Stellar Mass Distribution Research Articles

A DIRECT DYNAMICAL MEASUREMENT OF THE MILKY WAY'S DISK SURFACE DENSITY PROFILE, DISK SCALE LENGTH, AND DARK MATTER PROFILE AT 4 kpc ≲R≲ 9 kpc

We present and apply rigorous dynamical modeling with which we infer unprecedented constraints on the stellar and dark matter mass distribution within our Milky Way (MW), based on large sets of phase-space data on individual stars. Specifically, we model the dynamics of 16,269 G-type dwarfs from SEGUE, which sample 5 < R_GC/kpc < 12 and 0.3 < |Z|/kpc < 3. We independently fit a parameterized MW potential and a three-integral, action-based distribution function (DF) to the phase-space data of 43 separate abundance-selected sub-populations (MAPs), accounting for the complex selection effects affecting the data. We robustly measure the total surface density within 1.1 kpc of the mid-plane to 5% over 4.5 < R_GC/kpc < 9. Using metal-poor MAPs with small radial scale lengths as dynamical tracers probes 4.5 < R_GC/kpc < 7, while MAPs with longer radial scale lengths sample 7 < R_GC/kpc < 9. We measure the mass-weighted Galactic disk scale length to be R_d = 2.15+/-0.14 kpc, in agreement with the photometrically inferred spatial distribution of stellar mass. We thereby measure dynamically the mass of the Galactic stellar disk to unprecedented accuracy: M_* = 4.6+/-0.3+3.0x(R_0/kpc-8)x10^{10}Msun and a total local surface density of \Sigma_{R_0}(Z=1.1 kpc) = 68+/-4 Msun/pc^2 of which 38+/-4 Msun/pc^2 is contributed by stars and stellar remnants. By combining our surface density measurements with the terminal velocity curve, we find that the MW's disk is maximal in that V_{c,disk} / V_{c,total} = 0.83+/-0.04 at R=2.2 R_d. We also constrain for the first time the radial profile of the dark halo at such small Galactocentric radii, finding that \rho_{DM} (r;near R_0) \propto 1 / r^\alpha with \alpha < 1.53 at 95% confidence. Our results show that action-based distribution-function modeling of complex stellar data sets is now a feasible approach that will be fruitful for interpreting Gaia data.

Do gravitational lens galaxies have an excess of luminous substructure?

Strong gravitational lensing can be used to directly measure the mass function of their satellites, thus testing one of the fundamental predictions of cold dark matter cosmological models. Given the importance of this test it is essential to ensure that galaxies acting as strong lenses have dark and luminous satellites which are representative of the overall galaxy population. We address this issue by measuring the number and spatial distribution of luminous satellites in ACS imaging around lens galaxies from the Sloan Lens Advanced Camera for Surveys (SLACS) lenses, and comparing them with the satellite population in ACS imaging of non lens galaxies selected from COSMOS, which has similar depth and resolution to the ACS images of SLACS lenses. In order to compare the samples of lens and non lens galaxies, which have intrinsically different stellar mass distributions, we measure, for the first time, the number of satellites per host as a continuous function of host stellar mass for both populations. We find that the number of satellites as a function of host stellar mass, as well as the spatial distribution are consistent between the samples. Using these results, we predict the number of satellites we would expect to find around a subset of the Cosmic Lens All Sky Survey (CLASS) lenses, and find a result consistent with the the number observed by Jackson et al. 2010. Thus we conclude that within our measurement uncertainties there is no significant difference in the satellite populations of lens and non lens galaxies.

