Merging galaxies produce outliers from the Fundamental Metallicity Relation

From a large sample of $\approx 170,000$ local SDSS galaxies, we find that the Fundamental Metallicity Relation (FMR) has an overabundance of outliers, compared to what would be expected from a Gaussian distribution of residuals, with significantly lower metallicities than predicted from their stellar mass and star formation rate (SFR). This low-metallicity population has lower stellar masses, bimodial specific SFRs with enhanced star formation within the aperture and smaller half-light radii than the general sample, and is hence a physically distinct population. We show that they are consistent with being galaxies that are merging or have recently merged with a satellite galaxy. In this scenario, low-metallicity gas flows in from large radii, diluting the metallicity of star-forming regions and enhancing the specific SFR until the inflowing gas is processed and the metallicity has recovered. We introduce a simple model in which mergers with a mass ratio larger than a minimum dilute the central galaxy's metallicity by an amount that is proportional to the stellar mass ratio for a constant time, and show that it provides an excellent fit to the distribution of FMR residuals. We find the dilution time-scale to be $\tau=1.568_{-0.027}^{+0.029}$ Gyr, the average metallicity depression caused by a 1:1 merger to be $\alpha=0.2480_{-0.0020}^{+0.0017}$ dex and the minimum mass ratio merger that can be discerned from the intrinsic Gaussian scatter in the FMR to be $\xi_\text{min}=0.2030_{-0.0095}^{+0.0127}$ (these are statistical errors only). From this we derive that the average metallicity depression caused by a merger with mass ratio between 1:5 and 1:1 is 0.114 dex.Comment: 14 pages, 10 figures, published in MNRAS, updated to be essentially identical to the published versio

Living at a High Arctic Polynya: Inughuit Settlement and Subsistence around the North Water during the Thule Station Period, 1910–53

The settlement and subsistence patterns of the Inughuit of the Avanersuaq (Thule area) are described and analyzed for the years 1910 to 1953, when Knud Rasmussen’s trading station at Dundas was active. Inughuit subsistence was based on the rich biotic resources of the North Water polynya between Ellesmere Island and Greenland, but the analysis shows that trade, primarily with fox furs at the Thule Station, also played a major role in shaping the settlement pattern of the period. During the Thule Station Period, the named winter settlements amounted to c. 40 sites; however, only 10–15 of them were settled at any given time. The Inughuit settlement close to the station, Uummannaq, soon became the largest site in the area. The sources enable us to follow changes of residence of some hunting families over four decades. By moving their winter sites every second or third year, the families gained primary knowledge of the topography and seasonal variation of the hunting grounds in the entire Thule district during their active years. In the same way, they connected with diverse family networks through the years. Tracing the sledge routes that connected the sites over great distances reveals how decisive proximity to main and escape routes over the Ice Cap was for site location. Dog sledge technology, and thus capacity to transport people, gear, and stored food, boomed during the Thule Station Period with the wealth created from trade and access to raw materials. Mapping the main hunting grounds on the sea ice and modeling the hunters’ annual range of possibilities for accessing different game—mainly walrus, ringed seal, narwhal, and sea birds (plus some caribou)—showed that ringed seal formed the bread and butter of the subsistence economy. However, bulk resources, gained in particular from intensive spring walrus hunts at a few hot spots, as well as carefully timed consumption and sharing of the stored meat and blubber, were keys to life at the North Water polynya. Temporary settlement at the trading stations in the area—a couple of winters at a time—was also part of the risk management strategy of the Inughuit.Les modèles d’établissement et de subsistance des Inughuits de l’Avanersuaq (région de Thulé) font l’objet de descriptions et d’analyses pour les années allant de 1910 à 1953, à l’époque où la station de traite de Knud Rasmussen à Dundas était active. La subsistance des Inughuits reposait sur les riches ressources biotiques de la polynie des eaux du Nord, entre l’île d’Ellesmere et le Groenland, mais l’analyse montre que la traite, principalement celle de fourrure de renard à la station de Thulé, a également joué un grand rôle dans la formation du modèle d’établissement pendant cette période. Durant la période de la station de Thulé, le nombre d’établissements hivernaux nommés se chiffrait à environ 40 sites. Cependant, seulement 10 à 15 d’entre eux étaient fonctionnels en même temps. L’établissement des Inughuits près de la station, soit Uummannaq, a tôt fait de devenir le plus grand site de la région. Les sources nous permettent de suivre les changements de résidence de certaines familles de chasseurs sur quatre décennies. En déplaçant leurs sites hivernaux aux deux ou trois ans, les familles acquéraient des connaissances directes de la topographie et de la variation saisonnière des territoires de chasse de l’ensemble du district de Thulé pendant leurs années d’activité. De la même manière, ils entraient en contact avec divers réseaux familiaux au fil des ans. Le fait de retracer les chemins empruntés par les traîneaux qui reliaient les sites sur de grandes distances révèle à quel point la proximité des chemins principaux et des chemins d’évasion sur la calotte glaciaire jouait un grand rôle dans l’emplacement du site. La technologie des traîneaux à chiens et, par conséquent, la capacité à transporter des humains, de l’outillage et des vivres conservées, a battu son plein pendant la période de la station de Thulé, lorsque la richesse découlait de la traite et de l’accès aux matériaux bruts. Le mappage des principaux territoires de chasse sur la glace de mer et la modéli­sation de l’étendue annuelle des possibilités des chasseurs d’avoir accès à différents types de gibier — principalement le morse, le phoque annelé, le narval et les oiseaux marins (en plus d’un peu de caribou) — a permis de démontrer que le phoque annelé constituait l’essentiel des moyens d’existence de l’économie de subsistance. Cependant, les ressources en vrac, acquises par le biais de chasses printanières intensives du morse dans quelques points chauds, de même que la consommation et le partage soigneusement planifiés de la viande et du petit lard emmagasinés, étaient essentiels à la vie dans la polynie des eaux du Nord. L’établissement temporaire aux stations de traite de la région, environ deux hivers à la fois, faisait également partie de la stratégie de gestion des risques des Inughuits

