arXiv subjects

Bradley C. Whitmore

Publications and source records attributed to Bradley C. Whitmore.

At least 91 records · Page 5Linked to original sources

arXiv2011

Luminous Stars in Galaxies Beyond 3 Mpc

I am mainly interested in the formation and destruction of young star clusters in nearby star forming galaxies such as the Antennae, M83, and M51. One of the first analysis steps is to throw out all those pesky stars that keep contaminating my young cluster samples. Recently, spurred on by our new WFC3 Early Release Science data of galaxies including M83, NGC 4214, M82, NGC 2841, and Cen A, we began taking a closer look at the stellar component. Questions we are addressing are: 1) what are the most luminous stars, 2) how can we use them to help study the destruction of star clusters and the population of the field, 3) what fraction of stars, at least the bright stars, are formed in the field, in associations, and in compact clusters. In this contribution we describe some of the beginning steps in this process. More specifically, we describe how we separate stars from clusters in our galaxies, and describe how candidate Luminous Blue Variables (LBVs) and "Single Star" HII (SSHII) regions have been identified.

astro-ph.GA↗

arXiv2011

New Constraints on Mass-Dependent Disruption of Star Clusters in M51

We use UBVI,Ha images of the Whirlpool galaxy, M51, taken with the ACS and WFPC2 cameras on the Hubble Space Telescope (HST) to select star clusters, and to estimate their masses and ages by comparing their observed colors with predictions from population synthesis models. We construct the mass function of intermediate age (1-4x10^8 yr) clusters, and find that it is well described by a power law, psi(M) propto M^beta, with beta=-2.1 +/- 0.2, for clusters more massive than approximately 6x10^3 Msun. This extends the mass function of intermediate age clusters in M51 to masses lower by nearly a factor of five over previous determinations. The mass function does not show evidence for curvature at either the high or low mass end. This shape indicates that there is no evidence for the earlier disruption of lower mass clusters compared with their higher mass counterparts (i.e., no mass-dependent disruption) over the observed range of masses and ages, or for a physical upper mass limit Mc with which clusters in M51 can form. These conclusions differ from previous suggestions based on poorer-quality HST observations. We discuss their implications for the formation and disruption of the clusters. Ages of clusters in two "feathers," stellar features extending from the outer portion of a spiral arm, show that the feather with a larger pitch angle formed earlier, and over a longer period, than the other.

astro-ph.GA↗

arXiv2010

Anatomy of a post-starburst minor merger: a multi-wavelength WFC3 study of NGC 4150

(Abridged) We present a spatially-resolved near-UV/optical study of NGC 4150, using the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope. Previous studies of this early-type galaxy (ETG) indicate that it has a large reservoir of molecular gas, exhibits a kinematically decoupled core (likely indication of recent merging) and strong, central H_B absorption (indicative of young stars). The core of NGC 4150 shows ubiquitous near-UV emission and remarkable dusty substructure. Our analysis shows this galaxy to lie in the near-UV green valley, and its pixel-by-pixel photometry exhibits a narrow range of near-UV/optical colours that are similar to those of nearby E+A (post-starburst) galaxies. We parametrise the properties of the recent star formation (age, mass fraction, metallicity and internal dust content) in the NGC 4150 pixels by comparing the observed near-UV/optical photometry to stellar models. The typical age of the recent star formation (RSF) is around 0.9 Gyrs, consistent with the similarity of the near-UV colours to post-starburst systems, while the morphological structure of the young component supports the proposed merger scenario. The RSF metallicity, representative of the metallicity of the gas fuelling star formation, is around 0.3 - 0.5 Zsun. Assuming that this galaxy is a merger and that the gas is sourced mainly from the infalling companion, these metallicities plausibly indicate the gas-phase metallicity (GPM) of the accreted satellite. Comparison to the local mass-GPM relation suggests (crudely) that the mass of the accreted system is around 3x10^8 Msun, making NGC 4150 a 1:20 minor merger. A summation of the pixel RSF mass fractions indicates that the RSF contributes about 2-3 percent of the stellar mass. This work reaffirms our hypothesis that minor mergers play a significant role in the evolution of ETGs at late epochs.

