The Minimum Amount of Stars a Galaxy Will Form

We present an analysis of the atomic hydrogen and stellar properties of 38 late-type galaxies in the local Universe covering a wide range of HI mass-to-light ratios (M_HI/L_B), stellar luminosities, and surface brightnesses. From these data we have identified an upper envelope for the M_HI/L_B as a function of galaxy luminosity. This implies an empirical relation between the minimum amount of stars a galaxy will form and its initial baryonic mass. While the stellar mass of a galaxy seems to be only loosely connected to its baryonic mass, the latter quantity is strongly linked to the galaxy's dynamical mass as it is observed in the baryonic Tully-Fisher relation. We find that dwarf irregular galaxies with generally high M_HI/L_B-ratios follow the same trend as defined by lower M_HI/L_B giant galaxies, but are underluminous for their rotation velocity to follow the trend in a stellar mass Tully-Fisher relation, suggesting that the baryonic mass of the dwarf galaxies is normal but they have failed to produced a sufficient amount of stars. Finally, we present a three dimensional equivalent to the morphology-density relation which shows that high M_HI/L_B galaxies preferentially evolve and/or survive in low-density environments. We conclude that an isolated galaxy with a shallow dark matter potential can retain a large portion of its baryonic matter in the form of gas, only producing a minimum quantity of stars necessary to maintain a stable gas disk.Comment: 37 Pages, 7 Figures. Accepted for publication in A

Evidence for HI replenishment in massive galaxies through gas accretion from the cosmic web

We examine the H i -to-stellar mass ratio (H i fraction) for galaxies near filament backbones within the nearby Universe (d < 181 Mpc). This work uses the 6 degree Field Galaxy Survey (6dFGS) and the Discrete Persistent Structures Extractor (DisPerSE) to define the filamentary structure of the local cosmic web. H i spectral stacking of H i Parkes All Sky Survey (HIPASS) observations yield the H i fraction for filament galaxies and a field control sample. The H i fraction is measured for different stellar masses and 5th nearest neighbour projected densities (Σ5) to disentangle what influences cold gas in galaxies. For galaxies with stellar masses log(M⋆) ≤ 11 M⊙ in projected densities 0 ≤ Σ5 < 3 galaxies Mpc−2, all H i fractions of galaxies near filaments are statistically indistinguishable from the control sample. Galaxies with stellar masses log(M⋆) ≥ 11 M⊙ have a systematically higher H i fraction near filaments than the control sample. The greatest difference is 0.75 dex, which is 5.5σ difference at mean projected densities of 1.45 galaxies Mpc−2. We suggest that this is evidence for massive galaxies accreting cold gas from the intra-filament medium which can replenish some H i gas. This supports cold mode accretion where filament galaxies with a large gravitational potential can draw gas from the large scale structure

2D Bayesian automated tilted-ring fitting of disk galaxies in large HI galaxy surveys: 2DBAT

We present a novel algorithm based on a Bayesian method for 2D tilted-ring analysis of disk galaxy velocity fields. Compared to the conventional algorithms based on a chi-squared minimisation procedure, this new Bayesian-based algorithm suffers less from local minima of the model parameters even with highly multi-modal posterior distributions. Moreover, the Bayesian analysis, implemented via Markov Chain Monte Carlo (MCMC) sampling, only requires broad ranges of posterior distributions of the parameters, which makes the fitting procedure fully automated. This feature will be essential when performing kinematic analysis on the large number of resolved galaxies expected to be detected in neutral hydrogen (HI) surveys with the Square Kilometre Array (SKA) and its pathfinders. The so-called '2D Bayesian Automated Tilted-ring fitter' (2DBAT) implements Bayesian fits of 2D tilted-ring models in order to derive rotation curves of galaxies. We explore 2DBAT performance on (a) artificial HI data cubes built based on representative rotation curves of intermediate-mass and massive spiral galaxies, and (b) Australia Telescope Compact Array (ATCA) HI data from the Local Volume HI Survey (LVHIS). We find that 2DBAT works best for well-resolved galaxies with intermediate inclinations (20 deg < i < 70 deg), complementing three-dimensional techniques better suited to modelling inclined galaxies.Comment: Accepted for publication in MNRAS; 46 pages, 33 figure

