Galaxy formation and evolution

Abstract:

We present a generalization of surface photometry to the higher-order moments of the line-of-sight velocity distribution of galaxies observed with integral-field spectrographs. The generalization follows the approach of surface photometry by determining the best-fitting ellipses along which the profiles of the moments can be extracted and analysed by means of harmonic expansion. The assumption for the odd moments (e.g. mean velocity) is that the profile along an ellipse satisfies a simple cosine law. The assumption for the even moments (e.g. velocity dispersion) is that the profile is constant, as it is used in surface photometry. We test the method on a number of model maps and discuss the meaning of the resulting harmonic terms. We apply the method to the kinematic moments of an axisymmetric model elliptical galaxy and probe the influence of noise on the harmonic terms. We also apply the method to SAURON observations of NGC 2549, NGC 2974, NGC 4459 and NGC 4473 where we detect multiple co- and counter-rotating (NGC 2549 and NGC 4473, respectively) components. We find that velocity profiles extracted along ellipses of early-type galaxies are well represented by the simple cosine law (with 2 per cent accuracy), while possible deviations are carried in the fifth harmonic term which is sensitive to the existence of multiple kinematic components, and has some analogy to the shape parameter of photometry. We compare the properties of the kinematic and photometric ellipses and find that they are often very similar, but a study on a larger sample is necessary. Finally, we offer a characterization of the main velocity structures based only on the kinemetric parameters which can be used to quantify the features in velocity maps. © 2006 RAS.

Abstract:

We have demonstrated the first near-infrared multi-object spectrograph, CIRPASS, on the 4.2-m William Herschel Telescope (WHT) and the 3.9-m Anglo-Australian Telescope. We have conducted an H survey of 38 0.77 < z < 1 galaxies over ∼100 arcmin2 of the Hubble Deep Field North and Flanking Fields, to determine star formation rates (SFRs) using CIRPASS on the WHT. This represents the first successful application of this technique to observing high redshift galaxies (Doherty et al. 2004). Stacking the spectra in the rest-frame, we find a lower limit (uncorrected for dust reddening) on the star formation rate density at redshift z = 1 of 0.04 M yr1 Mpc 3 (Doherty et al. 2006). This implies rapid evolution in the star formation rate density from z = 0 to z = 1 which is proportional to (1 + z) 3.1. We intend to extend our work with FMOS on Subaru as the evolSMURF project (the Evolution of Star-formation and Metallicity in the Universe at high Redshift with FMOS). This will represent nearly two orders-of-magnitude improvement on previous work, and for the first time will provide a sample of sufficient size to measure accurately the H luminosity function, and so determine the global star formation rate using the same indicator as used in local surveys. Using [O II]3727 , H, [O III] 5007 and H redshifted into the z, J & H bands, we can chart the star formation history over 70% of the age of the Universe, affording complete coverage up to z = 1.6 with the same well-understood diagnostics. The line ratios will also allow the extinction and metallicity to be measured at z>1. This will resolve one of the long-standing puzzles in extragalactic astrophysics the true evolution of the Madau-Lilly diagram of star formation density. © 2007 International Astronomical Union.

Abstract:

We present the results of deep Westerbork Synthesis Radio Telescope observations of neutral hydrogen in 12 nearby elliptical and lenticular galaxies. The selected objects come from a representative sample of nearby galaxies earlier studied at optical wavelengths with the integralfield spectrograph SAURON (Spectrographic Areal Unit for Research on Optical Nebulae). They are field galaxies, or (in two cases) located in poor group environments. We detect HI - both in regular discs as well as in clouds and tails offset from the host galaxy - in 70 per cent of the galaxies. This detection rate is much higher than in previous, shallower single-dish surveys, and is similar to that for the ionized gas. The results suggest that at faint detection levels the presence of H I is a relatively common characteristic of field early-type galaxies, confirming what was suggested twenty years ago by Jura based on IRAS observations. The observed total Hi masses range between a few times 10 6 to just over 109 M⊙. The presence of regular disc-like structures is a situation as common as Hi in offset clouds and tails around early-type galaxies. All galaxies where Hi is detected also contain ionized gas, whereas no Hi is found around galaxies without ionized gas. Galaxies with regular Hi discs tend to have strong emission from ionized gas. In these cases, the similar kinematics of the neutral hydrogen and ionized gas suggest that they form one structure. The kinematical axis of the stellar component is nearly always misaligned with respect to that of the gas. We do not find a clear trend between the presence of H I and the global age of the stellar population or the global dynamical characteristics of the galaxies. More specifically, H I detections are uniformly spread through the (V/σ, ∈) diagram. If fast and slow rotators - galaxies with high and low specific angular momentum - represent the relics of different formation paths, this does not appear in the presence and characteristics of the H i. Our observations support the idea that gas accretion is common and does not happen exclusively in peculiar early-type galaxies. The links observed between the large-scale gas and the characteristics on the nuclear scale (e.g. the presence of kinematically decoupled cores, radio continuum emission etc.) suggest that for the majority of the cases the gas is acquired through merging, but the lack of correlation with the stellar population age suggests that smooth, cold accretion could be an alternative scenario, at least in some galaxies. In either case, the data suggest that early-type galaxies continue to build their mass up to the present. © 2006 RAS.

