Function Of Galactocentric Distance Research Articles

Mapping the Milky Way with LAMOST – IV. The large Galactic disc extending to 35 kpc

ABSTRACT We investigate extent of the disc in both vertical and radial directions by separately analysing the density and azimuthal velocity distributions using 31116 M giant stars selected from LAMOST DR5. We first measure the metallicities, radial velocities, and the distances with precision of 0.26 dex, 5.01 km s−1, and 20.8 per cent, respectively. Then, a Bayesian method is applied to model the distributions of the outer Galaxy including components of the thin, thick discs and the halo, to obtain the scale height and azimuthal velocity distributions for the thin and thick discs as a function of galactocentric distance R. The density profile of the thin disc shows that it extends out to R ∼ 35 kpc, beyond which there are no M giant stars in the halo except for Sagittarius Stream members. By using azimuthal velocity to trace the thin disc out to 26 kpc from the Galactic Centre, we show that both the density profile and azimuthal velocity decrease with Galactocentric distance R. In both the density and azimuthal velocity distributions, there is a significant breakpoint at a galactocentric distance of R ∼ 14 kpc. The scale height distribution shows that the thin disc is significantly flared, consistent with previous works. We also find that there is no significant thick disc (as traced by M giant stars) in the outer region beyond R ∼ 15.5 kpc.

Cosmological gas accretion history on to the stellar discs of Milky Way-like galaxies in the Auriga simulations – II. The inside–out growth of discs

ABSTRACT We investigate the growth of stellar discs in Milky Way-mass galaxies using the magnetohydrodynamical simulations of the Auriga Project in a full cosmological context. We focus on the gas accretion process along the discs, calculating the net, infall and outflow rates as a function of galactocentric distance, and investigate the relation between them and the star formation activity. The stellar distributions of around 70 per cent of the simulated galaxies exhibit an ‘inside–out’ pattern, with older (younger) stellar populations preferentially located in the inner (outer) disc regions. In all cases, we find a very tight correlation between the infall, outflow, and net accretion rates, as well as between these three quantities and the star formation rate. This is because the amount of gas which is ultimately available for star formation in each radial ring depends not only on the infall rates, but also on the amount of gas leaving the disc in outflows, which directly relates to the local star formation level. Therefore, any of these rates can be used to identify galaxies with inside–out growth. For these galaxies, the correlation between the dominant times of accretion/star formation and disc radius is well fitted by a linear function. We also find that, when averaged over galaxies with formation histories similar to the Milky Way, the simulated accretion rates show a similar evolution (both temporally and radially integrated) to the usual accretion prescriptions used in chemical evolution models, although some major differences arise at early times and in the inner disc regions.

Probing the Galactic halo with RR Lyrae stars – V. Chemistry, kinematics, and dynamically tagged groups

ABSTRACT We employ a sample of 135 873 RR Lyrae stars (RRLs) with precise photometric-metallicity and distance estimates from the newly calibrated P–ϕ31–R21–[Fe/H] and Gaia G band P–R21–[Fe/H] absolute magnitude–metallicity relations of Li et al., combined with available proper motions from Gaia EDR3, and 6955 systemic radial velocities from Gaia DR3 and other sources, in order to explore the chemistry and kinematics of the halo of the Milky Way (MW). This sample is ideally suited for characterization of the inner- and outer-halo populations of the stellar halo, free from the bias associated with spectroscopically selected probes, and for estimation of their relative contributions as a function of Galactocentric distance. The results of a Gaussian mixture model analysis of these contributions are broadly consistent with other observational studies of the halo, and with expectations from recent MW simulation studies. We apply the hdbscan clustering method to the specific energies and cylindrical actions (E, Jr, Jϕ, Jz), identifying 97 dynamically tagged groups (DTGs) of RRLs, and explore their associations with recognized substructures of the MW. The precise photometric-distance determinations (relative distance errors on the order of 5 per cent or better), and the resulting high-quality determination of dynamical parameters, yield highly statistically significant (low) dispersions of [Fe/H] for the stellar members of the DTGs compared to random draws from the full sample, indicating that they share common star-formation and chemical histories, influenced by their birth environments.

