Truncation of stellar disks in galaxies at z ~ 1 Isabel Pérez, Kapteyn Astronomical Institute
Is finding truncation at high z feasible ? Biases and systematic errors Comparison to the local sample (tricky!). Evolution? Why are some galaxies truncated and others are not? (A problem also locally) Summary and discussion Outline of the talk
Redshift range limited mainly by the cosmological dimming (~ (1+z)4) , reaching a restframe μB≈ 24.5 mag arcsec-2 (at z=1.1). z between 0.6 and 1.1. This implies a dimming between 2-3 mag with an isophotal threshold range between 25.5 – 24.5 mag arcsec-2. The first sample (Perez 2004) consists of 16 GOODS HST/ACS (Giavalisco et al. 2004). The new sample consists of 19 galaxies, UDF (Beckwith et al. 2005) z-band was used to ensure that the restframe corresponds to bands widely observed locally (in this case, V and B at z=0.6 and 1.1) The redshifts were obtained from ESO/VLT GOODS (Vanzella et al. 2004), and the Keck DEIMOS spectra (Wirth et al. 2004). Sample and profile fitting
Galaxies were visually selected to be relatively symmetric and not strongly interacting. Hubble types range from Sa-Sc, and inclinations less than 35°. Azimuthaly averaged profiles from ellipse fitting. The sky background was evaluated locally by taking the mean values of boxes placed around the galaxies. The standard deviation of the median of the distribution in the different boxes was adopted as the error in the background. We fit the sum of two exponential functions. The criterion for detection of truncation was that the outer exponential should extend at least 2 × hout before reaching the noise level. Sample and profile fitting
Sample and profile fitting • The break radius is determined as the radius where the two exponentials cross • While the errors in hin are dominated by the effect of the bulge, the errors in hout are dominated by the determination of the break radius and the sky subtraction • Average profiles of different image segments were checked for systematic differences; none was found • Unsharp masking was used to locate the position of the end of the spiral structure and compare it with the break radius.
Results and comparison to nearby objects • In the first sample from GOODS, six of 16 galaxies showed a two-slope profile. From the UDF sample (19 galaxies), 6 showed double exponential. • Around 34 % of the galaxies at high-z show truncation. • The mean value for the ratio • hin/hout is 2 ± 0.2 for local galaxies (Pohlen et al, 2002), remarkably similar to that found in high z galaxies. This results suggests that dust does not make Rbr/hin appear smaller at higher redshifts.
Results and comparison to nearby objects • The mean value for the ratio Rbr/hin in local galaxies is still controversial, due to differences in the analysis. New data (Pohlen & Trujillo in prep.), gives Rbr/hin 2.7 ± 0.7. • High z galaxies: Rbr/hin 1.8 ± 0.5 • In the nearby universe, the scale-lengths show a systematic increase at shorter wavelengths, but not steeply. Pohlen et al. 2002 + Kregel, van der Kruit & de Grijs 2002 High-z galaxies, From UDF and GOODS
Results and comparison to nearby objects Kregel, van der Kruit & de Grijs 2002 High z GOODS and UDF
Results and comparison to nearby objects • Kregel , van der Kruit & de Grijs 2002 • Pohlen & Trujillo in prep.. • High z GOODS and UDF
Results and comparison to nearby objects If the question about the Rbr/hin is settled at low redshift, it might imply that the ratio Rbr/ hin is not evolving with redshift, and that both the break radius and the scale length evolve in similar ways.
Truncated vs not (perhaps) truncated Untruncated sample galaxies Truncated sample galaxies
Truncated vs not (perhaps) truncated Trujillo & Pohlen sample This UDF sample
12 of 35 galaxies (from GOODS and the UDF fields, redshifts between 0.6 and 1.1) show truncation , with an average break radius of 7kpc. A double exponential provides a good fit to the profiles, similar to nearby galaxies Evolution with redshift is still an open question which needs further investigation; current data shows no evolution. Galaxies showing no truncation have similar scale-lengths and surface brightnesses to the truncated galaxies. These galaxies either do not show truncation or it occurs at lower surface brightness. Some evidence for bluer colour in the galaxies which show truncation. Summary