Showing papers by "Nate Bastian published in 2008"

On the star formation rate – brightest cluster relation: estimating the peak star formation rate in post-merger galaxies

Abstract: We further the recent discussion on the relation between the star formation rate (SFR) of a galaxy and the luminosity of its brightest star cluster (SFR versus M V brightest ). We first show that the observed trend of SFR versus M V brightest is due to the brightest cluster in a galaxy being preferentially young (≤15 Myr - for a constant SFR) and hence a good tracer of the current SFR, although we give notable exceptions to this rule. Archival Hubble Space Telescope (HST) imaging of high-SFR galaxies, as well as additional galaxies/clusters from the literature, is used to further confirm the observed trend. Using a series of Monte Carlo simulations, we show that a pure power-law mass function with index a = 2 is ruled out by the current data. Instead, we find that a Schechter function (i.e. a power law with an exponential truncation at the high-mass end) provides an excellent fit to the data. Additionally, these simulations show that bound cluster formation (in M ⊙ yr -1 ) represents only ∼8±3 per cent of the total star formation within a galaxy, independent of the SFR. From this, we conclude that there is only a single mode of cluster formation which operates over at least 6 orders of magnitude in the SFR. We provide a simple model of star/cluster formation feedback within dwarf galaxies (and star-forming complexes within spirals) which highlights the strong impact that a massive cluster can have on its surroundings. Using this relation, we can extrapolate backwards in time in order to estimate the peak SFR of major merger galaxies, such as NGC 7252,1316 and 3610. The derived SFRs for these galaxies are between a few hundred and a few thousand solar masses per year. The inferred far-infrared luminosity of the galaxies, from the extrapolated SFR, places them well within the range of ultraluminous infrared galaxies (ULIRGs) and for NGC 7252 within the hyperluminous infrared galaxy (HLIRG) regime. Thus, we provide evidence that these post-merger galaxies passed through a ULIRG/HLIRG phase and are now evolving passively. Using the current and extrapolated past SFR of NGC 34, we infer that the ULIRG phase of this galaxy has lasted for at least 150 Myr.

On the Star Formation Rate - Brightest Cluster Relation: Estimating the peak SFR in post-merger galaxies

Abstract: We further the recent discussion on the relation between the star-formation rate (SFR) of a galaxy and the luminosity of its brightest star-cluster (SFR vs. M$_{V}^{brightest}$). We first show that the observed trend between SFR vs. M$_{V}^{brightest}$ is due to the brightest cluster in a galaxy being preferentially young (<15 Myr - for a constant SFR) and hence a good tracer of the current SFR, although we give notable exceptions to this rule. Using a series of Monte Carlo simulations we show that a pure power-law mass function with index, alpha=2, is ruled out by the current data. Instead we find that a Schechter function (i.e. a power-law with an exponential truncation at the high mass end) provides an excellent fit to the data. Additionally, these simulations show that bound cluster formation (in Msun/yr) represents only ~8+-3% of the total star-formation within a galaxy, independent of the star-formation rate. From this we conclude that there is only a single mode of cluster formation which operates over at least six orders of magnitude in the SFR. Using this relation, we can extrapolate backwards in time in order to estimate the peak SFR of major merger galaxies, such as NGC7252, NGC1316 and NGC3610. The derived SFRs for these galaxies are between a few hundred and a few thousand solar masses per year. The inferred far infrared luminosity of the galaxies, from the extrapolated SFR, places them well within the range of Ultra-luminous galaxies (ULIRGs) and for NGC 7252 within the Hyper-luminous infrared galaxy regime. Thus, we provide evidence that these post merger galaxies passed through a ULIRG/HLIRG phase and are now evolving passively. Using the current and extrapolated past SFR of NGC 34, we infer that the ULIRG phase of this galaxy has lasted for at least 150 Myr.

The Spatial Evolution of Stellar Structures in the LMC

Abstract: We present an analysis of the spatial distribution of various stellar populations within the Large Magellanic Cloud. We combine mid-infrared selected young stellar objects, optically selected samples with mean ages between ~9 and ~1000 Myr, and existing stellar cluster catalogues to investigate how stellar structures form and evolve within the LMC. For the analysis we use Fractured Minimum Spanning Trees, the statistical Q parameter, and the two-point correlation function. Restricting our analysis to young massive (OB) stars we confirm our results obtained for M33, namely that the luminosity function of the groups is well described by a power-law with index -2, and that there is no characteristic length-scale of star-forming regions. We find that stars in the LMC are born with a large amount of substructure, consistent with a 2D fractal distribution with dimension ~1.8 and evolve towards a uniform distribution on a timescale of ~175 Myr. This is comparable to the crossing time of the galaxy and we suggest that stellar structure, regardless of spatial scale, will be eliminated in a crossing time. This may explain the smooth distribution of stars in massive/dense young clusters in the Galaxy, while other, less massive, clusters still display large amounts of structure at similar ages. By comparing the stellar and star cluster distributions and evolving timescales, we show that infant mortality of clusters (or 'popping clusters') have a negligible influence on galactic structure. Finally, we quantify the influence of the elongation, differential extinction, and contamination of a population on the measured Q value.

