arXiv subjects

S. Michael Fall

Publications and source records attributed to S. Michael Fall.

At least 55 records · Page 3Linked to original sources

arXiv2009

Large Area Survey for z=7 Galaxies in SDF and GOODS-N: Implications for Galaxy Formation and Cosmic Reionization

We present results of our large-area survey for z'-band dropout galaxies at z=7 in a 1568 arcmin^2 sky area covering the SDF and GOODS-N fields. Combining our ultra-deep Subaru/Suprime-Cam z'- and y-band (lambda_eff=1um) images with legacy data of Subaru and HST, we have identified 22 bright z-dropout galaxies down to y=26, one of which has a spectroscopic redshift of z=6.96 determined from Lya emission. The z=7 luminosity function (LF) yields the best-fit Schechter parameters of phi*=0.69 +2.62/-0.55 x10^(-3) Mpc^(-3), Muv*=-20.10 +/-0.76 mag, and alpha=-1.72 +/-0.65, and indicates a decrease from z=6 at a >95% confidence level. This decrease is beyond the cosmic variance in our two fields, which is estimated to be a factor of <~2. We have found that the cosmic star formation rate density drops from the peak at z=2-3 to z=7 roughly by a factor of ~10 but not larger than ~100. A comparison with the reionization models suggests either that the Universe could not be totally ionized by only galaxies at z=7, or more likely that properties of galaxies at z=7 are different from those at low redshifts having, e.g., a larger escape fraction (>~0.2), and/or a flatter IMF. Our SDF z-dropout galaxies appear to form 60-Mpc long filamentary structures, and the z=6.96 galaxy with Lya emission is located at the center of an overdense region consisting of four UV bright dropout candidates, which might suggest an existence of a well-developed ionized bubble at z=7.

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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.

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arXiv2008

Shaping the Globular Cluster Mass Function by Stellar-Dynamical Evaporation

We show that the globular cluster mass function (GCMF) in the Milky Way depends on cluster half-mass density (rho_h) in the sense that the turnover mass M_TO increases with rho_h while the width of the GCMF decreases. We argue that this is the expected signature of the slow erosion of a mass function that initially rose towards low masses, predominantly through cluster evaporation driven by internal two-body relaxation. We find excellent agreement between the observed GCMF -- including its dependence on internal density rho_h, central concentration c, and Galactocentric distance r_gc -- and a simple model in which the relaxation-driven mass-loss rates of clusters are approximated by -dM/dt = mu_ev ~ rho_h^{1/2}. In particular, we recover the well-known insensitivity of M_TO to r_gc. This feature does not derive from a literal ``universality'' of the GCMF turnover mass, but rather from a significant variation of M_TO with rho_h -- the expected outcome of relaxation-driven cluster disruption -- plus significant scatter in rho_h as a function of r_gc. Our conclusions are the same if the evaporation rates are assumed to depend instead on the mean volume or surface densities of clusters inside their tidal radii, as mu_ev ~ rho_t^{1/2} or mu_ev ~ Sigma_t^{3/4} -- alternative prescriptions that are physically motivated but involve cluster properties (rho_t and Sigma_t) that are not as well defined or as readily observable as rho_h. In all cases, the normalization of mu_ev required to fit the GCMF implies cluster lifetimes that are within the range of standard values (although falling towards the low end of this range). Our analysis does not depend on any assumptions or information about velocity anisotropy in the globular cluster system.

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arXiv2007

Density Dependence of the Mass Function of Globular Star Clusters in the Sombrero Galaxy and its Dynamical Implications

We have constructed the mass function of globular star clusters in the Sombrero galaxy in bins of different internal half-mass density rho_h and projected galactocentric distance R. This is based on the published measurements of the magnitudes and effective radii of the clusters by Spitler et al. (2006) in BVR images taken with the ACS on HST. We find that the peak of the mass function M_p increases with rho_h by a factor of about 4 but remains nearly constant with R. Our results are almost identical to those presented recently by McLaughlin & Fall (2007) for globular clusters in the Milky Way. The mass functions in both galaxies agree with a simple, approximate model in which the clusters form with a Schechter initial mass function and evolve subsequently by stellar escape driven by internal two-body relaxation. These findings therefore undermine recent claims that the present peak of the mass function of globular clusters must have been built into the initial conditions.

