ELT imaging and photometry in crowded fields

Publications of the Astronomical Society of the Pacific, 2012

The expected imaging capabilities of future Extremely Large Telescopes (ELTs) will offer the unique possibility to investigate the stellar population of distant galaxies from the photometry of the stars in very crowded fields. Using simulated images and photometric analysis we explore here two representative science cases aimed at recovering the characteristics of the stellar populations in the inner regions of distant galaxies. Specifically: case A) at the center of the disk of a giant spiral in the Centaurus Group, (µ B ∼ 21, distance of 4.6 Mpc); and, case B) at half of the effective radius of a giant elliptical in the Virgo Cluster (µ I ∼ 19.5, distance of 18 Mpc). We generate synthetic frames by distributing model stellar populations and adopting a representative instrumental set up, i.e. a 42 m Telescope operating close to the diffraction limit. The effect of crowding is discussed in detail showing how stars are measured preferentially brighter than they are as the confusion limit is approached. We find that (i) accurate photometry (σ ∼ 0.1, completeness ∼ > 90%) can be obtained for case B) down to I ∼ 28.5, J ∼ 27.5 allowing us to recover the stellar metallicity distribution in the inner regions of ellipticals in Virgo to within ∼ 0.1 dex; (ii) the same photometric accuracy holds for the science case A) down to J ∼ 28.0, K ∼ 27.0, enabling to reconstruct of the star formation history up to the Hubble time via simple star counts in diagnostic boxes. For this latter case we discuss the possibility of deriving more detailed information on the star formation history from the analysis of their Horizontal Branch stars. We show that the combined features of high sensitivity and angular resolution of ELTs may open a new era for our knowledge of the stellar content of galaxies of different morphological type up to the distance of the Virgo cluster.

2016

The expected imaging capabilities of future Extremely Large Telescopes (ELTs) will offer the unique possibility to investigate the stellar population of distant galaxies from the photometry of the stars in very crowded fields. Using simulated images and photometric analysis we explore here two representative science cases aimed at recovering the characteristics of the stellar populations in the inner regions of distant galaxies. Specifically: case A) at the center of the disk of a giant spiral in the Centaurus Group, (µ B ∼ 21, distance of 4.6 Mpc); and, case B) at half of the effective radius of a giant elliptical in the Virgo Cluster (µ I ∼ 19.5, distance of 18 Mpc). We generate synthetic frames by distributing model stellar populations and adopting a representative instrumental set up, i.e. a 42 m Telescope operating close to the diffraction limit. The effect of crowding is discussed in detail showing how stars are measured preferentially brighter than they are as the confusion limit is approached. We find that (i) accurate photometry (σ ∼ 0.1, completeness ∼ > 90%) can be obtained for case B) down to I ∼ 28.5, J ∼ 27.5 allowing us to recover the stellar metallicity distribution in the inner regions of ellipticals in Virgo to within ∼ 0.1 dex; (ii) the same photometric accuracy holds for the science case A) down to J ∼ 28.0, K ∼ 27.0, enabling to reconstruct of the star formation history up to the Hubble time via simple star counts in diagnostic boxes. For this latter case we discuss the possibility of deriving more detailed information on the star formation history from the analysis of their Horizontal Branch stars. We show that the combined features of high sensitivity and angular resolution of ELTs may open a new era for our knowledge of the stellar content of galaxies of different morphological type up to the distance of the Virgo cluster.

Astronomy & Astrophysics, 2011

One of the key science goals for a diffraction limited imager on an extremely large telescope (ELT) is the resolution and accurate photometry of individual stars down to faint magnitudes in a range of environments. The aim of this study is to test the proposed capabilities of a multi-conjugate adaptive optics (MCAO) assisted imager working at the diffraction limit, in IJHK s filters, on a 42 m diameter ELT to carry out accurate stellar photometry in crowded images in an elliptical-like galaxy at the distance of the Virgo cluster. As a basis for realistic simulations we have used the phase A studies of the European-ELT project, including the MICADO imager and the MAORY MCAO module. We convolved a complex resolved stellar population with the telescope and instrument performance expectations to create realistic images. We then tested the ability of the currently available photometric packages STARFINDER and DAOPHOT to handle these simulated images. Our results show that for a one hour exposure deep colour-magnitude diagrams (photometric error, ±0.25 at I ≥ 27.2; H ≥ 25. and K s ≥ 24.6) of old stellar populations in galaxies, at the distance of Virgo, are feasible at a maximum surface brightness, μ V ∼ 17 mag/arcsec 2 (down to M I > −4 and M H ∼ M K > −6), and significantly deeper (photometric error, ±0.25 at I ≥ 29.3; H ≥ 26.6 and K s ≥ 26.2) for μ V ∼ 21 mag/arcsec 2 (down to M I ≥ −2 and M H ∼ M K ≥ −4.5). The photometric errors, and thus also the depth of the photometry could potentially be improved with photometry packages specifically designed for an ELT MCAO point spread function. We also make a simple comparison between these simulations and what can be expected from a single conjugate adaptive optics feed to MICADO and also the NIRCAM imager on the James Webb space telescope.