The Rotating Nuclear Star Cluster in NGC 4244

Abstract: We present observations of the nuclear star cluster in the nearby edge-on spiral galaxy NGC 4244 using the Gemini Near-Infrared Integral Field Spectrograph (NIFS) with laser guide star adaptive optics. From a previous study of edge-on galaxies, this nuclear star cluster was found to be one of a sample of clusters that appear flattened along the plane of their host galaxies disks. Such clusters show evidence for multiple morphological components, with younger/bluer disk components and older/redder spheroidal components. Our new observations of NGC 4244 show clear rotation of 30 km s{sup -1} within the central 10 pc (0.5'') of the cluster. The central velocity dispersion is found to be 28 {+-} 2 km s{sup -1}. The multiple stellar populations inferred from the optical colors and spectra are seen as variations in the CO line strength in the NIFS spectra. The rotation is clearly detected even in the older, more spheroidal stellar component. We discuss evidence for similar structures and kinematics in the nuclear star clusters of other galaxies including M33 and the Milky Way. Our observations support two possible formation mechanisms: (1) episodic accretion of gas from the disk directly onto the nuclear star cluster; or (2) episodic accretion ofmore » young star clusters formed in the central part of the galaxy due to dynamical friction.« less

The early expansion of cluster cores

Abstract: The observed properties of young star clusters, such as the core radius and luminosity profile, change rapidly during the early evolution of the clusters. Here we present observations of 6 young clusters in M51 where we derive their sizes using HST imaging and ages using deep Gemini-North spectroscopy. We find evidence for a rapid expansion of the cluster cores during the first 20 Myr of their evolution. We confirm this trend by including data from the literature of both Galactic and extra-galactic embedded and young clusters, and possible mechanisms (rapid gas removal, stellar evolutionary mass-loss, and internal dynamical heating) are discussed. We explore the implications of this result, focussing on the fact that clusters were more concentrated in the past, implying that their stellar densities were much higher and relaxation times correspondingly shorter. Thus, when estimating if a particular cluster is dynamically relaxed, (i.e. when determining if a cluster's mass segregation is due to primordial or dynamical processes), the current relaxation time is only an upper-limit, with the relaxation time likely being significantly shorter in the past.

An alternative method to study star cluster disruption

Abstract: Many embedded star clusters do not evolve into long-lived bound clusters. The most popular explanation for this "infant mortality" of young clusters is the expulsion of natal gas by stellar winds and supernovae, which leaves up to 90% of them unbound. A cluster disruption model has recently been proposed in which this mass- independent disruption of clusters proceeds for another Gyr after gas expulsion. In this scenario, the survival chances of massive clusters are much smaller than in the traditional mass-dependent disruption models. The most common way to study cluster disruption is to use the cluster age distribution, which, however, can be heavily affected by incompleteness. To avoid this, we introduce a new method, based on size-of-sample effects, namely the relation between the most massive cluster, M_max, and the age range sampled. Assuming that clusters are sampled from a power-law initial mass function, with index -2 and that the cluster formation rate is constant, M_max scales with the age range sampled, such that the slope in a log(M_max) vs. log(age) plot is equal to unity. This slope decreases if mass-independent disruption is included. For 90% mass-independent cluster disruption per age dex, the predicted slope is zero. For the solar neighbourhood, SMC, LMC, M33, and M83, based on ages and masses taken from the literature, we find slopes consistent with the expected size-of-sample correlations for the first 100 Myr, hence ruling out the 90% mass-independent cluster disruption scenario. For M51, however, the increase of log(M_max) with log(age) is slightly shallower and for the Antennae galaxies it is flat. This simple method shows that the formation and/or disruption of clusters in the Antennae must have been very different from that of the other galaxies studied here, so it should not be taken as a representative case.

Evolution of stellar structure in the Small Magellanic Cloud

Abstract: The projected distribution of stars in the Small Magellanic Cloud (SMC) from the Magellanic Clouds Photometric Survey is analysed. Stars of different ages are selected via criteria based on V magnitude and V-I colour, and the degree of `grouping' as a function of age is studied. We quantify the degree of structure using the two-point correlation function and a method based on the Minimum Spanning Tree and find that the overall structure of the SMC is evolving from a high degree of sub-structure at young ages (~10 Myr) to a smooth radial density profile. This transition is gradual and at ~75 Myr the distribution is statistically indistinguishable from the background SMC distribution. This time-scale corresponds to approximately the dynamical crossing time of stars in the SMC. The spatial positions of the star clusters in the SMC show a similar evolution of spatial distribution with age. Our analysis suggests that stars form with a high degree of (fractal) sub-structure, probably imprinted by the turbulent nature of the gas from which they form, which is erased by random motions in the galactic potential on a time-scale of a galactic crossing time.