The effect of the Galactic magnetic field on gas accretion

Throughout cosmic time, the evolution of dark matter drives galaxy formation through the hierarchical merging of mass clumps. The accretion of baryons onto dark matter gave rise to galaxies, a complex process that continues to this day. I concentrate my efforts here on the role of large-scale magnetic fields (B-fields) when gas accretes onto galaxies through the use of magnetohydrodynamic (MHD) simulations. I probe the Galactic B-field through constrained Galactic simulations where the magnetic field is inserted at a specific epoch to observe how gaseous structures evolve. I adopt this approach with a view to comparing my simulations with new Galactic surveys, particularly of High-Velocity Clouds (HVCs) in the Galactic halo. The apparent longevity of HVCs is puzzling from a hydrodynamical perspective. They should be stripped and dispersed through their interaction with the halo gas. I show that B-fields in the halo can help explain this. As clouds move through the halo the field becomes amplified around them acting as a barrier resisting instabilities. This increases the amount of gas that can be accreted through HVCs. For clouds in the outskirts of the halo the effect is limited but closer to the disk at z < 20 kpc it is substantial. I show that the amplification of the B-fields around the clouds is well behaved so that observationally derived B-field strengths may be used to constrain their distances. I find that the B-field also strongly affects slower moving clouds that are part of the galactic fountain closer to the disk. The stripping of gas from these clouds can lead to condensation of material out of the halo gas through mixing. I find that the associated accretion rate is excessive compared to observational constraints when the B-field is ignored. However, for these clouds the magnetically reduced stripping, and so reduced mixing, lowers the efficiency of the associated accretion. These reduced accretion rates are in better agreement with the observations

Efficiency of thermal conduction in a magnetized circumgalactic medium

The large temperature difference between cold gas clouds around galaxies and the hot halos that they are moving through suggests that thermal conduction could play an important role in the circumgalactic medium. However, thermal conduction in the presence of a magnetic field is highly anisotropic, being strongly suppressed in the direction perpendicular to the magnetic field lines. This is commonly modelled by using a simple prescription that assumes that thermal conduction is isotropic at a certain efficiency $f<1$, but its precise value is largely unconstrained. We investigate the efficiency of thermal conduction by comparing the evolution of 3D hydrodynamical (HD) simulations of cold clouds moving through a hot medium, using artificially suppressed isotropic thermal conduction (with $f$), against 3D magnetohydrodynamical (MHD) simulations with (true) anisotropic thermal conduction. Our main diagnostic is the time evolution of the amount of cold gas in conditions representative of the lower (close to the disc) circumgalactic medium of a Milky Way-like galaxy. We find that in almost every HD and MHD run, the amount of cold gas increases with time, indicating that hot gas condensation is an important phenomenon that can contribute to gas accretion onto galaxies. For the most realistic orientations of the magnetic field with respect to the cloud motion we find that $f$ is in the range 0.03 -- 0.15. Thermal conduction is thus always highly suppressed, but its effect on the cloud evolution is generally not negligible