astro-ph.CO↗

arXiv2010

Hubble Space Telescope Imaging of Lyman Alpha Emission at z=4.4

We present the highest redshift detections of resolved Lyman alpha emission, using Hubble Space Telescope/ACS F658N narrowband-imaging data taken in parallel with the Wide Field Camera 3 Early Release Science program in the GOODS CDF-S. We detect Lyman alpha emission from three spectroscopically confirmed z = 4.4 Lyman alpha emitting galaxies (LAEs), more than doubling the sample of LAEs with resolved Lyman alpha emission. Comparing the light distribution between the rest-frame ultraviolet continuum and narrowband images, we investigate the escape of Lyman alpha photons at high redshift. While our data do not support a positional offset between the Lyman alpha and rest-frame ultraviolet (UV) continuum emission, the half-light radii in two out of the three galaxies are significantly larger in Lyman alpha than in the rest-frame UV continuum. This result is confirmed when comparing object sizes in a stack of all objects in both bands. Additionally, the narrowband flux detected with HST is significantly less than observed in similar filters from the ground. These results together imply that the Lyman alpha emission is not strictly confined to its indigenous star-forming regions. Rather, the Lyman alpha emission is more extended, with the missing HST flux likely existing in a diffuse outer halo. This suggests that the radiative transfer of Lyman alpha photons in high-redshift LAEs is complicated, with the interstellar-medium geometry and/or outflows playing a significant role in galaxies at these redshifts.

astro-ph.CO↗

arXiv2010

The Luminosity, Mass, and Age Distributions of Compact Star Clusters in M83 Based on HST/WFC3 Observations

The newly installed Wide Field Camera 3 (WFC3) on the Hubble Space Telescope has been used to obtain multi-band images of the nearby spiral galaxy M83. These new observations are the deepest and highest resolution images ever taken of a grand-design spiral, particularly in the near ultraviolet, and allow us to better differentiate compact star clusters from individual stars and to measure the luminosities of even faint clusters in the U band. We find that the luminosity function for clusters outside of the very crowded starburst nucleus can be approximated by a power law, dN/dL \propto L^{alpha}, with alpha = -2.04 +/- 0.08, down to M_V ~ -5.5. We test the sensitivity of the luminosity function to different selection techniques, filters, binning, and aperture correction determinations, and find that none of these contribute significantly to uncertainties in alpha. We estimate ages and masses for the clusters by comparing their measured UBVI,Halpha colors with predictions from single stellar population models. The age distribution of the clusters can be approximated by a power-law, dN/dt propto t^{gamma}, with gamma=-0.9 +/- 0.2, for M > few x 10^3 Msun and t < 4x10^8 yr. This indicates that clusters are disrupted quickly, with ~80-90% disrupted each decade in age over this time. The mass function of clusters over the same M-t range is a power law, dN/dM propto M^{beta}, with beta=-1.94 +/- 0.16, and does not have bends or show curvature at either high or low masses. Therefore, we do not find evidence for a physical upper mass limit, M_C, or for the earlier disruption of lower mass clusters when compared with higher mass clusters, i.e. mass-dependent disruption. We briefly discuss these implications for the formation and disruption of the clusters.

astro-ph.GA↗

arXiv2010

Progressive star formation in the young galactic super star cluster NGC 3603

Early release science observations of the cluster NGC3603 with the WFC3 on the refurbished HST allow us to study its recent star formation history. Our analysis focuses on stars with Halpha excess emission, a robust indicator of their pre-main sequence (PMS) accreting status. The comparison with theoretical PMS isochrones shows that 2/3 of the objects with Halpha excess emission have ages from 1 to 10 Myr, with a median value of 3 Myr, while a surprising 1/3 of them are older than 10 Myr. The study of the spatial distribution of these PMS stars allows us to confirm their cluster membership and to statistically separate them from field stars. This result establishes unambiguously for the first time that star formation in and around the cluster has been ongoing for at least 10-20 Myr, at an apparently increasing rate.

astro-ph.SR↗

arXiv2010

The Antennae Galaxies (NGC 4038/4039) Revisited: ACS and NICMOS Observations of a Prototypical Merger

