Multiwavelength Light Curve Parameters of Cepheid Variables

We present a comparative analysis of theoretical and observed light curves of Cepheid variables using Fourier decomposition. The theoretical light curves at multiple wavelengths are generated using stellar pulsation models for chemical compositions representative of Cepheids in the Galaxy and Magellanic Clouds. The observed light curves at optical ({\it VI}), near-infrared ({\it JHK}$_s$) and mid-infrared (3.6 $\&$ 4.5-$\mu$m) bands are compiled from the literature. We discuss the variation of light curve parameters as a function of period, wavelength and metallicity. Theoretical and observed Fourier amplitude parameters decrease with increase in wavelength while the phase parameters increase with wavelength. We find that theoretical amplitude parameters obtained using canonical mass-luminosity levels exhibit a greater offset with respect to observations when compared to non-canonical relations. We also discuss the impact of variation in convective efficiency on the light curve structure of Cepheid variables. The increase in mixing length parameter results in a zero-point offset in bolometric mean magnitudes and reduces the systematic large difference in theoretical amplitudes with respect to observations.Comment: 5 pages, 4 figures, proceedings for the 22nd Los Alamos Stellar Pulsation Conference Series Meeting "Wide field variability surveys: a 21st-century perspective", held in San Pedro de Atacama, Chile, Nov. 28 - Dec. 2, 201

Optical Counterparts of X-Ray Point Sources Observed by CHANDRA in NGC5128: 20 New Globular Cluster X-Ray Sources

VLT images in $BVI$ are used to identify the optical counterparts to bright CHANDRA X-ray points sources discovered by Kraft et al. (2001, ApJ, 560, 675) in NGC5128. Of a total of 111 X-ray point sources with L_X>2*10^{36} ergs/s present in a 56 arcmin^2 field centered on this galaxy, 58 have optical counterparts. Based on the sizes, optical magnitudes and colors, 20 new globular cluster counterparts of X-ray sources are identified, plus 3 identified based on their sizes. This brings the total number of globular cluster X-ray sources in this galaxy to 33, and establishes that 30% of the X-ray point sources in NGC5128 are associated with globular clusters. These X-ray globular clusters occupy the brightest end of the globular cluster luminosity function, indicating that bright low-mass X-ray binaries are preferentially found in massive clusters. Most of the globular clusters with X-ray sources have red colors, with 1.0<V-I<1.5, indicating that low-mass X-ray binaries are preferentially formed in metal-rich clusters. The NGC5128 X-ray globular cluster sources are brighter in comparison with the Milky Way sources: there are 24 globular clusters with X-ray sources of L_x>10^{37} erg/sec. There is, however, no globular cluster X-ray source in NGC5128 as bright as expected for an accreting black hole. In addition, 31 optical counterparts of X-ray point sources that are not associated with globular clusters are identified. Finally, 53 X-ray point sources (48% of the population), do not have any optical counterparts down to the faintest magnitude limits (B=25).Comment: 32 pages, 8 figures; replaced with the accepted version; to appear in ApJ, January 200

Large Magellanic Cloud Near-infrared Synoptic Survey. IV. Leavitt Laws for Type II Cepheid Variables

We present time-series observations of Population \rom{2} Cepheids in the Large Magellanic Cloud at near-infrared ($JHK_s$) wavelengths. Our sample consists of 81 variables with accurate periods and optical ($VI$) magnitudes from the OGLE survey, covering various subtypes of pulsators (BL Herculis, W Virginis and RV Tauri). We generate light curve templates using high-quality $I$-band data in the LMC from OGLE and $K_s$-band data in the Galactic Bulge from VVV and use them to obtain robust mean magnitudes. We derive Period-Luminosity (P-L) relations in the near-infrared and Period-Wesenheit (P-W) relations by combining optical and near-infrared data. Our P-L and P-W relations are consistent with published work when excluding long-period RV Tauris. We find that Pop II Cepheids and RR Lyraes follow the same P-L relations in the LMC. Therefore, we use trigonometric parallax from the {\it Gaia DR1} for VY~Pyx and the {\it Hubble Space Telescope} parallaxes for $k$~Pav and 5 RR Lyrae variables to obtain an absolute calibration of the Galactic $K_s$-band P-L relation, resulting in a distance modulus to the LMC of $\mu_{\rm LMC} = 18.54\pm0.08$~mag. We update the mean magnitudes of Pop~II Cepheids in Galactic globular clusters using our light curve templates and obtain distance estimates to those systems, anchored to a precise late-type eclipsing binary distance to the LMC. We find the distances to these globular clusters based on Pop~II Cepheids are consistent (within $2\sigma$) with estimates based on the $M_V-[\rm{Fe}/\rm{H}]$ relation for horizontal branch stars.Comment: Accepted for publication in The Astronomical Journa