The Herschel Virgo Cluster Survey – XIV. Transition-type dwarf galaxies in the Virgo cluster

We use dust scaling relations to investigate the hypothesis that Virgo cluster transition-type dwarfs are infalling star-forming field galaxies, which is argued based on their optical features (e.g. disks, spiral arms, bars) and kinematic properties similar to late-type galaxies. After their infall, environmental effects gradually transform them into early-type galaxies through the removal of their interstellar medium and quenching of all star formation activity. In this paper, we aim to verify whether this hypothesis holds using far-infrared diagnostics based on Herschel observations of the Virgo cluster taken as part of the Herschel Virgo Cluster Survey (HeViCS). We select transition-type objects in the nearest cluster, Virgo, based on spectral diagnostics indicative for their residual or ongoing star formation. We detect dust Md ~ 10^{5-6} Msun in 36% of the transition-type dwarfs located on the high end of the stellar mass distribution. This suggests that the dust reservoirs present in non-detections fall just below the Herschel detection limit (< 1.1x10^5 Msun). Dust scaling relations support the hypothesis of a transformation between infalling late-type galaxies to quiescent low-mass spheroids governed by environmental effects, with dust-to-stellar mass fractions for transition-type dwarfs in between values characteristic for late-type objects and the lower dust fractions observed in early-type galaxies. Several transition-type dwarfs demonstrate blue central cores, hinting at the radially outside-in removal of gas and quenching of star formation activity. The fact that dust is also confined to the inner regions suggests that metals are stripped in the outer regions along with the gas. In the scenario of most dust being stripped from the galaxy along with the gas, we argue that... (abridged)

Stellar masses of SDSS-III/BOSS galaxies at z ∼ 0.5 and constraints to galaxy formation models

We calculate stellar masses for massive luminous galaxies at redshift 0.2-0.7 using the first two years of data from the Baryon Oscillation Spectroscopic Survey (BOSS). Stellar masses are obtained by fitting model spectral energy distributions to u,g,r,i,z magnitudes, and simulations with mock galaxies are used to understand how well the templates recover the stellar mass. Accurate BOSS spectroscopic redshifts are used to constrain the fits. We find that the distribution of stellar masses in BOSS is narrow (Delta log M~0.5 dex) and peaks at about logM ~ 11.3 (for a Kroupa initial stellar mass function), and that the mass sampling is uniform over the redshift range 0.2 to 0.6, in agreement with the intended BOSS target selection. The galaxy masses probed by BOSS extend over ~10^{12} M, providing unprecedented measurements of the high-mass end of the galaxy mass function. We find that the galaxy number density above ~ 2.5 10^{11} M agrees with previous determinations. We perform a comparison with semi-analytic galaxy formation models tailored to the BOSS target selection and volume, in order to contain incompleteness. The abundance of massive galaxies in the models compare fairly well with the BOSS data, but the models lack galaxies at the massive end. Moreover, no evolution with redshift is detected from ~0.6 to 0.4 in the data, whereas the abundance of massive galaxies in the models increases to redshift zero. Additionally, BOSS data display colour-magnitude (mass) relations similar to those found in the local Universe, where the most massive galaxies are the reddest. On the other hand, the model colours do not display a dependence on stellar mass, span a narrower range and are typically bluer than the observations. We argue that the lack of a colour-mass relation for massive galaxies in the models is mostly due to metallicity, which is too low in the models.

Theoretical predictions for the effect of nebular emission on the broad-band photometry of high-redshift galaxies

By combining optical and near-IR observations from the Hubble Space Telescope with near-IR photometry from the Spitzer Space Telescope, it is possible to measure the rest-frame UV–optical colours of galaxies at z = 4–8. The UV–optical spectral energy distribution of star formation dominated galaxies is the result of several different factors. These include the joint distribution of stellar masses, ages and metallicities (solely responsible for the pure stellar spectral energy distribution), and the subsequent reprocessing by dust and gas in the interstellar medium. Using a large cosmological hydrodynamical simulation (MassiveBlack-II), we investigate the predicted spectral energy distributions of galaxies at high redshift with a particular emphasis on assessing the potential contribution of nebular emission. We find that the average (median) pure stellar UV–optical colour correlates with both luminosity and redshift such that galaxies at lower redshift and higher luminosity are typically redder. Assuming that the escape fraction of ionizing photons is close to zero, the effect of nebular emission is to redden the UV–optical 1500 − Vw colour by, on average, 0.4 mag at z = 8 declining to 0.25 mag at z = 4. Young and low-metallicity stellar populations, which typically have bluer pure stellar UV–optical colours, produce larger ionizing luminosities and are thus more strongly affected by the reddening effects of nebular emission. This causes the distribution of 1500 − Vw colours to narrow and the trends with luminosity and redshift to weaken. The strong effect of nebular emission leaves observed-frame colours critically sensitive to the redshift of the source. For example, increasing the redshift by 0.1 can result in observed-frame colours changing by up to ∼0.6. These predictions reinforce the need to include nebular emission when modelling the spectral energy distributions of galaxies at high redshift and also highlight the difficultly in interpreting the observed colours of individual galaxies without precise redshift information.

