arXiv subjects

Ralf Kotulla

Publications and source records attributed to Ralf Kotulla.

21 records · Page 2Linked to original sources

arXiv2008

Impact of sub-solar metallicities on photometric redshifts

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 solar metallicity 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 a systematic underestimate of the photometric redshift by typically Delta z ~ 0.1 ... 0.2 until z ~ 1.2, depending on galaxy type, of distant, faint and low-metallicity galaxies if analysed with solar-metallicity templates.

astro-ph↗

arXiv2008

Rescuing the Initial Mass Function for Arp 78

We present deep R and narrow-band Halpha images of Arp 78 obtained with the WIYN 3.5-m telescope on Kitt Peak. GALEX observations had shown a very extended UV structure for this system, reaching beyond the optical radius of Arp 78 and also beyond its previously known Halpha-radius. Our new Halpha data now show agreement not only with the spatial extent of the near- and far-UV maps, but also in terms of structural details. Star formation rates derived from L(Halpha) and L(FUV) are in reasonable agreement, indicating that in this case the upper stellar IMF in the UV-bright outer arm is relatively normal. The star forming sites in the outer arms are younger than ~15 Myr and massive enough to properly sample the IMF up to high masses; their low optical visibility evidently is a property of their youth.

astro-ph↗

arXiv2008

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

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. Our results prove the power of this approach to reveal the (violent) star formation and chemical enrichment histories of galaxies on the basis of combined optical and near-infrared photometry.

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