Average Stellar Mass Research Articles

Simulations of star formation in a gaseous disc around Sgr A* - a failed active galactic nucleus

We numerically model fragmentation of a gravitationally unstable gaseous disc under conditions that may be appropriate for the formation of the young massive stars observed in the central parsec of our Galaxy. In this study, we adopt a simple prescription with a locally constant cooling time. We find that, for cooling times just short enough to induce disc fragmentation, stars form with a top-heavy initial mass function (IMF), as observed in the Galactic Centre (GC). For shorter cooling times, the disc fragments much more vigorously, leading to lower average stellar masses. Thermal feedback associated with gas accretion on to protostars slows down disc fragmentation, as predicted by some analytical models. We also simulate the fragmentation of a gas stream on an eccentric orbit in a combined Sgr A* plus stellar cusp gravitational potential. The stream processes, self-collides and forms stars with a top-heavy IMF. None of our models produces large enough comoving groups of stars that could account for the observed 'ministar cluster' IRS 13E in the GC. In all of the gravitationally unstable disc models that we explored, star formation takes place too fast to allow any gas accretion on to the central supermassive black hole. While this can help to explain the quiescence of 'failed active galactic nucleus' such as Sgr A*, it poses a challenge for understanding the high gas accretion rates inferred for many quasars.

On the Similarity between Cluster and Galactic Stellar Initial Mass Functions

The stellar initial mass functions (IMFs) for the Galactic bulge, the Milky Way, other galaxies, clusters of galaxies, and the integrated stars in the universe are composites from countless individual IMFs in star clusters and associations where stars form. These galaxy-scale IMFs, reviewed in detail here, are not steeper than the cluster IMFs except in rare cases. This is true even though low-mass clusters generally outnumber high-mass clusters and the average maximum stellar mass in a cluster scales with the cluster mass. The implication is that the mass distribution function for clusters and associations is a power law with a slope of -2 or shallower. Steeper slopes, even by a few tenths, upset the observed equality between large- and small-scale IMFs. Such a cluster function is expected from the hierarchical nature of star formation, which also provides independent evidence for the IMF equality when it is applied on subcluster scales. We explain these results with analytical expressions and Monte Carlo simulations. Star clusters appear to be the relaxed inner parts of a widespread hierarchy of star formation and cloud structure. They are defined by their own dynamics rather than by preexisting cloud boundaries.

The Stellar, Gas, and Dynamical Masses of Star‐forming Galaxies atz∼ 2

We present analysis of the near-infrared spectra of 114 rest-frame UV-selected star-forming galaxies at z ~ 2. By combining the Hα spectra with photometric measurements from observed 0.3-8 μm, we assess the relationships among kinematics, dynamical masses, inferred gas fractions, and stellar masses and ages. The Hα line widths give a mean dynamical mass Mdyn = (6.9 ± 0.6) × 1010 M☉ within a typical radius of ~6 kpc, after excluding AGNs. The average dynamical mass is ~2 times larger than the average stellar mass, and the two agree to within a factor of several for most objects. However, ~15% of the sample has Mdyn ≫ M⋆. These objects are best fit by young stellar populations and tend to have high Hα equivalent widths, WHα ≳ 200 Å, suggesting that they are young starbursts with large gas masses. Rest-frame optical luminosity and velocity dispersion are correlated with 4 σ significance. Using the local empirical correlation between star formation rate per unit area and gas surface density, we estimate the mass of the gas associated with star formation and find a mean gas fraction of ~50% and a strong decrease in gas fraction with increasing stellar mass. The masses of gas and stars combined are considerably better correlated with the dynamical masses than are the stellar masses alone, and agree to within a factor of 3 for 85% of the sample. The combination of kinematic measurements, estimates of gas masses, and stellar population properties suggest that the factor of ~500 range in stellar mass across the sample cannot be fully explained by intrinsic differences in the total masses of the galaxies, which vary by a factor of ~40; the remaining variation is due to the evolution of the stellar population and the conversion of gas into stars.