Galaxy And Mass Assembly (GAMA): linking star formation histories and stellar mass growth

We present evidence for stochastic star formation histories in low-mass (M* < 10^10 Msun) galaxies from observations within the Galaxy And Mass Assembly (GAMA) survey. For ~73,000 galaxies between 0.05<z<0.32, we calculate star formation rates (SFR) and specific star formation rates (SSFR = SFR/M*) from spectroscopic Halpha measurements and apply dust corrections derived from Balmer decrements. We find a dependence of SSFR on stellar mass, such that SSFRs decrease with increasing stellar mass for star-forming galaxies, and for the full sample, SSFRs decrease as a stronger function of stellar mass. We use simple parametrizations of exponentially declining star formation histories to investigate the dependence on stellar mass of the star formation timescale and the formation redshift. We find that parametrizations previously fit to samples of z~1 galaxies cannot recover the distributions of SSFRs and stellar masses observed in the GAMA sample between 0.05<z<0.32. In particular, a large number of low-mass (M* < 10^10 Msun) galaxies are observed to have much higher SSFRs than can be explained by these simple models over the redshift range of 0.05<z<0.32, even when invoking mass-dependent staged evolution. For such a large number of galaxies to maintain low stellar masses, yet harbour such high SSFRs, requires the late onset of a weak underlying exponentially declining SFH with stochastic bursts of star formation superimposed.

Reaction of massive clusters to gas expulsion – The cluster density dependence

The expulsion of the unconverted gas at the end of the star formation process potentially leads to the expansion of the just formed stellar cluster and membership loss. The degree of expansion and mass loss depends largely on the star formation efficiency and scales with the mass and size of the stellar group as long as stellar interactions can be neglected. We investigate under which circumstances stellar interactions between cluster members become so important that the fraction of bound stars after gas expulsion is significantly altered. The Nbody6 code is used to simulate the cluster dynamics after gas expulsion for different SFEs. Concentrating on the most massive clusters observed in the Milky Way, we test to what extend the results depend on the model, i.e. stellar mass distribution, stellar density profile etc., and the cluster parameters, such as cluster density and size.We find that stellar interactions are responsible for up to 20% mass loss in the most compact massive clusters in the Milky Way, making ejections the prime mass loss process in such systems. Even in the loosely bound OB associations stellar interactions are responsible for at least ~5% mass loss. The main reason why the importance of encounters for massive clusters has been largely overlooked is the often used approach of a single-mass representation instead of a realistic distribution for the stellar masses. The density-dependence of the encounter-induced mass loss is shallower than expected because of the increasing importance of few-body interactions in dense clusters compared to sparse clusters where 2-body encounters dominate.

Keplerobservations of very low-mass stars

Observations of very low-mass stars with Kepler represent an excellent opportunity to search for planetary transits and to characterize optical photometric variability at the cool end of the stellar mass distribution. In this paper, we present low-resolution red optical spectra that allow us to identify 18 very low-mass stars that have Kepler light curves available in the public archive. Spectral types of these targets are found to lie in the range dM4.5--dM8.5, implying spectrophotometric distances from 17 pc to 80 pc. Limits to the presence of transiting planets are placed from modelling of the Kepler light curves. We find that the size of the planets detectable by Kepler around these small stars typically lie in the range 1 to 5 Earth radii within the habitable regions (P$\le$10 days). We identify one candidate transit with a period of 1.26 days that resembles the signal produced by a planet slightly smaller than the Moon. However, our pixel by pixel analysis of the Kepler data shows that the signal most likely arises from a background contaminating eclipsing binary. For 11 of these objects reliable photometric periods shorter than 7 days are derived, and are interpreted as rotational modulation of magnetic cool spots. For 3 objects we find possible photometric periods longer than 50 days that require confirmation. H$_\alpha$ emission measurements and flare rates are used as a proxies for chromospheric activity and transversal velocities are used as an indicator of dynamical ages. These data allow us to discuss the relationship between magnetic activity and detectability of planetary transits around very low-mass stars. We show that Super-Earth planets with sizes around 2 Earth radii are detectable with Kepler around about two thirds of the stars in our sample, independently from their level of chromospheric activity.