Gemini spectroscopy and hst imaging of the stellar cluster population in region b of m82

Abstract: We present new spectroscopic observations of the stellar cluster population of region B in the prototype starburst galaxy M82 obtained with the Gemini North 8.1 m telescope. By coupling the spectroscopy with UBVI photometry acquired with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST), we derive ages, extinctions, and radial velocities for seven young massive clusters (YMCs) in region B. We find the clusters to have ages between 80 and 200 Myr and velocities in the range 230-350 km s−1, while the extinctions AV vary between ~1 and 2.5 mag. We also find evidence of differential extinction across the faces of some clusters, which hinders the photometric determination of ages and extinctions in these cases. The cluster radial velocities indicate that the clusters are located at different depths within the disk and are on regular disk orbits. Our results overall contradict the findings of previous studies, in which region B was thought to be a bound region populated by intermediate-age clusters that formed in an independent, offset starburst episode that commenced 600 Myr-1 Gyr ago. Our findings instead suggest that region B is optically bright because of low-extinction patches, and that this allows us to view the cluster population of the inner M82 disk, which probably formed as a result of the last encounter with M81. This study forms part of a series of papers whose aim is to study the cluster population of M82 using deep optical spectroscopy and multiband photometry.

An alternative method to study star cluster disruption

Abstract: Many embedded star clusters do not evolve into long-lived bound clusters. The most popular explanation for this "infant mortality" of young clusters is the expulsion of natal gas by stellar winds and supernovae, which leaves up to 90% of them unbound. A cluster disruption model has recently been proposed in which this mass- independent disruption of clusters proceeds for another Gyr after gas expulsion. In this scenario, the survival chances of massive clusters are much smaller than in the traditional mass-dependent disruption models. The most common way to study cluster disruption is to use the cluster age distribution, which, however, can be heavily affected by incompleteness. To avoid this, we introduce a new method, based on size-of-sample effects, namely the relation between the most massive cluster, M_max, and the age range sampled. Assuming that clusters are sampled from a power-law initial mass function, with index -2 and that the cluster formation rate is constant, M_max scales with the age range sampled, such that the slope in a log(M_max) vs. log(age) plot is equal to unity. This slope decreases if mass-independent disruption is included. For 90% mass-independent cluster disruption per age dex, the predicted slope is zero. For the solar neighbourhood, SMC, LMC, M33, and M83, based on ages and masses taken from the literature, we find slopes consistent with the expected size-of-sample correlations for the first 100 Myr, hence ruling out the 90% mass-independent cluster disruption scenario. For M51, however, the increase of log(M_max) with log(age) is slightly shallower and for the Antennae galaxies it is flat. This simple method shows that the formation and/or disruption of clusters in the Antennae must have been very different from that of the other galaxies studied here, so it should not be taken as a representative case.

The spatial evolution of stellar structures in the LMC/SMC

Abstract: We present an analysis of the spatial distribution of various stellar populations within the Large and Small Magellanic Clouds. We use optically selected stellar samples with mean ages between ~9 and ~1000 Myr, and existing stellar cluster catalogues to investigate how stellar structures form and evolve within the LMC/SMC. We use two statistical techniques to study the evolution of structure within these galaxies, the $Q$-parameter and the two-point correlation function (TPCF). In both galaxies we find the stars are born with a high degree of substructure (i.e. are highly fractal) and that the stellar distribution approaches that of the 'background' population on timescales similar to the crossing times of the galaxy (~80/150 Myr for the SMC/LMC respectively). By comparing our observations to simple models of structural evolution we find that 'popping star clusters' do not significantly influence structural evolution in these galaxies. Instead we argue that general galactic dynamics are the main drivers, and that substructure will be erased in approximately the crossing time, regardless of spatial scale, from small clusters to whole galaxies. This can explain why many young Galactic clusters have high degrees of substructure, while others are smooth and centrally concentrated. We conclude with a general discussion on cluster 'infant mortality', in an attempt to clarify the time/spatial scales involved.