The dwarf galaxy satellite system of Centaurus A

Dwarf galaxy satellite systems are essential probes to test models of structure formation, making it necessary to establish a census of dwarf galaxies outside of our own Local Group. We present deep FORS2 VI band images from the ESO Very Large Telescope (VLT) for 15 dwarf galaxy candidates in the Centaurus group of galaxies. We confirm nine dwarfs to be members of Cen A by measuring their distances using a Bayesian approach to determine the tip of the red giant branch luminosity. We have also fit theoretical isochrones to measure their mean metallicities. The properties of the new dwarfs are similar to those in the Local Group in terms of their sizes, luminosities, and mean metallicities. Within our photometric precision, there is no evidence of a metallicity spread, but we do observe possible extended star formation in several galaxies, as evidenced by a population of asymptotic giant branch stars brighter than the red giant branch tip. The new dwarfs do not show any signs of tidal disruption. Together with the recently reported dwarf galaxies by the complementary PISCeS survey, we study the luminosity function and 3D structure of the group. By comparing the observed luminosity function to the high-resolution cosmological simulation IllustrisTNG, we find agreement within a 90% confidence interval. However, Cen A seems to be missing its brightest satellites and has an overabundance of the faintest dwarfs in comparison to its simulated analogs. In terms of the overall 3D distribution of the observed satellites, we find that the whole structure is flattened along the line-of-sight, with a root-mean-square (rms) height of 130 kpc and an rms semi-major axis length of 330 kpc. Future distance measurements of the remaining dwarf galaxy candidates are needed to complete the census of dwarf galaxies in the Centaurus group.O.M. is grateful to the Swiss National Science Foundation for financial support

Surface brightness fluctuation spectrum: a new probe of evolved stars in unresolved stellar populations

The surface brightness fluctuation (SBF) method measures spatial fluctuations due to the most luminous stars in a galaxy. Besides being useful for distance measurements, it also provides diagnostic power to investigate the contribution of the brightest stars to the underlying unresolved stellar population. We apply the SBF technique to every wavelength layer in integral field spectroscopy data obtained with the MUSE instrument. This yields the first SBF spectrum of a galaxy. SBF spectra combine the sensitivity of SBF to luminous stars with the physical information content of spectra. We show that the SBF spectrum of the S0 galaxy NGC 5102 is dominated by spectra of M-type giants (red giant branch, asymptotic giant branch, and red supergiant stars). We build the first stellar population synthesis tool that predicts SBF spectra. Through comparison of integrated spectra from our new tool with published model spectra, we confirm that it is mathematically correct. We use the integrated spectrum and a bootstrap method to derive an ensemble of (degenerate) star formation histories of NGC 5102. We compute a model SBF spectrum for each star formation history. Given that some models do not reproduce the observed SBF spectrum well, we are able to obtain additional constraints on the star formation history, leading to marginally reduced uncertainties on the mean age and metallicity. This result is confirmed by modelling mock spectra with different star formation histories

A 1D fluid model of the Centaurus A jet

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. Final published version available at: https://doi.org/10.1093/mnras/stz348.We implement a steady, one-dimensional flow model for the X-ray jet of Centaurus A in which entrainment of stellar mass loss is the primary cause of dissipation. Using over 260 ks of new and archival Chandra/ACIS data, we have constrained the temperature, density and pressure distributions of gas in the central regions of the host galaxy of Centaurus A, and so the pressure throughout the length of its jet. The model is constrained by the observed profiles of pressure and jet width, and conserves matter and energy, enabling us to estimate jet velocities, and hence all the other flow properties. Invoking realistic stellar populations within the jet, we find that the increase in its momentum flux exceeds the net pressure force on the jet unless only about one half of the total stellar mass loss is entrained. For self-consistent models, the bulk speed only falls modestly, from ~0.67c to ~0.52c over the range of 0.25-5.94 kpc from the nucleus. The sonic Mach number varies between ~5.3 and 3.6 over this range.Peer reviewe