The ACS and NICMOS have been used to obtain new HST images of NGC 4038/4039 ("The Antennae"). These new observations allow us to better differentiate compact star clusters from individual stars, based on both size and color. We use this ability to extend the cluster luminosity function by approximately two magnitudes over our previous WFPC2 results, and find that it continues as a single power law, dN/dL propto L^alpha with alpha=-2.13+/-0.07, down to the observational limit of Mv~-7. Similarly, the mass function is a single power law dN/dM propto M^beta with beta=-2.10+/-0.20 for clusters with ages t<3x10^8 yr, corresponding to lower mass limits that range from 10^4 to 10^5 Msun, depending on the age range of the subsample. Hence the power law indices for the luminosity and mass functions are essentially the same. The luminosity function for intermediate-age clusters (i.e., ~100-300 Myr old objects found in the loops, tails, and outer areas) shows no bend or turnover down to Mv~-6, consistent with relaxation-driven cluster disruption models which predict the turnover should not be observed until Mv~-4. An analysis of individual ~0.5-kpc sized areas over diverse environments shows good agreement between values of alpha and beta, similar to the results for the total population of clusters in the system. Several of the areas studied show evidence for age gradients, with somewhat older clusters appearing to have triggered the formation of younger clusters. The area around Knot B is a particularly interesting example, with an ~10-50 Myr old cluster of estimated mass ~10^6 Msun having apparently triggered the formation of several younger, more massive (up to 5x10^6 Msun) clusters along a dust lane.

astro-ph.EP↗

arXiv2010

A Comparison of Methods for Determining the Age Distribution of Star Clusters: Application to the Large Magellanic Cloud

The age distribution of star clusters in nearby galaxies plays a crucial role in evaluating the lifetimes and disruption mechanisms of the clusters. Two very different results have been found recently for the age distribution chi(t) of clusters in the Large Magellanic Cloud (LMC). We found that chi(t) can be described approximately by a power law chi(t) propto t^{gamma}, with gamma -0.8, by counting clusters in the mass-age plane, i.e., by constructing chi(t) directly from mass-limited samples. Gieles & Bastian inferred a value of gamma~, based on the slope of the relation between the maximum mass of clusters in equal intervals of log t, hereafter the M_max method, an indirect technique that requires additional assumptions about the upper end of the mass function. However, our own analysis shows that the M_max method gives a result consistent with our direct counting method for clusters in the LMC, namely chi(t) propto t^-0.8 for t<10^9 yr. The reason for the apparent discrepancy is that our analysis includes many massive (M>1.5x10^3 M_sol), recently formed (t<10^7 yr) clusters, which are known to exist in the LMC, whereas Gieles & Bastian are missing such clusters. We compile recent results from the literature showing that the age distribution of young star clusters in more than a dozen galaxies, including dwarf and giant galaxies, isolated and interacting galaxies, irregular and spiral galaxies, has a similar declining shape. We interpret this approximately "universal" shape as due primarily to the progressive disruption of star clusters over their first ~few x 10^8 yr, starting soon after formation, and discuss some observational and physical implications of this early disruption for stellar populations in galaxies.

astro-ph.GA↗

arXiv2010

New Tests for Disruption Mechanisms of Star Clusters: The Large and Small Magellanic Clouds

We compare the observed bivariate distribution of masses(M) and ages(t) of star clusters in the LMC with the predicted distributions g(M,t) from 3 idealized models for the disruption of star clusters: (1)sudden mass-dependent disruption;(2)gradual mass-dependent disruption; and (3)gradual mass-independent disruption. The model with mass-{\em in}dependent disruption provides a good, first-order description of these cluster populations, with g(M,t) propto M^{beta} t^{gamma}, beta=-1.8+/-0.2 and gamma=-0.8+/-0.2, at least for clusters with ages t<10^9 yr and masses M<10^3 M_sol (more specifically, t<10^7(M/10^2 M_sol)^{1.3} yr). This model predicts that the clusters should have a power-law luminosity function, dN/dL propto L^-1.8, in agreement with observations. The first two models, on the other hand, fare poorly when describing the observations, refuting previous claims that mass-dependent disruption of star clusters is observed in the LMC over the studied M-t domain. Clusters in the SMC can be described by the same g(M,t) distribution as for the LMC, but with smaller samples and hence larger uncertainties. The successful g(M,t) model for clusters in the Magellanic Clouds is virtually the same as the one for clusters in the merging Antennae galaxies, but extends the domain of validity to lower masses and to older ages. This indicates that the dominant disruption processes are similar in these very different galaxies over at least t<10^8 yr and possibly t<10^9 yr. The mass functions for young clusters in the LMC are power-laws, while that for ancient globular clusters is peaked. We show that the observed shapes of these mass functions are consistent with expectations from the simple evaporation model presented by McLaughlin & Fall.

