Calibration of the Fundamental Plane Zero-point in the Leo-I Group and an Estimate of the Hubble Constant

We derive new effective radii and total magnitudes for 5 E and S0 galaxies in the Leo-I group from wide-field CCD images. These are used in conjunction with recent literature velocity data to construct the fundamental plane (FP) of the Leo-I group. The rms scatter that we find is only 6 % in distance. The zero point of this relation provides a calibration of the FP as a distance indicator and directly determines the angular diameter distance ratio between the Leo-I group and more distant clusters. In the language of Jerjen and Tammann (1993) we determine a cosmic velocity of the Leo-I group of 757+-68 km/s relative to the Coma cluster, or 796+-57 km/s relative to a frame of 9 clusters. Combining this velocity with the Cepheid distance to M96, a member of Leo-I, we find the Hubble constant to be H_0=67+-8 km/s/Mpc or H_0=70+-7 km/s/Mpc for each case. The distance we obtain for the Coma cluster itself (108+-12 Mpc) is in good agreement with a number of other recent estimates.Comment: 19 pages, LaTeX, includes aaspp4.sty and 3 eps figures. To appear in ApJ. Also available at http://www.nordita.dk/~jens/preprints.htm

Detection of Intergalactic Red-Giant-Branch Stars in the Virgo Cluster

It has been suspected for nearly 50 years that clusters of galaxies contain a population of intergalactic stars, ripped from galaxies during cluster formation or when the galaxies’ orbits take them through the cluster center. Support for the existence of such a population of free-floating stars comes from measurements of the diffuse light in clusters and from recent detections of planetary nebulae with positions and/or velocities far removed from any observed cluster galaxy. 10 , 11 But estimates for the mass of the diffuse population and its distribution relative to the galaxies are still highly uncertain. Here we report the direct detection of intergalactic stars in deep images of a blank field in the Virgo Cluster. The data suggest that approximately 10% of the stellar mass of the cluster is in intergalactic stars. We observe a relatively homogeneous distribution of stars, with evidence of a slight gradient toward M87

GRB 170817A as a Refreshed Shock Afterglow viewed off-axis

Energy injection into the external shock system that generates the afterglow to a gamma-ray burst (GRB) can result in a re-brightening of the emission. Here we investigate the off-axis view of a re-brightened refreshed shock afterglow. We find that the afterglow light-curve, when viewed from outside of the jet opening angle, could be characterised by a slow rise, or long-plateau, with a maximum flux determined by the total system energy. Using the broadband afterglow data for GRB170817A, associated with the gravitational wave detected binary neutron star merger GW170817, we show that a refreshed shock model with a simple top-hat jet can reproduce the observed afterglow features. We consider two particular refreshed shock models: a single episode of energy injection; and a period of continuous energy injection. The best fit model parameters give a jet opening angle, for our first or second model of $\theta_j=5.2^{+1.1}_{-0.6}~$or$~6.3^{+1.7}_{-1.1}$ deg, an inclination to the line of sight $\iota=16.0^{+3.4}_{-1.1}~$or$~17.8^{+4.5}_{-2.9}$ deg, an initial isotropic equivalent kinetic energy $E_1 = (0.3^{+3.5}_{-0.3}~$or$~0.5^{+6.7}_{-0.2})\times10^{52}$erg and a total/final, refreshed shock energy $E_{\rm total}=(0.42^{+5.6}_{-0.4}~$or$~1.26^{+18.2}_{-0.7})\times10^{53}$erg. The first model fitting prefers an initial bulk Lorentz factor $\Gamma_{0,1}<60$, with a comparatively low central value of $\Gamma_{0,1}=19.5$, indicating that, in this case, the on-axis jet could have been a `failed-GRB'. Alternatively, our second model is consistent with a bright GRB for an on-axis observer, with $\Gamma_{0,1}=162.2^{+219.7}_{-122.1}$. Due to the low-Lorentz factor or the jet opening angles at $\theta_j\sim\iota/3$, both models are unable to reproduce the $\gamma$-ray emission observed in GRB170817A, which would therefore require an alternative explanation such as cocoon shock-breakout.Comment: 14 pages, 6 figures - Version accepted for publication in ApJ. Analysis now includes two refreshed shock models and expanded discussio

A search for neutron star-black hole binary mergers in the short Gamma-ray burst population

Short gamma-ray bursts (SGRBs) are now known to be the product of the merger of two compact objects. However, two possible formation channels exist: neutron star–neutron star (NS–NS) or NS–black hole (BH). The landmark SGRB 170817A provided evidence for the NS–NS channel, thanks to analysis of its gravitational wave signal. We investigate the complete population of SGRBs with an associated redshift (39 events) and search for any divisions that may indicate that an NS–BH formation channel also contributes. Though no conclusive dichotomy is found, we find several lines of evidence that tentatively support the hypothesis that SGRBs with extended emission (EE; seven events) constitute the missing merger population: they are unique in the large energy-band sensitivity of their durations and have statistically distinct energies and host galaxy offsets when compared to regular (non-EE) SGRBs. If this is borne out via future gravitational wave detections, it will conclusively disprove the magnetar model for SGRBs. Furthermore, we identify the first statistically significant anticorrelation between the offsets of SGRBs from their host galaxies and their prompt emission energies

