The Intrinsic Shapes of Stellar Systems

I compute the estimated distribution function f(q) for the apparent axis ratio q of various types of stellar systems, using a nonparametric kernel method. I then invert f(q) to find the distribution of intrinsic axis ratios, using two different hypotheses: first, that the stellar systems are all oblate, and second, that they are all prolate. The shapes of globular clusters in our own galaxy are consistent, at the 99% confidence level, with both the oblate and prolate hypothesis. The shapes of dwarf galaxies in the Virgo cluster are consistent, at the 99% confidence level, with the prolate hypothesis, but inconsistent with the oblate hypothesis. The shapes of star clusters in the Large Magellanic Cloud, of ordinary elliptical galaxies, of brightest cluster ellipticals, and of galaxy clusters are all inconsistent, at the 99% confidence level, with both the oblate and prolate hypotheses. The globular clusters in our galaxy are older than their half-mass relaxation time, and are most likely rotationally flattened oblate spheroids. The other stellar systems considered are generally younger than their half-mass relaxation time, and thus are triaxial bodies flattened by anisotropy of their velocity dispersion.Comment: 23 pages, uuencoded compressed PostScript, includes 6 figures, accepted for publication in the Ap

Inclination-Dependent Extinction Effects in Disk Galaxies in the Sloan Digital Sky Survey

We analyze the r-band absolute magnitude and u − r color of low-redshift (z 0.9) to ~0.26 for nearly edge-on galaxies (q < 0.3). When the dimming law ΔM_r ⍺ (log q)^2 is used to create an inclination-corrected sample of bright exponential galaxies, their apparent shapes are consistent with a distribution of mildly noncircular disks, with median short-to-long axis ratio γ approx 0.22 and median disk ellipticity ε approx 0.08

Formation of Massive Counterrotating Disks in Spiral Galaxies

We present results of numerical simulations of the formation of a massive counterrotating gas disk in a spiral galaxy. Using a hierarchical tree gravity solver combined with a sticky-particle gas dissipation scheme for our simulations, we have investigated three mechanisms: episodic and continuous gas infall, and a merger with a gas-rich dwarf galaxy. We find that both episodic and continuous gas infall work reasonably well and are able to produce a substantial gas counterrotating disk without upsetting the stability of the existing disk drastically, but it is very important for the gas to be well-dispersed in phase-space and not form concentrated clumps prior to its absorption by the disk galaxy. The initial angular momentum of the gas also plays a crucial role in determining the scale length of the counterrotating disk formed and the time it takes to form. The rate of infall, i.e. the mass of gas falling in per unit time, has to be small enough to preclude excessive heating of the preexisting disk. It is much easier in general to produce a smaller counterrotating disk than it is to produce an extensive disk whose scale length is similar to that of the original prograde disk. A gas-rich dwarf merger does not appear to be a viable mechanism to produce a massive counterrotating disk, because only a very small dwarf galaxy can produce a counterrotating disk without increasing the thickness of the existing disk by an order of magnitude, and the time-scale for this process is prohibitively long because it makes it very unlikely that several such mergers can accumulate a massive counterrotating disk over a Hubble time.Comment: Accepted by ApJ, 22 pages, uuencoded compressed Postscript. 18 Figures (compressed Postscript) available from anonymous ftp at ftp://bessel.mps.ohio-state.edu/pub/thakar/cr1/figs.ps.Z A complete (text+figs) compressed PostScript preprint is also available at ftp://bessel.mps.ohio-state.edu/pub/thakar/cr1/pp.ps.g

Galaxy Alignments with Surrounding Structure in the Sloan Digital Sky Survey

Using data from the Sloan Digital Sky Survey (SDSS) Legacy Survey, we study the alignment of relatively luminous galaxies with spectroscopic data with the surrounding larger-scale structure as defined by galaxies with only photometric data. We find that galaxies from the red sequence have a statistically significant tendency for their images to align parallel to the projected surrounding structure. Red galaxies brighter than the median of our sample ($M_r < -21.05$) have a mean alignment angle $\langle \Phi \rangle < 45^\circ$, indicating preferred parallel alignment, at a significance level $> 7.8 \sigma$ on projected scales $1\,\textrm{Mpc} < r_p < 30\,\textrm{Mpc}$. Fainter red galaxies have $\langle \Phi \rangle 3.4 \sigma$ only at scales $r_p > 18\,\textrm{Mpc}$. Galaxies from the blue sequence show no statistically significant ($3\sigma$) tendency for their images to align with larger-scale structure. No dependence of alignment angle is seen as a function of local overdensity or of offset from the local distribution of surrounding galaxies.Comment: 9 pages, 5 figures, 2 tables, submitted to Ap

