Uta Fritze

Bio: Uta Fritze is an academic researcher from University of Hertfordshire. The author has contributed to research in topics: Galaxy & Star formation. The author has an hindex of 7, co-authored 19 publications receiving 421 citations.

galev evolutionary synthesis models – I. Code, input physics and web interface

Abstract: GALEV (GALaxy EVolution) evolutionary synthesis models describe the evolution of stellar populations in general, of star clusters as well as of galaxies, both in terms of resolved stellar populations and of integrated light properties over cosmological time-scales of ≥13 Gyr from the onset of star formation shortly after the big bang until today. For galaxies, GALEV includes a simultaneous treatment of the chemical evolution of the gas and the spectral evolution of the stellar content, allowing for what we call a chemically consistent treatment: we use input physics (stellar evolutionary tracks, stellar yields and model atmospheres) for a large range of metallicities and consistently account for the increasing initial abundances of successive stellar generations. Here we present the latest version of the GALEV evolutionary synthesis models that are now interactively available at http://www.galev.org. We review the currently used input physics, and also give details on how this physics is implemented in practice. We explain how to use the interactive web interface to generate models for user-defined parameters and also give a range of applications that can be studied using GALEV, ranging from star clusters, undisturbed galaxies of various types E–Sd to starburst and dwarf galaxies, both in the local and the high-redshift Universe.

When efficient star formation drives cluster formation

Abstract: We investigate the impact of the star-formation efficiency (SFE) in cluster-forming cores (i.e., local SFE) on the evolution of mass in star clusters (SCs) over the age range 1-100 Myr, when SCs undergo their infant weight-loss/mortality phase. Our model builds on the N-body simulation grid of Baumgardt & Kroupa. Assuming a constant formation rate of gas-embedded clusters and a weak tidal field, we show that the ratio between the total mass in stars bound to the clusters over that age range and the total mass in stars initially formed in gas-embedded clusters is a strongly increasing function of the averaged local SFE, with little influence from any assumed core mass-radius relation. Our results suggest that, for young starbursts with estimated tidal field strength and known recent star-formation history, observed cluster-to-star mass ratios, once corrected for the undetected clusters, constitute promising probes of the local SFE without the need to resort to gas mass estimates. Similarly, the mass ratio of stars that remain in bound clusters at the end of the infant mortality/weight-loss phase (i.e., age 50 Myr) depends sensitively on the mean local SFE, although the impacts of the width of the SFE distribution function and of the core mass-radius relation require more careful assessment in this case. Following the recent finding by Bastian that galaxies form, on average, 8% of their stars in bound clusters regardless of their star-formation rate, we raise the hypothesis that star formation in the present-day universe is characterized by a near-universal distribution of the local SFE. A related potential application of our model is in tracing the evolution of the local SFE over cosmological lookback times by comparing the age distribution of the total mass in SCs to that in field stars in galaxies where field stars can be resolved and age dated. We describe aspects of our model which are still to be worked out before this goal can be achieved.

The role of E+A and post-starburst galaxies – I. Models and model results

Abstract: Different compositions of galaxy types in the field in comparison to galaxy clusters as described by the morphology-density relation in the local universe is interpreted as a result of transformation processes from late- to early-type galaxie s. This interpretation is supported by the Butcher-Oemler effect. We investigate E+A galaxies as an intermediate state between latetype galaxies in low density environments and early-type galaxies in high density environment to constrain the possible transformation processes. For this purpose we model a grid of post-starburst galaxies by inducing a burst and/ or a halting of star formation on the normal evolution of spiral galaxies with our galaxy evolution code GALEV. From our models we find that the common E+A criteria exclude a significant number of post-starburst galaxies and propose that comparing their spectral energy distributions leads to a more suffi cient method to investigate post-starbust galaxies. We predict that a higher number of E+A galaxies in the early universe can not be ascribed solely to a higher number of starburst, but is a result of a lower metallicity an d a higher burst strength due to more gas content of the galaxies in the early universe. We find that even galaxies with a normal evolution without a starburst have a Hδ-strong phase at early galaxy ages.

Young Globular Clusters in an old S0: Clues to the Formation History of NGC 4570

Abstract: We here present our first attempt to use Globular Clusters as tracers of their parent galaxy’s formation history. Globular Cluster Systems of most early-type galaxies feature two peaks in their optical colour distributions. Blue-peak Globular Clusters are generally believed to be old and metal-poor. The ages, metallicities, and the origin of the red-peak Globular Clusters are being debated. We here present our analysis of the ages and metallicities of the red peak Globular Clusters in the Virgo S0 NGC 4570 using deep Ks-band photometry from NTT/SOFI (ESO program ID 079.B-0511) for the red-peak Globular Clusters in combination with HST-ACS archival data to break the age-metallicity degeneracy. We analyze the combined g, z, and Ks spectral energy distribution by comparison with a large grid of GALEV evolutionary synthesis models for star clusters with different ages and metallicities. This analysis reveals a substantial population of intermediate-age (1–3 Gyr) and metal-rich (� solar metallicity) Globular Clusters. We discuss their age and metallicity distributions together with information on the parent galaxy from the literature to gain insight into the formation history of this galaxy.