Abstract:

We present two-degree field spectroscopic observations of a sample of 96 A-type stars selected from the Sloan Digital Sky Survey Data Release 3 (SDSS DR3). Our aim is to identify blue horizontal branch (BHB) stars in order to measure the kinematic properties of the tidal tails of the Sagittarius dwarf spheroidal galaxy. We confine our attention to the 44 classifiable stars with spectra of signal-to-noise ratio > 15 Å-1. Classification produces a sample of 29 BHB stars at distances of 5-47 kpc from the Sun. We split our sample into three bins based on their distance. We find 10 of the 12 stars at 14-25 kpc appear to have coherent, smoothly varying radial velocities which are plausibly associated with old debris in the Sagittarius tidal stream. Further observations along the orbit and at greater distances are required to trace the full extent of this structure on the sky. Three of our BHB stars in the direction of the globular cluster Palomar (Pal) 5 appear to be in an overdensity but are in the foreground of Pal 5. More observations are required around this overdensity to establish any relation to Pal 5 and/or the Sgr stream. We emphasize observations of BHB stars have unlimited potential for providing accurate velocity and distance information in old distant halo streams and globular clusters alike. The next-generation multi-object spectrographs provide an excellent opportunity to accurately trace the full extent of such structures. © 2006 RAS.

Abstract:

We seek to understand whether the stellar populations of galactic bulges show evidence of secular evolution triggered by the presence of the disc. For this purpose, we re-analyse the sample of Proctor and Sansom, deriving stellar population ages and element abundances from absorption-line indices as functions of central velocity dispersion and Hubble type. We obtain consistent constraints on ages from the three Balmer-line indices Hβ, Hγ and Hδ, based on stellar population models that take the abundance ratio effects on these indices into account. Emission-line contamination turns out to be a critical aspect, which favours the use of the higher order Balmer-line indices. Our derived ages are consistent with those of Proctor and Sansom based on a completely different method. In agreement with other studies in the literature, we find that bulges have relatively low luminosity weighted ages, the lowest age derived being 1.3 Gyr. Hence, bulges are not generally old, but actually rejuvenated systems. We discuss evidence that this might be true also for the bulge of the Milky Way. The data reveal clear correlations of all three parameters luminosity weighted age, total metallicity and α/Fe ratio with central velocity dispersion. The smallest bulges are the youngest with the lowest α/Fe ratios owing to late Fe enrichment from Type Ia supernovae. Using models combining recent minor star formation with a base old population, we show that the smallest bulges must have experienced significant star formation events involving 10-30 per cent of their total mass in the past 1-2 Gyr. No significant correlations of the stellar population parameters with Hubble type are found. We show that the above relationships with σ coincide perfectly with those of early-type galaxies. In other words, bulges are typically younger, metal poorer and less α/Fe enhanced than early-type galaxies because of their smaller masses. At a given velocity dispersion, bulges and elliptical galaxies are indistinguishable as far as their stellar populations are concerned. These results favour an inside-out formation scenario and indicate that the discs in spiral galaxies of Hubble types Sbc and earlier cannot have a significant influence on the evolution of the stellar populations in the bulge component. The phenomenon of pseudo-bulge formation must be restricted to spirals of types later than Sbc. © 2005 RAS.