Matteo Fossati

The KMOS3D survey: design, first results, and the evolution of galaxy kinematics from 0.7 ≤ z ≤ 2.7

We present the KMOS3D survey, a new integral field survey of over 600 galaxies at 0.7 1, implying that the star-forming main sequence is primarily composed of rotating galaxies at both redshift regimes. When considering additional stricter criteria, the Hα kinematic maps indicate that at least ~70% of the resolved galaxies are disk-like systems. Our high-quality KMOS data confirm the elevated velocity dispersions reported in previous integral field spectroscopy studies at z 0.7. For rotation-dominated disks, the average intrinsic velocity dispersion decreases by a factor of two from 50 km s–1at z ~ 2.3 to 25 km s–1at z ~ 0.9. Combined with existing results spanning z ~ 0-3, we show that disk velocity dispersions follow an evolution that is consistent with the dependence of velocity dispersion on gas fractions predicted by marginally stable disk theory.

Evidence for Wide-spread Active Galactic Nucleus-driven Outflows in the Most Massive z 1-2 Star-forming Galaxies

In this paper, we follow up on our previous detection of nuclear ionized outflows in the most massive (log(M */M ☉) ≥ 10.9) z ~ 1-3 star-forming galaxies by increasing the sample size by a factor of six (to 44 galaxies above log(M */M ☉) ≥ 10.9) from a combination of the SINS/zC-SINF, LUCI, GNIRS, and KMOS3Dspectroscopic surveys. We find a fairly sharp onset of the incidence of broad nuclear emission (FWHM in the Hα, [N II], and [S II] lines ~450-5300 km s–1), with large [N II]/Hα ratios, above log(M */M ☉) ~ 10.9, with about two-thirds of the galaxies in this mass range exhibiting this component. Broad nuclear components near and above the Schechter mass are similarly prevalent above and below the main sequence of star-forming galaxies, and at z ~ 1 and ~2. The line ratios of the nuclear component are fit by excitation from active galactic nuclei (AGNs), or by a combination of shocks and photoionization. The incidence of the most massive galaxies with broad nuclear components is at least as large as that of AGNs identified by X-ray, optical, infrared, or radio indicators. The mass loading of the nuclear outflows is near unity. Our findings provide compelling evidence for powerful, high-duty cycle, AGN-driven outflows near the Schechter mass, and acting across the peak of cosmic galaxy formation.

Assessing the Tendency of Fluorescent Proteins to Oligomerize under Physiologic Conditions

Several fluorescent proteins (FPs) are prone to forming low‐affinity oligomers. This undesirable tendency is exacerbated when FPs are confined to membranes or when fused to naturally oligomeric proteins. Oligomerization of FPs limits their suitability for creating fusions with proteins of interest. Unfortunately, no standardized method evaluates the biologically relevant oligomeric state of FPs. Here, we describe a quantitative visual assay for assessing whether FPs are sufficiently monomeric under physiologic conditions. Membrane‐associated FP‐fusion proteins, by virtue of their constrained planar geometry, achieve high effective concentrations. We exploited this propensity to develop an assay to measure FP tendencies to oligomerize in cells. FPs were fused on the cytoplasmic end of an endoplasmic reticulum (ER) signal‐anchor membrane protein (CytERM) and expressed in cells. Cells were scored based on the ability of CytERM to homo‐oligomerize with proteins on apposing membranes and restructure the ER from a tubular network into organized smooth ER (OSER) whorl structures. The ratio of nuclear envelope and OSER structures mean fluorescent intensities for cells expressing enhanced green fluorescent protein (EGFP) or monomeric green fluorescent protein (mGFP) CytERM established standards for comparison of uncharacterized FPs. We tested three FPs and identified two as sufficiently monomeric, while a third previously reported as monomeric was found to strongly oligomerize.