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arXiv2006

Relations Between the Luminosity, Mass, and Age Distributions of Young Star Clusters

We derive and interpret some relations between the luminosity, mass, and age distributions of star clusters, denoted here by phi(L), psi(M), and chi(tau), respectively. Of these, phi(L) is the easiest to determine observationally, whereas psi(M) and chi(tau) are more informative about formation and disruption processes. For populations of young clusters, with a relatively wide range of ages, phi(L) depends on both psi(M) and chi(tau) and thus cannot serve as a proxy for psi(M) in general. We demonstrate this explicitly by four illustrative examples with specific forms for either psi(M) or chi(tau). In the special case in which psi(M) is a power law and is independent of chi(tau), however, phi(L) is also a power law with the same exponent as psi(M). We conclude that this accounts for the observed similarity between phi(L) and psi(M) for the young clusters in the Antennae galaxies. This result reinforces our picture in which clusters form with psi(M) propto M^{-2} and are then disrupted rapidly at a rate roughly independent of their masses. The most likely disruptive process in this first stage is the removal of interstellar matter by the energy and momentum input from young stars (by photoionization, winds, jets, and supernovae). The few clusters that avoid this "infant mortality" are eventually disrupted in a second stage by the evaporation of stars driven by two-body relaxation, a process with a strong dependence on mass. We suspect this picture may apply to many, if not all, populations of star clusters, but this needs to be verified observationally by determinations of psi(M) and chi(tau) in more galaxies.

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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.

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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.

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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.

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arXiv2005

STIS Spectroscopy of Young Star Clusters in "The Antennae" Galaxies (NGC 4038/4039)

Long-slit spectra of several dozen young star clusters have been obtained at three positions in the Antennae galaxies with the Space Telescope Imaging Spectrograph (STIS) and its 52"x0.2" slit. Based on H-alpha emission-line measurements, the average cluster-to-cluster velocity dispersion in 7 different cluster aggregates ("knots") is <10 \kms. The fact that this upper limit is similar to the velocity dispersion of gas in the disks of typical spiral galaxies suggests that the triggering mechanism for the formation of young massive compact clusters ("super star clusters") is unlikely to be high velocity cloud-cloud collisions. On the other hand, models where preexisting giant molecular clouds in the disks of spiral galaxies are triggered into cluster formation are compatible with the observed low velocity dispersions. These conclusions are consistent with those reached by Zhang et al. (2001) based on comparisons between the positions of the clusters and the velocity and density structure of the nearby interstellar medium. We find evidence for systematically lower values of the line ratios [NII]/H-alpha and [SII]/H-alpha in the bright central regions of some of the knots, relative to their outer regions. This suggests that the harder ionizing photons are used up in the regions nearest the clusters, and the diffuse ionized gas farther out is photoionized by 'leakage' of the leftover low-energy photons. The low values of the [SII]/H-alpha line ratio, typically [SII]/H-alpha<0.4, indicates that the emission regions are photoionized rather than shock heated. The absence of evidence for shock-heated gas is an additional indication that high velocity cloud-cloud collisions are not playing a major role in the formation of the young clusters.

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arXiv2005

The Evolution of Damped Lyman-alpha Absorbers: Metallicities and Star Formation Rates

The damped Lyman-alpha (DLA) and sub-DLA quasar absorption lines provide powerful probes of the evolution of metals, gas, and stars in galaxies. One major obstacle in trying to understand the evolution of DLAs and sub-DLAs has been the small number of metallicity measurements at z < 1.5, an epoch spanning \~70 % of the cosmic history. In recent surveys with the Hubble Space Telescope and Multiple Mirror Telescope, we have doubled the DLA Zn sample at z < 1.5. Combining our results with those at higher redshifts from the literature, we find that the global mean metallicity of DLAs does not rise to the solar value at low redshifts. These surprising results appear to contradict the near-solar mean metallicity observed for nearby (z ~ 0) galaxies and the predictions of cosmic chemical evolution models based on the global star formation history. Finally, we discuss direct constraints on the star formation rates (SFRs) in the absorber galaxies from our deep Fabry-Perot Ly-alpha imaging study and other emission-line studies in the literature. A large fraction of the observed heavy-element quasar absorbers at 0 < z < 3.4 appear to have SFRs substantially below the global mean SFR, consistent with the low metallicities observed in the spectroscopic studies.