Hubble Space Telescope Observations of cD Galaxies and their Globular Cluster Systems

We have used WFPC2 on the Hubble Space Telescope to obtain F450W and F814W images of four cD galaxies (NGC 541 in Abell 194, NGC 2832 in Abell 779, NGC 4839 in Abell 1656 and NGC 7768 in Abell 2666) in the range 5400 < cz < 8100 km s^{-1}. For NGC 541, the HST data are supplemented by ground-based B and I images obtained with the FORS1 on the VLT. We present surface brightness and color profiles for each of the four galaxies, confirming their classification as cD galaxies. Isophotal analyses reveal the presence of subarcsecond-scale dust disks in the nuclei of NGC 541 and NGC 7768. Despite the extreme nature of these galaxies in terms of spatial extent and luminosity, our analysis of their globular cluster systems reveals no anomalies in terms of specific frequencies, metallicity gradients, average metallicities, or the metallicity offset between the globulars and the host galaxy. We show that the latter offset appears roughly constant at \Delta [Fe/H] ~ 0.8 dex for early-type galaxies spanning a luminosity range of roughly four orders of magnitude. We combine the globular cluster metallicity distributions with an empirical technique described in a series of earlier papers to investigate the form of the protogalactic mass spectrum in these cD galaxies. We find that the observed GC metallicity distributions are consistent with those expected if cD galaxies form through the cannibalism of numerous galaxies and protogalactic fragments which formed their stars and globular clusters before capture and disruption. However, the properties of their GC systems suggest that dynamical friction is not the primary mechanism by which these galaxies are assembled. We argue that cDs instead form rapidly, via hierarchical merging, prior to cluster virialization.Comment: 36 pages, 20 postscript figures, uses emulateapj. Accepted for publication in the Astronomical Journa

Metal-poor nuclear star clusters in two dwarf galaxies near Centaurus A suggesting formation from the in-spiraling of globular clusters

Studies of nucleated dwarf galaxies can constrain the scenarios for the formation and evolution of nuclear star clusters (NSC) in low-mass galaxies and give us insights on the origin of ultra compact dwarf galaxies (UCDs). We report the discovery of a NSC in the dwarf galaxy KKs58 and investigate its properties together with those of another NSC in KK197. Both NSCs are hosted by dwarf elliptical galaxies of the Centaurus group. Combining ESO VLT MUSE data with photometry from VLT FORS2, CTIO Blanco DECam, and HST ACS, as well as high-resolution spectroscopy from VLT UVES, we analyse the photometric, kinematic and stellar population properties of the NSCs and their host galaxies. We confirm membership of the NSCs based on their radial velocities and location close to the galaxy centres. We also confirm the membership of two globular clusters (GCs) and detect oblate rotation in the main body of KK197. Based on high signal-to-noise spectra taken with MUSE of the NSCs of both KKs58 and KK197 we measure low metallicities, [Fe/H] = −1.75 ± 0.06 dex and [Fe/H] = −1.84 ± 0.05 dex, and stellar masses of 7.3 × 105 M and 1.0 × 106 M , respectively. Both NSCs are more metal-poor than their hosts that have metallicities of −1.35±0.23 dex (KKs58) and −0.84±0.12 dex (KK197). This can be interpreted as NSC formation via the in-spiral of GCs. The masses, sizes and metallicities of the two NSCs place them among other NSCs, but also among the known UCDs of the Centaurus group. This indicates that NSCs might constitute the progenitors of a part of the low-mass UCDs, although their properties are almost indistinguishable from typical GCs.OM is grateful to the Swiss National Science Foundation for financial support. GvdV acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme under grant agreement No 724857 (Consolidator Grant ArcheoDyn). HJ acknowledges support from the Australian Research Council through the Discovery Project DP150100862