On the dependence of galaxy morphologies on galaxy mergers

The distribution of galaxy morphological types is a key test for models of galaxy formation and evolution, providing strong constraints on the relative contribution of different physical processes responsible for the growth of the spheroidal components. In this paper, we make use of a suite of semi-analytic models to study the efficiency of galaxy mergers in disrupting galaxy discs and building galaxy bulges. In particular, we compare standard prescriptions usually adopted in semi-analytic models, with new prescriptions proposed by Kannan et al., based on results from high-resolution hydrodynamical simulations, and we show that these new implementations reduce the efficiency of bulge formation through mergers. In addition, we compare our model results with a variety of observational measurements of the fraction of spheroid-dominated galaxies as a function of stellar and halo mass, showing that the present uncertainties in the data represent an important limitation to our understanding of spheroid formation. Our results indicate that the main tension between theoretical models and observations does not stem from the survival of purely disc structures (i.e. bulgeless galaxies), rather from the distribution of galaxies of different morphological types, as a function of their stellar mass.Comment: MNRAS in press, 11 pages, 5 figure

Lithium alters brain activation in bipolar disorder in a task- and state-dependent manner: an fMRI study

BACKGROUND: It is unknown if medications used to treat bipolar disorder have effects on brain activation, and whether or not any such changes are mood-independent. METHODS: Patients with bipolar disorder who were depressed (n = 5) or euthymic (n = 5) were examined using fMRI before, and 14 days after, being started on lithium (as monotherapy in 6 of these patients). Patients were examined using a word generation task and verbal memory task, both of which have been shown to be sensitive to change in previous fMRI studies. Differences in blood oxygenated level dependent (BOLD) magnitude between the pre- and post-lithium results were determined in previously defined regions of interest. Severity of mood was determined by the Hamilton Depression Scale for Depression (HAM-D) and the Young mania rating scale (YMRS). RESULTS: The mean HAM-D score at baseline in the depressed group was 15.4 ± 0.7, and after 2 weeks of lithium it was 11.0 ± 2.6. In the euthymic group it was 7.6 ± 1.4 and 3.2 ± 1.3 respectively. At baseline mean BOLD signal magnitude in the regions of interest for the euthymic and depressed patients were similar in both the word generation task (1.56 ± 0.10 and 1.49 ± 0.10 respectively) and working memory task (1.02 ± 0.04 and 1.12 ± 0.06 respectively). However, after lithium the mean BOLD signal decreased significantly in the euthymic group in the word generation task only (1.56 ± 0.10 to 1.00 ± 0.07, p < 0.001). Post-hoc analysis showed that these differences were statistically significant in Broca's area, the left pre-central gyrus, and the supplemental motor area. CONCLUSION: This is the first study to examine the effects of lithium on brain activation in bipolar patients. The results suggest that lithium has an effect on euthymic patients very similar to that seen in healthy volunteers. The same effects are not seen in depressed bipolar patients, although it is uncertain if this lack of change is linked to the lack of major improvements in mood in this group of patients. In conclusion, this study suggests that lithium may have effects on brain activation that are task- and state-dependent. Given the small study size and the mildness of the patient's depression these results require replication

The lack of star formation gradients in galaxy groups up to z~1.6

In the local Universe, galaxy properties show a strong dependence on environment. In cluster cores, early type galaxies dominate, whereas star-forming galaxies are more and more common in the outskirts. At higher redshifts and in somewhat less dense environments (e.g. galaxy groups), the situation is less clear. One open issue is that of whether and how the star formation rate (SFR) of galaxies in groups depends on the distance from the centre of mass. To shed light on this topic, we have built a sample of X-ray selected galaxy groups at 0<z<1.6 in various blank fields (ECDFS, COSMOS, GOODS). We use a sample of spectroscopically confirmed group members with stellar mass M >10^10.3 M_sun in order to have a high spectroscopic completeness. As we use only spectroscopic redshifts, our results are not affected by uncertainties due to projection effects. We use several SFR indicators to link the star formation (SF) activity to the galaxy environment. Taking advantage of the extremely deep mid-infrared Spitzer MIPS and far-infrared Herschel PACS observations, we have an accurate, broad-band measure of the SFR for the bulk of the star-forming galaxies. We use multi-wavelength SED fitting techniques to estimate the stellar masses of all objects and the SFR of the MIPS and PACS undetected galaxies. We analyse the dependence of the SF activity, stellar mass and specific SFR on the group-centric distance, up to z~1.6, for the first time. We do not find any correlation between the mean SFR and group-centric distance at any redshift. We do not observe any strong mass segregation either, in agreement with predictions from simulations. Our results suggest that either groups have a much smaller spread in accretion times with respect to the clusters and that the relaxation time is longer than the group crossing time.Comment: Accepted for publication in MNRA