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arXiv2004

Hubble Space Telescope Observations of Element Abundances in Low-redshift Damped Lyman-alpha Galaxies and Implications for the Global Metallicity-Redshift Relation

Most models of cosmic chemical evolution predict that the mass-weighted mean interstellar metallicity of galaxies should rise with time from a low value $\sim 1/30$ solar at $z \sim 3$ to a nearly solar value at $z = 0$. In the absence of any selection effects, the damped Lyman-alpha absorbers (DLAs) in quasar spectra are expected to show such a rise in global metallicity. However, it has been difficult to determine whether or not DLAs show this effect, primarily because of the very small number of DLA metallicity measurements at low redshifts. In an attempt to put tighter constraints on the low-redshift end of the DLA metallicity-redshift relation, we have observed Zn II and Cr II lines in four DLAs at $0.09 < z < 0.52$, using the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST). These observations have provided the first constraints on Zn abundances in DLAs with $z < 0.4$. In all the three DLAs for which our observations offer meaningful constraints on the metallicity, the data suggest that the metallicities are much lower than the solar value. These results are consistent with recent imaging studies indicating that these DLAs may be associated with dwarf or low surface brightness galaxies. We combine our results with higher redshift data from the literature to estimate the global mean metallicity-redshift relation for DLAs. We find that the global mean metallicity shows at most a slow increase with decreasing redshift. ...(Please see the paper for the complete abstract).

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arXiv2004

Dust Depletion and Extinction in a Gamma-Ray Burst Afterglow

We put stringent constraints for the first time on the dust properties in the circumburst medium of a gamma-ray burst (GRB) afterglow. This is based on the optical spectrum of GRB 020813 (z=1.255), obtained with Keck I LRIS 4.65 h after the burst. From the absorption lines in the spectrum, we derive very high column densities for six heavy elements with different refractory properties. The relative abundances resemble the dust depletion patterns in the Milky Way, from which we infer a visual extinction of A_V=0.4 and A_V>0.3 at 95% confidence level. However, the high columns of metals and dust contrast with an observed UV continuum spectrum that is remarkably close to a power law of the form F_nu \propto nu^{-0.9}, with no sign of curvature, or a 2200 A extinction feature, suggesting low reddening. The Milky Way or Magellanic Cloud reddenings are possible only for very low visual extinctions (A_V<0.08 or A_V<0.2, respectively at 95% confidence), inconsistent with the A_V values inferred from the depletion analysis. If we assume a GRB intrinsic spectrum and an extinction law of the forms F_lambda^i = F_V (5500/lambda)^alpha and A_lambda= A_V (5500/lambda)^gamma, we obtain (for A_V=0.4) the constraints from continuum spectrum: gamma<0.85 and alpha<1.72.

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arXiv2004

High-Redshift Extremely Red Objects in the HST Ultra Deep Field Revealed by the GOODS IRAC Observations

Using early data from the Infrared Array Camera (IRAC) on the Spitzer Space Telescope, taken for the Great Observatories Origins Deep Survey (GOODS), we identify and study objects that are well-detected at 3.6um, but are very faint (and in some cases, invisible) in the Hubble Ultra Deep Field (HUDF) ACS and NICMOS images and in very deep VLT Ks-band imaging. We select a sample of 17 objects with f_nu(3.6um)/f_nu(z_{850})>20. The analysis of their spectral energy-distributions (SEDs) from 0.4 to 8.0um shows that the majority of these objects cannot be satisfactorily explained without a well-evolved stellar population. We find that most of them can be well fitted by a simple two-component model, where the primary component represents a massive, old population that dominates the strong IR emission, while the secondary component represents a low amplitude, on-going star-formation process that accounts for the weak optical fluxes. Their estimated photometric redshifts (z_p) range from 1.6 to 2.9, with the median at z_p=2.4. For the simple star formation histories considered here, their corresponding stellar masses range from 0.1 to 1.6e11 M_sun for a Chabrier Initial Mass Function (IMF). Their median rest-frame Ks-band absolute magnitude is -22.9 mag in AB system, or 1.5xL*(K) for present--day elliptical galaxies. In the scenario of pure luminosity evolution, such objects may be direct progenitors for at least 14 to 51% of the local population of early--type galaxies. Due to the small cosmic volume of the HUDF, however, this simple estimate could be affected by other effects such as cosmic variance and the strong clustering of massive galaxies. A full analysis of the entire GOODS area is now underway to assess such effects.

