A New Method for Estimating Dark Matter Halo Masses using Globular Cluster Systems

All galaxies are thought to reside within large halos of dark matter, whose properties can only be determined from indirect observations. The formation and assembly of galaxies is determined from the interplay between these dark matter halos and the baryonic matter they host. Although statistical relations can be used to approximate how massive a galaxy's halo is, very few individual galaxies have direct measurements of their halo masses. We present a method to directly estimate the total mass of a galaxy's dark halo using its system of globular clusters. The link between globular cluster systems and halo masses is independent of a galaxy's type and environment, in contrast to the relationship between galaxy halo and stellar masses. This trend is expected in models where globular clusters form in early, rare density peaks in the cold dark matter density field and the epoch of reionisation was roughly coeval throughout the Universe. We illustrate the general utility of this relation by demonstrating that a galaxy's supermassive black hole mass and global X-ray luminosity are directly proportional to their host dark halo masses, as inferred from our new method.Comment: 6 pages, 4 colour figures. Accepted by MNRAS Letters. Data catalogue available from the first autho

Extending the baseline: Spitzer Mid-Infrared Photometry of Globular Cluster Systems in the Centaurus A and Sombrero Galaxies

Spitzer IRAC mid-infrared photometry is presented for the globular cluster (GC) systems of the NGC 5128 ("Centaurus A") and NGC 4594 ("Sombrero") galaxies. Existing optical photometric and spectroscopic are combined with this new data in a comprehensive optical to mid-IR colour catalogue of 260 GCs. Empirical colour-metallicity relationships are derived for all optical to mid-IR colour combinations. These colours prove to be very effective quantities to test the photometric predictions of simple stellar population (SSP) models. In general, four SSP models show larger discrepancies between each other and the data at bluer wavelengths, especially at high metallicities. Such differences become very important when attempting to use colour-colour model predictions to constrain the ages of stellar populations. Furthermore, the age-substructure determined from colour-colour diagrams and 91 NGC 5128 GCs with spectroscopic ages from Beasley et al. (2008) are inconsistent, suggesting any apparent GC system age-substructure implied by a colour-colour analysis must be verified independently. Unlike blue wavebands, certain optical to mid-IR colours are insensitive to the flux from hot horizontal branch stars and thus provide an excellent metallicity proxy. The NGC 5128 GC system shows strong bimodality in the optical R-band to mid-IR colour distributions, hence proving it is bimodal in metallicity. In this new colour space, a colour-magnitude trend, a "blue tilt", is found in the NGC 5128 metal-poor GC data. The NGC 5128 young GCs do not contribute to this trend. [abridged]Comment: 16 pages, 12 colour figures. To be published in MNRAS. Catalogue available from the first author. Full resolution copy available here http://lee.spitler.googlepages.com/spitzer_spitler.pd

A selection of H{\alpha} emitters at z = 2.1-2.5 using the Ks-band photometry of ZFOURGE

Large and less-biased samples of star-forming galaxies are essential to investigate galaxy evolution. H{\alpha} emission line is one of the most reliable tracers of star-forming galaxies because its strength is directly related to recent star formation. However, it is observationally expensive to construct large samples of H{\alpha} emitters by spectroscopic or narrow-band imaging survey at high-redshifts. In this work, we demonstrate a method to extract H{\alpha} fluxes of galaxies at z = 2.1-2.5 from Ks broad-band photometry of ZFOURGE catalog. Combined with 25-39 other filters, we estimate the emission line fluxes by SED fitting with stellar population models that incorporate emission-line strengths. 2005 galaxies are selected as H{\alpha} emitters by our method and their fluxes show good agreement with previous measurements in the literature. On the other hand, there are more H{\alpha} luminous galaxies than previously reported. The discrepancy can be explained by extended H{\alpha} profiles of massive galaxies and a luminosity dependence of dust attenuation, which are not taken into account in the previous work. We also find that there are a large number of low-mass galaxies with much higher specific star formation rate (sSFR) than expected from the extrapolated star formation main sequence. Such low-mass galaxies exhibit larger ratios between H{\alpha} and UV fluxes compared to more massive high sSFR galaxies. This result implies that a "starburst" mode may differ among galaxies: low-mass galaxies appear to assemble their stellar mass via short-duration bursts while more massive galaxies tend to experience longer-duration (> 10 Myr) bursts.Comment: 18 pages, 19 figures, Resubmitted to ApJ after addressing reviewer's comment

