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We present a detailed study of the Circinus Galaxy, investigating its star formation, dust and gas properties both in the inner and outer disk. To achieve this, we obtained high-resolution Spitzer mid-infrared images with the IRAC (3.6, 5.8, 4.5, 8.0 micron) and MIPS (24 and 70 micron) instruments and sensitive HI data from the Australia Telescope Compact Array (ATCA) and the 64-m Parkes telescope. These were supplemented by CO maps from the Swedish-ESO Submillimetre Telescope (SEST). Because Circinus is hidden behind the Galactic Plane, we demonstrate the careful removal of foreground stars as well as large- and small-scale Galactic emission from the Spitzer images. We derive a visual extinction of Av = 2.1 mag from the Spectral Energy Distribution of the Circinus Galaxy and total stellar and gas masses of 9.5 x 10^{10} Msun and 9 x 10^9 Msun, respectively. Using various wavelength calibrations, we find obscured global star formation rates between 3 and 8 Msun yr^{-1}. Star forming regions in the inner spiral arms of Circinus, which are rich in HI, are beautifully unveiled in the Spitzer 8 micron image. The latter is dominated by polycyclic aromatic hydrocarbon (PAH) emission from heated interstellar dust. We find a good correlation between the 8 micron emission in the arms and regions of dense HI gas. The (PAH 8 micron) / 24 micron surface brightness ratio shows significant variations across the disk of Circinus.
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We employ MUSE/VLT data to study the ionised and highly ionised gas phases of the feedback in Circinus, the closest Seyfert 2 galaxy to us. The analysis of the nebular emission allowed us to detect a remarkable high-ionisation gas outflow beyond the galaxy plane traced by the coronal lines [Fe VII] $lambda$6089 and [Fe X] $lambda$6374, extending up to 700 pc and 350 pc NW from the nucleus, respectively. This is the first time that the [Fe X] emission is observed at such distances from the central engine in an AGN. The gas kinematics reveals expanding gas shells with velocities of a few hundred km s$^{-1}$, spatially coincident with prominent hard X-ray emission detected by Chandra. Density and temperature sensitive line ratios show that the extended high-ionisation gas is characterized by a temperature reaching 25000 K and an electron density > 10$^2$ cm$^{-3}$. We found that local gas excitation by shocks produced by the passage of a radio jet leads to the spectacular high-ionisation emission in this object. This hypothesis is fully supported by photoionisation models that accounts for the combined effects of the central engine and shocks. They reproduce the observed emission line spectrum at different locations inside and outside of the NW ionisation cone. The energetic outflow produced by the radio jet is spatially located close to an extended molecular outflow recently reported using ALMA which suggests that they both represent different phases of the same feedback process acting on the AGN.
We report the first characterization of an extended outflow of high ionized gas in the Circinus Galaxy by means of the coronal line [FeVII] $lambda$6087 AA. This emission is located within the ionization cone already detected in the [OIII] $lambda$5007 AA line and is found to extend up to a distance of 700 pc from the AGN. The gas distribution appears clumpy, with several knots of emission. Its kinematics is complex, with split profiles and line centroids shifted from the systemic velocity. The physical conditions of the gas show that the extended coronal emission is likely the remnants of shells inflated by the passage of a radio-jet. This scenario is supported by extended X-ray emission, which is spatially coincident with the morphology and extension of the [FeVII] $lambda$6087~AA gas in the NW side of the galaxy. The extension of the coronal gas in the Circinus galaxy is unique among active galaxies and demonstrates the usefulness of coronal lines for tracing the shock ionization component in these objects.
Interacting galaxies surrounded by HI tidal debris are ideal sites for the study of young clusters and tidal galaxy formation. The process that triggers star formation in the low-density environments outside galaxies is still an open question. New clusters and galaxies of tidal origin are expected to have high metallicities for their luminosities. Spectroscopy of such objects is, however, at the limit of what can be done with existing 8-10m class telescopes, which has prevented statistical studies of these objects. NGC2865 is an UV-bright merging elliptical galaxy with shells and extended HI tails. The regions observed in this work were previously detected using multi-slit imaging spectroscopy. We obtain new multislit spectroscopy of six young star-forming regions around NGC2865, to determine their redshifts and metallicities. The six emission-line regions are located 16-40 kpc from NGC2865 and they have similar redshifts. They have ages of ~10Myears and an average metallicity of 12+log(O/H) ~ 8.6, suggesting a tidal origin for the regions. It is noted that they coincide with an extended HI tail, which has projected density of N$_{HI}$ < 10$^{19}$ cm$^{-2}$, and displays a low surface brightness counterpart. These regions may represent the youngest of the three populations of star clusters already identified in NGC2865. The high, nearly-solar, oxygen abundances found for the six regions in the vicinity of NGC2865 suggest that they were formed by pre-enriched material from the parent galaxy, from gas removed during the last major merger. Given the mass and the location of the HII regions, we can speculate that these young star-forming regions are potential precursors of globular clusters that will be part of the halo of NGC2865 in the future. Our result supports the use of the multi-slit imaging spectroscopy as a useful tool for finding nearly-formed stellar systems around galaxies.
