Age-velocity Dispersion Relation Research Articles

A unified model for age–velocity dispersion relations in Local Group galaxies: disentangling ISM turbulence and latent dynamical heating

We analyze age-velocity dispersion relations (AVRs) from kinematics of individual stars in eight Local Group galaxies ranging in mass from Carina ($M_{*} \sim 10^{6}$) to M31 ($M_{*} \sim 10^{11}$). Observationally the $\sigma$ vs. stellar age trends can be interpreted as dynamical heating of the stars by GMCs, bars/spiral arms, or merging subhalos; alternatively the stars could have simply been born out of a more turbulent ISM at high redshift and retain that larger velocity dispersion till present day - consistent with recent IFU studies. To ascertain the dominant mechanism and better understand the impact of instabilities and feedback, we develop models based on observed SFHs of these Local Group galaxies in order to create an evolutionary formalism which describes the ISM velocity dispersion due to a galaxy's evolving gas fraction. These empirical models relax the common assumption that the stars are born from gas which has constant velocity dispersion at all redshifts. Using only the observed SFHs as input, the ISM velocity dispersion and a mid-plane scattering model fits the observed AVRs of low mass galaxies without fine tuning. Higher mass galaxies above $M_{vir} > 10^{11}$ need a larger contribution from latent dynamical heating processes (for example minor mergers), in excess of the ISM model. Using the SFHs we also find that supernovae feedback does not appear to be a dominant driver of the gas velocity dispersion compared to gravitational instabilities - at least for dispersions $\sigma \gtrsim 25$ km/s. Together our results point to stars being born with a velocity dispersion close to that of the gas at the time of their formation, with latent dynamical heating operating with a galaxy mass-dependent efficiency. These semi-empirical relations may help constrain the efficiency of feedback and its impact on the physics of disk settling in galaxy formation simulations.

Migration and kinematics in growing disc galaxies with thin and thick discs

We analyse disc heating and radial migration in N-body models of growing disc galaxies with thick and thin discs. Similar to thin-disc-only models, galaxies with appropriate non-axisymmetric structures reproduce observational constraints on radial disc heating in and migration to the Solar Neighbourhood (Snhd). The presence of thick discs can suppress non-axisymmetries and thus higher baryonic-to-dark matter fractions are required than in models that only have a thin disc. Models that are baryon-dominated to roughly the Solar radius R_0 are favoured, in agreement with data for the Milky Way. For inside-out growing discs, today's thick-disc stars at R_0 are dominated by outwards migrators. Whether outwards migrators are vertically hotter than non-migrators depends on the radial gradient of the thick disc vertical velocity dispersion. There is an effective upper boundary in angular momentum that thick disc stars born in the centre of a galaxy can reach by migration, which explains the fading of the high-alpha sequence outside R_0. Our models compare well to Snhd kinematics from RAVE-TGAS. For such comparisons it is important to take into account the azimuthal variation of kinematics at R ~ R_0 and biases from survey selection functions. The vertical heating of thin disc stars by giant molecular clouds is only mildly affected by the presence of thick discs. Our models predict higher vertical velocity dispersions for the oldest stars than found in the Snhd age-velocity dispersion relation, possibly because of measurement uncertainties or an underestimation of the number of old cold stars in our models.

The kinematics of the white dwarf population from the SDSS DR12

We use the Sloan Digital Sky Survey Data Release 12, which is the largest available white dwarf catalog to date, to study the evolution of the kinematical properties of the population of white dwarfs in the Galactic disc. We derive masses, ages, photometric distances and radial velocities for all white dwarfs with hydrogen-rich atmospheres. For those stars for which proper motions from the USNO-B1 catalog are available the true three-dimensional components of the stellar space velocity are obtained. This subset of the original sample comprises 20,247 objects, making it the largest sample of white dwarfs with measured three-dimensional velocities. Furthermore, the volume probed by our sample is large, allowing us to obtain relevant kinematical information. In particular, our sample extends from a Galactocentric radial distance $R_{\rm G}=7.8$~kpc to 9.3~kpc, and vertical distances from the Galactic plane ranging from $Z=-0.5$~kpc to 0.5~kpc. We examine the mean components of the stellar three-dimensional velocities, as well as their dispersions with respect to the Galactocentric and vertical distances. We confirm the existence of a mean Galactocentric radial velocity gradient, $\partial\langle V_{\rm R}\rangle/\partial R_{\rm G}=-3\pm5$~km~s$^{-1}$~kpc$^{-1}$. We also confirm North-South differences in $\langle V_{\rm z}\rangle$. Specifically, we find that white dwarfs with $Z>0$ (in the North Galactic hemisphere) have $\langle V_{\rm z}\rangle<0$, while the reverse is true for white dwarfs with $Z<0$. The age-velocity dispersion relation derived from the present sample indicates that the Galactic population of white dwarfs may have experienced an additional source of heating, which adds to the secular evolution of the Galactic disc.