HST/WFPC2 morphologies and color maps of distant luminous infrared galaxies

Using HST/WFPC2 imaging in F606W (or F450W) and F814W filters, we obtained the color maps in observed frame for 36 distant (0.4 < z < 1.2) luminous infrared galaxies (LIRGs, L IR (8-1000 μm) ≥ 10 11 L ○. ), with average star formation rates of ∼100 M ○. yr -1 . Stars and compact sources are taken as references to align images after correction of geometric distortion. This leads to an alignment accuracy of 0.15 pixel, which is a prerequisite for studying the detailed color properties of galaxies with complex morphologies. A new method is developed to quantify the reliability of each pixel in the color map without any bias against very red or blue color regions. Based on analyses of two-dimensional structure and spatially resolved color distribution, we carried out morphological classification for LIRGs. About 36% of the LIRGs were classified as disk galaxies and 22% as irregulars. Only 6 (17%) systems are obvious ongoing major mergers. An upper limit of 58% was found for the fraction of mergers in LIRGs with all the possible merging/interacting systems included. Strikingly, the fraction of compact sources is as high as 25%, similar to that found in optically selected samples. From their K band luminosities, LIRGs are relatively massive systems, with an average stellar mass of about 1.1 x 10 11 M ○. . They are related to the formation of massive and large disks, from their morphologies and also from the fact that they represent a significant fraction of distant disks selected by their sizes. If sustained at such large rates, their star formation can double their stellar masses in less than I Gyr. The compact LIRGs show blue cores, which could be associated with the formation of the central region of these galaxies. We find that all LIRGs are distributed along a sequence which relate their central color to their concentration index. This sequence links compact objects with blue central color to extended ones with relatively red central color, which are closer to the local disks. We suggest that there are many massive disks which have been forming a large fraction of their stellar mass since z = 1. For most of them, their central parts (bulge?) were formed prior to the formation of their disks.

A Direct Measurement of Major Galaxy Mergers atz3

This paper presents direct evidence for hierarchical galaxy assembly out to redshifts z ~ 3. We identify major mergers using the model-independent CAS (concentration, asymmetry, clumpiness) physical morphological system on galaxies detected, and photometrically selected, in the WFPC2 and NICMOS Hubble Deep Field North. We specifically use the asymmetric distributions of rest-frame optical light measured through the asymmetry parameter (A) to determine the fraction of galaxies undergoing major mergers as a function of redshift (z), stellar mass (M*), and absolute magnitude (MB). We find that the fraction of galaxies consistent with undergoing a major merger increases with redshift for all galaxies, but most significantly, at 5?10 ? confidence, for the most luminous and massive systems. The highest merger fractions we find are 40%?50% for galaxies with MB 1010 M? at z > 2.5, e.g., objects identified as Lyman-break galaxies. Using these results, we model the merger fraction evolution in the form fm(A, M*, MB, z) = f0 ? (1 + z). We find mA values ~4?6 for the most luminous and massive galaxies, while lower mass and less luminous galaxies have smaller mA values. We use these merger fractions, combined with merger timescales calculated from N-body simulations, to derive galaxy merger rates to z ~ 3. We also use stellar masses of HDF-N galaxies to determine the mass accretion rate of field galaxies involved in major mergers. We find an average stellar mass accretion rate of G ~ 4 ? 108 M? Gyr-1 per galaxy at z ~ 1 for galaxies with stellar masses M* > 109 M?. This accretion rate changes with redshift as G = 1.6 ? 108(1 + z)0.99?0.32 M? Gyr-1 per galaxy. We also find that the fraction of stellar mass density in galaxies involved in major mergers increases with redshift, with a peak mass fraction ~0.5 for the brightest, MB 1010 M?, systems near z ~ 2.5. By comparing merger fractions predicted in cold dark matter semianalytic models with our results we find a reasonably good agreement for the largest and brightest systems, although we find more low-mass galaxy mergers at lower redshifts than what these models predict.