astro-ph.GA↗

arXiv2010

Supernova Remnants and the Interstellar Medium of M83: Imaging & Photometry with WFC3 on HST

We present Wide Field Camera 3 images taken with the Hubble Space Telescope within a single field in the southern grand design star-forming galaxy M83. Based on their size, morphology and photometry in continuum-subtracted H$α$, [\SII], H$β$, [\OIII] and [\OII] filters, we have identified 60 supernova remnant candidates, as well as a handful of young ejecta-dominated candidates. A catalog of these remnants, their sizes and, where possible their H$α$ fluxes are given. Radiative ages and pre-shock densities are derived from those SNR which have good photometry. The ages lie in the range $2.62 < log(τ_{\rm rad}/{\rm yr}) < 5.0$, and the pre-shock densities at the blast wave range over $0.56 < n_0/{\rm cm^{-3}} < 1680$. Two populations of SNR have been discovered. These divide into a nuclear and spiral arm group and an inter-arm population. We infer an arm to inter-arm density contrast of 4. The surface flux in diffuse X-rays is correlated with the inferred pre-shock density, indicating that the warm interstellar medium is pressurised by the hot X-ray plasma. We also find that the interstellar medium in the nuclear region of M83 is characterized by a very high porosity and pressure and infer a SNR rate of one per 70-150 yr for the nuclear ($R<300 $pc) region. On the basis of the number of SNR detected and their radiative ages, we infer that the lower mass of Type II SNe in M83 is $M_{\rm min} = 16^{+7}_ {-5}$ M$_{\odot}$. Finally we give evidence for the likely detection of the remnant of the historical supernova, SN1968L.

astro-ph.CO↗

arXiv2009

New Tests for Disruption Mechanisms of Star Clusters: Methods and Application to the Antennae Galaxies

We present new tests for disruption mechanisms of star clusters based on the bivariate mass-age distribution g(M,τ). In particular, we derive formulae for g(M,τ) for two idealized models in which the rate of disruption depends on the masses of the clusters and one in which it does not. We then compare these models with our Hubble Space Telescope observations of star clusters in the Antennae galaxies over the mass-age domain in which we can readily distinguish clusters from individual stars: τ\la10^7(M/10^4 M_{\odot})^{1.3} yr. We find that the models with mass-dependent disruption are poor fits to the data, even with complete freedom to adjust several parameters, while the model with mass-independent disruption is a good fit. The successful model has the simple form g(M,τ) \propto M^{-2} τ^{-1}, with power-law mass and age distributions, dN/dM propto M^{-2} and dN/dτ\proptoτ^{-1}. The predicted luminosity function is also a power law, dN/dL \propto L^{-2}, in good agreement with our observations of the Antennae clusters. The similarity of the mass functions of star clusters and molecular clouds indicates that the efficiency of star formation in the clouds is roughly independent of their masses. The age distribution of the massive young clusters is plausibly explained by the following combination of disruption mechanisms: (1) removal of interstellar material by stellar feedback, τ\la 10^7$ yr; (2) continued stellar mass loss, 10^7 yr \la τ\la 10^8 yr; (3), tidal disturbances by passing molecular clouds, τ\ga 10^8 yr. None of these processes is expected to have a strong dependence on mass, consistent with our observations of the Antennae clusters. We speculate that this simple picture also applies--at least approximately--to the clusters in many other galaxies.

astro-ph.GA↗

arXiv2009

Further Definition of the Mass-Metallicity Relation in Globular Cluster Systems Around Brightest Cluster Galaxies

