Measurements of the electron density of populations of extragalactic HII regions in nearby galaxies remain limited, despite the relevance of this quantity for characterizing the porosity of the interstellar medium and the escape of the ionizing radiation. We initiated a project aimed at analysing the root-mean-square electron density (n(e))(rms), the in situ density (n(e)) and the volume filling factor (phi) of extragalactic HII regions, investigating the dependence of these attributes on nebular and host galaxy properties. We present an image-segmentation methodology for constructing homogeneous HII region catalogues, and apply it to two pilot galaxies: NGC 2403 and NGC 628. We derive (n(e))(rms) from their H alpha luminosities and equivalent radii (R-eq), and obtain n(e) and phi for spectroscopic subsamples. While ne is below 300 cm(-3) , (n(e))(rms) is typically one to two orders of magnitude lower, implying that 4 is in the range similar to 10(-4) to 10(-1) . The two galaxies exhibit a similar size-density relation, (n(e))(rms) alpha R-eq(-0.3), which breaks for R-eq greater than or similar to 50 pc, show at most a weak dependence of (n(e))(rms) on galactocentric radius for NGC 2403, and no clear dependence of ne or 4 on these parameters. Combining these results with published data, (n(e))rms presents tentative scaling relations with the median HII region size, the fraction of large regions in the parent galaxy, and the star formation rate surface density. These trends, if confirmed, would provide new constraints for massive cluster formation models and important clues for interpreting dependencies observed at high redshift, underscoring the necessity of consistently extending this analysis to larger samples.

The Whirlpool Galaxy is a well-studied grand design galaxy with two major spiral arms, and a large satellite NGC 5195. The arms both show long uniform sections with perturbations (“kinks” or sharp turns) in specific regions. Comparing the two arms shows a small radial offset between the main kinked regions. We analyzed the morphology and also the velocity field in the disk of M51 using kinematic maps based on H α and CO line emission. These sample complementary radial ranges, with the CO map covering the central zone and the H α map extending to cover the outer zone. We looked for indicators of density wave resonance, zones where radial flows of gas in the disk plane reverse their sign. These were present in both velocity maps; their 2D localization placed them along or closely parallel to the spiral arms, at a set of well-defined galactocentric radii, and notably more concentrated along the southern, stronger arm. The results can be well interpreted quantitatively, using a numerical model of the interaction of M51 and NGC 5195 in which the satellite has made two relatively recent passes through the disk plane of M51. During the first pass the pair of dominant spiral arms was stimulated, and during the second pass the strong kinks in both arms were formed at about the same time. The second interaction is particularly well characterized, because the timescale corresponding to the production of the kinks and the recovery of the original pitch angle is identical for the two arms.

A popular science account of multimessenger astronomy, this book gives an attractive insight into 21st century methods of observational astronomy

Abstract In this contribution we present our recent study of the resonant structure of M51, using kinematic mapping with the CO molecule from PAWS supplemented with our own velocity map in Hα, which is complemented with the analysis of the morphology of M51 in terms of the variation of the pitch angle along the spiral arms, which unveil the existence of two kinks in each arm. We demonstrate the role of its interacting satellite NGC 5195 in stimulating the formation of the spiral arms and kinks, and demonstrate that it has twice passed through the plane of M51, calculating the time interval between passages. Our results are strengthened by comparison with simulations of this interaction.

We present photometric and morphological analyses of nuclear star clusters (NSCs)—very dense, massive star clusters present in the central regions of most galaxies—in a sample of 33 massive disk galaxies within 20 Mpc, part of the “Composite Bulges Survey.” We use data from the Hubble Space Telescope including optical (F475W and F814W) and near-IR (F160W) images from the Wide Field Camera 3. We fit the images in 2D to take into account the full complexity of the inner regions of these galaxies (including the contributions of nuclear disks and bars), isolating the NSC and bulge components. We derive NSC radii and magnitudes in all three bands, which we then use to estimate NSC masses. Our sample significantly expands the sample of massive late-type galaxies with measured NSC properties. We clearly identify NSCs in nearly 80% of our galaxies, putting a lower limit on the nucleation fraction in these galaxies that is higher than previous estimates. We find that the NSCs in our massive disk galaxies are consistent with previous NSC mass–NSC radius and galaxy mass–NSC mass relations. However, we also find a large spread in NSC masses, with a handful of galaxies hosting very low-mass, compact clusters. Our NSCs are aligned in PA with their host galaxy disks but are less flattened. They show no correlations with bar or bulge properties. Finally, we find the ratio of NSC to BH mass in our massive disk galaxy sample spans a factor of ∼300.