The Assembly of Massive Galaxies from Near-Infrared Observations of the Hubble Deep Field-South

We use a deep KAB ≤ 25 galaxy sample in the Hubble Deep Field-South to trace the evolution of the cosmological stellar mass density from z 0.5 to z 3. We find clear evidence for a decrease of the average stellar mass density at high redshift, 2 ≤ z ≤ 3.5, that is 15% of the local value, 2 times higher than observed in the Hubble Deep Field-North. To take into account for the selection effects, we define a homogeneous subsample of galaxies with 1010 M☉ ≤ M* ≤ 1011 M☉: in this sample, the mass density at z > 2 is 20% of the local value. In the mass-limited subsample at z > 2, the fraction of passively fading galaxies is at most 25%, although they can contribute up to about 40% of the stellar mass density. On the other hand, star-forming galaxies at z > 2 form stars with an average specific rate of at least 4 × 10-10 yr-1, 3 times higher than the z ≤ 1 value. This implies that UV-bright star-forming galaxies are substancial contributors to the rise of the stellar mass density with cosmic time. Although these results are globally consistent with Λ-CDM scenarios, the present rendition of semianalytic models fails to match the stellar mass density produced by more massive galaxies present at z > 2.

Quantitative Interpretation of Quasar Microlensing Light Curves

We develop a general method for analyzing the light curves of microlensed quasars and apply it to the OGLE light curves of the four-image lens Q2237+0305. We simultaneously estimate the effective source velocity, the average stellar mass, the stellar mass function, and the size and structure of the quasar accretion disk. The light curves imply an effective source-plane velocity of 10,200 km s-1 ≲ veh⟨M/M☉⟩-1/2 ≲ 39,600 km s-1 (68% confidence). Given an independent estimate for the source velocity, found by combining estimates for the peculiar velocity of the lens galaxy with its measured stellar velocity dispersion, we obtain a mean stellar mass of ⟨M⟩ ≃ 0.037 h2 M☉ (0.0059 h2 M☉ ≲ ⟨M⟩ ≲ 0.20 h2 M☉). We were unable to distinguish a Salpeter mass function from one in which all stars had the same mass, but we do find a strong lower bound of κ*/κ ≳ 0.5 on the fraction of the surface mass density represented by the microlenses. Our models favor a standard thin accretion disk model as the source structure over a simple Gaussian source. For a face-on, thin disk radiating as a blackbody with temperature profile Ts ∝ R-3/4, the radius rs where the temperature matches the filter pass band [2000 Å or Ts(rs) ≃ 7 × 104 K] is 1.4 × 1015 h-1 cm ≲ rs ≲ 4.5 × 1015 h-1 cm. The flux predicted by the disk model agrees with the observed flux of the quasar, so nonthermal or optically thin emission processes are not required. From the disk structure we estimate a black hole mass of MBH ≃ 1.1 × 109 h-3/2η(L/LE)-1/2 M☉, consistent with the mass estimated under the assumption that the quasar is radiating at the Eddington luminosity (L/LE = 1).

Astrometric Detection of Extrasolar Planets: Results of a Feasibility Study with the Palomar 5 Meter Telescope

The detection of extrasolar planets around stars like the Sun remains an important goal of astronomy. We present results from Palomar 5 m observations of the open cluster NGC 2420 in which we measure some of the sources of noise that will be present in an astrometric search for extrasolar planets. This is the first time that such a large aperture has been used for high-precision astrometry. We find that the atmospheric noise is 150 micro-arcsec hr(exp 1/2) across a 90 sec field of view and that differential chromatic refraction (DCR) can be calibrated to 128 micro-arcsec for observations within 1 hr of the meridian and 45 deg of zenith. These results confirm that a model for astrometric measurements can be extrapolated to large apertures. We demonstrate, based upon these results, that a large telescope achieves the sensitivity required to perform a statistically significant search for extra planets. We describe an astrometric technique to detect planets, the astrometric signals expected, the role of reference stars, and the sources of measurement noise: photometric noise, atmospheric motion between stars, sky background, instrumental noise, and DCR. For the latter, we discuss a method to reduce the noise further to 66 micro-arcsecond for observations within 1 hr of the meridian and 45 deg of zenith. We discuss optimal lists of target stars taken from the latest Gliese & Jahreiss catalog of nearby stars with the largest potential astrometric signals, declination limits for both telescope accessibility and reduced DCR, and galactic latitude limits for a sufficiant number of reference stars. Two samples are described from which one can perform statistically significant searches for gas giant planets around nearby stars. One sample contains 100 solar class stars with an average stellar mass of 0.82 mass; the other maximizes the number of stars, 574, by searching mainly low-mass M stars. We perform Monte Carlo simulations of the statistical significance of the expected results by using measured and estimated noise quantities. We show the semimajor axis parameter spaces that are searched for each star and how an increase in the length of the observing program expands these spaces. The search over semimajor axis parameter space relates to the theory of gas giant planet formation.

