Galaxy formation and evolution

Abstract:

We determine the abundance of i′-band drop-outs in the HST/ACS GOODS surveys and the Hubble Ultra Deep Field (UDF). The majority of these sources are likely to be z ≈ 6 galaxies whose flux decrement between the F775W i′-band and F850LP z′-band arises from Lyman-α absorption. We have shown with Keck/DEIMOS and Gemini/GMOS spectroscopy that this technique does indeed select high redshift galaxies, and we discovered Lyman-α emission in the expected redshift range for about a third of the galaxies with z′AB < 25.6 in the 150 arcmin2 of the GOODS-South field. The i-drop number counts in the GOODS-North field are consistent, so cosmic variance is possibly not be the dominant uncertainty. The increased depth of UDF enables us to reach a ∼10σ limiting magnitude of z′AB = 28.5 (equivalent to 1.5 h70-2 M⊙ yr-1 at z = 6.1, or 0.1 LUV* for the z ≈ 3 U-drop population). The star formation rate at z ≈ 6 was approximately ×6 less than at z ≈ 3. This declining comoving star formation rate (0.005 h70 M⊙ yr-1 Mpc-3 at z ≈ 6 at LUV > 0.1L* for a Salpeter IMF) poses an interesting challenge for models which suggest that LUV > 0.1L* star forming galaxies at z ≃ 6 reionized the universe. The short-fall in ionizing photons might be alleviated by galaxies fainter than our limit, or a radically different IMF. Alternatively, the bulk of reionization might have occurred at z ≫ 6. We have recently discovered evidence of an early epoch of star formation in some of the i′-drops at z ≈ 6. Spitzer images with IRAC at 3.6-4.5 μm show evidence of the age-sensitive Balmer/4000 Å, dominated by stars older than 100 Myr (and most probably 400 Myr old). This pushes the formation epoch for these galaxies to zform = 7.5-13.5. There are at least some galaxies already assembled with stellar masses ≈3 × 1010 M⊙ (equivalent to 0.2 M* today) within the first billion years. The early formation of such systems may have played a key role in reionizing the Universe at z ∼ 10. © 2005 Elsevier B.V. All rights reserved.

Abstract:

We present integral-field observations of kinematically decoupled components (KDCs) in a sample of 24 Sa galaxies part of the SAURON Survey. We show how the V/ maps can be used to highlight the presence of the KDC and investigate their stellar populations (i.e. age) to understand their origin (i.e. secular vs hierarchical scenarios). We find that KDCs can be long-lived and are typically well aligned with the galaxies kinematical major axis, supporting mounting evidence that secular evolution becomes important at intermediate redshifts. Dynamically colder ones seem to host younger stellar populations. We also relate these features to the ones found in our sample of 48 elliptical and lenticular galaxies. We find that there is no age vs size relation for these decoupled components in Sa galaxies, suggesting that the dominant factor determining their size is the amount of star formation they have been subject to during their life time. © 2007 International Astronomical Union.

Abstract:

We present absorption line strength maps of 48 representative elliptical and lenticular galaxies obtained as part of a survey of nearby galaxies using our custom-built integral-field spectrograph, SAURON, operating on the William Herschel Telescope. Using high-quality spectra, spatially binned to a constant signal-to-noise ratio, we measure four key age, metallicity and abundance ratio sensitive indices from the LickIDS system over a two-dimensional field extending up to approximately one effective radius. A discussion of calibrations and offsets is given, along with a description of error estimation and nebular emission correction. We modify the classical Fe5270 index to define a new index, , which maximizes the useable spatial coverage of SAURON. Maps of Hβ, Fe5015, Mg b and are presented for each galaxy. We use the maps to compute average line strengths integrated over circular apertures of one-eighth effective radius, and compare the resulting relations of index versus velocity dispersion with previous long-slit work. The metal line strength maps show generally negative gradients with increasing radius roughly consistent with the morphology of the light profiles. Remarkable deviations from this general trend exist, particularly the Mg b isoindex contours appear to be flatter than the isophotes of the surface brightness for about 40 per cent of our galaxies without significant dust features. Generally, these galaxies exhibit significant rotation. We infer from this that the fast-rotating component features a higher metallicity andor an increased MgFe ratio as compared to the galaxy as a whole. The Hβ maps are typically flat or show a mild positive outwards radial gradient, while a few galaxies show strong central peaks andor elevated overall Hβ strength likely connected to recent star formation activity. For the most prominent post-starburst galaxies, even the metal line strength maps show a reversed gradient. © 2006 RAS.