The structure of the outskrits of galaxies provides valuable information about their past and evolution. Due to their projected orientation, edge-on isolated galaxies effectively serve as test labs in which to study the three-dimensional structures of galaxies including warps and flares, and to explore the possible sources of souch distortions. We analyzed the structure of the apparently isolated edge-on ultra-thin galaxy UGC11859 to look for the presence of disortions. The deep optical imaging observations we acquired with the GTC (Gran Telescopio Canarias) are used to derive the radial and vertical surface brightness profiles and g-r color radial profile. We find that the galaxy disk display a significant gravitational distortion. A warp is clearly detected on one side of the disk, and the galactic plane on both sides of the centre shows increasing scale height with increasing galactocentric radius, indicating the presence of a flare in the stellar distribution. The surface brightness profile of the disk shows a sharp break at 24 kiloparsecs galactocentric radius, and a steep decline to larger radii, and edge-on truncation, which we associate with the presence of the flare. The present study is the first observational support for a connection between truncations and flares. Just beyond the warped side of the disk a faint galaxy is observed within a small angular distance, identified as a potential interacting companion. Bases on ultra-deep g and r photometry we estimate that if the potential companion is at the same distance as UGC11859, the stellar mas of the satellite galaxy is approximately 6.33 log(MSol)

We present the analysis of the magnetic field (B-field) structure of galaxies measured with far-infrared (FIR) and radio (3 and 6 cm) polarimetric observations. We use the first data release of the Survey of extragALactic magnetiSm with SOFIA of 14 nearby (<20 Mpc) galaxies with resolved (5 '' -18 ''; 90 pc-1 kpc) imaging polarimetric observations using SOFIA/HAWC+ from 53 to 214 mu m. We compute the magnetic pitch-angle (psi(B)) profiles as a function of the galactocentric radius. We introduce a new magnetic alignment parameter (.) to estimate the disordered-to-ordered ratio of spiral B-fields. We find FIR and radio wavelengths to not generally trace the same B-field morphology in galaxies. The psi(B) profiles tend to be more ordered across all galactocentric radii in radio (zeta(6cm) = 0.93 +/- 0.03) than in FIR (zeta(154 mu m) = 0.84 +/- 0.14). For spiral galaxies, FIR B-fields are 2%-75% more turbulent than the radio B-fields. For starburst galaxies, we find that FIR polarization is a better tracer of the B-fields along the galactic outflows than radio polarization. Our results suggest that the B-fields associated with dense, dusty, turbulent star-forming regions (those traced at FIR) are less ordered than warmer, less dense regions (those traced at radio) of the interstellar medium. The FIR B-fields seem to be more sensitive to the activity of the star-forming regions and molecular clouds within a vertical height of a few hundred parsecs in the disk of spiral galaxies than the radio B-fields.

We describe the data processing of the Survey on extragALactic magnetiSm with SOFIA (SALSA Legacy Program). This first data release presents 33% (51.34 hr out of 155.7 hr, including overheads) of the total awarded time from 2020 January to 2021 December. Our observations were performed using the newly implemented on-the-fly mapping (OTFMAP) technique in the polarimetric mode. We present the pipeline steps to obtain homogeneously reduced high-level data products of polarimetric maps of galaxies for use in scientific analysis. Our approach has a general design and can be applied to sources smaller than the field of view of the HAWC+ array in any given band. We estimate that the OTFMAP polarimetric mode offers a reduction of observing overheads by a factor 2.34 and an improvement in sensitivity by a factor 1.80 when compared to the same on-source time polarimetric observations using the chopping and nodding mode. The OTFMAP is a significant optimization of the polarimetric mode of HAWC+, as it ultimately reduces the cost of operations of HAWC+/SOFIA by increasing the science collected per hour of observation up to an overall factor of 2.49. The OTFMAP polarimetric mode is the standard observing strategy of SALSA. The results and quantitative analysis of this first data release are presented in Papers IV and V of the series.

This is the “odd chapter out” because it does not deal with a particular messenger or its results. Instead you will see what kind of measurements have tranformed cosmology into a quantitative discipline within the last 50 years. The starting point was the detection at short radio wavelengths of the comic background radiation in 1964, which gave strong support to Big Bang cosmology. This was followed by observational work, mostly by the optical astronomers, on the production from hydrogen of the elements heliium, deuterium and lithium in the primordial fireball, which permitted the prediction of the ratio of baryons to photons in the universe as a whole, as well as giving information about the number of types of neutrinos and the mean lifetime of free neutrons. This work showed that the mean density of baryons (protons plus electrons) in the universe is only some 4% of the quantity needed to make it “closed” by its own gravity. This links to the problem of dark matter, needed to explain the rotation curves of galaxies, which for two decades was understood to make up the remaining 96%. Then the optical observers detected that the universe is accelerating, and the dark energy needed to produce this acceleration is estimated at over 70% of the “closure density” leaving 26% as dark matter, and 4% as “ordinary” baryonic matter. This was all tied together by increasingly precise measurements of the variation over the sky of the cosmic background radiation, which shows that the universe is indeed “just closed”. The only problems are that we don’t know what the dark matter or dark energy are. Experiments to detect dark matter particles form the latter part of the chapter.