Direct measurements of carbon and sulfur isotope ratios in the Milky Way

Context. Isotope abundance ratios provide a powerful tool for tracing stellar nucleosynthesis, evaluating the composition of stellar ejecta, and constraining the chemical evolution of the Milky Way. Aims. We aim to measure the 12C/13C, 32S/34S, 32S/33S, 32S/36S, 34S/33S, 34S/36S, and 33S/36S isotope ratios across the Milky Way. Methods. With the IRAM 30 meter telescope, we performed observations of the J = 2−1 transitions of CS, C33S, C34S, C36S, 13CS, 13C33S, and 13C34S as well as the J = 3−2 transitions of C33S, C34S, C36S, and 13CS toward a large sample of 110 high-mass star-forming regions. Results. We measured the 12C/13C, 32S/34S, 32S/33S, 32S/36S, 34S/33S, 34S/36S, and 33S/36S abundance ratios with rare isotopologs of CS, thus avoiding significant saturation effects. With accurate distances obtained from parallax data, we confirm previously identified 12C/13C and 32S/34S gradients as a function of galactocentric distance. In the central molecular zone, 12C/13C ratios are higher than suggested by a linear fit to the disk values as a function of galactocentric radius. While 32S/34S ratios near the Galactic center and in the inner disk are similar, this is not the case for 12C/13C, when comparing central values with those near galactocentric radii of 5 kpc. As was already known, there is no 34S/33S gradient but the average ratio of 4.35 ± 0.44 derived from the J = 2−1 transition lines of C34S and C33S is well below previously reported values. A comparison between solar and local interstellar 32S/34S and 34S/33S ratios suggests that the Solar System may have been formed from gas with a particularly high 34S abundance. For the first time, we report positive gradients of 32S/33S, 34S/36S, 33S/36S, and 32S/36S in our Galaxy. The predicted 12C/13C ratios from the latest Galactic chemical-evolution models are in good agreement with our results. While 32S/34S and 32S/36S ratios show larger differences at larger galactocentric distances, 32S/33S ratios show an offset across the entire inner 12 kpc of the Milky Way.

An MCMC approach to the three-dimensional structure of the Milky Way bulge using OGLE-IV δ Scuti stars

ABSTRACT We present an analysis of high-latitude δ Scuti stars (|b| > 1°) in the Galactic bulge region (−8${_{.}^{\circ}}$3 < l < 9${_{.}^{\circ}}$4) using a clean sample of the photometric data of 7440 stars recently released by the OGLE-IV project. The geometrical parameters of the bulge are determined based on Maximum Likelihood (ML) analysis in five-dimensional parameter space. More refined values of these parameters as well as their uncertainties are obtained from a fully Bayesian Markov chain Monte Carlo (MCMC) analysis. Approximating the bulge as an ellipsoid, the distribution of the number density of stars as a function of Galacto-centric distance has been modelled using three distribution functions: two Exponential ($\rm E_{1},\rm E_{2}$) types and one Gaussian ($\rm G$) type. Based on the AIC and BIC values, the exponential model $\rm E_{1}$ is chosen as the best statistical model for the parameter values obtained from the MCMC analysis. The MCMC analysis yields the following results: the mean distance to the Galactic Centre (GC) is found to be R0 = 8.034 ± 0.012stat ± 0.586sys kpc; the bulge δ Scuti distribution has a triaxial shape with normalized (a ≡ 1) axes ratios (a: b: c) as 1.000 ± 0.005: 0.348 ± 0.002: 0.421 ± 0.002. Here a is the semi-major axis lying in the Galactic plane and pointing towards us; b and c are the two semi-minor axes, the former lying in the Galactic plane and the later perpendicular to it. Smaller values of b as compared to a obtained for Galacto-centric distances R ≥ 2.0 kpc indicate the presence of a bar-like structure of the bulge with a bar angle of 22${_{.}^{\circ}}$006 ± 2${_{.}^{\circ}}$078.

