ALBERT

Description: Context. Recent surveys of the Galactic plane in the dust continuum and CO emission lines reveal that large (≳50 pc) and massive (≳105 M⊙) filaments, know as giant molecular filaments (GMFs), may be linked to Galactic dynamics and trace the mid-plane of the gravitational potential in the Milky Way. Yet our physical understanding of GMFs is still poor. Aims. We investigate the dense gas properties of one GMF, with the ultimate goal of connecting these dense gas tracers with star formation processes in the GMF. Methods. We imaged one entire GMF located at l ~ 52–54° longitude, GMF54 (~68 pc long), in the empirical dense gas tracers using the HCN(1–0), HNC(1–0), and HCO+(1–0) lines, and their 13C isotopologue transitions, as well as the N2H+(1–0) line. We studied the dense gas distribution, the column density probability density functions (N-PDFs), and the line ratios within the GMF. Results. The dense gas molecular transitions follow the extended structure of the filament with area filling factors between 0.06 and 0.28 with respect to 13CO(1–0). We constructed the N-PDFs of H2 for each of the dense gas tracers based on their column densities and assumed uniform abundance. The N-PDFs of the dense gas tracers appear curved in log–log representation, and the HCO+ N-PDF has the flattest power-law slope index. Studying the N-PDFs for sub-regions of GMF54, we found an evolutionary trend in the N-PDFs that high-mass star-forming and photon-dominated regions have flatter power-law indices. The integrated intensity ratios of the molecular lines in GMF54 are comparable to those in nearby galaxies. In particular, the N2H+/13CO ratio, which traces the dense gas fraction, has similar values in GMF54 and all nearby galaxies except Ultraluminous Infrared Galaxies. Conclusions. As the largest coherent cold gaseous structure in our Milky Way, GMFs, are outstanding candidates for connecting studies of star formation on Galactic and extragalactic scales. By analyzing a complete map of the dense gas in a GMF we have found that: (1) the dense gas N-PDFs appear flatter in more evolved regions and steeper in younger regions, and (2) its integrated dense gas intensity ratios are similar to those of nearby galaxies.

Description: One of the key goals of the Bluedisk survey is to characterize the impact of gas accretion in disc galaxies in the context of galaxy evolution. It contains 50 disc galaxies in the stellar mass range 10 10 –10 11 M , of which half are bluer and more H i -rich galaxies than their H i -normal (control) counterparts. In this paper, we investigate how ongoing disc growth affects the molecular gas distribution and the star formation efficiency in these galaxies. We present 12 CO observations from the IRAM 30-m telescope in 26 galaxies of the Bluedisk survey. We compare the amount and spatial distribution of the molecular gas to key quantities such as atomic gas, stellar mass and surface density, star formation rate (SFR) and metallicity. We analyse the SFR per unit gas (SFR/H i and SFR/H 2 ) and relate all those parameters to general galaxy properties (H i -rich/control disc, morphology, etc.). We find that the H i -rich galaxies have similar H 2 masses as the control galaxies. In their centres, H i -rich galaxies have lower H 2 /H i ratios and marginally shorter molecular gas depletion times. However, the main differences between the two samples occur in the outer parts of the discs, with the H i -rich galaxies having slightly smaller CO discs (relative to the optical radius R 25 ) and steeper CO and metallicity gradients than the control galaxies. The ongoing accretion of H i at large radii has thus not led to an appreciable growth of the CO discs in our sample. Based on depletion times, we estimate that this gas will contribute to star formation on time-scales of at least 5 Gyr.

Description: As part of the Bluedisk survey, we analyse the radial gas-phase metallicity profiles of 50 late-type galaxies. We compare the metallicity profiles of a sample of H i -rich galaxies against a control sample of H i -‘normal’ galaxies. We find the metallicity gradient of a galaxy to be strongly correlated with its H i mass fraction ( $\textrm {M(H\,\small {I})} / \textrm {M}_{\ast }$ ). We note that some galaxies exhibit a steeper metallicity profile in the outer disc than in the inner disc. These galaxies are found in both the H i -rich and control samples. This contradicts a previous indication that these outer drops are exclusive to H i -rich galaxies. These effects are not driven by bars, although we do find some indication that barred galaxies have flatter metallicity profiles. By applying a simple analytical model, we are able to account for the variety of metallicity profiles that the two samples present. The success of this model implies that the metallicity in these isolated galaxies may be in a local equilibrium, regulated by star formation. This insight could provide an explanation of the observed local mass–metallicity relation.