Infrared-Faint Radio Sources are at high redshifts

Context: Infrared-Faint Radio Sources (IFRS) are characterised by relatively high radio flux densities and associated faint or even absent infrared and optical counterparts. The resulting extremely high radio-to-infrared flux density ratios up to several thousands were previously known only for High-redshift Radio Galaxies (HzRGs), suggesting a link between the two classes of object. Prior to this work, no redshift was known for any IFRS in the Australia Telescope Large Area Survey (ATLAS) fields which would help to put IFRS in the context of other classes of object, especially of HzRGs. Aims: This work aims at measuring the first redshifts of IFRS in the ATLAS fields. Further, we test the hypothesis that IFRS are similar to HzRGs, as higher-redshift or dust-obscured versions of these massive galaxies. Methods: A sample of IFRS was spectroscopically observed using the Focal Reducer and Low Dispersion Spectrograph 2 (FORS2) at the Very Large Telescope (VLT). The data were calibrated based on the Image Reduction and Analysis Facility (IRAF) and redshifts extracted. This information was then used to calculate rest-frame luminosities, and to perform the first spectral energy distribution modelling of IFRS based on redshifts. Results: We found redshifts of 1.84, 2.13, and 2.76, for three IFRS, confirming the suggested high-redshift character of this class of object. These redshifts as well as the resulting luminosities show IFRS to be similar to HzRGs. We found further evidence that fainter IFRS are at even higher redshifts. Conclusions: Considering the similarities between IFRS and HzRGs substantiated in this work, the detection of IFRS, which have a significantly higher sky density than HzRGs, increases the number of Active Galactic Nuclei in the early universe and adds to the problems of explaining the formation of supermassive black holes shortly after the Big Bang.Comment: 7 pages, 4 figures; version in prin

A Blue Tilt in the Globular Cluster System of the Milky Way-like Galaxy NGC 5170

Here we present HST/ACS imaging, in the B and I bands, of the edge-on Sb/Sc galaxy NGC 5170. Excluding the central disk region region, we detect a 142 objects with colours and sizes typical of globular clusters (GCs). Our main result is the discovery of a `blue tilt' (a mass-metallicity relation), at the 3sigma level, in the metal-poor GC subpopulation of this Milky Way like galaxy. The tilt is consistent with that seen in massive elliptical galaxies and with the self enrichment model of Bailin & Harris. For a linear mass-metallicity relation, the tilt has the form Z ~ L^{0.42 +/- 0.13}. We derive a total GC system population of 600 +/- 100, making it much richer than the Milky Way. However when this number is normalised by the host galaxy luminosity or stellar mass it is similar to that of M31. Finally, we report the presence of a potential Ultra Compact Dwarf of size ~ 6 pc and luminosity M_I ~ -12.5, assuming it is physically associated with NGC 5170.Comment: Accepted for publication in MNRAS, 11 pages, 10 figure

A Study of Selection Methods for H alpha Emitting Galaxies at z~1.3 for the Subaru/FMOS Galaxy Redshift Survey for Cosmology (FastSound)

The efficient selection of high-redshift emission galaxies is important for future large galaxy redshift surveys for cosmology. Here we describe the target selection methods for the FastSound project, a redshift survey for H alpha emitting galaxies at z=1.2-1.5 using Subaru/FMOS to measure the linear growth rate f\sigma 8 via Redshift Space Distortion (RSD) and constrain the theory of gravity. To select ~400 target galaxies in the 0.2 deg^2 FMOS field-of-view from photometric data of CFHTLS-Wide (u*g'r'i'z'), we test several different methods based on color-color diagrams or photometric redshift estimates from spectral energy distribution (SED) fitting. We also test the improvement in selection efficiency that can be achieved by adding near-infrared data from the UKIDSS DXS (J). The success rates of H alpha detection with FMOS averaged over two observed fields using these methods are 11.3% (color-color, optical), 13.6% (color-color, optical+NIR), 17.3% (photo-z, optical), and 15.1% (photo-z, optical+NIR). Selection from photometric redshifts tends to give a better efficiency than color-based methods, although there is no significant improvement by adding J band data within the statistical scatter. We also investigate the main limiting factors for the success rate, by using the sample of the HiZELS H alpha emitters that were selected by narrow-band imaging. Although the number density of total H alpha emitters having higher H alpha fluxes than the FMOS sensitivity is comparable with the FMOS fiber density, the limited accuracy of photometric redshift and H alpha flux estimations have comparable effects on the success rate of <~20% obtained from SED fitting.Comment: 12 pages, 7 figures, accepted to PAS