The outermost stellar halo of NGC 5128 (Centaurus A): Radial structure

Context.The extended stellar halos of galaxies contain important clues for investigating their assembly history and evolution.Aims.We investigate the resolved stellar content and the extended halo of NGC 5128 as a function of galactocentric distance, and trace the halo outward to its currently detectable limits.Methods.We usedHubbleSpace Telescope images obtained with the WFPC2, ACS, and WFC3 cameras equipped withF606WandF814Wfilters to resolve individual red giant branch (RGB) stars in 28 independent pointings across the halo of NGC 5128. The stellar halo analysis for 14 of these pointings is presented here for the first time. Star counts from deepVIcolor-magnitude diagrams reaching at least 1.5 mag below the tip of the RGB are used to derive the surface density distribution of the halo. The contamination by Milky Way stars is assessed with a new control field, with models, and by combining optical and near-IR photometry.Results.We present a new calibration of the WFC3F606W + F814Wphotometry to the ground-basedVIphotometric system. The photometry shows that the stellar halo of NGC 5128 is dominated by old RGB stars that are present in all fields. TheV-band surface brightness of fields changes from 23 to 32 mag arcsec−2between the innermost field only 8.3 kpc from the galaxy center to our outermost halo fields, which are located 140 kpc away from the center along the major axis and 92 kpc along the minor axis. Within the inner ∼30 kpc, we also find evidence for a 2 − 3 Gyr old population traced by asymptotic giant branch stars that are brighter than the tip of the RGB. This population contributes only up to 10% in total stellar mass if it is 2 Gyr old, but a larger fraction of 30 − 40% is required if its age is 3 Gyr. The stellar surface density profile is well fit by a classicr1/4curve or a simple power-law form ∼r−3.1over the full radial range, with no obvious break in the slope, but with large field-to-field scatter. The ellipticity measured from integrated-light photometry in the inner parts,e = (b/a) = 0.77, flattens toe = 0.54 ± 0.02 beyond 30 kpc. Considering the flattening of the outer halo, the projection of the elliptical isophote on the semimajor axis for our most distant field reaches nearly 30 effective radii.

Mild radial variations of the stellar IMF in the bulge of M31

ABSTRACT Using new, homogeneous, long-slit spectroscopy in the wavelength range from ∼0.35 to $\sim 1 \, \mu$m, we study radial gradients of optical and near-infrared (NIR) initial mass function (IMF)-sensitive features along the major axis of the bulge of M31, out to a galactocentric distance of ∼200 arcsec (∼800 pc). Based on state-of-the-art stellar population synthesis models with varying Na abundance ratio, we fit a number of spectral indices, from different chemical species (including TiO’s, Ca, and Na indices), to constrain the low-mass (≲0.5 M⊙) end slope (i.e. the fraction of low-mass stars) of the stellar IMF, as a function of galactocentric distance. Outside a radial distance of ∼10 arcsec, we infer an IMF similar to a Milky Way-like distribution, while at small galactocentric distances, an IMF radial gradient is detected, with a mildly bottom-heavy IMF in the few inner arcsec. We are able to fit Na features (both NaD and $\rm Na\,{\small I}8190$), without requiring extremely high Na abundance ratios. $\rm [Na/Fe]$ is ∼0.4 dex for most of the bulge, rising up to ∼0.6 dex in the innermost radial bins. Our results imply an overall, luminosity-weighted, IMF and mass-to-light ratio for the M31 bulge, consistent with those for a Milky Way-like distribution, in contrast to results obtained, in general, for most massive early-type galaxies.