Description: As part of the Bluedisk survey, we analyse the radial gas-phase metallicity profiles of 50 late-type galaxies. We compare the metallicity profiles of a sample of H i -rich galaxies against a control sample of H i -‘normal’ galaxies. We find the metallicity gradient of a galaxy to be strongly correlated with its H i mass fraction ( $\textrm {M(H\,\small {I})} / \textrm {M}_{\ast }$ ). We note that some galaxies exhibit a steeper metallicity profile in the outer disc than in the inner disc. These galaxies are found in both the H i -rich and control samples. This contradicts a previous indication that these outer drops are exclusive to H i -rich galaxies. These effects are not driven by bars, although we do find some indication that barred galaxies have flatter metallicity profiles. By applying a simple analytical model, we are able to account for the variety of metallicity profiles that the two samples present. The success of this model implies that the metallicity in these isolated galaxies may be in a local equilibrium, regulated by star formation. This insight could provide an explanation of the observed local mass–metallicity relation.

Description: We report on observations of the hydroxyl radical (OH) within The H i , OH, Recombination line survey (THOR) pilot region. The region is bounded approximately between Galactic coordinates l = 29 $_{.}^{\circ}$ 2 to 31 $_{.}^{\circ}$ 5 and b = –1 $_{.}^{\circ}$ 0 to +1 $_{.}^{\circ}$ 0 and includes the high-mass star-forming region W43. We identify 103 maser sites, including 72 with 1612 MHz masers, 42 showing masers in either of the main-line transitions at 1665 and 1667 MHz and four showing 1720 MHz masers. Most maser sites with either main-line or 1720 MHz emission are associated with star formation, whereas most of the 1612 MHz masers are associated with evolved stars. We find that nearly all of the main-line maser sites are co-spatial with an infrared source, detected by GLIMPSE. We also find diffuse OH emission, as well as OH in absorption towards selected unresolved or partially resolved sites. Extended OH absorption is found towards the well-known star-forming complex W43 Main.

Description: We estimate the parameters of the Kennicutt–Schmidt (KS) relationship, linking the star formation rate ( SFR ) to the molecular gas surface density ( mol ), in the Survey Toward Infrared-Bright Nearby Galaxies sample of nearby disc galaxies using a hierarchical Bayesian method. This method rigorously treats measurement uncertainties, and provides accurate parameter estimates for both individual galaxies and the entire population. Assuming standard conversion factors to estimate SFR and mol from the observations, we find that the KS parameters vary between galaxies, indicating that no universal relationship holds for all galaxies. The KS slope of the whole population is 0.76, with the 2 range extending from 0.58 to 0.94. These results imply that the molecular gas depletion time is not constant, but varies from galaxy-to-galaxy, and increases with the molecular gas surface density. Therefore, other galactic properties besides just mol affect SFR , such as the gas fraction or stellar mass. The non-universality of the KS relationship indicates that a comprehensive theory of star formation must take into account additional physical processes that may vary from galaxy to galaxy.

Description: The study of 21 cm line observations of atomic hydrogen allows detailed insight into the kinematics of spiral galaxies. We use sensitive high-resolution Very Large Array data from The H i Nearby Galaxy Survey (THINGS) to search for radial gas flows primarily in the outer parts (up to 3  x   r 25 ) of 10 nearby spiral galaxies. Inflows are expected to replenish the gas reservoir and fuel star formation under the assumption that galaxies evolve approximately in steady state. We carry out a detailed investigation of existing tilted ring fitting schemes and discover systematics that can hamper their ability to detect signatures of radial flows. We develop a new Fourier decomposition scheme that fits for rotational and radial velocities and simultaneously determines position angle and inclination as a function of radius. Using synthetic velocity fields we show that our novel fitting scheme is less prone to such systematic errors and that it is well suited to detect radial inflows in discs. We apply our fitting scheme to 10 THINGS galaxies and find clear indications of, at least partly previously unidentified, radial gas flows, in particular for NGC 2403 and NGC 3198 and to a lesser degree for NGC 7331, NGC 2903 and NGC 6946. The mass flow rates are of the same order but usually larger than the star formation rates. At least for these galaxies a scenario in which continuous mass accretion feeds star formation seems plausible. The other galaxies show a more complicated picture with either no clear inflow, outward motions or complex kinematic signatures.