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arXiv2004

Mass, Age, and Space Distributions of Star Clusters

This article reviews some recent studies of the mass, age, and space distributions of star clusters, with a primary focus on the large population of clusters in the interacting Antennae galaxies. Some of the highlights of these studies are the following: 1. The mass function of the young clusters (with ages τ< 10^8 yr) has power-law form, dN/dM propto M^{-2}, over the observed range of masses, 10^4 < M < 10^6 solar masses. 2. The age distribution of the clusters declines steeply at all ages, roughly as dN/d tau propto tau^{-1} for mass-limited samples, indicating rapid disruption of most clusters. 3. At least 20% and possibly all stars form in clusters and/or associations, including those that are unbound and short-lived. 4. Many of the clusters that remain bound just after their formation are disrupted on longer timescales by a combination of mass loss by stellar evolution and several stellar dynamical processes. 5. The young clusters have a clumpy space distribution and are located preferentially in regions of high interstellar density, at least when averaged over scales of about a kpc. 6. The positions of the young clusters, however, are not correlated with the local velocity gradients or velocity dispersions in the interstellar medium. There are growing indications that most of these conclusions also apply to the populations of young star clusters in other galaxies, both interacting and quiescent.

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arXiv2003

The Size Evolution of High Redshift Galaxies

Hubble Space Telescope images of high-redshift galaxies selected via color and photometric redshifts are used to examine the size and axial-ratio distribution of galaxies as a function of redshift at lookback times t > 8 Gyr. These parameters are measured at rest-frame UV wavelengths (1200-2000 Angstroms) on images with a rest-frame resolution of less than 0.8 kpc. Galaxy radii are found to scale with redshift approximately as the inverse of the Hubble parameter H(z). This is in accord with the theoretical expectation that the typical sizes of the luminous parts of galaxies should track the expected evolution in the virial radius of dark-matter halos. The mean ratio of semi-major to semi-minor axis for a bright well-resolved sample of galaxies at z ~ 4 is b/a = 0.65, suggesting that these Lyman break galaxies are not drawn from a spheroidal population. However the median concentration index of this sample is C = 3.5, which is closer to the typical concentration indices, C ~ 4, of nearby elliptical galaxies than to the values, C<2 for local disk galaxies of type Sb and later.

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arXiv2002

Metallicity Evolution of Damped Lyman-Alpha Galaxies

We have reanalyzed the existing data on Zinc abundances in damped Ly-alpha (DLA) absorbers to investigate whether their mean metallicity evolves with time. Most models of cosmic chemical evolution predict that the mass- weighted mean interstellar metallicity of galaxies should rise with time from a low value ~ 1/30 solar at z ~ 3 to a nearly solar value at z ~ 0. However, several previous analyses have suggested that there is little or no evolution in the global metallicity of DLAs. We have used a variety of statistical techniques to quantify the global metallicity-redshift relation and its uncertainties, taking into account both measurement and sampling errors. Three new features of our analysis are: (a) an unbinned N(H I)-weighted nonlinear chi-square fit to an exponential relation; (b) survival analysis to treat the large number of limits in the existing data; and (c) a comparison of the data with several models of cosmic chemical evolution based on an unbinned N(H I)-weighted chi-square. We find that a wider range of evolutionary rates is allowed by the present data than claimed in previous studies. The slope of the exponential fit to the N(H I)-weighted mean Zn metallicity vs. redshift relation is -0.20 plus minus 0.11 counting limits as detections and -0.27 plus minus 0.12 counting limits as zeros. Similar results are also obtained if the data are binned in redshift, and if survival analysis is used. These slopes are marginally consistent with no evolution, but are also consistent with the rates predicted by several models of cosmic chemical evolution. Finally, we outline some future measurements necessary to improve the statistics of the global metallicity-redshift relation.

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arXiv2002

Formation of Galactic Disks

We review progress in understanding the formation of galactic disks in the standard cosmogonic scenario involving gravitational clustering of baryons and dark matter and dissipative collapse of the baryons. This scenario accounts remarkably well for the observed properties of galactic disks if they have retained most of the specific angular momentum they acquired by tidal torques. Early simulations, which included cooling of the gas but not star formation and the associated feedback, indicated instead that most of the angular momentum of the baryons would be transferred to the dark matter. Recent simulations indicate that this angular-momentum problem can be solved partially, and in some cases entirely, by feedback and other effects.

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arXiv2002

The Unique History of the Globular Cluster Omega Centauri

Using current observational data and simple dynamical modeling, we demonstrate that Omega Cen is not special among the Galactic globular clusters in its ability to produce and retain the heavy elements dispersed in the AGB phase of stellar evolution. The multiple stellar populations observed in Omega Cen cannot be explained if it had formed as an isolated star cluster. The formation within a progenitor galaxy of the Milky Way is more likely, although the unique properties of Omega Cen still remain a mystery.

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