Resolving the Metallicity Distribution of the Stellar Halo with the H3 Survey

Abstract The Galactic stellar halo is predicted to have formed at least partially from the tidal disruption of accreted dwarf galaxies. This assembly history should be detectable in the orbital and chemical properties of stars. The H3 Survey is obtaining spectra for 200,000 stars and, when combined with Gaia data, is providing detailed orbital and chemical properties of Galactic halo stars. Unlike previous surveys of the halo, the H3 target selection is based solely on magnitude and Gaia parallax; the survey therefore provides a nearly unbiased view of the entire stellar halo at high latitudes. In this paper we present the distribution of stellar metallicities as a function of Galactocentric distance and orbital properties for a sample of 4232 kinematically selected halo giants to 100 kpc. The stellar halo is relatively metal-rich, , and there is no discernible metallicity gradient over the range 6 < R gal < 100 kpc. However, the halo metallicity distribution is highly structured, including distinct metal-rich and metal-poor components at R gal < 10 kpc and R gal > 30 kpc, respectively. The Sagittarius stream dominates the metallicity distribution at 20–40 kpc for stars on prograde orbits. The Gaia–Enceladus merger remnant dominates the metallicity distribution for radial orbits to ≈30 kpc. Metal-poor stars with [Fe/H] < −2 are a small population of the halo at all distances and orbital categories. We associate the “in situ” stellar halo with stars displaying thick disk chemistry on halo-like orbits; such stars are confined to . The majority of the stellar halo is resolved into discrete features in chemical–orbital space, suggesting that the bulk of the stellar halo formed from the accretion and tidal disruption of dwarf galaxies. The relatively high metallicity of the halo derived in this work is a consequence of the unbiased selection function of halo stars and, in combination with the recent upward revision of the total stellar halo mass, implies a Galactic halo metallicity that is typical for its mass.

IMF radial gradients in most massive early-type galaxies

ABSTRACT Using new long-slit spectroscopy obtained with X-Shooter at ESO-VLT, we study, for the first time, radial gradients of optical and near-infrared initial mass function (IMF)-sensitive features in a representative sample of galaxies at the very high mass end of the galaxy population. The sample consists of seven early-type galaxies (ETGs) at z ∼ 0.05, with central velocity dispersion in the range 300 ≲ σ ≲ 350 km s−1. Using state-of-the-art stellar population synthesis models, we fit a number of spectral indices, from different chemical species (including TiO and Na indices), to constrain the IMF slope (i.e. the fraction of low-mass stars), as a function of galactocentric distance, over a radial range out to ∼4 kpc. ETGs in our sample show a significant correlation of IMF slope and surface mass density. The bottom-heavy population (i.e. an excess of low-mass stars in the IMF) is confined to central galaxy regions with surface mass density above $\rm \sim 10^{10}\, M_\odot \, kpc^{-2}$, or, alternatively, within a characteristic radius of ∼2 kpc. Radial distance, in physical units, and surface mass density are the best correlators to IMF variations, with respect to other dynamical (e.g. velocity dispersion) and stellar population (e.g. metallicity) properties. Our results for the most massive galaxies suggest that there is no single parameter that fully explains variations in the stellar IMF, but IMF radial profiles at z ∼ 0 rather result from the complex formation and mass accretion history of galaxy inner and outer regions.

The velocity anisotropy of the Milky Way satellite system

We analyse the orbital kinematics of the Milky Way (MW) satellite system utilizing the latest systemic proper motions for 38 satellites based on data from Gaia Data Release 2. Combining these data with distance and line-of-sight velocity measurements from the literature, we use a likelihood method to model the velocity anisotropy, $\beta$, as a function of Galactocentric distance and compare the MW satellite system with those of simulated MW-mass haloes from the APOSTLE and Auriga simulation suites. The anisotropy profile for the MW satellite system increases from $\beta\sim -2$ at $r\sim20$ kpc to $\beta\sim 0.5$ at $r\sim200$ kpc, indicating that satellites closer to the Galactic centre have tangentially-biased motions while those farther out have radially-biased motions. The motions of satellites around APOSTLE host galaxies are nearly isotropic at all radii, while the $\beta(r)$ profiles for satellite systems in the Auriga suite, whose host galaxies are substantially more massive in baryons than those in APOSTLE, are more consistent with that of the MW satellite system. This shape of the $\beta(r)$ profile may be attributed to the central stellar disc preferentially destroying satellites on radial orbits, or intrinsic processes from the formation of the Milky Way system.