We combine the globular cluster data for fifteen Brightest Cluster Galaxies and use this material to trace the mass-metallicity relations (MMR) in their globular cluster systems (GCSs). This work extends previous studies which correlate the properties of the MMR with those of the host galaxy. Our combined data sets show a mean trend for the metal-poor (MP) subpopulation which corresponds to a scaling of heavy-element abundance with cluster mass Z ~ M^(0.30+/-0.05). No trend is seen for the metal-rich (MR) subpopulation which has a scaling relation that is consistent with zero. We also find that the scaling exponent is independent of the GCS specific frequency and host galaxy luminosity, except perhaps for dwarf galaxies. We present new photometry in (g',i') obtained with Gemini/GMOS for the globular cluster populations around the southern giant ellipticals NGC 5193 and IC 4329. Both galaxies have rich cluster populations which show up as normal, bimodal sequences in the colour-magnitude diagram. We test the observed MMRs and argue that they are statistically real, and not an artifact caused by the method we used. We also argue against asymmetric contamination causing the observed MMR as our mean results are no different from other contamination-free studies. Finally, we compare our method to the standard bimodal fitting method (KMM or RMIX) and find our results are consistent. Interpretation of these results is consistent with recent models for globular cluster formation in which the MMR is determined by GC self-enrichment during their brief formation period.

astro-ph.CO↗

arXiv2007

Dynamical Evolution of Globular Cluster Systems formed in Galaxy Mergers: Deep HST/ACS Imaging of Old and Intermediate-Age Globular Clusters in NGC 3610

(ABRIDGED) The ACS camera on board the Hubble Space Telescope has been used to obtain deep images of the giant elliptical galaxy NGC 3610, a well-established dissipative galaxy merger remnant. These observations supersede previous WFPC2 images which revealed the presence of a population of metal-rich globular clusters (GCs) of intermediate age (~1.5-4 Gyr). We detect a total of 580 GC candidates, 46% more than from the previous WFPC2 images. The new photometry strengthens the significance of the previously found bimodality of the color distribution of GCs. Peak colors in V-I are 0.93 +/-0.01 and 1.09 +/- 0.01 for the blue and red subpopulations, respectively. The luminosity function (LF) of the inner 50% of the metal-rich (`red') population of GCs differs markedly from that of the outer 50%. In particular, the LF of the inner 50% of the red GCs shows a flattening consistent with a turnover that is about 1.0 mag fainter than the turnover of the blue GC LF. This is consistent with predictions of recent models of GC disruption for the age range mentioned above and for metallicities that are consistent with the peak color of the red GCs as predicted by population synthesis models. We determine the specific frequency of GCs in NGC 3610 and find a present-day value of S_N = 1.4 +/- 0.6. We estimate that this value will increase to S_N = 3.8 +/- 1.7 at an age of 10 Gyr, which is consistent with typical S_N values for `normal' ellipticals. Our findings constitute further evidence in support of the notion that metal-rich GC populations formed during major mergers involving gas-rich galaxies can evolve dynamically (through disruption processes) into the red, metal-rich GC populations that are ubiquitous in `normal' giant ellipticals.

astro-ph↗

arXiv2006

Massive stars and star clusters in the Antennae galaxies

Large numbers of young stars are formed in merging galaxies, such as the Antennae galaxies. Most of these stars are formed in compact star clusters (i.e., super star clusters), which have been the focus of a large number of studies. However, an increasing number of projects are beginning to focus on the individual stars as well. In this contribution, we examine a few results relevant to the triggering of star and star cluster formation; ask what fraction of stars form in the field rather than in clusters; and begin to explore the demographics of both the massive stars and star clusters in the Antennae.

astro-ph↗

arXiv2006

Star Cluster Demographics. I. A General Framework and Application to the Antennae Galaxies

We present a framework for understanding the demographics of star cluster systems, and develop a toy model which incorporates a universal initial power law mass function, selected formation histories, selected disruption laws, and a convolution with common artifacts and selection effects found in observational data. The model confirms that the observed correlation between the brightest young cluster in a galaxy and the total number of young clusters can be understood as a statistical size-of-sample effect, rather than a difference in the physical process responsible for the formation of the clusters. A comparison is made between different cluster disruption laws and it is shown that the break in the dN/dτdiagram used to determine the parameters in the Boutloukos & Lamers model may be produced by incompleteness near the breakpoint. A model of the Antennae galaxies is developed and compared with the observational data. An important component of our model is the use of a "two-stage" disruption process, with a very high "infant mortality" rate for the clusters with ages less than \approx10^8 yrs (i.e., roughly 80%-90% are lost each factor of ten in time, τ, independent of mass), and two-body relaxation, which becomes the dominant disruption mechanism at older ages, preferentially removing the lower mass clusters. Hence, in our model, stars from the dissolved clusters form the field population. We note that a 90% infant mortality rate for each factor of ten in τ(i.e., dN/dτ\propto τ^{-1}) is consistent with all measured young cluster populations, including those in the Antennae, Small Magellanic Cloud, and the Milky Way.