Atacama Large Millimeter/submillimeter Array (ALMA) 12CO(J=1-0) observations are used to study the cold molecular ISM of the Cartwheel ring galaxy and its relation to HI and massive star formation (SF). CO moment maps find $(2.69pm0.05)times10^{9}$ M$_{odot}$ of H$_2$ associated with the inner ring (72%) and nucleus (28%) for a Galactic I(CO)-to-N(H2) conversion factor ($alpha_{rm CO}$). The spokes and disk are not detected. Analysis of the inner rings CO kinematics show it to be expanding ($V_{rm exp}=68.9pm4.9$ km s$^{-1}$) implying an $approx70$ Myr age. Stack averaging reveals CO emission in the starburst outer ring for the first time, but only where HI surface density ($Sigma_{rm HI}$) is high, representing $M_{rm H_2}=(7.5pm0.8)times10^{8}$ M$_{odot}$ for a metallicity appropriate $alpha_{rm CO}$, giving small $Sigma_{rm H_2}$ ($3.7$ M$_{odot}$ pc$^{-2}$), molecular fraction ($f_{rm mol}=0.10$), and H$_2$ depletion timescales ($tau_{rm mol} approx50-600$ Myr). Elsewhere in the outer ring $Sigma_{rm H_2}lesssim 2$ M$_{odot}$ pc$^{-2}$, $f_{rm mol}lesssim 0.1$ and $tau_{rm mol}lesssim 140-540$ Myr (all $3sigma$). The inner ring and nucleus are H$_2$-dominated and are consistent with local spiral SF laws. $Sigma_{rm SFR}$ in the outer ring appears independent of $Sigma_{rm H_2}$, $Sigma_{rm HI}$ or $Sigma_{rm HI+H_2}$. The ISMs long confinement in the robustly star forming rings of the Cartwheel and AM0644-741 may result in either a large diffuse H$_2$ component or an abundance of CO-faint low column density molecular clouds. The H$_2$ content of evolved starburst rings may therefore be substantially larger. Due to its lower $Sigma_{rm SFR}$ and age the Cartwheels inner ring has yet to reach this state. Alternately, the outer ring may trigger efficient SF in an HI-dominated ISM.
We investigate the connection between star formation and molecular gas properties in galaxy mergers at low redshift (z$leq$0.06). The study we present is based on IRAM 30-m CO(1-0) observations of 11 galaxies with a close companion selected from the Sloan Digital Sky Survey (SDSS). The pairs have mass ratios $leq$4, projected separations r$_{mathrm{p}} leq$30 kpc and velocity separations $Delta$V$leq$300 km s$^{-1}$, and have been selected to exhibit enhanced specific star formation rates (sSFR). We calculate molecular gas (H$_{2}$) masses, assigning to each galaxy a physically motivated conversion factor $alpha_{mathrm{CO}}$, and we derive molecular gas fractions and depletion times. We compare these quantities with those of isolated galaxies from the extended CO Legacy Data base for the GALEX Arecibo SDSS Survey sample (xCOLDGASS, Saintonge et al. 2017) with gas quantities computed in an identical way. Ours is the first study which directly compares the gas properties of galaxy pairs and those of a control sample of normal galaxies with rigorous control procedures and for which SFR and H$_{2}$ masses have been estimated using the same method. We find that the galaxy pairs have shorter depletion times and an average molecular gas fraction enhancement of 0.4 dex compared to the mass matched control sample drawn from xCOLDGASS. However, the gas masses (and fractions) in galaxy pairs and their depletion times are consistent with those of non-mergers whose SFRs are similarly elevated. We conclude that both external interactions and internal processes may lead to molecular gas enhancement and decreased depletion times.
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