Lyman break and ultraviolet-selected galaxies at z ∼ 1 – I. Stellar populations from the ALHAMBRA survey

We take advantage of the exceptional photometric coverage provided by the combination of GALEX data in the UV and the ALHAMBRA survey in the optical and near-IR to analyze the physical properties of a sample of 1225 GALEX-selected Lyman break galaxies (LBGs) at $0.8 \lesssim z \lesssim 1.2$ located in the COSMOS field. This is the largest sample of LBGs studied at that redshift range so far. According to a spectral energy distribution (SED) fitting with synthetic stellar population templates, we find that LBGs at $z \sim 1$ are mostly young galaxies with a median age of 341 Myr and have intermediate dust attenuation, $\ < E_s (B-V) \ > \sim 0.20$. Due to their selection criterion, LBGs at $z \sim 1$ are UV-bright galaxies and have high dust-corrected total SFR, with a median value of 16.9 $M_\odot {\rm yr}^{-1}$. Their median stellar mass is $\log{\left(M_*/M_\odot \right)} = 9.74$. We obtain that the dust-corrected total SFR of LBGs increases with stellar mass and the specific SFR is lower for more massive galaxies. Only 2% of the galaxies selected through the Lyman break criterion have an AGN nature. LBGs at $z \sim 1$ are mostly located over the blue cloud of the color-magnitude diagram of galaxies at their redshift, with only the oldest and/or the dustiest deviating towards the green valley and red sequence. Morphologically, 69% of LBGs are disk-like galaxies, with the fraction of interacting, compact, or irregular systems being much lower, below 12%. LBGs have a median effective radius of 2.5 kpc and bigger galaxies have higher total SFR and stellar mass. Comparing to their high-redshift analogues, we find evidence that LBGs at lower redshifts are bigger, redder in the UV continuum, and have a major presence of older stellar populations in their SEDs. However, we do not find significant difference in the distributions of stellar mass or dust attenuation.

Three-dimensional modeling of ionized gas

The first generation of stars that formed directly from the primordial gas played a crucial role in the early phase of the reionization of the universe. Because of the short lifetimes of these stars the metals produced in their cores were quickly returned to the environment, from which early PopII stars with a different initial mass function and different SEDs were formed, already much earlier than the time at which the universe became completely reionized at a redshift of z~6. Using a state-of-the-art model atmosphere code we calculate realistic SEDs of very massive stars (VMSs) of different metallicities to serve as input for the 3-dimensional multi-frequency radiative transfer code we have developed to simulate the temporal evolution of the ionization of the inhomogeneous interstellar and intergalactic medium, using multiple stellar clusters as sources of ionizing radiation. Our tool handles distributions of numerous radiative sources characterized by high resolution synthetic SEDs, and yields occupation numbers of the required energy levels of the most important elements which are treated in NLTE consistently with the 3d radiative transfer. We further demonstrate that the increasing metallicity of the radiative sources in the transition from PopIII stars to PopII stars has a strong impact on the hardness of the emitted spectrum, and hence on the reionization history of helium. A top-heavy stellar mass distribution characterized by VMSs forming in chemically evolved clusters of high core mass density may not only provide the progenitors of intermediate-mass and supermassive black holes (SMBHs), but also play an important role for the reionization of HeII. The number of VMSs required to reionize HeII by a redshift of z~2.5 is astonishingly close to the number of VMSs required to explain galactic SMBHs if one assumes that these have been formed by mergers of smaller black holes.

