We investigate the utility of the Tunable Filters (TFs) for obtaining flux calibrated emission line maps of extended objects such as galactic nebulae and nearby galaxies, using the OSIRIS instrument at the 10.4-m GTC. Despite a relatively large field
of view of OSIRIS (8x8), the change in the wavelength across the field (~80 Ang) and the long-tail of Tunable Filter (TF) spectral response function, are hindrances for obtaining accurate flux calibrated emission-line maps of extended sources. The purpose of this article is to demonstrate that emission-line maps useful for diagnostics of nebula can be generated over the entire field of view of OSIRIS, if we make use of theoretically well-understood characteristics of TFs. We have successfully generated the flux-calibrated images of the nearby, large late-type spiral galaxy M101 in the emission lines of Halpha, [NII]6583, [SII]6716 and [SII]6731. We find that the present uncertainty in setting the central wavelength of TFs (~1 Ang), is the biggest source of error in the emission-line fluxes. By comparing the Halpha fluxes of HII regions in our images with the fluxes derived from Halpha images obtained using narrow-band filters, we estimate an error of ~11% in our fluxes. The flux calibration of the images was carried out by fitting the SDSS griz magnitudes of in-frame stars with the stellar spectra from the SDSS spectral database. This method resulted in an accuracy of 3% in flux calibration of any narrow-band image, which is as good as, if not better, to that is feasible using the observations of spectrophotometric standard stars. Thus time-consuming calibration images need not be taken. A user-friendly script under the IRAF environment was developed and is available on request.
We here present the ages of four compact stellar clusters (CSCs) in the nearby spiral galaxy M81, using long-slit optical spectra obtained with the 10.4-m Gran Telescopio Canarias (GTC). All the four CSCs, including the brightest in this galaxy, are
found to have ages between 5 to 6 Myr, with one of them showing Wolf-Rayet spectral features. The photometric masses of these clusters, calculated using their spectroscopically-derived ages, lie between 3000 and 18000 Msun. The observed clusters are among the brightest objects, and hence the most massive, in the entire disk of M81. This implies the absence of massive (1.0e5 Msun) compact stellar clusters in M81.
We present the results on the star formation history and extinction in the disk of M82 over spatial scales of 10 (~180 pc). Multi-band photometric data covering from the far ultraviolet to the near infrared bands were fitted to a grid of synthetic sp
ectral energy distributions. We obtained distribution functions of age and extinction for each of the 117 apertures analyzed, taking into account observational errors through Monte-Carlo simulations. These distribution functions were fitted with gaussian functions to obtain the mean ages and extinctions along with errors on them. The analyzed zones include the high surface brightness complexes defined by OConnell & Mangano (1978). We found that these complexes share the same star formation history and extinction as the field stellar populations in the disk. There is an indication that the stellar populations are marginally older at the outer disk (450 Myr at ~3 kpc) as compared to the inner disk (100 Myr at 0.5 kpc). For the nuclear regions (radius less than 500 pc), we obtained an age of less than 10 Myr. The results obtained in this work are consistent with the idea that the 0.5-3 kpc part of the disk of M82 formed around 90% of the stellar mass in a star-forming episode that started around 450 Myr ago lasting for about 350 Myr. We found that field stars are the major contributors to the flux over the spatial scales analyzed in this study, with stellar cluster contribution being 7% in the nucleus and 0.7% in the disk.
We study the population of compact stellar clusters (CSCs) in M81, using the HST/ACS images in the filters F435W, F606W and F814W covering, for the first time, the entire optical extent of the galaxy. Our sample contains 435 clusters of FWHM less tha
n 10 ACS pixels (9 pc). The sample shows the presence of two cluster populations, a blue group of 263 objects brighter than B=22 mag, and a red group of 172 objects, brighter than B=24 mag. Based on the analysis of colour magnitude diagrams and making use of simple stellar population models, we find the blue clusters are younger than 300 Myr with some clusters as young as few Myr, and the red clusters are as old as globular clusters. The luminosity function of the blue group follows a power-law distribution with an index of 2.0, typical value for young CSCs in other galaxies. The power-law shows unmistakable signs of truncation at I=18.0 mag (M_I=-9.8 mag), which would correspond to a mass-limit of 4x10^4 M_solar if the brightest clusters are younger than 10 Myr. The red clusters have photometric masses between 10^5 to 2x10^7 M_solar for the adopted age of 5 Gyr and their luminosity function resembles very much the globular cluster luminosity function in the Milky Way. The brightest GC in M81 has M_B^0=-10.3 mag, which is ~0.9 mag brighter than w-Cen, the most massive GC in the Milky Way.
We present the results obtained from an objective search for stellar clusters, both in the currently active nuclear starburst region, and in the post-starburst disk of M82. Images obtained with the HST/ACS in F435W(B), F555W(V), and F814W(I) filters
were used in the search for the clusters. We detected 653 clusters of which 393 are located outside the central 450 pc in the post-starburst disk of M82. The luminosity function of the detected clusters show an apparent turnover at B=22 mag (M_B=-5.8), which we interpret from Monte Carlo simulations as due to incompleteness in the detection of faint clusters, rather than an intrinsic log-normal distribution. We derived a photometric mass of every detected cluster from models of simple stellar populations assuming a mean age of either an 8 (nuclear clusters) or 100 (disk clusters) million years old. The mass functions of the disk (older) and the nuclear (younger) clusters follow power-laws, the former being marginally flatter (alpha=1.5+/-0.1) than the latter (alpha=1.8+/-0.1). The distribution of sizes (Full Width at Half Maximum) of clusters brighter than the apparent turn-over magnitude (mass>2E+4 Mo) can be described by a log-normal function. This function peaks at 10 pc for clusters more massive than 1E+5 Mo, whereas for lower masses, the peak is marginally shifted to larger values for the younger, and smaller values for the older clusters. The observed trend towards flattening of the mass function with age, together with an over-abundance of older compact clusters, imply that cluster disruption in M82 is both dependent on the mass and size of the clusters.
