Welcome to the Homepage of 
Dr. Christian Maier
Extragalactic Astrophysics Group

Recently Awarded Scientific Programs with Large Telescopes

PI of program 084.B-0232 SINFONI at the VLT,
Chemical evolution: metallicities of vigorously star-forming galaxies at z ~ 2.3,
37 hours in service mode

PI of program 084.B-0312 ISAAC at the VLT,
Establishing the evolutionary status of candidates low-metallicity luminous galaxies at z ~ 0.7,
23.5 hours in service mode

PI of program S09B-013 MOIRCS at the SUBARU telescope,
Chemical evolution: metallicities of vigorously star-forming galaxies at z ~ 2.3,
2.5 nights in visitor mode

PI of program 085.B-0317 ISAAC at the VLT,
Establishing the role of ENVIRONMENT on METALLICITIES of galaxies at 0.5 < z< 0.7,
23.5 hours in service mode

Metallicities at the Peak of the Cosmic Star Formation Rate:
Implications for the Evolution of Galaxies

Gas metallicities are a particularly important diagnostic of galaxy evolution. The rather unexplored 1 < z < 2 redshift regime is one of particular importance to trace the evolution of the metal content in galaxies: there, the star formation and metal production rates for the universe as a whole, as measured by the the integrated luminosity density in the ultraviolet and far-infrared, appear to peak, i.e., are a factor of about 6 higher relative to the local value. Furthermore, this is the redshift regime where the galaxy population clearly undergoes a transition in properties: it is beyond z~1 that luminous ultraviolet star-forming galaxies with ``unobscured'' star-formation rates above ~10 Msol/yr appear in optically-selected galaxy samples. Such galaxies are not detected below z~0.8-1.0. Studies of the metal content of the star forming galaxies at these key epochs are however sparse.

Figure 1. We have used VLT-ISAAC near-infrared spectroscopy for a sample of five [OII]-selected, M_B,AB<~-21.5, z~1.4 galaxies, to measure their Hbeta, [OIII]5007, Halpha emission line fluxes, and upper limits for [NII]6584 fluxes. These have allowed us to determine accurate [O/H] abundances for the z~1.4 galaxies, which we have compared with those of galaxies at lower redshifts and with chemical evolution models. Not surpringsingly, we see a relationship between redshift and inferred chemical age. For example, despite the large scatter, the bright, M_B,AB<-19.5, z~1.4 galaxies (black filled squares) appear to be ``younger'' than 0.7 < z < 0.9 galaxies (red filled squares), in the sense that they lie towards the beginning of the luminosity-metallicity track. The 0.7 < z < 0.9 galaxies appear in turn to be on average ``younger'' than most 0.5 < z < 0.7 galaxies (green filled squares), which themselves overlap on the diagram with the metallicity-luminosity relation traced by nearby galaxies.

The tracks of the chemical evolution models in Fig. 1 suggest that the bright star forming z~1.4 galaxies are likely to evolve into the population of less luminous but nonetheless rather massive, metal-rich galaxies that appear in the 0.5 < z < 0.9 galaxy population. The broad range of galaxy morphologies suggests that the metal-enriched reservoirs of star forming gas that we are probing at intermediate redshifts are being mostly consumed to build up both the disk and the bulge components of spiral galaxies.

Our analysis of the metallicity-luminosity relation at 0 < z < 1.5 suggests that the period of rapid chemical evolution takes place progressively in lower mass systems as the universe ages. The Figure shows the signatures of this "downsizing" effect: (a) at z < 0.7 (green symbols) nearly all galaxies with M_B,AB<-20 are fairly close to the low-z [O/H]-M_B relation (with one obvious exception); (b) at 0.7 < z < 0.9, this is true only for M_B,AB<-21.3; and (c) at z~1.4 even the most luminous galaxies are evolved off of the low redshift [O/H]-M_B,AB relation. As the Universe ages, particular signatures of "youth" (e.g., high [OIII]/[OII] or low [O/H]) are seen in progressively less luminous, less massive systems!

Which galaxies contribute to the star formation rate density over the past 8 billion years?

One of the key unanswered questions in the study of galaxy evolution is what physical processes inside galaxies drive the changes in the star formation rates in individual galaxies that, taken together, produce the large decline in the global star-formation rate density to redshifts since z ~ 2 (Lilly et al. 1996, Hippelein, Maier et al. 2003). Studies using the local SDSS sample have argued that the surface mass density may be more important than stellar mass in regulating star formation. Using the SDSS sample Brinchmann et al. (2004) found that the low specific star formation rate (SSFR, star formation rate / unit stellar mass) peak is more prominent at high surface density than at high stellar mass, and therefore concluded that the surface density of stars is more important than stellar mass in regulating star formation.