DUST-OBSCURED GALAXIES IN THE LOCAL UNIVERSE

We use Wide-field Infrared Survey Explorer (WISE), AKARI, and Galaxy Evolution Explorer (GALEX) data to select local analogs of high-redshift (z~2) dust obscured galaxies (DOGs). We identify 47 local DOGs with S_{12\mu m}/S_{0.22 \mu m}>892 and S_{12\mu m}>20 mJy at 0.05<z<0.08 in the Sloan Digital Sky Survey data release 7. The infrared luminosities of these DOGs are in the range 3.4x10^{10} (L_\odot)<L_{IR}<7.0x10^{11} (L_\odot) with a median L_{IR} of 2.1x10^{11} (L_\odot). We compare the physical properties of local DOGs with a control sample of galaxies that have lower $S_{12\mu m}/S_{0.22 \mu m}$ but have similar redshift, IR luminosity, and stellar mass distributions. Both WISE 12 micron and GALEX near-ultraviolet (NUV) flux densities of DOGs differ from the control sample of galaxies, but the difference is much larger in the NUV. Among the 47 DOGs, 36\pm7% have small axis ratios in the optical (i.e., b/a<0.6), larger than the fraction among the control sample (17\pm3%). There is no obvious sign of interaction for many local DOGs. No local DOGs have companions with comparable optical magnitudes closer than ~50 kpc. The large- and small-scale environments of DOGs are similar to the control sample. Many physical properties of local DOGs are similar to those of high-z DOGs, even though the IR luminosities of local objects are an order of magnitude lower than for the high-z objects: the presence of two classes (active galactic nuclei- and star formation-dominated) of DOGs, abnormal faintness in the UV rather than extreme brightness in the mid-infrared, and diverse optical morphology. These results suggest a common underlying physical origin of local and high-z DOGs. Both seem to represent the high-end tail of the dust obscuration distribution resulting from various physical mechanisms rather than a unique phase of galaxy evolution.

GAMA/H-ATLAS: linking the properties of submm detected and undetected early-type galaxies – I. z ≤ 0.06 sample

We present two large, nearby (0.013$\le$z$\le$0.06) samples of Early-Type Galaxies (ETGs): a visually classified sample of 220 ETGs, created using source-matched data from the Galaxy and Mass Assembly (GAMA) database with FIR/sub-mm detections from $Herschel$-ATLAS; and a visually classified sample of 551 ETGs which are undetected with $Herschel$-ATLAS. Active galactic nuclei (AGN) are removed from our samples using optical emission line diagnostics. These samples are scrutinised to determine characteristics of sub-mm detected versus undetected ETGs. We find similarities in the stellar mass distributions of the two ETG samples but testing other properties uncovers significant differences. The sub-mm detected sample is shown to have lower concentration and S\'ersic indices than those in the undetected sample - a result which may be linked to the presence of dust in the former. Optical and UV-optical colours are also shown to be much bluer, indicating that the dust is linked with recent star formation. The intrinsic effective radii are on average 1.5 times larger for the sub-mm detected ETGs. Surface densities and groups data from the GAMA database are examined for the two samples, leading to the conclusion that dusty ETGs inhabit sparser environments than non-dusty ETGs in the nearby universe, although environments of the brightest ETGs are shown to differ the least. Modified Planck functions are fit to the H-ATLAS detected PACS and SPIRE fluxes for ETGs with sub-mm flux densities of at least 3$\sigma$ in the 350$\mu$m SPIRE band, giving a resultant mean cold dust temperature of T$_{d}$=22.1K, with a range of 9-30K. The corresponding mean dust mass is 1.8$\times10^{7}$M$_{\odot}$, with a range of (0.08-35.0)$\times10^{7}$M$_{\odot}$. The dust masses calculated from these fits, normalised by stellar mass, are shown to increase with decreasing stellar mass and bluer colours.