The kinematics of the white dwarf population from the SDSS DR12

AbstractWe use the Sloan Digital Sky Survey Data Release 12, which is the largest available white dwarf catalog to date, to study the evolution of the kinematical properties of the population of white dwarfs in the Galactic disc. We derive masses, ages, photometric distances and radial velocities for all white dwarfs with hydrogen-rich atmospheres. For those stars for which proper motions from the USNO-B1 catalog are available the true three-dimensional components of the stellar space velocity are obtained. This subset of the original sample comprises 20,247 objects, making it the largest sample of white dwarfs with measured three-dimensional velocities. Furthermore, the volume probed by our sample is large, allowing us to obtain relevant kinematical information. In particular, our sample extends from a Galactocentric radial distance RG = 7.8 kpc to 9.3 kpc, and vertical distances from the Galactic plane ranging from Z = −0.5 kpc to 0.5 kpc. We examine the mean components of the stellar three-dimensional velocities, as well as their dispersions with respect to the Galactocentric and vertical distances. We confirm the existence of a mean Galactocentric radial velocity gradient, ∂〈VR〉/∂RG = −3 ± 5 km s−1 kpc−1. We also confirm North-South differences in 〈Vz〉. Specifically, we find that white dwarfs with Z &gt; 0 (in the North Galactic hemisphere) have 〈Vz〉 &lt; 0, while the reverse is true for white dwarfs with Z &lt; 0. The age-velocity dispersion relation derived from the present sample indicates that the Galactic population of white dwarfs may have experienced an additional source of heating, which adds to the secular evolution of the Galactic disc.

Imprints of zero-age velocity dispersions and dynamical heating on the age–velocity dispersion relation

Abstract Observations of stars in the the solar vicinity show a clear tendency of old stars to have larger velocity dispersions. This relation is called the age–velocity dispersion relation (AVR) and it is believed to provide insight into the heating history of the Milky Way galaxy. Here, in order to investigate the origin of the AVR, we performed smoothed particle hydrodynamic simulations of the self-gravitating multiphase gas disks in the static disk-halo potentials. Star formation from cold and dense gas is taken into account, and we analyze the evolution of these star particles. We find that exponents of simulated AVR and the ratio of the radial to vertical velocity dispersion are close to the observed values. We also find that the simulated AVR is not a simple consequence of dynamical heating. The evolution tracks of stars with different epochs evolve gradually in the age–velocity dispersion plane as a result of: (1) the decrease in velocity dispersion in star-forming regions, and (2) the decrease in the number of cold/dense/gas as scattering sources. These results suggest that the AVR involves not only the heating history of a stellar disk, but also the historical evolution of the ISM in a galaxy.

Age–velocity dispersion relations and heating histories in disc galaxies

We analyse the heating of stellar discs by non axisymmetric structures and giant molecular clouds (GMCs) in N-body simulations of growing disc galaxies. The analysis resolves long-standing discrepancies between models and data by demonstrating the importance of distinguishing between measured age-velocity dispersion relations (AVRs) and the heating histories of the stars that make up the AVR. We fit both AVRs and heating histories with formulae proportional to t^beta and determine the exponents beta_R and beta_z derived from in-plane and vertical AVRs and ~beta_R and ~beta_z from heating histories. Values of beta_z are in almost all simulations larger than values of ~beta_z, whereas values of beta_R are similar to or mildly larger than values of ~beta_R. Moreover, values of beta_z (~beta_z) are generally larger than values of beta_R (~beta_R). The dominant cause of these relations is the decline over the life of the disc in importance of GMCs as heating agents relative to spiral structure and the bar. We examine how age errors and biases in solar neighbourhood surveys influence the measured AVR: they tend to decrease beta values by smearing out ages and thus measured dispersions. We compare AVRs and velocity ellipsoid shapes sigma_z/sigma_R from simulations to Solar neighbourhood data. We conclude that for the expected disc mass and dark halo structure, combined GMC and spiral/bar heating can explain the AVR of the Galactic thin disc. Strong departures of the disc mass or the dark halo structure from expectation spoil fits to the data.

