMA observations of molecular gas. Upon further dynamical analysis, we find that the Kelvin–Helmholtz (KH) instability (KHI) results in an unavoidable catastrophe for the disc developed at the interface between the reversely rotating parts. We demonstrate that a close binary of supermassive black holes (CB-SMBHs) provides tidal torques to prevent the disc from the KH catastrophe and are led to the conclusion that there is a CB-SMBH at the centre of NGC 1068. The binary is composed of black holes with a separation of $0.1\,$ pc from GRAVITY/VLTI observations, a total mass of 1.3 × 107 M⊙ and a mass ratio of ∼0.3 estimated from the angular momentum (AM) budget of the global system. The KHI gives rise to a gap without cold gas at the velocity interface that overlaps with the observed gap of hot and cold dust regions. Releases of kinetic energies from the KHI of the disc are in agreement with observed emissions in radio and γ-rays. Such a binary is shrinking on a time-scale much longer than the local Hubble time via gravitational waves, however, the KHI leads to an efficient annihilation of the orbital AM and a speed-up merge of the binary, providing a new mechanism for solving the long-standing issue of ‘final parsec problem’. Future observations of GRAVITY+/VLTI are expected to be able to spatially resolve the CB-SMBHs suggested in this paper.

Multifrequency study of HH 137 and HH 138: discovering new knots and molecular outflows with Gemini and APEX

ABSTRACT We present a multiwavelength study of two Herbig–Haro (HH) objects (137 and 138) that may be associated. We use Gemini H2 (2.12 μm) and K (2.2 μm) images, as well as Atacama Pathfinder EXperiment molecular line observations and Spitzer image archives. Several H2 knots, linked to the optical chain of knots of HH 137, are identified in the Gemini and Spitzer 4.5 μm images. New shock excited regions related to the optical knots delineating HH 138 are also reported. In addition, a bright 4.5 μm 0.09 pc-long arc-shaped structure, roughly located mid-way between HH 137 and HH 138, is found to be associated with two Spitzer Class I/II objects, which are likely to be the exciting stars. These sources are almost coincident with a high-density molecular clump detected in 12CO(3 − 2), 13CO(3 − 2), C18O(3 − 2), HCO+(3 − 2), and HCN(3 − 2) molecular lines with an local thermodynamic equilibrium (LTE) mass of 36 M⊙. The 12CO(3 − 2) emission distribution over the observed region reveals molecular material underlying three molecular outflows. Two of them (outflows 1 and 2) are linked to all optical knots of HH 137 and HH 138 and to the H2 and 4.5 μm shock emission knots. In fact, the outflow 2 shows an elongated 12CO blue lobe that coincides with all the H2 knots of HH 137, which end at a terminal H2 bow shock. We propose a simple scenario that connects the outflows to the dust clumps detected in the region. A third possible outflow is located to the north-east projected towards a secondary weak and cold dust clump.

Illuminating a tadpole’s metamorphosis II: observing the ongoing transformation with ALMA

ABSTRACT We present new Atacama Large Millimeter/submillimeter Array observations of the tadpole, a small globule in the Carina Nebula that hosts the HH 900 jet+outflow system. Our data include 12CO, 13CO, C18O J=2–1, 13CO, C18O J=3–2, and serendipitous detections of DCN J=3–2 and CS J=7–6. With angular resolution comparable to the Hubble Space Telescope, our data reveal for the first time the bipolar molecular outflow in CO, seen only inside the globule, that is launched from the previously unseen jet-driving protostar (the HH 900 YSO). The biconical morphology joins smoothly with the externally irradiated outflow seen in ionized gas tracers outside the globule, tracing the overall morphology of a jet-driven molecular outflow. Continuum emission at the location of the HH 900 YSO appears to be slightly flattened perpendicular to outflow axis. Model fits to the continuum have a best-fitting spectral index of ∼2, suggesting cold dust and the onset of grain growth. In position–velocity space, 13CO and C18O gas kinematics trace a C-shaped morphology, similar to infall profiles seen in other sources, although the global dynamical behaviour of the gas remains unclear. Line profiles of the CO isotopologues display features consistent with externally heated gas. We estimate a globule mass of ∼1.9 M⊙, indicating a remaining lifetime of ∼4 Myr, assuming a constant photoevaporation rate. This long globule lifetime will shield the disc from external irradiation perhaps prolonging its life and enabling planet formation in regions where discs are typically rapidly destroyed.