Spectroastrometry of rotating gas disks for the detection of supermassive black holes in galactic nuclei

We present new CRIRES spectroscopic observations of BrGamma in the nuclear region of the Circinus galaxy, obtained with the aim of measuring the black hole (BH) mass with the spectroastrometric technique. The Circinus galaxy is an ideal benchmark for the spectroastrometric technique given its proximity and secure BH measurement obtained with the observation of its nuclear H2O maser disk. The kinematical data have been analyzed both with the classical method based on the analysis of the rotation curves and with the new method developed by us and based on spectroastrometry. The classical method indicates that the gas disk rotates in the gravitational potential of an extended stellar mass distribution and a spatially unresolved mass of (1.7 +- 0.2) 10^7 Msun, concentrated within r < 7 pc. The new method is capable of probing gas rotation at scales which are a factor ~3.5 smaller than those probed by the rotation curve analysis. The dynamical mass spatially unresolved with the spectroastrometric method is a factor ~2 smaller, 7.9 (+1.4 -1.1) 10^6 Msun indicating that spectroastrometry has been able to spatially resolve the nuclear mass distribution down to 2 pc scales. This unresolved mass is still a factor ~4.5 larger than the BH mass measurement obtained with the H2O maser emission indicating that it has not been possible to resolve the sphere of influence of the BH. Based on literature data, this spatially unresolved dynamical mass distribution is likely dominated by molecular gas and it has been tentatively identified with the circum-nuclear torus which prevents a direct view of the central BH in Circinus. This mass distribution, with a size of ~2pc, is similar in shape to that of the star cluster of the Milky Way suggesting that a molecular torus, forming stars at a high rate, might be the earlier evolutionary stage of the nuclear star clusters which are common in late type spirals.

Galactic archaeology: mapping and dating stellar populations with asteroseismology of red-giant stars

Our understanding of how the Galaxy was formed and evolves is severely hampered by the lack of precise constraints on basic stellar properties such as distances, masses, and ages. Here, we show that solar-like pulsating red giants represent a well-populated class of accurate distance indicators, spanning a large age range, which can be used to map and date the Galactic disc in the regions probed by observations made by the CoRoT and Kepler space telescopes. When combined with photometric constraints, the pulsation spectra of such evolved stars not only reveal their radii, and hence distances, but also provide well-constrained estimates of their masses, which are reliable proxies for the ages of the stars. As a first application we consider red giants observed by CoRoT in two different parts of the Milky Way, and determine precise distances for ~2000 stars spread across nearly 15,000 pc of the Galactic disc, exploring regions which are a long way from the solar neighbourhood. We find significant differences in the mass distributions of these two samples which, by comparison with predictions of synthetic models of the Milky Way, we interpret as mainly due to the vertical gradient in the distribution of stellar masses (hence ages) in the disc. In the future, the availability of spectroscopic constraints for this sample of stars will not only improve the age determination, but also provide crucial constraints on age-velocity and age-metallicity relations at different Galactocentric radii and heights from the plane.

On the evolution of irradiated turbulent clouds: a comparative study between modes of triggered star formation

Here we examine the evolution of irradiated clouds using the Smoothed Particle Hydrodynamics ({\small SPH}) algorithm coupled with a ray-tracing scheme that calculates the position of the ionisation-front at each timestep. We present results from simulations performed for three choices of {\small IR}-flux spanning the range of fluxes emitted by a typical {\small B}-type star to a cluster of {\small OB}-type stars. The extent of photo-ablation, of course, depends on the strength of the incident flux and a strong flux of {\small IR} severely ablates a {\small MC}. Consequently, the first star-formation sites appear in the dense shocked layer along the edges of the irradiated cloud. Radiation-induced turbulence readily generates dense filamentary structure within the photo-ablated cloud although several new star-forming sites also appear in some of the densest regions at the junctions of these filaments. Prevalent physical conditions within a {\small MC} play a crucial role in determining the mode, i.e., filamentary as compared to isolated pockets, of star-formation, the timescale on which stars form and the distribution of stellar masses. The probability density functions ({\small PDF}s) derived for irradiated clouds in this study are intriguing due to their resemblance with those presented in a recent census of irradiated {\small MC}s. Furthermore, irrespective of the nature of turbulence, the protostellar mass-functions({\small MF}s) derived in this study follow a power-law distribution. When turbulence within the cloud is driven by a relatively strong flux of {\small IR} such as that emitted by a massive {\small O}-type star or a cluster of such stars, the {\small MF} approaches the canonical form due to Salpeter, and even turns-over for protostellar masses smaller than $\sim$0.2 M$_{\odot}$.