Testing modified gravity theory in the Milky Way

We perform a test of John Moffat's Modified Gravity theory (MOG) within the Milky Way, adopting the well known "Rotation Curve" method. We use the dynamics of observed tracers within the disk to determine the gravitational potential as a function of galactocentric distance, and compare that with the potential that is expected to be generated by the visible component only (stars and gas) under different "flavors" of the MOG theory, making use of a state-of-the-art setup for both the observed tracers and baryonic morphology. Our analysis shows that in both the original and the modified version (considering a self-consistent evaluation of the Milky Way mass), the theory fails to reproduce the observed rotation curve. We conclude that in none of its present formulation, the MOG theory is able to explain the observed Rotation Curve of the Milky Way.

Circumnuclear Star Formation and AGN Activity: Clues from Surface Brightness Radial Profile of PAHs and [ S IV

Abstract We studied the circumnuclear mid-IR emission in a sample of 19 local active galactic nuclei (AGNs) with high spatial resolution spectra using T-ReCS (Gemini) and CanariCam (GTC), together with Spitzer/IRS observations. We measured the flux and the equivalent width for the 11.3 μm PAH feature and the [S iv] line emission as a function of galactocentric distance. This allowed us to study the star formation (SF) at subkiloparsec scales from the nucleus for a large sample of nearby AGNs. The [S iv] line emission could be tracing the AGN radiation field within a few thousand times the sublimation radius (R sub), but it often peaks at distances greater than 1000 R sub. One possibility is that the SF is contributing to the [S iv] total flux. We found an 11.3 μm PAH emission deficit within the inner few tens of parsecs from the AGN. This deficit might be due to the destruction of the molecules responsible for this feature or the lack of SF at these distances. We found a sensible agreement in the expected shift of the relation of the AGN bolometric luminosity and the SF rate. This indicates that numerical models attributing the link between AGN activity and host galaxy growth to mergers are in agreement with our data, for most inner galaxy parts.

The young star cluster population of M51 with LEGUS – II. Testing environmental dependences

It has recently been established that the properties of young star clusters (YSCs) can vary as a function of the galactic environment in which they are found. We use the cluster catalogue produced by the Legacy Extragalactic UV Survey (LEGUS) collaboration to investigate cluster properties in the spiral galaxy M51. We analyse the cluster population as a function of galactocentric distance and in arm and inter-arm regions. The cluster mass function exhibits a similar shape at all radial bins, described by a power law with a slope close to $-2$ and an exponential truncation around $10^5\ \rm{M}_{\odot}$ . While the mass functions of the YSCs in the spiral arm and inter-arm regions have similar truncation masses, the inter-arm region mass function has a significantly steeper slope than the one in the arm region; a trend that is also observed in the giant molecular cloud mass function and predicted by simulations. The age distribution of clusters is dependent on the region considered, and is consistent with rapid disruption only in dense regions, while little disruption is observed at large galactocentric distances and in the inter-arm region. The fraction of stars forming in clusters does not show radial variations, despite the drop in the $H_2$ surface density measured as function of galactocentric distance. We suggest that the higher disruption rate observed in the inner part of the galaxy is likely at the origin of the observed flat cluster formation efficiency radial profile.

The need for speed: escape velocity and dynamical mass measurements of the Andromeda galaxy

Our nearest large cosmological neighbour, the Andromeda galaxy (M31), is a dynamical system, and an accurate measurement of its total mass is central to our understanding of its assembly history, the life-cycles of its satellite galaxies, and its role in shaping the Local Group environment. Here, we apply a novel approach to determine the dynamical mass of M31 using high velocity Planetary Nebulae (PNe), establishing a hierarchical Bayesian model united with a scheme to capture potential outliers and marginalize over tracers unknown distances. With this, we derive the escape velocity run of M31 as a function of galacto-centric distance, with both parametric and non-parametric approaches. We determine the escape velocity of M31 to be $470\pm{40}$ km s$^{-1}$ at a galacto-centric distance of 15 kpc, and also, derive the total potential of M31, estimating the virial mass and radius of the galaxy to be $0.8\pm{0.1}\times10^{12}\,M_\odot$ and $240\pm{10}$ kpc, respectively. Our M31 mass is on the low-side of the measured range, this supports the lower expected mass of the M31-Milky Way system from the timing and momentum arguments, satisfying the HI constraint on circular velocity between $10\lesssim R/\textrm{kpc}<35$, and agreeing with the stellar mass Tully-Fisher relation. To place these results in a broader context, we compare them to the key predictions of the $\Lambda{\rm CDM}$ cosmological paradigm, including the stellar-mass-halo-mass and the dark matter halo concentration-virial mass correlation, and finding it to be an outlier to this relation.