The Shapes of Dense Cores and Bok Globules

The shapes of isolated Bok globules and embedded dense cores of molecular clouds are analyzed using a nonparametric method, under the alternate hypotheses that they are randomly oriented prolate objects or that they are randomly oriented oblate objects. In all cases, the prolate hypothesis gives a better fit to the data. If Bok globules are oblate, they must be very flat; the average axis ratio is b/a = 0.3, and few or no globules can have b/a > 0.7. If Bok globules are prolate, then the mean axis ratio is b/a = 0.5. For most data samples of dense cores, the randomly-oriented oblate hypothesis can be rejected at the 99% confidence level. If the dense cores are prolate, their mean axis ratio is approximately 0.4 to 0.5. Dense cores are significantly different in shape from the clouds in which they are embedded; clouds have flatter apparent shapes, and are inconsistent with a population of randomly oriented axisymmetric objects.Comment: 26 pages (LaTeX) including 8 postscript figures; to appear in Ap

Dependence of Galaxy Shape on Environment in the Sloan Digital Sky Survey

Using a sample of galaxies from the Sloan Digital Sky Survey (SDSS) Data Release 4, we study the trends relating surface brightness profile type and apparent axis ratio to the local galaxy environment. We use the SDSS parameter `fracDeV' to quantify the profile type. We find that galaxies with M_r > -18 are mostly described by exponential profiles in all environments. Galaxies with -21 < M_r < -18 mainly have exponential profiles in low density environments and de Vaucouleurs profiles in high density environments. The most luminous galaxies, with M_r < -21, are mostly described by de Vaucouleurs profiles in all environments. For galaxies with M_r < -19, the fraction of de Vaucouleurs galaxies is a monotonically increasing function of local density, while the fraction of exponential galaxies is monotonically decreasing. For a fixed surface brightness profile type, apparent axis ratio is frequently correlated with environment. As the local density of galaxies increases, we find that for -20 < M_r < -18, galaxies of all profile types become slightly rounder, on average; for -22 < M_r < -20, galaxies with exponential profiles tend to become flatter, while galaxies with de Vaucouleurs profiles become rounder; for M_r < -22, galaxies with exponential profiles become flatter, while the de Vaucouleurs galaxies become rounder in their inner regions, yet exhibit no change in their outer regions. We comment on how the observed trends relate to the merger history of galaxies.Comment: 23 pages, 7 figures, accepted by Ap

NGC 4138 - A Case Study in Counterrotating Disk Formation

The Sa(r) galaxy NGC 4138 has been recently found to contain an extensive counterrotating disk which appears to be still forming. Up to a third of the stars in the disk system may be on retrograde orbits. A counterrotating ring of H II regions, along with extended counterrotating H I gas, suggests that the retrograde material has been recently acquired in the gas phase and is still trickling in. Using numerical simulations, we have attempted to model the process by which the counterrotating mass has been accreted by this galaxy. We investigate two possibilities: continuous retrograde infall of gas, and a retrograde merger with a gas-rich dwarf galaxy. Both processes are successful in producing a counterrotating disk of the observed mass and dimensions without heating up the primary significantly. Contrary to our experience with a fiducial cold, thin primary disk, the gas-rich merger works well for the massive, compact primary disk of NGC 4138 even though the mass of the dwarf galaxy is a significant fraction of the mass of the primary disk. Although we have restricted ourselves mainly to coplanar infall and mergers, we report on one inclined infall simulation as well. We also explore the possibility that the H-alpha ring seen in the inner half of the disk is a consequence of counterrotating gas clouds colliding with corotating gas already present in the disk and forming stars in the process.Comment: To appear in ApJ, 21 pages, LaTeX (aaspp4) format, 17 figs (gzipped tar file) also available at ftp://bessel.mps.ohio-state.edu/pub/thakar/cr2/ or at http://www-astronomy.mps.ohio-state.edu/~thakar