Systematic variation of the stellar initial mass function with velocity dispersion in early-type galaxies

Abstract An essential component of galaxy formation theory is the stellar initial mass function (IMF) that describes the parent distribution of stellar mass in star-forming regions. We present observational evidence in a sample of early-type galaxies (ETGs) of a tight correlation between central velocity dispersion and the strength of several absorption features sensitive to the presence of low-mass stars. Our sample comprises ∼40 000 ETGs from the Spheroids Panchromatic Investigation in Different Environmental Regions survey (z ≲ 0.1). The data – extracted from the Sloan Digital Sky Survey – are combined, rejecting both noisy data, and spectra with contamination from telluric lines, resulting in a set of 18 stacked spectra at high signal-to-noise ratio (S/N ≳ 400 Å−1). A combined analysis of IMF-sensitive line strengths and spectral fitting is performed with the latest state-of-the-art population synthesis models (an extended version of the MILES models). A significant trend is found between IMF slope and velocity dispersion, towards an excess of low-mass stars in the most massive galaxies. Although we emphasize that accurate values of the IMF slope will require a detailed analysis of chemical composition (such as [α/Fe] or even individual element abundance ratios), the observed trends suggest that low-mass ETGs are better fitted by a Kroupa-like IMF, whereas massive galaxies require bottom-heavy IMFs, exceeding the Salpeter slope at σ ≳ 200 km s−1.

The properties of a large volume-limited sample of face-on low surface brightness disk galaxies

We select a large volume-limited sample of low surface brightness galaxies (LSBGs, 2021) to investigate in detail their statistical properties and their differences from high surface brightness galaxies (HSBGs, 3639). The distributions of stellar masses of LSBGs and HSBGs are nearly the same and they have the same median values. Thus this volume-limited sample has good completeness and is further removed from the effect of stellar masses on their other properties when we compare LSBGs to HSBGs. We found that LSBGs tend to have lower stellar metallicities and lower effective dust attenuations, indicating that they have lower dust than HSBGs. The LSBGs have relatively higher stellar mass-to-light ratios, higher gas fractions, lower star forming rates (SFRs), and lower specific SFRs than HSBGs. Moreover, with the decreasing surface brightness, gas fraction increases, but the SFRs and specific SFRs decrease rapidly for the sample galaxies. This could mean that the star formation histories between LSBGs and HSBGs are different, and HSBGs may have stronger star forming activities than LSBGs.

A weak lensing mass reconstruction of the large-scale filament feeding the massive galaxy cluster MACS J0717.5+3745