Giant H i hole inside the 3 kpc ring and the North Polar Spur—The Galactic crater

Abstract Applying a newly developed tangent-circle method (TCM), we derive a volume density map of H i gas in the inner Galaxy as a function of galactocentric distance R and height Z. The H i hole around the Galactic Center (GC) is shown to have a crater-shaped wall, which coincides with the brightest ridge of the North Polar Spur and emanates from the 3 kpc expanding ring. The crater structure is explained by sweeping of the halo gas by a shock-wave from the GC. The unperturbed H i halo outside 3 kpc is shown to be in hydrostatic equilibrium, obeying the sech2Z/h density law with a scale height h ∼ 450 pc.

Multitemperature mapping of dust structures throughout the Galactic Plane using the PPMAP tool with Herschel Hi-GAL data

We describe new Hi-GAL based maps of the entire Galactic Plane, obtained using continuum data in the wavelength range 70-500 $\mu$m. These maps are derived with the PPMAP procedure, and therefore represent a significant improvement over those obtained with standard analysis techniques. Specifically they have greatly improved resolution (12 arcsec) and, in addition to more accurate integrated column densities and mean dust temperatures, they give temperature-differential column densities, i.e., separate column density maps in twelve distinct dust temperature intervals, along with the corresponding uncertainty maps. The complete set of maps is available on-line. We briefly describe PPMAP and present some illustrative examples of the results. These include (a) multi-temperature maps of the Galactic HII region W5-E, (b) the temperature decomposition of molecular cloud column-density probability distribution functions, and (c) the global variation of mean dust temperature as a function of Galactocentric distance. Amongst our findings are: (i) a strong localised temperature gradient in W5-E in a direction orthogonal to that towards the ionising star, suggesting an alternative heating source and providing possible guidance for models of the formation of the bubble complex, and (ii) the overall radial profile of dust temperature in the Galaxy shows a monotonic decrease, broadly consistent both with models of the interstellar radiation field and with previous estimates at lower resolution. However, we also find a central temperature plateau within ~ 6 kpc of the Galactic centre, outside of which is a pronounced steepening of the radial profile. This behaviour may reflect the greater proportion of molecular (as opposed to atomic) gas in the central region of the Galaxy.

Measuring the Galactic Distribution of Transiting Planets with WFIRST

The WFIRST microlensing mission will measure precise light curves and relative parallaxes for millions of stars, giving it the potential to characterize short-period transiting planets all along the line of sight and into the galactic bulge. These light curves will enable the detection of more than 100,000 transiting planets whose host stars have measured distances. Although most of these planets cannot be followed up, several thousand hot Jupiters can be confirmed directly by detection of their secondary eclipses in the WFIRST data. Additionally, some systems of small planets may be confirmed by detecting transit timing variations over the duration of the WFIRST microlensing survey. Finally, many more planets may be validated by ruling out potential false positives. The combination of WFIRST transits and microlensing will provide a complete picture of planetary system architectures, from the very shortest periods to unbound planets, as a function of galactocentric distance.

