arXiv subjects

Karl Rakos

Publications and source records attributed to Karl Rakos.

5 recordsLinked to original sources

arXiv2014

Comparative studies of Population Synthesis Models in the frame work of modified Strömgren filters

Evolutionary models form a vital part of stellar population research to understand their evolution, but despite their long history of development, they often misrepresent and misinterpret the properties of stellar population observed through broadband and spectroscopic measurements. With the growing numbers of these synthesis models, model comparison becomes an important analysis to choose a suitable model for upgrade. Along with the model comparison, we reinvestigate the technique of modified Strömgren photometry to measure reliable parameter-sensitive colours and estimate precise model ages and metallicities. The assessment of Rakos/Schulz models with GALEV and Worthey's Lick/IDS model find smaller colour variation: Δ(uz-vz) $\leq$ 0.056, Δ(bz-yz) $\leq$ -0.05 and Δ(vz-yz) $\leq$ 0.061. The study conveys a good agreement of GALEV models with the modified Strömgren colours but with poor UV model predictions with the observed globular cluster data, while the spectroscopic models perform badly due to the use of older isochrone and stellar spectral libraries with inaccurate/insufficient knowledge of various stellar phases and their treatment. Overall, the assessment finds modified Strömgren photometry well suited to study different types stellar populations by mitigating the effects of the age-metallicity degeneracy.

astro-ph.GA↗

arXiv2009

The Age of Ellipticals and the Color-Magnitude Relation

Using new narrowband color observations of early-type galaxies in clusters, we reconstruct the color-magnitude relation (CMR) with a higher degree of accuracy than previous work. We then use the spectroscopically determined ages and metallicities from three samples (Trager et al 2008, Thomas et al 2005, Gallazzi et al 2006), combined with multi-metallicity SED models, to compare predicted colors for galaxies with young ages (less than 8 Gyr) with the known CMR. We find that the CMR cannot by reproduced by the spectroscopically determined ages and metallicities in any of the samples despite the high internal accuracies to the spectroscopic indices. In contrast, using only the $<$Fe$>$ index to determine [Fe/H], and assuming a mean age of 12 Gyr for a galaxy's stellar population, we derive colors that exactly match not only the color zeropoint of the CMR but also its slope. We consider the source of young age estimates, the H$β$ index, and examine the conflict between red continuum colors and large H$β$ values in galaxy spectra. We conclude that our current understanding of stellar populations is insufficient to correctly interpret H$β$ values and that the sum of our galaxy observations supports an old and monolithic scenario of galaxy formation. This result has a devastating impact on every study that has used the H$β$ index to calculate galaxy age, as the use of the H$β$ versus MgFe diagram will result in incorrectly deduced young ages.

astro-ph.CO↗

arXiv2000

Stromgren Photometry from z=0 to z~1. The Method

We use rest-frame Stromgren photometry to observe clusters of galaxies in a self-consistent manner from z=0 to z=0.8. Stromgren photometry of galaxies is an efficient compromise between standard broad-band photometry and spectroscopy, in the sense that it is more sensitive to subtle variations in spectral energy distributions than the former, yet much less time-consuming than the latter. Principal Component Analysis (PCA) is used to extract maximum information from the Stromgren data. By calibrating the Principal Components using well-studied galaxies (and stellar population models), we develop a purely empirical method to detect, and subsequently classify, cluster galaxies at all redshifts smaller than 0.8. Interlopers are discarded with unprecedented efficiency (up to 100%). The first Principal Component essentially reproduces the Hubble Sequence, and can thus be used to determine the global star formation history of cluster members. The (PC2, PC3) plane allows us to identify Seyfert galaxies (and distinguish them from starbursts) based on photometric colors alone. In the case of E/S0 galaxies with known redshift, we are able to resolve the age-dust- metallicity degeneracy, albeit at the accuracy limit of our present observations. This technique will allow us to probe galaxy clusters well beyond their cores and to fainter magnitudes than spectroscopy can achieve. We are able to directly compare these data over the entire redshift range without a priori assumptions because our observations do not require k-corrections. The compilation of such data for different cluster types over a wide redshift range is likely to set important constraints on the evolution of galaxies and on the clustering process.

astro-ph↗

arXiv1996

Color Evolution from z=0 to z=1

Rest frame Stromgren photometry (3500A, 4100A, 4750A and 5500A) is presented for 509 galaxies in 17 rich clusters between z=0 and z=1 as a test of color evolution. Our observations confirm a strong, rest frame, Butcher-Oemler effect where the fraction of blue galaxies increases from 20% at z=0.4 to 80% at z=0.9. We also find that a majority of these blue cluster galaxies are composed of normal disk or post-starbursts systems based on color criteria. When comparing our colors to the morphological results from HST imaging, we propose that the blue cluster galaxies are a population of late-type, LSB objects who fade and are then destroyed by the cluster tidal field. After isolating the red objects from Butcher-Oemler objects, we have compared the mean color of these old, non-star forming objects with SED models in the literature as a test for passive galaxy evolution in ellipticals. We find good agreement with single burst models which predict a mean epoch of galaxy formation at z=5. Tracing the red envelope for ellipticals places the earliest epoch of galaxy formation at z=10.

astro-ph↗

arXiv1996

Photometric Signatures of Starbursts in Interacting Galaxies and the Butcher-Oemler Effect

This paper presents new and synthetic narrow band photometry of ellipticals, spirals, Seyferts and interacting galaxies in an attempt to identify the cause of the unusually high fraction of blue cluster galaxies in distant clusters (Butcher-Oemler effect). The properties and distribution of the low redshift sample specifically points to starbursts as the origin of the blue narrow band colors in interacting Arp galaxies. Comparison between theoretical models and multicolor diagrams, particularly 4000A break colors, indicates a photometric signature which differs from both normal disk galaxy star formation and nonthermal components. This photometric signature is absent for the Butcher-Oemler galaxies whose general color distribution, compared to present-day clusters, is consistent with a majority of the blue population involved in normal star formation rates (spiral-like) with the addition of a small fraction of bright, blue interacting/merger systems. This photometric picture of the Butcher- Oemler galaxies is in agreement with the morophological evidence from HST imaging.

astro-ph↗