On the universality of microlensing in quadruple gravitational lenses

It is well known that multiply imaged quasars are likely to be affected by microlensing. Quadruply imaged systems are especially useful laboratories for studying microlensing because their macrolens models are relatively well constrained. We begin with analytical results for a simple family of galaxy models. These results can be used to estimate the magnifications and time delays for the quadruple systems. We compute expected brightness fluctuations due to microlensing in several such systems for a range of source sizes. Among these we treat for the first time the limiting case of a completely unresolved source. For the case of MG 0414+0534 we interpret the discrepant optical-to-radio flux ratios for the different components as the result of microlensing. On the assumption that the lensing galaxy is roughly isothermal this gives a 95% confidence upper limit on the optical source size of 10(exp 16) cm x (mean value of M)/0.1 solar mass(exp 1/2) where (M) is the average stellar mass of the lensing galaxy. For more centrally concentrated macromodels the upper limit is greater. Sufficiently long strings of photometric observations can in principle constrain the degree of concentration.

The Dynamics of Spherical Stellar Systems: III. The Relative Loss of Stars WithDifferent Mass

A distribution function that includes a velocity-space anisotropy and that satisfies the Boltzmann equation for both large and small energy is used to calculate the relative rate of stellar mass loss from spherical systems. The calculations are based on the velocity-space flux vector, and do not employ the use of a relaxation time. The loss of stars of different mass at varying distances throughout the system from different spherical models demonstrates the dynanmical importance of the low-mass stars. The stars with mass approximates 1/4 the average stellar mass may leave the system, as a result of distant encounters, from 10 to over 100 times faster than the average stars, depending on the model. The depletion of low luminosity stars in galactic clusters and the distribution of white dwarfs in globular clusters is briefly discussed.

Dynamics of Spherical Stellar Systems: Properties of Theoretical Models, and Comparison with Clusters and Elliptical Galaxies.

view Abstract Citations (2) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dynamics of Spherical Stellar Systems: Properties of Theoretical Models, and Comparison with Clusters and Elliptical Galaxies. Michie, Richard W. Abstract A wide range of spherical models are now available and result from a distribution function which for the inner regions is obtained from the Boltzmann equation with encounters. It is assumed that encounters and orbital mixing were important in the early stages of the formation and development of the star clusters and elliptical galaxies, and, since this time period, the structure of these systems has not significantly changed. The orbital mixing allows an extension of the distribution function to the regions far from the center, where mathematically it satisfies the collisionless Boltzmann equation. The gravitational potential is obtained from Poisson's equation. The stellar orbits are not restricted to an isotropic velocity distribution everywhere the models range from complete isotropy to highly radial orbits. While the distribution function is obtained for stars of different mass, the comparison with observation has assumed all stars to have the average stellar mass. In a log density versus log distance plot, it is found that for certain degrees of velocity space anisotropy there is an inflection in the curve, and this is a result of the orbits becoming elongated as well as the energy cutoff becoming important. All the models have finite mass, are self-supporting and satisfy the virial theorem. They are all obtained from an analysis of the Boltzmann equation with particular emphasis placed at the high energies. In dimensionless variables the models are characterized by a single model parameter C. The model parameter involves three physical parameters which indicate the total mass, total energy, and degree of velocity space anisotropy. A comparison with rich galactic clusters, globular clusters, dwarf galaxies and spherical galaxies has been made. The theoretical models appear to be in agreement with the observed systems. For example, the log density versus log distance inflection observed in systems such as NGC 3379 is exactly as predicted by the theory. There may be a small inflection for the globular cluster 47 Tuc. Both star clusters and spherical galaxies show a fairly wide range in the model parameter. The star clusters may favor a somewhat larger value of C, but the data are probably not sufficient to firmly establish this trend. Publication: The Astronomical Journal Pub Date: June 1962 DOI: 10.1086/108880 Bibcode: 1962AJ.....67R.582M full text sources ADS |