astro-ph↗

arXiv2006

Connection between the Age Distributions of Star Clusters and Field Stars: A First Application to the Small Magellanic Cloud

We present the age distributions for star clusters and individual stars in the Small Magellanic Cloud (SMC) based on data from the Magellanic Clouds Photometric Survey by Zaritsky and collaborators. The age distribution of the SMC clusters shows a steep decline, dN_{cluster}/dt \propto t^{-0.85\pm0.15}, over the period 10^7 < t <10^9 yr. This decline is essentially identical to that observed previously for more massive clusters in the merging Antennae galaxies, and also for lower-mass embedded clusters in the solar neighborhood. The SMC cluster age distribution therefore provides additional evidence for the rapid disruption of star clusters (``infant mortality''). These disrupted clusters deliver their stars to the general field population, implying that the field star age distribution, dN_{fld star}/dt, should have an inverse relation to dN_{cluster}/dt if most stars form initially in clusters. We make specific predictions for dN_{fldstar}/dt based on our cluster disruption models, and compare them with current data available for stars in the SMC. While these data do not extend to sufficiently young ages for a definitive test, they are consistent with a scenario wherein most SMC stars formed in clusters. Future analyses of dN_{fldstar}/dt that extend down to ages of approximately few million years are needed to verify the age relationship between stars residing in clusters and in the field.

astro-ph↗

arXiv2005

Properties of Resolved Star Clusters in M51

We present a study of compact star clusters in the nearby pair of interacting galaxies NGC 5194/95 (M51), based on multifilter Hubble Space Telescope WFPC2 archival images. We have detected ~400 isolated clusters. Our requirement that clusters be detected based only on their morphology results in the selection of relatively isolated objects, and we estimate that we are missing the majority (by a factor 4-6) of <10 Myr clusters due to crowding. Hence we focus on the cluster population older than 10 Myr. An age distribution shows a broad peak between 100-500 Myr, which is consistent with the crossing times of NGC 5195 through the NGC 5194 disk estimated in both single and multiple-passage dynamical models. We estimate that the peak contains approximately 2.2-2.5 times more clusters than expected from a constant rate of cluster formation over this time interval. We estimate the effective radii of our sample clusters and find a median value of 3-4 pc. Additionally, we see correlations of increasing cluster size with cluster mass (with a best fit slope of 0.14\pm0.03) at the 4sigma level, and with cluster age (0.06\pm0.02) at the 3sigma level. Finally, we report for the first time the discovery of faint, extended star clusters in the companion, NGC 5195, an SB0 galaxy. These have red [(V-I)>1.0] colors, effective radii >7 pc, and are scattered over the disk of NGC 5195. Our results indicate that NGC 5195 is therefore currently the third known barred lenticular galaxy to have formed so-called "faint fuzzy" star clusters. (abridged)

astro-ph↗

arXiv2005

The Age Distribution of Massive Star Clusters in the Antennae Galaxies

We determine the age distribution of star clusters in the Antennae galaxies (NGC 4038/9) for two mass-limited samples (M > 3 x 10^4 M_{\odot} and M > 2 x 10^5 M_{\odot}). This is based on integrated broadband UBVI and narrowband H-alpha photometry from deep images taken with the Hubble Space Telescope. We find that the age distribution of the clusters declines steeply, approximately as dN/dτ\propto τ^{-1}. The median age of the clusters is ~10^7 yr, which we interpret as evidence for rapid disruption ("infant mortality"). It is very likely that most of the young clusters are not gravitationally bound and were disrupted near the times they formed by the energy and momentum input from young stars to the interstellar matter of the protoclusters. At least 20% and possibly all stars form in clusters and/or associations, including those that are unbound and short-lived.

astro-ph↗