We report the first weak lensing detection of a large-scale filament funnelling matter on to the core of the massive galaxy cluster MACS J0717.5+3745. Our analysis is based on a mosaic of 18 multipassband images obtained with the Advanced Camera for Surveys aboard the Hubble Space Telescope, covering an area of ∼10 × 20 arcmin2. We use a weak lensing pipeline developed for the Cosmic Evolution Survey, modified for the analysis of galaxy clusters, to produce a weak lensing catalogue. A mass map is then computed by applying a weak gravitational lensing multiscale reconstruction technique designed to describe irregular mass distributions such as the one investigated here. We test the resulting mass map by comparing the mass distribution inferred for the cluster core with the one derived from strong lensing constraints and find excellent agreement. Our analysis detects the MACS J0717.5+3745 filament within the 3σ detection contour of the lensing mass reconstruction, and underlines the importance of filaments for theoretical and numerical models of the mass distribution in the cosmic web. We measure the filament's projected length as ∼4.5 Mpc, and its mean density as (2.92 ± 0.66) × 108 h74 M⊙ kpc−2. Combined with the redshift distribution of galaxies obtained after an extensive spectroscopic follow-up in the area, we can rule out any projection effect resulting from the chance alignment on the sky of unrelated galaxy group-scale structures. Assuming plausible constraints concerning the structure's geometry based on its galaxy velocity field, we construct a three-dimensional (3D) model of the large-scale filament. Within this framework, we derive the 3D length of the filament to be 18 Mpc. The filament's deprojected density in terms of the critical density of the Universe is measured as (206 ± 46) ρcrit, a value that lies at the very high end of the range predicted by numerical simulations. Finally, we study the distribution of stellar mass in the field of MACS J0717.5+3749 and, adopting a mean mass-to-light ratio 〈M*/LK〉 of 0.73 ± 0.22 and assuming a Chabrier initial mass function, measure a stellar mass fraction along the filament of (0.9 ± 0.2) per cent, consistent with previous measurements in the vicinity of massive clusters.

STAR FORMATION RATE DISTRIBUTIONS: INADEQUACY OF THE SCHECHTER FUNCTION

In this paper we posit that galaxy luminosity functions (LFs) come in two fundamentally different types depending on whether the luminosity traces galaxy stellar mass or its current star formation rate (SFR). Mass function types reflect the older stars and therefore the stellar mass distribution, while SFR function types arise from the young stars and hence the distribution of SFRs. Optical and near-infrared LFs are of the mass function type, and are well fit by a Schechter function (power law with an exponential cutoff at the bright end). In contrast, LFs of the SFR function type are of a different form, one that cannot be adequately described by a Schechter function. We demonstrate this difference by generating SFR distributions for mock samples of galaxies drawn from a Schechter stellar mass distribution along with established empirical relations between the SFR and stellar mass. Compared with the Schechter function, SFR distributions have a shallower decline at the bright end, which can be traced to the large intrinsic scatter of SFRs at any given stellar mass. A superior description of SFR distributions is given by the "Saunders" function, which combines a power law with a Gaussian at the high end. We show that the Schechter-like appearance of UV and H alpha LFs, although they are LFs of SFR function type, results when luminosities are not corrected for dust, or when average statistical corrections are used because individual attenuation measurements are not available. We thus infer that the non-Schechter form of the far-IR LFs is a true reflection of the underlying SFR distribution, rather than the purported artifact of AGN contamination.

Stellar mass map and dark matter distribution in M 31

Stellar mass distribution in M31 is estimated using optical and infrared imaging data. Combining the derived stellar mass model with various kinematical data, properties of the DM halo of the galaxy are constrained. SDSS observations through the ugriz filters and the Spitzer imaging at 3.6 microns are used to sample the SED of the galaxy at each imaging pixel. Intrinsic dust extinction effects are taken into account by using far-infrared observations. Synthetic SEDs created with different stellar population synthesis models are fitted to the observed SEDs, providing estimates for the stellar mass surface density. The stellar mass distribution of the galaxy is described with a 3D model consisting of a nucleus, a bulge, a disc, a young disc and a halo component, each following the Einasto density distribution (relations between different functional forms of the Einasto density distribution are given in App. B). By comparing the stellar mass distribution to the observed rotation curve and kinematics of outer globular clusters and satellite galaxies, the DM halo parameters are estimated. Stellar population synthesis models suggest that M31 is dominated by old stars throughout the galaxy. The total stellar mass is (10-15)10^10Msun, 30% of which is in the bulge and 56% in the disc. None of the tested DM distribution models can be falsified on the basis of the stellar matter distribution and the rotation curve of the galaxy. The virial mass of the DM halo is (0.8-1.1)10^12Msun and the virial radius is 189-213kpc, depending on the DM distribution. The central density of the DM halo is comparable to that of nearby dwarf galaxies, low-surface-brightness galaxies and distant massive disc galaxies, thus the evolution of central DM halo properties seems to be regulated by similar processes for a broad range of halo masses, environments, and cosmological epochs.