Impact of subsolar metallicities on photometric redshifts

Abstract: With the advent of deep photometric surveys the use of photometric redshifts, obtained with a variety of techniques, has become more and more widespread. Giving access to galaxies with a wide range of luminosities out to high redshifts, these surveys include many faint galaxies with significantly sub-solar metallicities. We use our chemically consistent galaxy evolutionary synthesis code GALEV to produce a large grid of template Spectral Energy Distributions (SEDs) for galaxies of spectral types E and Sa through Sd – one accounting in a chemically consistent way for the increasing initial metallicities of successive stellar generations, the other one for exclusively solar metallicities – for comparison. We use our new photometric redshift code GAZELLE based on the comparison of observed and model SEDs. Comparing the photometric redshifts obtained using solarmetallicity templates when working on a catalogue of artificially created chemically consistent SEDs, typical for low-metallicity local late-type galaxies and for intrinsically low-luminosity, and hence low-metallicity, galaxies in the high-redshift universe, we find a significant bias resulting from this metallicity mismatch. This bias consists in

UV luminosity functions at redshifts z ∼ 4 to z ∼ 10: 10,000 galaxies from HST legacy fields

Abstract: The remarkable Hubble Space Telescope?(HST) data sets from the CANDELS, HUDF09, HUDF12, ERS, and BoRG/HIPPIES programs have allowed us to map the evolution of the rest-frame UV luminosity function (LF) from to . We develop new color criteria that more optimally utilize the full wavelength coverage from the optical, near-IR, and mid-IR observations over our search fields, while simultaneously minimizing the incompleteness and eliminating redshift gaps. We have identified 5859, 3001, 857, 481, 217, and 6 galaxy candidates at , , , , , and , respectively, from the ?1000 arcmin2 area covered by these data sets. This sample of >10,000 galaxy candidates at is by far the largest assembled to date with HST. The selection of 4?8 candidates over the five CANDELS fields allows us to assess the cosmic variance; the largest variations are at . Our new LF determinations at and span a 6 mag baseline and reach to ?16 AB mag. These determinations agree well with previous estimates, but the larger samples and volumes probed here result in a more reliable sampling of galaxies and allow us to reassess the form of the UV LFs. Our new LF results strengthen our earlier findings to significance for a steeper faint-end slope of the UV LF at , with ? evolving from at to at (and at ), consistent with that expected from the evolution of the halo mass function. We find less evolution in the characteristic magnitude M* from to the observed evolution in the LF is now largely represented by changes in . No evidence for a non-Schechter-like form to the z ? 4?8 LFs is found. A simple conditional LF model based on halo growth and evolution in the M/L ratio of halos provides a good representation of the observed evolution.

UV Luminosity Functions at redshifts z~4 to z~10: 10000 Galaxies from HST Legacy Fields

Abstract: The remarkable HST datasets from the CANDELS, HUDF09, HUDF12, ERS, and BoRG/HIPPIES programs have allowed us to map out the evolution of the UV LF from z~10 to z~4. We have identified 5859, 3001, 857, 481, 217, and 6 galaxy candidates at z~4, z~5, z~6, z~7, z~8, and z~10, respectively from the ~1000 arcmin**2 area probed. The selection of z~4-8 galaxies over the five CANDELS fields allows us to assess the cosmic variance; the largest variations are apparent at z>=7. Our new LF determinations at z~4 and z~5 span a 6-mag baseline (-22.5 to -16 AB mag). These determinations agree well with previous estimates, but the larger samples and volumes probed here result in a more reliable sampling of >L* galaxies and allow us to reassess the form of the UV LFs. Our new LF results strengthen our earlier findings to 3.4 sigma significance for a steeper faint-end slope to the UV LF at z>4, with alpha evolving from alpha=-1.64+/-0.04 at z~4 to alpha=-2.06+/-0.13 at z~7 (and alpha = -2.02+/-0.23 at z~8), consistent with that expected from the evolution of the halo mass function. With our improved constraints at the bright end, we find less evolution in the characteristic luminosity M* over the redshift range z~4 to z~7; the observed evolution in the LF is now largely represented by changes in phi*. No evidence for a non-Schechter-like form to the z~4-8 LFs is found. A simple conditional LF model based on halo growth and evolution in the M/L ratio of halos ((1+z)**-1.5) provides a good representation of the observed evolution.