Galactic bars are frequent in disk galaxies and they may support the transfer of matter towards the central engine of active nuclei. The barred galaxy NGC 1097 has magnetic forces controlling the gas flow at several kpc scales, which suggest that magnetic fields (B-fields) are dynamically important along the bar and nuclear ring. However, the effect of the B-field on the gas flows in the central kpc scale has not been characterized. Using thermal polarized emission at 89 μm with HAWC+/SOFIA, here, we measure that the polarized flux is spatially located at the contact regions of the outer-bar with the starburst ring. The linear polarization decomposition analysis shows that the 89 μm and radio (3.5 and 6.2 cm) polarization traces two different modes, m, of the B-field: a constant B-field orientation and dominated by m=0 at 89 μm, and a spiral B-field dominated by m=2 at radio. We show that the B-field at 89 μm is concentrated in the warmest region of a shock driven by the galactic-bar dynamics in the contact regions between the outer-bar with the starburst ring. Radio polarization traces a superposition of the spiral B-field outside and within the starburst ring. According to Faraday rotation measures between 3.5 and 6.2 cm, the radial component of the B-field along the contact regions points toward the galaxy's center on both sides. We conclude that gas streams outside and within the starburst ring follow the B-field, which feeds the black hole with matter from the host galaxy.

We present detailed morphological, photometric, and stellar-kinematic analyses of the central regions of two massive, early-type barred galaxies with nearly identical large-scale morphologies. Both have large, strong bars with prominent inner photometric excesses that we associate with boxy/peanut-shaped (B/P) bulges; the latter constitute similar to 30 per cent of the galaxy light. Inside its B/P bulge, NGC 4608 has a compact, almost circular structure (half-light radius R-e approximate to 310 pc, Sersic n = 2.2) we identify as a classical bulge, amounting to 12.1 per cent of the total light, along with a nuclear star cluster (R-e similar to 4 pc). NGC 4643, in contrast, has a nuclear disc with an unusual broken-exponential surface-brightness profile (13.2 per cent of the light), and a very small spheroidal component (R-e approximate to 35 pc, n = 1.6; 0.5 per cent of the light). IFU stellar kinematics support this picture, with NGC 4608's classical bulge slowly rotating and dominated by high velocity dispersion, while NGC 4643's nuclear disc shows a drop to lower dispersion, rapid rotation, V-h(3) anticorrelation, and elevated h(4). Both galaxies show at least some evidence for V-h(3)correlation in the bar (outside the respective classical bulge and nuclear disc), in agreement with model predictions. Standard two-component (bulge/disc) decompositions yield B/T similar to 0.5-0.7 (and bulge n > 2) for both galaxies. This overestimates the true 'spheroid' components by factors of 4 (NGC 4608) and over 100 (NGC 4643), illustrating the perils of naive bulge-disc decompositions applied to massive barred galaxies.

The recent availability of high-resolution far-infrared (FIR) polarization observations of galaxies using HAWC+/SOFIA has facilitated studies of extragalactic magnetic fields in the cold and dense molecular disks.We investigate if any significant structural differences are detectable in the kpc-scale magnetic field of the grand design face-on spiral galaxy M51 when traced within the diffuse (radio) and the dense and cold (FIR) interstellar medium (ISM). Our analysis reveals a complex scenario where radio and FIR polarization observations do not necessarily trace the same magnetic field structure. We find that the magnetic field in the arms is wrapped tighter at 154um than at 3 and 6 cm; statistically significant lower values for the magnetic pitch angle are measured at FIR in the outskirts (R > 7 kpc) of the galaxy. This difference is not detected in the interarm region. We find strong correlations of the polarization fraction and total intensity at FIR and radio with the gas column density and 12CO(1-0) velocity dispersion. We conclude that the arms show a relative increase of small-scale turbulent B-fields at regions with increasing column density and dispersion velocities of the molecular gas. No correlations are found with HI neutral gas. The star formation rate shows a clear correlation with the radio polarized intensity, which is not found in FIR, pointing to a small-scale dynamo-driven B-field amplification scenario. This work shows that multi-wavelength polarization observations are key to disentangling the interlocked relation between star formation, magnetic fields, and gas kinematics in the multi-phase ISM.

