Galaxy formation and evolution

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.

Abstract:

We present the emission-line fluxes and kinematics of 48 representative elliptical and lenticular galaxies obtained with our custom-built integral-field spectrograph, SAURON, operating on the William Herschel Telescope. Hβ, [O III] λλ.4959,5007 and [N I] λλ5198,5200 emission lines were measured using a new procedure that simultaneously fits both the stellar spectrum and the emission lines. Using this technique we can detect emission lines down to an equivalent width of 0.1 Å set by the current limitations in describing galaxy spectra with synthetic and real stellar templates, rather than by the quality of our spectra. Gas velocities and velocity dispersions are typically accurate to within 14 and 20 km s ^-1, respectively, and at worse to within 25 and 40 km s~L. The errors on the flux of the [O III] and Hβ lines are on average 10 and 20 per cent, respectively, and never exceed 30 per cent. Emission is clearly detected in 75 per cent of our sample galaxies, and comes in a variety of resolved spatial distributions and kinematic behaviours. A mild dependence on the Rubble type and galactic environment is observed, with higher detection rates in lenticular galaxies and field objects. More significant is the fact that only 55 per cent of the galaxies in the Virgo cluster exhibit clearly detected emission. The ionized-gas kinematics is rarely consistent with simple coplanar circular motions. However, the gas almost never displays completely irregular kinematics, generally showing coherent motions with smooth variations in angular momentum. In the majority of the cases, the gas kinematics is decoupled from the stellar kinematics, and in half of the objects this decoupling implies a recent acquisition of gaseous material. Over the entire sample however, the distribution of the mean misalignment values between stellar and gaseous angular momenta is inconsistent with a purely external origin. The distribution of kinematic misalignment values is found to be strongly dependent on the apparent flattening and the level of rotational support of galaxies, with flatter, fast rotating objects hosting preferentially corotating gaseous and stellar systems. In a third of the cases, the distribution and kinematics of the gas underscore the presence of nonaxisymmetric perturbations of the gravitational potential. Consistent with previous studies, the presence of dust features is always accompanied by gas emission while the converse is not always true. A considerable range of values for the [O III]/Hβ ratio is found both across the sample and within single galaxies. Despite the limitations of this ratio as an emission-line diagnostic, this finding suggests either that a variety of mechanisms is responsible for the gas excitation in E and S0 galaxies or that the metallicity of the interstellar material is quite heterogeneous. © 2006 RAS.

Abstract:

Samples of Extremely Red Galaxies (ERGs) have generally been seen to comprise a mix of actively star-forming galaxies with significant dust reddening and evolved, passive galaxies, at redshifts about z 1 2. Initial results from deep Keck spectroscopy of ERGs (Doherty et al. 2005) revealed dominant old stellar populations in 75% of our spectroscopic sample, but only 28% have spectra with no evidence of recent star formation activity, such as would be expected for a strictly passively-evolving population. This study suggests that the bulk of the ERGs are luminous, spheroidal, evolved galaxies, but undergoing intermittent activity consistent with continued growth. Through a detailed investigation of individual galaxies in our sample we aim to address various outstanding questions. What fraction of their mass is produced in ongoing star formation? Is there a characteristic mass at which star formation is abruptly truncated? What mechanism provokes a secondary burst of star formation in evolved galaxies? We fit Bruzual & Charlot (2003; BC03) simple stellar population models to the broad band SEDs over a wide baseline, using a reduced 2 minimisation, to investigate ages, stellar masses and star formation histories. The fits for the early types agree well with information in the spectra and return ages of 23 Gyr and masses in the range 10 111012M. The objects with recent star formation episodes are more complex. Some are fit well by continuous star formation models, accounting for the effects of dust. We are now in the process of exploring multi-population fits to investigate the effects of episodic bursts. Previous morphological studies of ERGs have revealed a diverse mix of galaxies a combination of pure bulges, disks and a small fraction of irregular or interacting systems. We are curious to determine whether a morphological analysis produces results consistent with the spectroscopic properties of our sample. We are investigating a sub-sample of our galaxies which have HST imaging publically available. Initial results from a quantitative analysis using bulge/disk decomposition with GALFIT and GIM2D indicate that most galaxies with Early type spectra are bulge dominated. In contrast, a significant fraction of the galaxies showing spectroscopic signatures of on-going star formation on top of underlying old stellar populations appear to have a well-established classical spiral morphology, wih knots of star formation located in spiral arms around a central bulge. There is tenuous evidence (under further investigation) that at least half of the post-starbursts in our sample are barred spirals, lending support to theories relating post-starbursts to recent mergers. © 2007 International Astronomical Union.

Abstract:

We have conducted an Hα survey of 38 0.77 < z < 1 galaxies over ∼100arcmin2 of the Hubble Deep Field-North and Flanking Fields, to determine star formation rates (SFRs), with the near-IR multi-object spectrograph Cambridge Infrared Panoramic Survey Spectrograph (CIRPASS) on the William Herschel Telescope (WHT). This represents the first successful application of this technique to observing high-redshift galaxies. Stacking the spectra in the rest frame to infer a total SFR for the field, we find a lower limit (uncorrected for dust reddening) on the SFR density at redshift z = 1 of 0.04 M⊙ yr-1 Mpc-3. This implies rapid evolution in the SFR density from z = 0 to 1 which is proportional to (1+ z)3.1 © 2006 RAS.