Abstract:

We present high spatial resolution integral-field spectroscopy of 28 elliptical (E) and lenticular (SO) galaxies from the SAURON representative survey obtained with the OASIS spectrograph during its operation at the Canada-France-Hawaii Telescope. These seeing-limited observations explore the central 8 × 10 arcsec2 (typically 1 kpc diameter) regions of these galaxies using a spatial sampling four times higher than SAURON (0.27-arcsec versus 0.94-arcsec spatial elements), resulting in almost a factor of 2 improvement in the median point spread function. These data allow accurate study of the central regions to complement the large-scale view provided by SAURON. Here we present the stellar and gas kinematics, stellar absorption-line strengths and nebular emission-line strengths for this sample. We also characterize the stellar velocity maps using the 'kinemetry' technique, and derive maps of the luminosity-weighted stellar age, metallicity and abundance ratio via stellar population models. We give a brief review of the structures found in our maps, linking also to larger-scale structures measured with SAURON. We present two previously unreported kinematically decoupled components (KDCs) in the centres of NGC 3032 and NGC 4382. We compare the intrinsic size and luminosity-weighted stellar age of all the visible KDCs in the full SAURON sample, and find two types of components: kiloparsec-scale KDCs, which are older than 8 Gyr, and are found in galaxies with little net rotation; and compact KDCs, which have intrinsic diameters of less than a few hundred parsec, show a range of stellar ages from 0.5 to 15 Gyr (with 5/6 younger than 5 Gyr), are found exclusively in fast-rotating galaxies, and are close to counter-rotating around the same axis as their host. Of the seven galaxies in the SAURON sample with integrated luminosity-weighted ages less than 5 Gyr, five show such compact KDCs, suggesting a link between counter-rotation and recent star formation. We show that this may be due to a combination of small sample size at young ages, and an observational bias, since young KDCs are easier to detect than their older and/or corotating counterparts. © 2006 RAS.

Abstract:

We present observations of the stellar and gas kinematics for a representative sample of 24 Sa galaxies obtained with our custom-built integral-field spectrograph SAURON operating on the William Herschel Telescope. The data have been homogeneously reduced and analysed by means of a dedicated pipeline. All resulting data cubes were spatially binned to a minimum mean signal-to-noise ratio of 60 per spatial and spectral resolution element. Our maps typically cover the bulge-dominated region. We find a significant fraction of kinematically decoupled components (12/24), many of them displaying central velocity dispersion minima. They are mostly aligned and co-rotating with the main body of the galaxies, and are usually associated with dust discs and rings detected in unsharp-masked images. Almost all the galaxies in the sample (22/24) contain significant amounts of ionized gas which, in general, is accompanied by the presence of dust. The kinematics of the ionized gas are consistent with circular rotation in a disc co-rotating with respect to the stars. The distribution of mean misalignments between the stellar and gaseous angular momenta in the sample suggests that the gas has an internal origin. The [O III]/Hβ ratio is usually very low, indicative of current star formation, and shows various morphologies (ring-like structures, alignments with dust lanes or amorphous shapes). The star formation rates (SFRs) in the sample are comparable with that of normal disc galaxies. Low gas velocity dispersion values appear to be linked to regions of intense star formation activity. We interpret this result as stars being formed from dynamically cold gas in those regions. In the case of NGC 5953, the data suggest that we are witnessing the formation of a kinematically decoupled component from cold gas being acquired during the ongoing interaction with NGC 5954. © 2006 RAS.