The quiescent phase of galactic disc growth

We perform a series of controlled N-body simulations of growing disc galaxies within non-growing, live dark matter haloes of varying mass and concentration. Our initial conditions include either a low-mass disc or a compact bulge. New stellar particles are continuously added on near-circular orbits to the existing disc, so spiral structure is continuously excited. To study the effect of combined spiral and giant molecular cloud (GMC) heating on the discs we introduce massive, short-lived particles that sample a GMC mass function. An isothermal gas component is introduced for a subset of the models. We perform a resolution study and vary parameters governing the GMC population, the histories of star formation and radial scale growth. Models with GMCs and standard values for the disc mass and halo density provide the right level of self-gravity to explain the age velocity dispersion relation of the Solar neighbourhood (Snhd). GMC heating generates remarkably exponential vertical profiles with scaleheights that are radially constant and agree with observations of galactic thin discs. GMCs are also capable of significantly delaying bar formation. The amount of spiral induced radial migration agrees with what is required for the metallicity distribution of the Snhd. However, in our standard models the outward migrating populations are not hot enough vertically to create thick discs. Thick discs can form in models with high baryon fractions, but the corresponding bars are too long, the young stellar populations too hot and the discs flare considerably.

Vertical disc heating in Milky Way-sized galaxies in a cosmological context

Vertically extended, high velocity dispersion stellar distributions appear to be a ubiquitous feature of disc galaxies, and both internal and external mechanisms have been proposed to be the major driver of their formation. However, it is unclear to what extent each mechanism can generate such a distribution, which is likely to depend on the assembly history of the galaxy. To this end, we perform 16 high resolution cosmological-zoom simulations of Milky Way-sized galaxies using the state-of-the-art cosmological magneto-hydrodynamical code \textlcsc{AREPO}, and analyse the evolution of the vertical kinematics of the stellar disc in connection with various heating mechanisms. We find that the bar is the dominant heating mechanism in most cases, whereas spiral arms, radial migration, and adiabatic heating from mid-plane density growth are all sub-dominant. The strongest source, though less prevalent than bars, originates from external perturbations from satellites/sub-halos of masses log$_{10} (M/\rm M_{\odot}) \gtrsim 10$. However, in many simulations the orbits of newborn star particles become cooler with time, such that they dominate the shape of the age-velocity dispersion relation and overall vertical disc structure unless a strong external perturbation takes place.

Investigation into the Origin of Age-Velocity dispersion Relation by High Resolution Simulation

AbstractWe focused on the stellar age-velocity dispersion relation (AVR). We performed the N-body/SPH simulations to investigate the origin of AVR. As a results, we found that AVR is not consistent with simple stellar heating.

Evidence for temporal evolution in the M33 disc as traced by its star clusters

We present precision radial velocities and stellar population parameters for 77 star clusters in the Local Group galaxy M33. Our GTC and WHT observations sample both young, massive clusters and known/candidate globular clusters, spanning ages � 10 6 10 10 yr, and metallicities, [M/H] � 1.7 to solar. The cluster system exhibits an age-metallicity relation; the youngest clusters are the most metal-rich. When compared to HI data, clusters with [M/H]� 1.0 and younger than � 4 Gyr are clearly identified as a disc population. The clusters show evidence for strong time evolution in the disc radial metallicity gradient (d[M/H]dt / dR = 0.03 dex/kpc/Gyr). The oldest clusters have stronger, more negative gradients than the youngest clusters in M33. The clusters also show a clear age-velocity dispersion relation. The line of sight velocity dispersions of the clusters increases with age similar to Milky Way open clusters and stars. The general shape of the relation is reproduced by disc heating simulations, and the similarity between the relations in M33 and the Milky Way suggests that heating by substructure, and cooling of the ISM both play a role in shaping this relation. We identify 12 “classical” GCs, six of which are newly identified GC candidates. The GCs are more metal-rich than Milky Way halo clusters, and show weak rotation. The inner (R < 4.5 kpc) GCs exhibit a steep radial metallicity gradient (d[M/H]/dR = 0.29 ± 0.11 dex/kpc) and an exponential-like surface density profile. We argue that these inner GCs are thick disc rather than halo objects.