Proceedings of the International Astronomical Union, 2005

A key science driver for future ELTs is to chronicle the complete formation and evolutionary histories of a meaningful number of nearby galaxies through their resolved stars. The goal will be to measure the entire star formation and chemical enrichment histories of a sample of galaxies that includes all Hubble types and covers all of their components, demanding photometry of stars in regions with high surface brightness at distances of up to 20Mpc. We present simulations that compare the abilities of 20, 30, 50, and 100m telescopes to recover the correct stellar population mix represented in field star color-magnitude diagrams observed with J, H, and K filters. As input, our simulations use scenes containing stars drawn from a mix of model isochrones with differing ages and metallicities, with surface densities set to match that found in the M31 bulge and at the effective radius of NGC 3379. We convolve these scenes with PSFs corresponding to the projected performance of MCAO systems containing two deformable mirrors, including the effect of realistic variations in the atmospheric turbulence profile over the span of the observations. These simulations provide a way to evaluate the scientific advances enabled by ELTs of differing apertures in the area of extragalactic stellar populations.

arXiv: Instrumentation and Methods for Astrophysics, 2014

We discuss the impact that the next generation of Extremely Large Telescopes will have on the open astrophysical problems of resolved stellar populations. In particular, we address the interplay between multiband photometry and spectroscopy.

New Astronomy, 2003

EAS Publications Series, 2007

We discuss how a wide field imager with very high spatial resolution on a relatively small telescope-but located in an observing site with very good seeing-might constrain the fraction of binary stars in Galactic globular clusters. We also mention the role that the new data might play in investigating advanced evolutionary phases of lowmass stars. Moreover, we discuss the role that an Extremely Large Telescope might have on the longstanding problem of cosmic distances and stellar populations. In particular, we discuss the impact that the detection of Cepheids in galaxies at the distance of the Coma cluster and of RR Lyrae stars in galaxies at the distance of the Virgo cluster might have on the calibration of secondary distance indicators.

arXiv (Cornell University), 2018

We discuss the predicted performance of the HARMONI spectrograph and the ELT in the context of two specific science cases: resolved stellar populations of Local Volume galaxies and Galactic archaeology in dense environments. We have produced and analysed a set of simulated data-cubes using the HSIM software, mimicking observations across the giant elliptical galaxy Centaurus A and in the nuclear bulge of the Milky Way. We use our results to demonstrate the instruments capabilities to perform stellar absorption line spectroscopy in a large number of stars which will allow us to study the detailed kinematics and stellar population characteristics of these high density regions.

Astronomy & Astrophysics, 2014

The unprecedented sensitivity and spatial resolution of next-generation, ground-based, extremely large telescopes (ELTs) will open a completely new window on the study of resolved stellar populations. In this paper we study the feasibility of the analysis of nuclear star cluster (NSC) stellar populations with ELTs. To date, NSC stellar population studies are based on the properties of their integrated light. NSCs are in fact observed as unresolved sources even with the HST. We explore the possibility of obtaining direct estimates of the age of NSC stellar populations from the photometry of main-sequence turn-off stars. We simulated ELT observations of NSCs at different distances and with different stellar populations. Photometric measurements on each simulated image were analysed in detail and results about photometric accuracy and completeness are reported here. We found that the main-sequence turn-off is detectableand therefore the age of stellar populations can be directly estimated-up to 2 Mpc for old, up to 3 Mpc for intermediate-age and up to 4−5 Mpc for young stellar populations. We found that for this particular science case, the performances of TMT and E-ELT are of comparable quality.

The Astrophysical Journal, 2014

We utilize 16-band Hubble Space Telescope (HST ) observations of 18 lensing clusters obtained as part of the Cluster Lensing And Supernova survey with Hubble (CLASH) Multi-Cycle Treasury program to search for z ∼ 6 − 8 galaxies. We report the discovery of 206, 45, and 13 Lyman-break galaxy (LBG) candidates at z ∼ 6, z ∼ 7, and z ∼ 8, respectively, identified from purely photometric redshift selections. This large sample, representing nearly an order of magnitude increase in the number of magnified star-forming galaxies at z ∼ 6 − 8 presented to date, is unique in that we have observations in four WFC3/UVIS UV, seven ACS/WFC optical and all five WFC3/IR broad-band filters, which enable very accurate photometric redshift selections. We construct detailed lensing models for all 18 clusters (although some are preliminary) to estimate object magnifications and to identify two new multiply-lensed z 6 candidates. The median magnifications for these 18 clusters are 4, 4, and 5 for the z ∼ 6, z ∼ 7, and z ∼ 8 samples, respectively, over an average area of 4.5 arcmin 2 per cluster. We compare our observed number counts with expectations based on convolving "blank" field UV luminosity functions through our cluster lens models and find agreement down to ∼ 27 mag, where we begin to suffer significant incompleteness. In all three redshift bins we find a higher number density at brighter observed magnitudes than the field predictions, in excellent agreement with the lensed expectations and clearly demonstrating the enhanced efficiency of lensing clusters over field surveys. Once again, lensing clusters appear to be a powerful tool in the discovery and study of high-redshift galaxies and allow for the first glimpse of faint galaxies beyond the reach of the deepest HST legacy field surveys, a technique that will continue be exploited with the upcoming ultradeep Hubble Frontier Fields campaign.