Dust reddening and extinction curves towards gamma-ray bursts at z > 4

Dust is known to be produced in the envelopes of AGB stars, the expanded shells of supernova (SN) remnants, and in situ grain growth in the ISM, although the corresponding efficiency of each of these dust formation mechanisms at different redshifts remains a topic of debate. During the first Gyr after the Big Bang, it is widely believed that there was not enough time to form AGB stars in high numbers, so that the dust at this epoch is expected to be purely from SNe, or subsequent grain growth in the ISM. The time period corresponding to z ~5-6 is thus expected to display the transition from SN-only dust to a mixture of both formation channels as we know it today. Here we aim to use afterglow observations of GRBs at redshifts larger than $z > 4$ in order to derive host galaxy dust column densities along their line-of-sight and to test if a SN-type dust extinction curve is required for some of the bursts. GRB afterglow observations were performed with the 7-channel GROND Detector at the 2.2m MPI telescope in La Silla, Chile and combined with data gathered with XRT. We increase the number of measured $A_V$ values for GRBs at z > 4 by a factor of ~2-3 and find that, in contrast to samples at mostly lower redshift, all of the GRB afterglows have a visual extinction of $A_V$ < 0.5 mag. Analysis of the GROND detection thresholds and results from a Monte-Carlo simulation show that, although we partly suffer from an observational bias against highly extinguished sight-lines, GRB host galaxies at 4 < z < 6 seem to contain on average less dust than at z ~ 2. Additionally, we find that all of the GRBs can be modeled with locally measured extinction curves and that the SN-like dust extinction curve provides a better fit for only two of the afterglow SEDs. For the first time we also report a photometric redshift of $z = 7.88$ for GRB 100905A, making it one of the most distant GRBs known to date.Comment: 26 pages, 37 figure

A Minor Axis Surface Brightness Profile for M31

We use data from the Isaac Newton Telescope Wide Field Camera survey of M31 to determine the surface brightness profile of M31 along the south-east minor axis. We combine surface photometry and faint red giant branch star counts to trace the profile from the innermost regions out to a projected radius of 4 degrees (~55 kpc) where the V-band surface brightness is 32 mag per square arcsec; this is the first time the M31 minor axis profile has been mapped over such a large radial distance using a single dataset. We confirm the finding by Pritchet & van den Bergh (1994) that the minor axis profile can be described by a single de Vaucouleurs law out to a projected radius of 1.4 degrees or ~20 kpc. Beyond this, the surface brightness profile flattens considerably and is consistent with either a power-law of index -2.3 or an exponential of scalelength 14 kpc. The fraction of the total M31 luminosity contained in this component is ~2.5%. While it is tempting to associate this outer component with a true Population II halo in M31, we find that the mean colour of the stellar population remains approximately constant at V-i~1.6 from 0.5-3.5 degrees along the minor axis. This result suggests that the same metal-rich stellar population dominates both structural components.Comment: 11 pages, 3 figures, ApJ Letters in press, extremely minor modification

Intergalactic Globular Clusters

We confirm and extend our previous detection of a population of intergalactic globular clusters in Abell 1185, and report the first discovery of an intergalactic globular cluster in the nearby Virgo cluster of galaxies. The numbers, colors and luminosities of these objects can place constraints on their origin, which in turn may yield new insights to the evolution of galaxies in dense environments.Comment: 2 pages, no figures. Talk presented at JD6, IAU General Assembly XXV, Sydney, Australia, July 2003, to appear in Highlights of Astronomy, Vol. 1

The Distance to NGC 4993: The Host Galaxy of the Gravitational-wave Event GW170817

The historic detection of gravitational waves from a binary neutron star merger (GW170817) and its electromagnetic counterpart led to the first accurate (sub-arcsecond) localization of a gravitational-wave event. The transient was found to be $\sim$10" from the nucleus of the S0 galaxy NGC 4993. We report here the luminosity distance to this galaxy using two independent methods. (1) Based on our MUSE/VLT measurement of the heliocentric redshift ($z_{\rm helio}=0.009783\pm0.000023$) we infer the systemic recession velocity of the NGC 4993 group of galaxies in the cosmic microwave background (CMB) frame to be $v_{\rm CMB}=3231 \pm 53$ km s$^{-1}$. Using constrained cosmological simulations we estimate the line-of-sight peculiar velocity to be $v_{\rm pec}=307 \pm 230$ km s$^{-1}$, resulting in a cosmic velocity of $v_{\rm cosmic}=2924 \pm 236$ km s$^{-1}$ ($z_{\rm cosmic}=0.00980\pm 0.00079$) and a distance of $D_z=40.4\pm 3.4$ Mpc assuming a local Hubble constant of $H_0=73.24\pm 1.74$ km s$^{-1}$ Mpc$^{-1}$. (2) Using Hubble Space Telescope measurements of the effective radius (15.5" $\pm$ 1.5") and contained intensity and MUSE/VLT measurements of the velocity dispersion, we place NGC 4993 on the Fundamental Plane (FP) of E and S0 galaxies. Comparing to a frame of 10 clusters containing 226 galaxies, this yields a distance estimate of $D_{\rm FP}=44.0\pm 7.5$ Mpc. The combined redshift and FP distance is $D_{\rm NGC 4993}= 41.0\pm 3.1$ Mpc. This 'electromagnetic' distance estimate is consistent with the independent measurement of the distance to GW170817 as obtained from the gravitational-wave signal ($D_{\rm GW}= 43.8^{+2.9}_{-6.9}$ Mpc) and confirms that GW170817 occurred in NGC 4993.Comment: 9 pages, 5 figure