UCDs as Probes of the Major and Minor Merger Histories of Galaxies

Two competing theories posit that Ultra Compact Dwarfs (UCDs) form either as the stripped nuclei of dwarf galaxies or as giant globular clusters (GGCs) associated with the largest globular cluster (GC) systems. By focussing on the field and group environments where young UCDs may be most common, we have discovered the first UCD that is clearly the result of recent (<4 Gyr ago) stripping of a companion galaxy. However, we have also found a definitive case of a multiple-UCD system created via GC formation processes, which are likely associated with major galaxy mergers. We demonstrate that it is possible to reliably distinguish the two types of UCD, thereby probing both the major and minor merger histories of individual galaxies.Comment: 2 pages, 1 figure, to appear in the proceedings of the conference "A Universe of Dwarf Galaxies" (Lyon, June 14-18, 2010

Forming young bulges within existing disks: Statistical evidence for external drivers

Contrary to traditional models of galaxy formation, recent observations suggest that some bulges form within preexisting disk galaxies. Such late-epoch bulge formation within disks seems to be linked to disk gas inflow and central star formation, caused by either internal secular processes or galaxy mergers and interactions. We identify a population of galaxies likely to be experiencing active bulge growth within disks, using the criterion that the color within the half-light radius is bluer than the outer disk color. Such blue-centered galaxies make up more than 10% of star-forming disk galaxies within the Nearby Field Galaxy Survey, a broad survey designed to represent the natural diversity of the low-z galaxy population over a wide range of luminosities and environments. Blue-centered galaxies correlate at 99% confidence with morphological peculiarities suggestive of minor mergers and interactions. From this and other evidence, we argue that external drivers rather than internal secular processes probably account for the majority of blue-centered galaxies. We go on to discuss quantitative plausibility arguments indicating that blue-centered evolutionary phases may represent an important mode of bulge growth for most disk galaxies, leading to significant changes in bulge-to-disk ratio without destroying disks. If this view is correct, bulge growth within disks may be a natural consequence of the repeated galaxy mergers and interactions inherent in hierarchical galaxy formation

Addressing the [O III]/Hβ offset of dwarf galaxies in the RESOLVE survey

Metal-poor dwarf galaxies in the local universe, such as those found in the RESOLVE galaxy survey, often produce high [O III]/Hβ ratios close to the star-forming demarcation lines of the diagnostic BPT diagram. Modelling the emission from these galaxies at lower metallicities generally underpredicts this line ratio, which is typically attributed to a deficit of photons >35 eV. We show that applying a model that includes empirical abundances scaled with metallicity strongly influences the thermal balance in HII regions and preserves the [O III]/Hβ offset even in the presence of a harder radiation field generated by interacting binaries. Additional heating mechanisms are more successful in addressing the offset. In accordance with the high sSFR typical of dwarf galaxies in the sample, we demonstrate that cosmic ray heating serves as one mechanism capable of aligning spectral synthesis predictions with observations. We also show that incorporating a range of physical conditions in our modelling can create even better agreement between model calculations and observed emission-line ratios. Together these results emphasize that both the hardness of the incident continuum and the variety of physical conditions present in nebular gas clouds must be accurately accounted for prior to drawing conclusions from emission-line diagnostic diagrams

Angular size measurements of carbon Miras and S-type stars

In our continuing investigation of highly evolved stars, we report new interferometric angular diameter observations of 5 carbon and 4 S-type Mira variable stars, and 4 non-Mira S stars. From the data, effective temperatures and linear radii are calculated. We compare the values of these parameters obtained for stars discussed in this paper with the same parameters for oxygen-rich giants/supergiants, oxygen-rich Mira variables, and non-Mira carbon stars presented in Dyck et al. (1996a, AJ, 111, 1705), van Belle et al. (1996, AJ, 112, 2147), and Dyck et al. (1996b, AJ, 112, 294), respectively. There are two principal findings from a synthesis of these studies. First, the non-Mira variables of each chemical class are consistently hotter and smaller than their Mira-variable counterparts. Second, the S stars lie between the oxygen-rich and the carbon-rich stars in both effective temperature and linear radius, for both the Mira-type and non-Mira stars

SN 2005cg: Explosion physics and circumstellar interaction of a normal type la supernova in a low-luminosity host

We present the spectral evolution, light curve, and corresponding interpretation for the "normal-bright" Type la supernova 2005cg discovered by ROTSE-IIIc. The host is a low-luminosity (Mr = -16.75) blue galaxy with strong indications of active star formation and an environment similar to that expected for SNe la at high redshifts. Earlytime (t ̃ -10 days) optical spectra obtained with the HET reveal an asymmetric, triangular-shaped Si II absorption feature at about 6100 Å with a sharp transition to the continuum at a blueshift of about 24,000 km s-1. By 4 days before maximum, the Si n absorption feature becomes symmetric with smoothly curved sides. Similar Si n profile evolution has previously been observed in other supemovae and is predicted by some explosion models, but its significance has not been fully recognized. Although the spectra predicted by pure deflagration and delayed detonation models are similar near maximum light, they predict qualitatively different chemical abundances in the outer layers and thus give qualitatively different spectra at the earliest phases. The Si line observed in SN 2005cg at early times requires the presence of burning products at high velocities, and the triangular shape is likely to be formed in an extended region of slowly declining Si abundance that characterizes delayed detonation models. The spectra show a high-velocity Ca n IR feature that coincides in velocity space with the Si n cutoff. This supports the interpretation that the Ca n is formed when the outer layers of the SN ejecta sweep up about 5 × 10-3 M, of material within the progenitor system. We compare our results with other "Branch-normal" SNe la with early-time spectra, namely, SN 2003du, 1999ee, and 1994D. Although the expansion velocities based on their Si n absorption minima differ, all show triangular-shaped profiles and velocity cutoffs between 23,000 and 25,000 km s-1, which are consistent with the Doppler shifts of their respective high-velocity Ca II IR features. SN 1990N-like objects, however, showed distinctly different behavior, which may suggest separate progenitor subclasses

