Neutral-atomic and molecular outflows are a common occurrence in galaxies, near and far. They operate over the full extent of their galaxy hosts, from the innermost regions of galactic nuclei to the outermost reaches of galaxy halos. They carry a substantial amount of material that would otherwise have been used to form new stars. These cool outflows may have a profound impact on the evolution of their host galaxies and environments. This article provides an overview of the basic physics of cool outflows, a comprehensive assessment of the observational techniques and diagnostic tools used to characterize them, a detailed description of the best-studied cases, and a more general discussion of the statistical properties of these outflows in the local and distant universe. The remaining outstanding issues that have not yet been resolved are summarized at the end of the review to inspire new research directions.

Cool outflows in galaxies and their implications

The `Local Volume HI Survey' (LVHIS) comprises deep HI spectral line and 20-cm radio continuum observations of 82 nearby, gas-rich galaxies, supplemented by multi-wavelength images. Our sample consists of all galaxies with Local Group velocities v_LG < 550 km/s or distances D < 10 Mpc that are detected in the HI Parkes All Sky Survey (HIPASS). Using full synthesis observations in at least three configurations of the Australia Telescope Compact Array (ATCA), we obtain detailed HI maps for a complete sample of gas-rich galaxies with Dec < -30 deg. Here we present a comprehensive LVHIS Galaxy Atlas, including the overall gas distribution, mean velocity field, velocity dispersion, and position-velocity diagrams, together with a homogeneous set of measured and derived galaxy properties. Our primary goal is to investigate the HI morphologies, kinematics, and environment at high resolution and sensitivity. LVHIS galaxies represent a wide range of morphologies and sizes; our measured HI masses range from ~10^7 to 10^10 Msun, based on independent distance estimates. The LVHIS Galaxy Atlas (including FITS files) is available on-line.

The Local Volume HI Survey (LVHIS)

We present the S-Band Polarization All Sky Survey (S-PASS), a survey of polarized radio emission over the southern sky at Dec~$ 3$). The largest depolarization areas are in the inner Galaxy, associated with the Sagittarius Arm. We have also computed a Rotation Measure map combining S-PASS with archival data from the WMAP and Planck experiments. A Stokes $I$ map has been generated, with a sensitivity limited to the confusion level of 9 mK.

S-band Polarization All-Sky Survey (S-PASS): survey description and maps

We present spatially-resolved spectroscopy from the Keck Cosmic Web Imager (KCWI) of a star-forming galaxy at z=0.6942, which shows emission from the Mg II 2796, 2803 Angstrom doublet in the circumgalactic medium (CGM) extending ~37 kpc at 3-sigma significance in individual spaxels (1-sigma detection limit 4.8 x 10^{-19} erg s^-1 cm^-2 arcsec^-2). After deconvolution with the seeing, we obtain 5-sigma detections extending for ~31 kpc measured in 7-spaxel (1.1 arcsec^2) apertures. Spaxels covering the galaxy stellar regions show clear P-Cygni-like emission/absorption profiles with the blueshifted absorption extending to relative velocities of v = -800 km/s; however, the P-Cygni profiles give way to pure emission at large radii from the central galaxy. We have performed three-dimensional radiative transfer modeling to infer the geometry and velocity and density profiles of the outflowing gas. Our observations are most consistent with an isotropic outflow rather than biconical wind models with half-opening angles phi <= 80 deg. Furthermore, our modeling suggests that a wind velocity profile that decreases with radius is necessary to reproduce the velocity widths and strengths of Mg II line emission profiles at large circumgalactic radii. The extent of the Mg II emission we measure directly is further corroborated by our modeling, where we rule out outflow models with extent <30 kpc.

Circumgalactic Mg ii Emission from an Isotropic Starburst Galaxy Outflow Mapped by KCWI

Context . Radio loud active galactic nuclei (AGN) are episodic in nature, cycling through periods of activity and quiescence. The study of this duty cycle has recently gained new relevance because of the importance of AGN feedback for galaxy evolution.Aims . In this work we investigate the duty cycle of the radio galaxy B2 0258+35, which was previously suggested to be a restarted radio galaxy based on its morphology. The radio source consists of a pair of kpc-scale jets embedded in two large-scale lobes (∼240 kpc) with relaxed shape and very low surface brightness, which resemble remnants of a past AGN activity.Methods . We have combined new LOFAR data at 145 MHz and new Sardinia Radio Telescope data at 6600 MHz with available WSRT data at 1400 MHz to investigate the spectral properties of the outer lobes and derive their age.Results . Interestingly, the spectrum of both the outer northern and southern lobes is not ultra-steep as expected for an old ageing plasma with spectral index values equal to and in the outer northern lobe, and in the outer southern lobe. Moreover, despite the wide frequency coverage available for the outer northern lobe (145–6600 MHz), we do not identify a significant spectral curvature (SPC ≃ 0.2 ± 0.2).Conclusions . While mechanisms such as in-situ particle reacceleration, mixing or compression can temporarily play a role in preventing the spectrum from steepening, in no case seem the outer lobes to be compatible with being very old remnants of past activity as previously suggested (with age ≳ 80 Myr). We conclude that either the large-scale lobes are still fuelled by the nuclear engine or the jets have switched off no more than a few tens of Myr ago, allowing us to observe both the inner and outer structure simultaneously. Our study shows the importance of combining morphological and spectral properties to reliably classify the evolutionary stage of low surface brightness, diffuse emission that low frequency observations are revealing around a growing number of radio sources.

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Duty cycle of the radio galaxy B2 0258+35

We present new radio and optical images of the nearest radio galaxy Centaurus A and its host galaxy NGC 5128. We focus our investigation on the northern transition region, where energy is transported from the similar to 5 kpc (similar to 5 arcmin) scales of the northern inner lobe (NIL) to the similar to 30 kpc (similar to 30 arcmin) scales of the northern middle lobe (NML). Our Murchison Widefield Array observations at 154 MHz and our Parkes radio telescope observations at 2.3 GHz show diffuse radio emission connecting the NIL to the NML, in agreement with previous Australia Telescope Compact Array observations at 1.4 GHz. Comparison of these radio data with our wide-field optical emission-line images show the relationship between the NML radio emission and the ionized filaments that extend north from the NIL, and reveal a new ionized filament to the east, possibly associated with a galactic wind. Our deep optical images show clear evidence for a bipolar outflow from the central galaxy extending to intermediate scales, despite the non-detection of a southern radio counterpart to the NML. Thus, our observational overview of Centaurus A reveals a number of features proposed to be associated with active galactic nucleus feedback mechanisms, often cited as likely to have significant effects in galaxy evolution models. As one of the closest galaxies to us, Centaurus A therefore provides a unique laboratory to examine feedback mechanisms in detail.

M. Sidonio

Papers

The jet/wind outflow in Centaurus A: A local laboratory for AGN feedback