Description: We analyse the radial distribution of H i gas for 23 disc galaxies with unusually high H i content from the Bluedisk sample, along with a similar-sized sample of ‘normal’ galaxies. We propose an empirical model to fit the radial profile of the H i surface density, an exponential function with a depression near the centre. The radial H i surface density profiles are very homogeneous in the outer regions of the galaxy; the exponentially declining part of the profile has a scalelength of ~0.18 R1, where R1 is the radius where the column density of the H i is 1 M  pc –2 . This holds for all galaxies, independent of their stellar or H i mass. The homogenous outer profiles, combined with the limited range in H i surface density in the non-exponential inner disc, results in the well-known tight relation between H i size and H i mass. By comparing the radial profiles of the H i -rich galaxies with those of the control systems, we deduce that in about half the galaxies, most of the excess gas lies outside the stellar disc, in the exponentially declining outer regions of the H i disc. In the other half, the excess is more centrally peaked. We compare our results with existing smoothed particle hydrodynamical simulations and semi-analytic models of disc galaxy formation in a cold dark matter universe. Both the hydro simulations and the semi-analytic models reproduce the H i surface density profiles and the H i size–mass relation without further tuning of the simulation and model inputs. In the semi-analytic models, the universal shape of the outer H i radial profiles is a consequence of the assumption that infalling gas is always distributed exponentially.

Description: We introduce the ‘Bluedisk’ project, a large programme at the Westerbork Synthesis Radio Telescope that has mapped the H i in a sample of 23 nearby galaxies with unusually high H i mass fractions, along with a similar-sized sample of control galaxies. This paper presents the sample selection, observational set-up, data reduction strategy and a first analysis of the sizes and structural properties of the H i discs. We find that the H i -rich galaxies lie on the same H i mass versus H i size relation as normal spiral galaxies, extending it to total H i masses of 2  x 10 10 M and radii R1 of ~100 kpc. The H i -rich galaxies have significantly larger values of H i -to-optical size ratio and more clumpy H i discs than those of normal spirals. There is no evidence that the discs of H i -rich galaxies are more disturbed. In fact, the centre of the H i distribution corresponds more closely with the centre of the optical light in the H i -rich galaxies than in the controls. All these results argue against a scenario in which new gas has been brought in by mergers. It is possible that they may be more consistent with cooling from a surrounding quasi-static halo of warm/hot gas.

Description: For investigating the relationship between the star formation rate and gas surface density, we develop a Bayesian linear regression method that rigorously treats measurement uncertainties and accounts for hierarchical data structure. The hierarchical Bayesian method simultaneously estimates the intercept, slope and scatter about the regression line of each individual subject (e.g. a galaxy) and the population (e.g. an ensemble of galaxies). Using synthetic data sets, we demonstrate that the method accurately recovers the underlying parameters of both the individuals and the population, especially when compared to commonly employed ordinary least squares techniques, such as the bisector fit. We apply the hierarchical Bayesian method to estimate the Kennicutt–Schmidt (KS) parameters of a sample of spiral galaxies compiled by Bigiel et al. We find significant variation in the KS parameters, indicating that no single KS relationship holds for all galaxies. This suggests that the relationship between molecular gas and star formation differs from galaxy to galaxy, possibly due to the influence of other physical properties within a given galaxy, such as metallicity, molecular gas fraction, stellar mass and/or magnetic fields. In four of the seven galaxies the slope estimates are sublinear, especially for M51, where unity is excluded at the 2 level. We estimate the mean index of the KS relationship for the population to be 0.84, with 2 range [0.63, 1.0]. For the galaxies with sublinear KS relationships, a possible interpretation is that CO emission is tracing some molecular gas that is not directly associated with star formation. Equivalently, a sublinear KS relationship may be indicative of an increasing gas depletion time at higher surface densities, as traced by CO emission. The hierarchical Bayesian method can account for all sources of uncertainties, including variations in the conversion of observed intensities to star formation rates and gas surface densities (e.g. the X CO factor), and is therefore well suited for a thorough statistical analysis of the KS relationship.