The Array Camera for Optical to Near-IR Spectrophotometry, or ARCONS, is a camera based on Microwave Kinetic Inductance Detectors (MKIDs), a new technology that has the potential for broad application in astronomy. Using an array of MKIDs, the instru
ment is able to produce time-resolved imaging and low-resolution spectroscopy constructed from detections of individual photons. The arrival time and energy of each photon are recorded in a manner similar to X-ray calorimetry, but at higher photon fluxes. The technique works over a very large wavelength range, is free from fundamental read noise and dark-current limitations, and provides microsecond-level timing resolution. Since the instrument reads out all pixels continuously while exposing, there is no loss of active exposure time to readout. The technology requires a different approach to data reduction compared to conventional CCDs. We outline here the prototype data reduction pipeline developed for ARCONS, though many of the principles are also more broadly applicable to energy-resolved photon counting arrays (e.g., transition edge sensors, superconducting tunnel junctions). We describe the pipelines current status, and the algorithms and techniques employed in taking data from the arrival of photons at the MKID array to the production of images, spectra, and time-resolved light curves.
We present a new method for the determination of the two-dimensional (2D) projected spatial distribution of globular clusters (GCs) in external galaxies. This method is based on the K-Nearest Neighbor density estimator of Dressler (1980), complemente
d by MonteCarlo simulations to establish the statistical significance of the results. We apply this method to NGC4261, a test galaxy where significant 2D anisotropy in the GC distribution has been reported. We confirm that the 2D distribution of GC is not azimuthally isotropic. Moreover, we demonstrate that the 2D distribution departures from the average GC radial distribution results in highly significant spiral-like or broken shell features. Overall, the same perturbations are found in red and blue GCs, but with some differences. In particular, we observe a central feature, roughly aligned with the minor axis of NGC4261, composed of red and most luminous GCs. Blue and fainter GCs are more frequent at large radial distances and follow the spiral-like features of the overall density structure. These results suggest a complex merging history for NGC4261.
Aims: We describe our newly developed approach to detailed abundance analysis from integrated-light high-dispersion spectra of star clusters. As a pilot project, we measure abundances of several elements for three globular clusters (GCs) in the Forna
x dSph, using VLT/UVES spectra. Methods: We divide the cluster colour-magnitude diagrams into about 100 bins and compute synthetic spectra for each bin. The individual model spectra are co-added and the abundances are iteratively adjusted until the best match to the observed spectra is achieved. Results: We find [Fe/H] = -2.3, -1.4 and -2.1 for Fornax 3, 4 and 5, with +/-0.1 dex uncertainties. Fornax 3 and 5 are thus similar in metallicity to the most metal-poor Milky Way GCs and fall near the extreme metal-poor end of the field star metallicity distribution in Fornax. The [alpha/Fe] ratios, as traced by Ca and Ti, are enhanced with respect to the Solar composition at the level of about +0.25 dex for Fornax 3 and 5, and possibly slightly less (about +0.12 dex) for Fornax 4. For all three clusters the [Mg/Fe] ratio is significantly less elevated than [Ca/Fe] and [Ti/Fe], possibly an effect of the abundance anomalies that are well-known in Galactic GCs. We thus confirm that Mg may be a poor proxy for the overall alpha-element abundances for GCs. The abundance patterns of heavy elements (Y, Ba and Eu) indicate a dominant contribution to nucleosynthesis from the r-process in all three clusters, with a mean [Ba/Eu]=-0.7, suggesting rapid formation of the GCs. Conclusions: Combining our results with literature data for Fornax 1 and 2, four of the five Fornax GCs fall in the range -2.5<[Fe/H]<-2, while Fornax 4 is substantially more metal-rich than the others. The indications that abundance anomalies are detectable in integrated light are encouraging, particularly for the prospects of detecting such anomalies in young, massive star clusters.
Recently, high-dispersion spectroscopy has demonstrated conclusively that four of the five globular clusters (GCs) in the Fornax dwarf spheroidal galaxy are very metal-poor with [Fe/H]<-2. The remaining cluster, Fornax 4, has [Fe/H]=-1.4. This is in
stark contrast to the field star metallicity distribution which shows a broad peak around [Fe/H]=-1 with only a few percent of the stars having [Fe/H]<-2. If we only consider stars and clusters with [Fe/H]<-2 we thus find an extremely high GC specific frequency, SN=400, implying by far the highest ratio of GCs to field stars known anywhere. We estimate that about 1/5-1/4 of all stars in the Fornax dSph with [Fe/H]<-2 belong to the four most metal-poor GCs. These GCs could, therefore, at most have been a factor of 4-5 more massive initially. Yet, the Fornax GCs appear to share the same anomalous chemical abundance patterns known from Milky Way GCs, commonly attributed to the presence of multiple stellar generations within the clusters. The extreme ratio of metal-poor GC- versus field stars in the Fornax dSph is difficult to reconcile with scenarios for self-enrichment and early evolution of GCs in which a large fraction (90%-95%) of the first-generation stars have been lost. It also suggests that the GCs may not have formed as part of a larger population of now disrupted clusters with an initial power-law mass distribution. The Fornax dSph may be a rosetta stone for constraining theories of the formation, self-enrichment and early dynamical evolution of star clusters.
We present deep wide-field photometry of three recently discovered faint Milky Way satellites: Leo V, Pisces II, and Canes Venatici II. Our main goals are to study the structure and star formation history of these dwarfs; we also search for signs of
tidal disturbance. The three satellites have similar half-light radii ($sim 60-90$ pc) but a wide range of ellipticities. Both Leo V and CVn II show hints of stream-like overdensities at large radii. An analysis of the satellite color-magnitude diagrams shows that all three objects are old ($>$ 10 Gyr) and metal-poor ([Fe/H] $sim -2$), though neither the models nor the data have sufficient precision to assess when the satellites formed with respect to cosmic reionization. The lack of an observed younger stellar population ($la 10$ Gyr) possibly sets them apart from the other satellites at Galactocentric distances $ga 150$ kpc. We present a new compilation of structural data for all Milky Way satellite galaxies and use it to compare the properties of classical dwarfs to the ultra-faints. The ellipticity distribution of the two groups is consistent at the $sim$2-$sigma$ level. However, the faintest satellites tend to be more aligned toward the Galactic center, and those satellites with the highest ellipticity ($ga 0.4$) have orientations ($Delta theta_{GC}$) in the range $20^{circ} lesssim Delta theta_{GC} lesssim 40^{circ}$. This latter observation is in rough agreement with predictions from simulations of dwarf galaxies that have lost a significant fraction of their dark matter halos and are being tidally stripped.