Abundance ratios and IMF slopes in the dwarf elliptical galaxy NGC 1396 with MUSE

Deep observations of the dwarf elliptical (dE) galaxy NGC 1396 (M$_V = -16.60$, Mass $\sim 4\times10^8$ M$_\odot$), located in the Fornax cluster, have been performed with the VLT/ MUSE spectrograph in the wavelength region from $4750-9350$ \AA{}. In this paper we present a stellar population analysis studying chemical abundances, the star formation history (SFH) and the stellar initial mass function (IMF) as a function of galacto-centric distance. Different, independent ways to analyse the stellar populations result in a luminosity-weighted age of $\sim$ 6 Gyr and a metallicity [Fe/H]$\sim$ $-0.4$, similar to other dEs of similar mass. We find unusually overabundant values of [Ca/Fe] $\sim +0.1$, and under-abundant Sodium, with [Na/Fe] values around $-0.1$, while [Mg/Fe] is overabundant at all radii, increasing from $\sim+0.1$ in the centre to $\sim +0.2$ dex. We notice a significant metallicity and age gradient within this dwarf galaxy. To constrain the stellar IMF of NGC 1396, we find that the IMF of NGC 1396 is consistent with either a Kroupa-like or a top-heavy distribution, while a bottom-heavy IMF is firmly ruled out. An analysis of the abundance ratios, and a comparison with galaxies in the Local Group, shows that the chemical enrichment history of NGC 1396 is similar to the Galactic disc, with an extended star formation history. This would be the case if the galaxy originated from a LMC-sized dwarf galaxy progenitor, which would lose its gas while falling into the Fornax cluster.

Properties of the cluster population of NGC 1566 and their implications

We present results of a photometric study into the cluster population of NGC 1566, a nearby grand design spiral galaxy, sampled out to a Galactocentric radius of $\approx 5.5$ kpc. The shape of the mass-limited age distribution shows negligible variation with radial distance from the centre of the galaxy, and demonstrates three separate sections, with a steep beginning, flat middle and steep end. The luminosity function can be approximated by a power law at lower luminosities with evidence of a truncation at higher luminosity. The power law section of the luminosity function of the galaxy is best fitted by an index $\approx -2$, in agreement with other studies, and is found to agree with a model luminosity function, which uses an underlying Schechter mass function. The recovered power law slope of the mass distribution shows a slight steepening as a function of galactocentric distance, but this is within error estimates. It also displays a possible truncation at the high mass end. Additionally, the cluster formation efficiency ($\Gamma$) and the specific U-band luminosity of clusters ($T_L(U)$) are calculated for NGC 1566 and are consistent with values for similar galaxies. A difference in NGC 1566, however, is that the fairly high star formation rate is in contrast with a low $\Sigma_{SFR}$ and $\Gamma$, indicating that $\Gamma$ can only be said to depend strongly on $\Sigma_{SFR}$, not the star formation rate.

Probing the role of the galactic environment in the formation of stellar clusters, using M83 as a test bench

We present a study of the M83 cluster population, covering the disc of the galaxy between radii of 0.45 and 4.5 kpc. We aim to probe the properties of the cluster population as a function of distance from the galactic centre. We observe a net decline in cluster formation efficiency ($\Gamma$, i.e. the amount of star formation happening in bound clusters) from about 26 % in the inner region to 8 % in the outer part of the galaxy. The recovered $\Gamma$ values within different regions of M83 follow the same $\Gamma$ versus star formation rate density relation observed for entire galaxies. We also probe the initial cluster mass function (ICMF) as a function of galactocentric distance. We observe a significant steepening of the ICMF in the outer regions (from $-1.90\pm0.11$ to $-2.70\pm0.14$) and for the whole galactic cluster population (slope of $-2.18\pm0.07$) of M83. We show that this change of slope reflects a more fundamental change of the 'truncation mass' at the high-mass end of the distribution. This can be modelled as a Schechter function of slope $-2$ with an exponential cut-off mass ($M_{\rm c}$) that decreases significantly from the inner to the outer regions (from 4.00 to $0.25\times 10^5$ M$_\odot$) while the galactic $M_{\rm c}$ is $\approx1.60\times10^5$ M$_\odot$. The trends in \Gamma and ICMF are consistent with the observed radial decrease of the $\Sigma({\rm H}_2)$, hence in gas pressure. As gas pressure declines cluster formation becomes less efficient. We conclude that the host galaxy environment appears to regulate 1) the fraction of stars locked in clusters; 2) the upper mass limit of the ICMF, consistently described by a near-universal slope $-2$ truncated at the high-mass end.