The baryonic collapse efficiency of galaxy groups in the RESOLVE and ECO surveys

We examine the z = 0 group-integrated stellar and cold baryonic (stars + cold atomic gas) mass functions (group SMF and CBMF) and the baryonic collapse efficiency (group cold baryonic to dark matter halo mass ratio) using the RESOLVE and ECO survey galaxy group catalogs and a GALFORM semi-analytic model (SAM) mock catalog. The group SMF and CBMF fall off more steeply at high masses and rise with a shallower low-mass slope than the theoretical halo mass function (HMF). The transition occurs at the group-integrated cold baryonic mass Mbary cold ~ 1011 M. The SAM, however, has significantly fewer groups at the transition mass ∼1011 M and a steeper low-mass slope than the data, suggesting that feedback is too weak in low-mass halos and conversely too strong near the transition mass. Using literature prescriptions to include hot halo gas and potential unobservable galaxy gas produces a group BMF with a slope similar to the HMF even below the transition mass. Its normalization is lower by a factor of ∼2, in agreement with estimates of warm-hot gas making up the remaining difference. We compute baryonic collapse efficiency with the halo mass calculated two ways, via halo abundance matching (HAM) and via dynamics (extended all the way to three-galaxy groups using stacking). Using HAM, we find that baryonic collapse efficiencies reach a flat maximum for groups across the halo mass range of Mhalo ~ 1011.4 - 12 M, which we label “nascent groups.” Using dynamics, however, we find greater scatter in baryonic collapse efficiencies, likely indicating variation in group hot-to-cold baryon ratios. Similarly, we see higher scatter in baryonic collapse efficiencies in the SAM when using its true groups and their group halo masses as opposed to friends-of-friends groups and HAM masses

Void galaxies follow a distinct evolutionary path in the environmental context catalog

We measure the environmental dependence, where environment is defined by the distance to the third nearest neighbor, of multiple galaxy properties inside the Environmental COntext (ECO) catalog. We focus primarily on void galaxies, which we define as the 10% of galaxies having the lowest local density. We compare the properties of void and non-void galaxies: baryonic mass, color, fractional stellar mass growth rate (FSMGR), morphology, and gas-to-stellar-mass ratio (estimated from a combination of H I data and photometric gas fractions calibrated with the REsolved Spectroscopy Of a Local VolumE survey). Our void galaxies typically have lower baryonic masses than galaxies in denser environments, and they display the properties expected of a lower mass population: they have more late types, are bluer, have a higher FSMGR, and are more gas-rich. We control for baryonic mass and investigate the extent to which void galaxies are different at fixed mass. Void galaxies are bluer, more gas-rich, and more star-forming at fixed mass than non-void galaxies, which is a possible signature of galaxy assembly bias. Furthermore, we show that these trends persist even at fixed mass and morphology, and we find that voids host a distinct population of early types that are bluer and more star-forming than the typical red and quenched early types. In addition to these empirical observational results, we also present theoretical results from mock catalogs with built-in galaxy assembly bias. We show that a simple matching of galaxy properties to (sub)halo properties, such as mass and age, can recover the observed environmental trends in ECO galaxies

UBVRI Light curves of 44 Type Ia supernovae

We present UBVRI photometry of 44 Type la supernovae (SNe la) observed from 1997 to 2001 as part of a continuing monitoring campaign at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics. The data set comprises 2190 observations and is the largest homogeneously observed and reduced sample of SNe la to date, nearly doubling the number of well-observed, nearby SNe la with published multicolor CCD light curves. The large sample of [U-band photometry is a unique addition, with important connections to SNe la observed at high redshift. The decline rate of SN la U-band light curves correlates well with the decline rate in other bands, as does the U - B color at maximum light. However, the U-band peak magnitudes show an increased dispersion relative to other bands even after accounting for extinction and decline rate, amounting to an additional ∼40% intrinsic scatter compared to the B band

LADUMA: looking at the distant universe with the MeerKAT array

The cosmic evolution of galaxies’ neutral atomic gas content is a major science driver for the Square Kilometre Array (SKA), as well as for its South African (MeerKAT) and Australian (ASKAP) precursors. Among the H I large survey programs (LSPs) planned for ASKAP and MeerKAT, the deepest and narrowest tier of the “wedding cake” will be defined by the combined L-band+UHF-band Looking At the Distant Universe with the MeerKAT Array (LADUMA) survey, which will probe H I in emission within a single “cosmic vuvuzela” that extends to z = 1.4, when the universe was only a third of its present age. Through a combination of individual and stacked detections (the latter relying on extensive multi-wavelength studies of the survey’s target field), LADUMA will study the redshift evolution of the baryonic Tully–Fisher relation and the cosmic H I density, the variation of the H I mass function with redshift and environment, and the connection between H I content and galaxies’ stellar properties (mass, age, etc.). The survey will also build a sample of OH megamaser detections that can be used to trace the cosmic merger history. This proceedings contribution provides a brief introduction to the survey, its scientific aims, and its technical implementation, deferring a more complete discussion for a future article after the implications of a recent review of MeerKAT LSP project plans are fully worked out

Estimating Dwarf Galaxy Gas Masses

Dwarf galaxies dominate the galaxy luminosity function, yet their contribution to the baryonic mass function is not well determined. Photometric gas fraction estimators allow us to obtain neutral atomic gas masses for statistical samples of galaxies to help constrain the baryonic mass function. We adapt the photometric gas fraction technique to be more useful for predicting gas masses of dwarf galaxies