Spectra of globular clusters in the Sombrero galaxy: evidence for spectroscopic metallicity bimodality

We present a large sample of over 200 integrated-light spectra of confirmed globular clusters (GCs) associated with the Sombrero (M104) galaxy taken with the DEIMOS instrument on the Keck telescope. A significant fraction of the spectra have signal-to-noise levels high enough to allow measurements of GC metallicities using the method of Brodie & Huchra (1990). We find a distribution of spectroscopic metallicities ranging from -2.2 < [Fe/H] < +0.1 that is bimodal, with peaks at [Fe/H] ~ -1.4 and -0.6. Thus the GC system of the Sombrero galaxy, like a few other galaxies now studied in detail, reveals a bimodal spectroscopic metallicity distribution supporting the long-held belief that colour bimodality reflects two metallicity subpopulations. This further suggests that the transformation from optical colour to metallicity for old stellar populations, such as GCs, is not strongly non-linear. We also explore the radial and magnitude distribution with metallicity for GC subpopulations but small number statistics prevent any clear trends in these distributions.Comment: 18 pages, 10 figures, 3 tables, MNRAS accepte

ZFIRE: The Evolution of the Stellar Mass Tully-Fisher Relation to Redshift 2.0 < Z < 2.5 with MOSFIRE

Using observations made with MOSFIRE on Keck I as part of the ZFIRE survey, we present the stellar mass Tully-Fisher relation at 2.0 < z < 2.5. The sample was drawn from a stellar mass limited, Ks-band selected catalog from ZFOURGE over the CANDELS area in the COSMOS field. We model the shear of the Halpha emission line to derive rotational velocities at 2.2X the scale radius of an exponential disk (V2.2). We correct for the blurring effect of a two-dimensional PSF and the fact that the MOSFIRE PSF is better approximated by a Moffat than a Gaussian, which is more typically assumed for natural seeing. We find for the Tully-Fisher relation at 2.0 < z < 2.5 that logV2.2 =(2.18 +/- 0.051)+(0.193 +/- 0.108)(logM/Msun - 10) and infer an evolution of the zeropoint of Delta M/Msun = -0.25 +/- 0.16 dex or Delta M/Msun = -0.39 +/- 0.21 dex compared to z = 0 when adopting a fixed slope of 0.29 or 1/4.5, respectively. We also derive the alternative kinematic estimator S0.5, with a best-fit relation logS0.5 =(2.06 +/- 0.032)+(0.211 +/- 0.086)(logM/Msun - 10), and infer an evolution of Delta M/Msun= -0.45 +/- 0.13 dex compared to z < 1.2 if we adopt a fixed slope. We investigate and review various systematics, ranging from PSF effects, projection effects, systematics related to stellar mass derivation, selection biases and slope. We find that discrepancies between the various literature values are reduced when taking these into account. Our observations correspond well with the gradual evolution predicted by semi-analytic models.Comment: 21 pages, 14 figures, 1 appendix. Accepted for publication by Apj, February 28, 201

Evidence for inhomogeneous reionization in the local Universe from metal-poor globular cluster systems

Exploiting a fundamental characteristic of galaxy assembly in the Λ cold dark matter paradigm, the observed spatial biasing and kinematics of metal-poor globular star clusters are used to constrain the local reionization epoch around individual galaxies. Selecting three galaxies located in different environments, the first attempt at constraining the environmental propagation of reionization in the local Universe is carried out. The joint constraint from the three galaxies () agrees remarkably well with the latest Wilkinson Microwave Anisotropy Probe constraint on zreion for a simple instantaneous reionization model. More importantly, the range of zreion values found here is consistent with the global range of zreion estimates from other observations. We furthermore find a 1.7σ indication that reionization completed in low-density environments before the intergalactic medium in high-density environments was reionized. This is consistent with certain theoretical models that predict that reionization was globally prolonged in duration, with neutral hydrogen pockets surviving in high-density environments, even after the surrounding regions were reionized. More generally, this work provides a useful constraint on the formation history of galaxy stellar haloe