BULGE-FORMING GALAXIES with AN EXTENDED ROTATING DISK at z ∼ 2

We present 0".2-resolution Atacama Large Millimeter/submillimeter Array observations at 870 um for 25 Halpha-seleced star-forming galaxies (SFGs) around the main-sequence at z=2.2-2.5. We detect significant 870 um continuum emission in 16 (64%) of these SFGs. The high-resolution maps reveal that the dust emission is mostly radiated from a single region close to the galaxy center. Exploiting the visibility data taken over a wide $uv$ distance range, we measure the half-light radii of the rest-frame far-infrared emission for the best sample of 12 massive galaxies with logM*>11. We find nine galaxies to be associated with extremely compact dust emission with R_{1/2,870um}<1.5 kpc, which is more than a factor of 2 smaller than their rest-optical sizes, R_{1/2,1.6um}=3.2 kpc, and is comparable with optical sizes of massive quiescent galaxies at similar redshifts. As they have an exponential disk with Sersic index of n=1.2 in the rest-optical, they are likely to be in the transition phase from extended disks to compact spheroids. Given their high star formation rate surface densities within the central 1 kpc of Sigma SFR1kpc=40 Msol/yr/kpc^2, the intense circumnuclear starbursts can rapidly build up a central bulge with Sigma M*1kpc>1e10 Msol/kpc^2 in several hundred Myr, i.e. by z~2. Moreover, ionized gas kinematics reveal that they are rotation-supported with an angular momentum as large as that of typical SFGs at z=1-3. Our results suggest bulges are commonly formed in extended rotating disks by internal processes, not involving major mergers.Comment: 11 pages, 6 figures, 2 tables, accepted for publication in Ap

The association between gas and galaxies II: the two-point correlation function

We measure the two-point correlation function, ξAG, between galaxies and quasar absorption-line systems at z 1017 cm2. For C iv absorbers, the peak strength of ξAG is roughly comparable to that of H i absorbers with NH I > 1016.5 cm2, consistent with the finding that the C iv absorbers are associated with strong H i absorbers. We do not reproduce the differences reported by Chen et al. between 1D ξAG measurements using galaxy subsamples of different spectral types. However, the full impact on the measurements of systematic differences in our samples is hard to quantify. We compare the observations with smoothed particle hydrodynamical (SPH) simulations and discover that in the observations ξAG is more concentrated to the smallest separations than in the simulations. The latter also display a 'finger of god' elongation of ξAG along the LOS in redshift space, which is absent from our data, but similar to that found by Ryan-Weber for the cross-correlation of quasar absorbers and H i-emission-selected galaxies. The physical origin of these 'fingers of god' is unclear, and we thus highlight several possible areas for further investigation

The influence of the environmental history on quenching star formation in a Λ cold dark matter universe

International audienceWe present a detailed analysis of the influence of the environment and of the environmental history on quenching star formation in central and satellite galaxies in the local Universe. We take advantage of publicly available galaxy catalogues obtained from applying a galaxy formation model to the Millennium simulation. In addition to halo mass, we consider the local density of galaxies within various fixed scales. Comparing our model predictions to observational data [Sloan Digital Sky Survey (SDSS)], we demonstrate that the models are failing to reproduce the observed density dependence of the quiescent galaxy fraction in several aspects: for most of the stellar mass ranges and densities explored, models cannot reproduce the observed similar behaviour of centrals and satellites, they slightly underestimate the quiescent fraction of centrals and significantly overestimate that of satellites. We show that in the models, the density dependence of the quiescent central galaxies is caused by a fraction of `backsplash' centrals which have been satellites in the past. The observed stronger density dependence on scales of 0.2-1 Mpc may, however, indicate additional environmental processes working on central galaxies. Turning to satellite galaxies, the density dependence of their quiescent fractions reflects a dependence on the time spent orbiting within a parent halo, correlating strongly with halo mass and distance from the halo centre. Comparisons with observational estimates suggest relatively long gas consumption time-scales of roughly 5 Gyr in low-mass satellite galaxies. The quenching time-scales decrease with increasing satellite stellar mass. Overall, a change in modelling both internal processes and environmental processes is required for improving currently used galaxy formation models

The stellar mass content of distant galaxy groups

We have obtained near-infrared imaging of 58 galaxy groups, in the redshift range 0.1 &lt; z &lt; 0.6, from the William Herschel Telescope and from the Spitzer IRAC data archive. The groups are selected from the CNOC2 redshift survey, with additional spectroscopy from the Baade telescope (Magellan). Our group samples are statistically complete to KVega = 17.7 (INGRID) and [3.6µm]AB = 19.9 (IRAC). From these data we construct near-infrared luminosity functions, for groups in bins of velocity dispersion, up to 800 km s −1, and redshift. The total amount of near-infrared luminosity per group is compared with the dynamical mass, estimated from the velocity dispersion, to compute the mass-to-light ratio, M200/LK. We find that the M200/LKvalues in these groups are in good agreement with those of their statistical descendants at z = 0, with no evidence for evolution beyond that expected for a passively evolving population. There is a trend of M200/LK with group mass, which increases from M200/LK ≈ 10 for groups with σ &lt; 250 km s −1 to M200/LK ≈ 100 for 425 km s −1 &lt; σ &lt; 800 km s −1. This trend is weaker, but still present, if we estimate the total mass from weak lensing measurements. In terms of stellar mass, stars make up&gt