Cosmic rays are not rays. They are high energy particles arriving from outside the atmosphere, produced by the Sun and a number of different types of high energy astronomical sources. I first explain how, in the early twentieth century Victor Hess in a high altititude balloon showed that they do not have a terrrestrial origin, and how Robert Millikan gave them the misleading name of cosmic rays, which has persisted. You will then read about their composition and how it has been determined with the AMS experiment on the International Space Station. Between 1930 and 1960 cosmic ray experiments were major players in particle physics. The positron, the pi meson (or pion) the kaon, and the ʌ hyperon were all discovered in this way. One feature of cosmic rays is their energies; the highest energy cosmic rays have 30 million times more energy than the particles in the CERN particle accelerator. These particles, as well as the highest energy gamma-rays, are detected via the showers of secondary particles they produce when they enter the upper atmosphere, using detectors on very large telescopes or distributed on the ground, which are described in this chapter. Their detection gives us information about supernovae within the galaxy, as well as the powerful sources around the central black holes in other galaxies, and the mergers of dense stellar objects such as back holes. We look forward to a future in which information from cosmic rays, neutrinos, and gravitational waves will combine with electromagnetic detection to probe the highest energy most distant processes in the universe.

We have applied stellar population synthesis to 500 pc sized regions in a sample of 102 galaxy discs observed with the MUSE spectrograph. We derived the star formation history and analyse specifically the "recent" ($20\rm{Myr}$) and "past" ($570\rm{Myr}$) age bins. Using a star formation self-regulator model we can derive local mass-loading factors, $\eta$ for specific regions, and find that this factor depends on the local stellar mass surface density, $\Sigma_*$, in agreement with the predictions form hydrodynamical simulations including supernova feedback. We integrate the local $\eta$-$\Sigma_*$ relation using the stellar mass surface density profiles from the Spitzer Survey of Stellar Structure in Galaxies (S4G) to derive global mass-loading factors, $\eta_{\rm{G}}$, as a function of stellar mass, $M_*$. The $\eta_{\rm{G}}$-$M_*$ relation found is in very good agreement with hydrodynamical cosmological zoom-in galaxy simulations. The method developed here offers a powerful way of testing different implementations of stellar feedback, to check on how realistic are their predictions.

ABSTRACT Stellar feedback has a notable influence on the formation and evolution of galaxies. However, direct observational evidence is scarce. We have performed stellar population analysis using MUSE optical spectra of the spiral galaxy NGC 628 and find that current maximum star formation in spatially resolved regions is regulated according to the level of star formation in the recent past. We propose a model based on the self-regulator or ‘bathtub’ models, but for spatially resolved regions of the galaxy. We name it the ‘resolved self-regulator model’ and show that the predictions of this model are in agreement with the presented observations. We observe star formation self-regulation and estimate the mass-loading factor, η = 2.5 ± 0.5, consistent with values predicted by galaxy formation models. The method described here will help provide better constraints on those models.

A method, which we have developed for determining corotation radii, has allowed us to map in detail the radial resonant structures of barred spiral galaxies. Here, we have combined this information with new determinations of the bar strength and the pitch angle of the innermost segment of the spiral arms to find relationships between these parameters of relevance to the dynamical evolution of the galaxies. We show how (1) the bar mass fraction, (2) the scaled bar angular momentum, (3) the pitch angle, and (4) the shear parameter vary along the Hubble sequence, and we also plot along the Hubble sequence (5) the scaled bar length, (6) the ratio of bar corotation radius to bar length, (7) the scaled bar pattern speed, and (8) the bar strength. It is of interest to note that the parameters (2), (5), (6), (7), and (8) all show breaks in their behaviour at type Scd. We find that bars with high shear have only small pitch angles, while bars with large pitch angles must have low shear; we also find a generally inverse trend of the pitch angle with bar strength. An inference that at first seems counter-intuitive is that the most massive bars rotate most slowly but have the largest angular momenta. Among a further set of detailed results, we pick out here the 2:1 ratio between the number of spiral arms and the number of corotations outside the bar. These results give a guideline to theories of disc–bar

A method which we have developed for determining corotation radii, has allowed us to map in detail the radial resonant structures of barred spiral galaxies. Here we have combined this information with new determinations of the bar strength and the pitch angle of the innermost segment of the spiral arms to find relationships between these parameters of relevance to the dynamical evolution of the galaxies. We show how (1) the bar mass fraction, (2) the scaled bar angular momentum, (3) the pitch angle, and (4) the shear parameter vary along the Hubble sequence, and we also plot along the Hubble sequence (5) the scaled bar length, (6) the ratio of bar corotation radius to bar length, (7) the scaled bar pattern speed, and (8) the bar strength. It is of interest to note that the parameters (2), (5), (6), (7), and (8) all show breaks in their behaviour at type Scd. We find that bars with high shear have only small pitch angles, while bars with large pitch angles must have low shear; we also find a generally inverse trend of pitch angle with bar strength. An inference which at first seems counter-intuitive is that the most massive bars rotate most slowly but have the largest angular momenta. Among a further set of detailed results we pick out here the 2:1 ratio between the number of spiral arms and the number of corotations ouside that of the bar. These results give a guideline to theories of disc-bar evolution.