Numerical simulation of a possible origin of the positive radial metallicity gradient of the thick disk

We analyze the radial and vertical metallicity and [α/Fe] gradients of the disk stars of a disk galaxy simulated in a fully cosmological setting with the chemo-dynamical galaxy evolution code GCD+. We study how the radial abundance gradients vary as a function of height above the plane and find that the metallicity ([α/Fe]) gradient becomes more positive (negative) with increasing height, changing sign around 1.5 kpc above the plane. At the largest vertical height (2 < |z| < 3 kpc), our simulated galaxy shows a positive radial metallicity gradient. We find that the positive metallicity gradient is caused by the age-metallicity and age-velocity dispersion relation, where younger stars have higher metallicity and lower velocity dispersion. Due to the age-velocity dispersion relation, a greater fraction of younger stars reaches |z| > 2 kpc at the outer region, because of the lower gravitational restoring force of the disk, i.e. flaring. As a result, the fraction of younger stars with higher metallicity due to the age-metallicity relation becomes higher at the outer radii, which makes the median metallicity higher at the outer radii. Combining this result with the recently observed age-metallicity and age-velocity dispersion relation for the Milky Way thick disk stars suggested by Haywood et al., we argue that the observed (small) positive radial metallicity gradient at large heights of the Milky Way disk stars can be explained by flaring of the younger thick and/or thin disk stars.

Properties of simulated Milky Way-mass galaxies in loose group and field environments

We test the validity of comparing simulated field disk galaxies with the empirical properties of systems situated within environments more comparable to loose groups, including the Milky Way's Local Group. Cosmological simulations of Milky Way-mass galaxies have been realised in two different environment samples: in the field and in environments with similar properties to the Local Group. Apart from the environments of the galaxies, the samples are kept as homogeneous as possible with equivalent ranges in last major merger time, halo mass and halo spin. Comparison of these two samples allow for systematic differences in the simulations to be identified. Metallicity gradients, disk scale lengths, colours, magnitudes and age-velocity dispersion relations are studied for each galaxy in the suite and the strength of the link between these and environment of the galaxies is studied. The bulge-to-disk ratio of the galaxies show that these galaxies are less spheroid dominated than many other simulated galaxies in literature with the majority of both samples being disk dominated. We find that secular evolution and mergers dominate the spread of morphologies and metallicity gradients with no visible differences between the two environment samples. In contrast with this consistency in the two samples there is tentative evidence for a systematic difference in the velocity dispersion-age relations of galaxies in the different environments. Loose group galaxies appear to have more discrete steps in their velocity dispersion-age relations. We conclude that at the current resolution of cosmological galaxy simulations field environment galaxies are sufficiently similar to those in loose groups to be acceptable proxies for comparison with the Milky Way provided that a similar assembly history is considered.

The role of collective effects and secular mass migration on galactic transformation

AbstractDuring the lifetime of a galaxy, secular radial mass redistribution is expected to gradually build up a bulge and transform the Hubble type from late to early. The dominant dynamical process responsible for this transformation is a collective instability mediated by density-wave collisionless shocks (Zhang 1996, 1998, 1999). The ability of this new mechanism to secularly redistribute the STELLAR mass provides a general pathway for the formation and evolution of the majority of Hubble types, ranging from late type disk galaxiess to disky ellipticals. ATLAS3D results (Cappellari et al. 2013) showed that spirals and S0s and disky ellipticals form a continuous trend of evolution which also coincides with the aging of the stellar population of galactic disks. The importance of stellar accretion is also revealed in the results of the COSMOS team which showed that the evolution of the black-hole-mass/bulge-mass correlation since z = 1 was mainly due to the mass redistribution on pre-existing STELLAR disks which were already in place by z = 1 (Cisternas et al. 2011). The weaker correlation between the masses of late-type bulges and AGNs observed at any given epoch in our view is a result of the quicker initial onset of accretion events in AGN disks compared to that in galactic disks, since the dynamical timescale is shorter for smaller AGN accretion disks.The same secular dynamical process can produce and maintain the well-known scaling relations and universal rotation curves of observed galaxies during their Hubble-type transformation (Zhang 2008), as well as reproduce many other observed structural and kinematic properties of galaxies such as the size-line-width relation of the interstellar medium and the age-velocity dispersion relation of solar neighborhood stars in our own Galaxy. A by-product of this analysis is a powerful new method for locating the multiple corotation resonances in galaxies (Zhang &amp; Buta 2007; Buta &amp; Zhang 2009).The current work also highlights the connection between collective effects in galactic dynamics and nonequilibrium phase transition processes in other branches of physics such as fluid turbulence and spontaneous breaking of gauge symmetry in high-energy physics. The continuous build-up of the Hubble sequence of galaxies through secular mass accretion also hints at the baryonic nature of galactic dark matter and poses challenges to the existing LCDM paradigm, since the well-known adiabatic compression process during baryonic mass inflow produced by secular evolution would lead to a concentration of the cold dark matter to the central region of early-type galaxies, which is not observed.