Figure 2. The shape of the specific SFR (SSFR) versus stellar mass surface density relation for relatively massive zCOSMOS z~0.7 galaxies is very similar to that of local SDSS galaxies (left panel). There is a roughly uniform increase in the average SSFR by a factor of 5-6 that is broadely independent of surface mass density, and which occurs for both late-type (middle panel), and early-type (right panel) galaxies. This emphasizes that galaxies of all types are contributing, proportionally, to the global increase in star formation rate density in the Universe back to these redshifts. Disk galaxies have a SSFR that is almost independent of surface mass density, and the same is probably also true of high Sersic index galaxies once obvious disk systems are excluded (red squares in the right panel).

Using the HST/ACS images of the COSMOS field, plus star formation rate information from emission lines measured in large numbers of zCOSMOS spectra we can study the changes that have occured in the SSFR - surface mass density relation between redshifts approaching z~1 and the present epoch, as sampled by the SDSS studies. The requirement is that we can select comparable samples at the different redshifts, and therefore we derive star formation rates, stellar masses, and structural parameters in a consistent way for both zCOSMOS and SDSS samples, and apply them to samples that are complete down to the same stellar mass at both redshifts.

Figure 3. The SSFR - surface mass density step-function (Fig.1) is clearly due to the change-over of different structural types from disk-dominated low Sersic galaxies (n<1.5) to bulge-dominated high Sersic galaxies (n>2.5), as the surface mass density increases. The cross-over point shifts to higher surface mass density in zCOSMOS compared to SDSS, because of a modest differential evolution in the size-mass relations of disk and spheroid galaxies.

Selected Papers

• Peng, Y., Lilly, S. J., ..., Maier, C. et al., Mass and environment as drivers of galaxy evolution in SDSS and zCOSMOS and the origin of the Schechter function, 2010, ApJ, 721, 193

• Maier, C., Lilly, S.J., Zamorani, G. et al., The Dependence of Star Formation Activity on Stellar Mass Surface Density and Sersic Index in zCOSMOS Galaxies at 0.5 < z < 0.9 Compared with SDSS Galaxies at 0.04 < z < 0.08, 2009, ApJ, 694, 1099

• Silverman, J. D., Lamareille, F., Maier, C. et al., Ongoing and co-evolving star formation in zCOSMOS galaxies hosting Active Galactic Nuclei, 2009, ApJ, 696, 396

• Lilly, S. J., Le Brun, V., Maier, C. et al., The zCOSMOS 10k-bright spectroscopic sample, 2009, ApJS, 184, 218

• Caputi, K. I, Lilly, S. J., Aussel, H., Le Floc'h, E., Sanders, D., Maier, C. et al., The optical spectra of Spitzer 24 microns galaxies in the COSMOS field: II. Faint infrared sources in the zCOSMOS-bright 10k catalogue, 2009, ApJ, 707, 1387

• Maier, C., Lilly, S.J., Carollo, C. M. et al., Oxygen Gas Abundances at z~1.4: Implications for the Chemical Evolution History of Galaxies, 2006, ApJ 639, 858

• Maier, C., Lilly, S.J., Carollo, C. M. et al., Near Infrared Spectroscopy of 0.4 < z < 1.0 Galaxies: Oxygen Abundances, SFRs and Dust, 2005, ApJ, 634, 849

• Maier, C., Meisenheimer, K., and Hippelein, H., The metallicity-luminosity relation at medium redshift based on faint CADIS emission line galaxies, 2004, A&A, 418, 475

• Maier, C., Meisenheimer, K., Thommes, E. et al., Constraints to the evolution of Lyman-Alpha bright galaxies between z=3 and z=6, 2003, A&A, 402, 79

• Hippelein, H., Maier, C., Meisenheimer, K. et al., Star Forming Rates between z=0.25 and z=1.2 from the CADIS Emission Line Survey, 2003, A&A, 402, 65

Conferences 2004 - 2008

• Maier, C., Lilly, S.J., and zCOSMOS Team, Poster presented at "When the Universe formed Stars: Understanding the Most Active Period of Star Fomation in the History of the Universe Down to its Fall", November, 2008, Martinique, France

• Maier, C., and zCOSMOS Team, Proceedings of the first Subaru International Conference: "Panoramic Views of Galaxy Formation and Evolution", Hayama, Japan, December, 2007: The Evolution of the Star Formation of zCOSMOS Galaxies at z<0.7 as a Function of Mass and Structural Parameters,2008, ASPC, 399, 231, astro-ph/0901.0921

• Maier, C., Lilly, S.J., Carollo, C. M., Meisenheimer, K., Hippelein, H., and Stockton, A., Poster presented at "Galaxies and Structures through Cosmic Times", 2006, Venice

• Maier, C., Lilly, S.J., Carollo, C. M., Proceedings contribution for "The Fabulous Destiny of Galaxies; Bridging Past and Present", 2005, Marseille: Oxygen Gas Abundances at 0.4 < z < 1.5: Implications for the Chemical Evolution History of Galaxies, astro-ph/0509114

• Maier, C., Lilly, S.J., Carollo, C. M., Proceedings contribution for "Starbursts: From 30 Doradus to Lyman Break Galaxies", Cambridge, UK, 6-10 September 2004: Oxygen abundances, SFRs and dust of CFRS galaxies at intermediate redshift, 2005sdlb.procP..43M

• Link to ADS published papers