The propagation of uncertainties in stellar population synthesis modeling. iii. model calibration, comparison, and evaluation

Abstract: Stellar population synthesis (SPS) provides the link between the stellar and dust content of galaxies and their observed spectral energy distributions. In the present work, we perform a comprehensive calibration of our own flexible SPS (FSPS) model against a suite of data. These data include ultraviolet, optical, and near-IR photometry, surface brightness fluctuations, and integrated spectra of star clusters in the Magellanic Clouds (MCs), M87, M31, and the Milky Way (MW), and photometry and spectral indices of both quiescent and post-starburst galaxies at z {approx} 0. Several public SPS models are intercompared, including the models of Bruzual and Charlot (BC03), Maraston (M05), and FSPS. The relative strengths and weaknesses of these models are evaluated, with the following conclusions: (1) the FSPS and BC03 models compare favorably with MC data at all ages, whereas M05 colors are too red and the age dependence is incorrect; (2) all models yield similar optical and near-IR colors for old metal-poor systems, and yet they all provide poor fits to the integrated J - K and V - K colors of both MW and M31 star clusters; (3) FSPS is able to fit all of the ultraviolet data because both the post-asymptotic giant branch (post-AGB)more » and horizontal branch evolutionary phases are handled flexibly, while the BC03 and M05 models fail in the far-UV, and both far- and near-UV, respectively; (4) all models predict ugr colors too red, D{sub n}4000 strengths too strong, and Hdelta{sub A} strengths too weak compared to massive red sequence galaxies, under the assumption that such galaxies are composed solely of old metal-rich stars; and (5) FSPS and, to a lesser extent, BC03 can reproduce the optical and near-IR colors of post-starburst galaxies, while M05 cannot. Reasons for these discrepancies are explored. The failure at predicting the ugr colors, D{sub n}4000, and Hdelta{sub A} strengths can be explained by some combination of a minority population of metal-poor stars, young stars, blue straggler and/or blue horizontal branch (HB) stars, but not by appealing to inadequacies in either theoretical stellar atmospheres or canonical evolutionary phases (e.g., the main-sequence turnoff). The different model predictions in the near-IR for intermediate age systems are due to different treatments of the thermally pulsating asymptotic giant branch stellar evolutionary phase. We emphasize that due to a lack of calibrating star cluster data in regions of the metallicity-age plane relevant for galaxies, all of these models continue to suffer from serious uncertainties that are difficult to quantify.« less

A Critical Assessment of Photometric Redshift Methods: A CANDELS Investigation

Abstract: We present results from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) photometric redshift methods investigation. In this investigation, the results from 11 participants, each using a different combination of photometric redshift code, template spectral energy distributions (SEDs), and priors, are used to examine the properties of photometric redshifts applied to deep fields with broadband multi-wavelength coverage. The photometry used includes U-band through mid-infrared filters and was derived using the TFIT method. Comparing the results, we find that there is no particular code or set of template SEDs that results in significantly better photometric redshifts compared to others. However, we find that codes producing the lowest scatter and outlier fraction utilize a training sample to optimize photometric redshifts by adding zero-point offsets, template adjusting, or adding extra smoothing errors. These results therefore stress the importance of the training procedure. We find a strong dependence of the photometric redshift accuracy on the signal-to-noise ratio of the photometry. On the other hand, we find a weak dependence of the photometric redshift scatter with redshift and galaxy color. We find that most photometric redshift codes quote redshift errors (e.g., 68% confidence intervals) that are too small compared to that expected from the spectroscopic control sample. We find that all codes show a statistically significant bias in the photometric redshifts. However, the bias is in all cases smaller than the scatter; the latter therefore dominates the errors. Finally, we find that combining results from multiple codes significantly decreases the photometric redshift scatter and outlier fraction. We discuss different ways of combining data to produce accurate photometric redshifts and error estimates.

The impact of nebular emission on the ages of z~6 galaxies

Abstract: We examine the influence of nebular continuous and line emission in high redshift star forming galaxies on determinations of their age, formation redshift and other properties from SED fits. We include nebular emission consistently with the stellar emission in our SED fitting tool and analyse differentially a sample of 10 z~6 galaxies in the GOODS-S field studied earlier by Eyles et al. (2007). We find that the apparent Balmer/4000 Ang breaks observed in a number of z~6 galaxies detected at >~3.6 micron with IRAC/Spitzer can be mimicked by the presence of strong restframe optical emission lines, implying in particular younger ages than previously thought. Applying these models to the small sample of z~6 galaxies, we find that this effect may lead to a typical downward revision of their stellar ages by a factor ~3. In consequence their average formation redshift may drastically be reduced, and these objects may not have contributed to cosmic reionisation at z>6. Extinction and stellar mass estimates may also be somewhat modified, but to a lesser extent. Careful SED fits including nebular emission and treating properly uncertainties and degeneracies are necessary for more accurate determinations of the physical parameters of high-z galaxies.