GALAXIA: A CODE TO GENERATE A SYNTHETIC SURVEY OF THE MILKY WAY

We present here a fast code for creating a synthetic survey of the Milky Way. Given one or more color-magnitude bounds, a survey size and geometry, the code returns a catalog of stars in accordance with a given model of the Milky Way. The model can be specified by a set of density distributions or as an N-body realization. We provide fast and efficient algorithms for sampling both types of models. As compared to earlier sampling schemes which generate stars at specified locations along a line of sight, our scheme can generate a continuous and smooth distribution of stars over any given volume. The code is quite general and flexible and can accept input in the form of a star formation rate, age metallicity relation, age velocity dispersion relation and analytic density distribution functions. Theoretical isochrones are then used to generate a catalog of stars and support is available for a wide range of photometric bands. As a concrete example we implement the Besancon Milky Way model for the disc. For the stellar halo we employ the simulated stellar halo N-body models of Bullock & Johnston (2005). In order to sample N-body models, we present a scheme that disperses the stars spawned by an N-body particle, in such a way that the phase space density of the spawned stars is consistent with that of the N-body particles. The code is ideally suited to generating synthetic data sets that mimic near future wide area surveys such as GAIA, LSST and HERMES. As an application we study the prospect of identifying structures in the stellar halo with a simulated GAIA survey. We plan to make the code publicly available at http://galaxia.sourceforge.net.

Towards a fully consistent Milky Way disc model - I. The local model based on kinematic and photometric data

We present a fully consistent evolutionary disc model of the solar cylinder. The model is based on a sequence of stellar sub-populations described by the star formation history (SFR) and the dynamical heating law (given by the age-velocity dispersion relation AVR). The combination of kinematic data from Hipparcos and the finite lifetimes of main sequence (MS) stars enables us to determine the detailed vertical disc structure independent of individual stellar ages and only weakly dependent on the IMF. The disc parameters are determined by applying a sophisticated best fit algorithm to the MS star velocity distribution functions in magnitude bins. We find that the AVR is well constrained by the local kinematics, whereas for the SFR the allowed range is larger. A simple chemical enrichment model is included in order to fit the local metallicity distribution of G dwarfs. In our favoured model A the power law index of the AVR is 0.375 with a minimum and maximum velocity dispersion of 5.1 km/s and 25.0 km/s, respectively. The SFR shows a maximum 10 Gyr ago and declines by a factor of four to the present day value of 1.5 M_sun/pc^2/Gyr. A best fit of the IMF leads to power-law indices of -1.46 below and -4.16 above 1.72 M_sun avoiding a kink at 1 M_sun. An isothermal thick disc component with local density of ~6% of the stellar density is included. A thick disc containing more than 10% of local stellar mass is inconsistent with the local kinematics of K and M dwarfs.

Revisiting the relations: Galactic thin disc age–velocity dispersion relation

The velocity dispersion of stars in the solar neighbourhood thin disc increases with time after star formation. NordstrA¶m et al. performed the most recent observations to constrain the age–velocity dispersion relation. They fitted the age–velocity dispersion relations of each Galactic cardinal direction space velocity component, U (towards the Galactic Centre), V (in the direction of Galactic rotation) and W (towards the North Galactic Pole), with power laws and interpreted these as evidence for continuous heating of the disc in all directions throughout its lifetime. We revisit these relations with their data and use the results of Famaey et al. to show that structure in the local velocity distribution function distorts the in-plane (U and V) velocity distributions away from Gaussian so that a dispersion is not an adequate parametrization of their functions. The age'? W relation can however be constrained because the sample is well phase-mixed vertically. We do not find any local signature of the stellar warp in the Galactic disc. Vertical disc heating does not saturate at an early stage. Our new result is that a power law is not required by the data: disc heating models that saturate after ?4.5 Gyr are equally consistent with observations.