HI absorption associated with Norma's brightest cluster galaxy

ESO 137-G006 is the brightest cluster galaxy (BCG) of the cool-core and dynamically young Norma cluster. We discover an atomic hydrogen (HI) absorption line associated with this BCG using the Australia Telescope Compact Array. We estimate a gas column density of $\approx (1.3 \pm 0.2) \times 10^{20}\,T_{\rm{spin}}$ atoms cm$^{-2}$ with spin temperature, $T_{\rm{spin}} \leq 194$ K, consistent with the HI properties of other early-type galaxies and cool-core cluster BCGs. The relationship between the presence of cold gas and a cluster cooling flow is unclear. Our results support the scenario that ESO 137-G006 may be a recent arrival to the cluster centre and not the original BCG. This scenario is consistent with the observed spatial alignment of the BCG's wide-angle tail radio lobes with Norma's X-ray sub-cluster and the significant line-of-sight velocity offset between the mean velocity of Norma and that of the BCG.Comment: Accepted for publication in MNRAS. 10 pages, 3 figures (additional 4 pages, 3 figures in appendix

Cleaning up Eta Carinae: Detection of Ammonia in the Homunculus

We report the first detection of ammonia in the Homunculus nebula around eta Carinae, which is also the first detection of emission from a polyatomic molecule in this or any other luminous blue variable (LBV) nebula. Observations of the NH3 (J,K)=(3,3) inversion transition made with the Australia Telescope Compact Array reveal emission at locations where infrared H2 emission had been detected previously, near the strongest dust emission in the core of the Homunculus. We also detect ammonia emission from the so-called ``strontium filament'' in the equatorial disk. The presence of NH3 around eta Car hints that molecular shells around some Wolf-Rayet stars could have originated in prior LBV eruptions, rather than in cool red supergiant winds or the ambient interstellar medium. Combined with the lack of any CO detection, NH3 seems to suggest that the Homunculus is nitrogen rich like the ionized ejecta around eta Car. It also indicates that the Homunculus is a unique laboratory in which to study unusual molecule and dust chemistry, as well as their rapid formation in a nitrogen-rich environment around a hot star. We encourage future observations of other transitions like NH3 (1,1) and (2,2), related molecules like N2H+, and renewed attempts to detect CO.Comment: 4 pages, accepted to ApJ letter

An HI study of the collisional ring galaxy NGC 922

We present new atomic hydrogen (HI) observations of the collisional ring galaxy NGC 922 obtained using the Australia Telescope Compact Array. Our observations reveal for the first time the vast extent of the HI disc of this galaxy. The HI morphology and kinematics of NGC 922 show that this galaxy is not the product of a simple drop-through interaction, but has a more complex interaction history. The integrated HI flux density of NGC 922 from our observations is 24.7 Jy km s$^{-1}$, which is within the error of the flux value obtained using the $64$-m Parkes radio telescope. This flux density translates to a total HI mass of $1.1*10^{10}$ M$_{\circ}$ and corresponds to an HI to total mass fraction (M$_{HI}$/M$_{tot}$) of approximately $0.11$. The gaseous structures of NGC 922 are more extended to the north and include an HI tail that has a projected physical length of $8$ kpc. Gas warps are also evident in the velocity field of NGC 922 and are more prominent on the approaching and the western side of the disc. In comparison with a large sample of star-forming galaxies in the local Universe, NGC 922 possesses a high gas fraction relative to galaxies with a similar stellar mass of ~$10^{10.4}$ M$_{\circ}$, and exhibits a high specific star formation rate.Comment: 11 pages, 8 figures, published in MNRA

Insights on the origin of ORCs from cosmological simulations

We investigate shock structures driven by merger events in high-resolution simulations that result in a galaxy with a virial mass M ~ 1e12 Msol. We find that the sizes and morphologies of the internal shocks resemble remarkably well those of the newly-detected class of odd radio circles (ORCs). This would highlight a so-far overlooked mechanism to form radio rings, shells and even more complex structures around elliptical galaxies. Mach numbers of M = 2-3 for such internal shocks are in agreement with the spectral indices of the observed ORCs. We estimate that ~5 percent of galaxies could undergo merger events which occasionally lead to such prominent structures within the galactic halo during their lifetime, explaining the low number of observed ORCs. At the time when the shock structures are matching the physical sizes of the observed ORCs, the central galaxies are typically classified as early-type galaxies, with no ongoing star formation, in agreement with observational findings. Although the energy released by such mergers could potentially power the observed radio luminosity already in Milky-Way-like halos, our predicted luminosity from a simple, direct shock acceleration model is much smaller than the observed one. Considering the estimated number of candidates from our cosmological simulations and the higher observed energies, we suggest that the proposed scenario is more likely for halo masses around 1e13 Msol in agreement with the observed stellar masses of the galaxies at the center of ORCs. Such shocks might be detectable with next generation X-ray instruments like the Line Emission Mapper (LEM).Comment: modified figure 4 to match published versio