The Rotating Nuclear Star Cluster in NGC 4244

Abstract: We present observations of the nuclear star cluster in the nearby edge-on spiral galaxy NGC 4244 using the Gemini Near-Infrared Integral Field Spectrograph (NIFS) with laser guide star adaptive optics. From a previous study of edge-on galaxies, this nuclear star cluster was found to be one of a sample of clusters that appear flattened along the plane of their host galaxies disks. Such clusters show evidence for multiple morphological components, with younger/bluer disk components and older/redder spheroidal components. Our new observations of NGC 4244 show clear rotation of 30 km/sec within the central 10 pc (0.5") of the cluster. The central velocity dispersion is found to be 28+/-2 km/sec. The multiple stellar populations inferred from the optical colors and spectra are seen as variations in the CO line strength in the NIFS spectra. The rotation is clearly detected even in the older, more spheroidal stellar component. We discuss evidence for similar structures and kinematics in the nuclear star clusters of other galaxies including M33 and the Milky Way. Our observations support two possible formation mechanisms: (1) episodic accretion of gas from the disk directly onto the nuclear star cluster, or (2) episodic accretion of young star clusters formed in the central part of the galaxy due to dynamical friction.

The Early Expansion of Cluster Cores

Abstract: The observed properties of young star clusters, such as the core radius and luminosity profile, change rapidly during the early evolution of the clusters. Here we present observations of 6 young clusters in M51 where we derive their sizes using HST imaging and ages using deep Gemini-North spectroscopy. We find evidence for a rapid expansion of the cluster cores during the first 20 Myr of their evolution. We confirm this trend by including data from the literature of both Galactic and extra-galactic embedded and young clusters, and possible mechanisms (rapid gas removal, stellar evolutionary mass-loss, and internal dynamical heating) are discussed. We explore the implications of this result, focussing on the fact that clusters were more concentrated in the past, implying that their stellar densities were much higher and relaxation times correspondingly shorter. Thus, when estimating if a particular cluster is dynamically relaxed, (i.e. when determining if a cluster's mass segregation is due to primordial or dynamical processes), the current relaxation time is only an upper-limit, with the relaxation time likely being significantly shorter in the past.

Evolution of stellar structure in the Small Magellanic Cloud

Abstract: The projected distribution of stars in the Small Magellanic Cloud (SMC) from the Magellanic Clouds Photometric Survey is analysed. Stars of different ages are selected via criteria based on V magnitude and V-I colour, and the degree of `grouping' as a function of age is studied. We quantify the degree of structure using the two-point correlation function and a method based on the Minimum Spanning Tree and find that the overall structure of the SMC is evolving from a high degree of sub-structure at young ages (~10 Myr) to a smooth radial density profile. This transition is gradual and at ~75 Myr the distribution is statistically indistinguishable from the background SMC distribution. This time-scale corresponds to approximately the dynamical crossing time of stars in the SMC. The spatial positions of the star clusters in the SMC show a similar evolution of spatial distribution with age. Our analysis suggests that stars form with a high degree of (fractal) sub-structure, probably imprinted by the turbulent nature of the gas from which they form, which is erased by random motions in the galactic potential on a time-scale of a galactic crossing time.

A Multiwavelength View of Star Formation in Interacting Galaxies in the Pavo Group

Abstract: We combine Spitzer IRAC mid-infrared (MIR) and Chandra X-ray observations of the dominant galaxies NGC6872 and NGC6876 in the Pavo group with archival optical and HI data to study interaction-induced star formation. In spiral galaxy NGC6872, 8.0 and 5.8 micron nonstellar emission having colors consistent with polycyclic aromatic hydrocarbons (PAHs) is concentrated in clumps in three regions: in a 5 kpc radius outer ring about the center of the spiral galaxy, in a bridge of emission connecting NGC6872's northern spiral arm to IC4970, and along the full extent of NGC6872's tidal arms. PAH emission is correlated with young star clusters and dense HI regions. We find no strong differences in the MIR colors of star-forming regions in the spiral galaxy NGC6872 as a function of position relative to the tidally interacting companion galaxy IC4970. We find 11 very luminous X-ray sources (>~ (0.5 - 5) x 10^{39} ergs/s) clustered to the southwest in NGC6872, near bright star-forming regions. In NGC6872's tidal features, young star clusters form at the boundaries of diffuse X-ray gas, suggesting that stars form as gas stripped by the interactions cools. The nucleus of NGC6872 is a weak X-ray point source (0.5-8 keV luminosity of 8.5 x 10^{39} ergs/s), but there is little evidence in the inner 1 kpc of NGC6872 for PAH emission from recent star formation or nuclear activity. However, a 4 kpc `stream', leading from the outer ring of NGC6872 to the nucleus, may signal transport of interstellar matter into NGC6872's nuclear region. Nonstellar emission, consistent with PAH emission, is also found in the central region of elliptical galaxy NGC6877, companion to dominant Pavo group elliptical NGC6876. However, in the central region of NGC6876, the dust emission is more likely due to silicate emission from old AGB stars.