The stellar velocity distribution in the solar neighbourhood

We explore the heating of the velocity distribution in the solar neighbourhood by stochastic spiral waves. Our investigation is based on direct numerical integration of initially circular test-particle orbits in the sheared sheet. We confirm the conclusion of other investigators that heating by spiral structure can explain the principal features of the age-velocity dispersion relation and other parameters of the velocity distribution in the solar neighbourhood. In addition, we find that heating by strong transient spirals can naturally explain the presence of small-scale structure in the velocity distribution (``moving groups''). Heating by spiral structure also explains why the stars in a single velocity-space moving group have a wide range of ages, a result which is difficult to understand in the traditional model that these structures result from inhomogeneous star formation. Thus we suggest that old moving groups arise from irregularities in the Galactic potential rather than irregularities in the star-formation rate.

Secular Evolution of Spiral Galaxies. III. The Hubble Sequence as a Temporal Evolution Sequence

In the first two papers in this series, we have presented a dynamical mechanism by which an unstable spiral mode in a galactic disk can remain quasi-stationary on the order of a Hubble time at the expense of a continuous dissipative basic state evolution, manifesting as the secular redistribution of disk matter, as well as the secular heating of disk stars. In the current paper, we consider in detail the astrophysical consequences of this spiral-induced basic state evolution process and show that it could explain, among a spectrum of observational facts, the variation of the disk surface-density profiles across the Hubble types, as well as the well-known age-velocity dispersion relation of the solar-neighborhood stars. We seek to establish that the Hubble sequence for the classification of galaxies, when viewed in the reverse direction, is essentially a temporal evolution sequence. The spiral-induced secular mass redistribution process could lead to the evolution of the Hubble type of a spiral galaxy from late (Sd and Sc) to early (Sb and Sa). Furthermore, since similar radial mass-accretion processes could also be mediated by central stellar bars, or other central nonaxisymmetric structures, this leads to the continued galaxy evolution across Hubble type S0 and into disky elliptical galaxies. The secular evolution mechanism presented in this series of papers, together with other previously proposed evolution mechanisms, provides a framework for understanding the results of Hubble Space Telescope Deep Field and Medium Deep Survey, which showed that there is significant evolution of the number statistics of the Hubble types of galaxies with redshift, consistent with an evolution along the Hubble sequence from late to early. It also provides possible solutions to several long-standing problems in the theory and observation of galaxy formation and evolution, among them the cause of the steepening of the galaxy morphology-environment relation; the similarity between the Tully-Fisher and Faber-Jackson relations for spiral and elliptical galaxies, respectively; the existence of a fundamental-plane relation for elliptical galaxies; and the mass-angular momentum anticorrelation observed along the Hubble sequence.

Different Laws of Increasing of Stellar Dispersion: Observational Flaws or Natural Property of Stellar Population?

Observations show that in the solar neighborhood the velocity dispersions of disk stars increase with their age. In this work we present the results of a critical analysis of the existing interpretations of the data, as well as of previous theoretical explanations of the heating phenomenon. It is shown that different relaxation mechanisms based on star-cloud collisions can result in a wide set of age–velocity dispersion relations (AVDR). Thus the observed differing power laws of the heating of the stellar component can be a consequence of the different relaxation mechanisms.

The Vertical Age Distribution of Stellar Disk Populations in Edge-on Galaxies

The scale height of the disk stars in our Galaxy increases continously with age, which is also seen in the age-velocity dispersion relation of the solar vicinity. In external galaxies a similar evolution of stellar disks can be deduced from the colour distribution of the light in edge-on galaxies. Using physical disk models and stellar population synthesis we compute the luminosity and colour profiles in optical and infrared bands. For diffusion coefficient and SFR constant in time the vertical density profile is exponential. The colour profiles are mainly affected by dust extinction, and the age and metallicity distribution of the stars. The comparison of a basic model with the observed profiles of IC 2531 shows, that these influences can be disentangled by analyzing the systmatic deviations in the colour profiles.