A VAPB mutant linked to amyotrophic lateral sclerosis generates a novel form of organized smooth endoplasmic reticulum

VAPB (vesicle‐associated membrane protein‐associated protein B) is an endoplasmic reticulum (ER)‐resident tail‐anchored adaptor protein involved in lipid transport. A dominantly inherited mutant, P56S‐VAPB, causes a familial form of amyotrophic lateral sclerosis (ALS) and forms poorly characterized inclusion bodies in cultured cells. To provide a cell biological basis for the understanding of mutant VAPB pathogenicity, we investigated its biogenesis and the inclusions that it generates. Translocation assays in cell‐free systems and in cultured mammalian cells were used to investigate P56S‐VAPB membrane insertion, and the inclusions were characterized by confocal imaging and electron microscopy. We found that mutant VAPB inserts post‐translationally into ER membranes in a manner indistinguishable from the wild‐type protein but that it rapidly clusters to form inclusions that remain continuous with the rest of the ER. Inclusions were induced by the mutant also when it was expressed at levels comparable to the endogenous wild‐type protein. Ultrastructural analysis revealed that the inclusions represent a novel form of organized smooth ER (OSER) consisting in a limited number of parallel cisternae (usually 2 or 3) interleaved by a ~30 nm‐thick electron‐dense cytosolic layer. Our results demonstrate that the ALS‐linked VAPB mutant causes dramatic ER restructuring that may underlie its pathogenicity in motoneurons.—Fasana, E., Fossati, M., Ruggiano, A., Brambillasca, S., Hoogenraad, C. C., Navone, F., Francolini, M., Borgese, N. A VAPB mutant linked to amyotrophic lateral sclerosis generates a novel form of organized smooth endoplasmic reticulum. FASEB J. 24, 1419–1430 (2010). www.fasebj.org

MUSE sneaks a peek at extreme ram-pressure stripping events – I. A kinematic study of the archetypal galaxy ESO137−001

We present Multi Unit Spectroscopic Explorer (MUSE) observations of ESO137−001, a spiral galaxy infalling towards the centre of the massive Norma cluster at z ∼ 0.0162. During the high-velocity encounter of ESO137−001 with the intracluster medium, a dramatic ram-pressure stripping event gives rise to an extended gaseous tail, traced by our MUSE observations to >30 kpc from the galaxy centre. By studying the Hα surface brightness and kinematics in tandem with the stellar velocity field, we conclude that ram pressure has completely removed the interstellar medium from the outer disc, while the primary tail is still fed by gas from the inner regions. Gravitational interactions do not appear to be a primary mechanism for gas removal. The stripped gas retains the imprint of the disc rotational velocity to ∼20 kpc downstream, without a significant gradient along the tail, which suggests that ESO137−001 is fast moving along a radial orbit in the plane of the sky. Conversely, beyond ∼20 kpc, a greater degree of turbulence is seen, with velocity dispersion up to ≳100 km s−1. For a model-dependent infall velocity of vinf ∼ 3000 km s−1, we conclude that the transition from laminar to turbulent flow in the tail occurs on time-scales ≥6.5 Myr. Our work demonstrates the terrific potential of MUSE for detailed studies of how ram-pressure stripping operates on small scales, providing a deep understanding of how galaxies interact with the dense plasma of the cluster environment.

Strongly baryon-dominated disk galaxies at the peak of galaxy formation ten billion years ago

In the cold dark matter cosmology, the baryonic components of galaxies—stars and gas—are thought to be mixed with and embedded in non-baryonic and non-relativistic dark matter, which dominates the total mass of the galaxy and its dark-matter halo. In the local (low-redshift) Universe, the mass of dark matter within a galactic disk increases with disk radius, becoming appreciable and then dominant in the outer, baryonic regions of the disks of star-forming galaxies. This results in rotation velocities of the visible matter within the disk that are constant or increasing with disk radius—a hallmark of the dark-matter model. Comparisons between the dynamical mass, inferred from these velocities in rotational equilibrium, and the sum of the stellar and cold-gas mass at the peak epoch of galaxy formation ten billion years ago, inferred from ancillary data, suggest high baryon fractions in the inner, star-forming regions of the disks. Although this implied baryon fraction may be larger than in the local Universe, the systematic uncertainties (owing to the chosen stellar initial-mass function and the calibration of gas masses) render such comparisons inconclusive in terms of the mass of dark matter. Here we report rotation curves (showing rotation velocity as a function of disk radius) for the outer disks of six massive star-forming galaxies, and find that the rotation velocities are not constant, but decrease with radius. We propose that this trend arises because of a combination of two main factors: first, a large fraction of the massive high-redshift galaxy population was strongly baryon-dominated, with dark matter playing a smaller part than in the local Universe; and second, the large velocity dispersion in high-redshift disks introduces a substantial pressure term that leads to a decrease in rotation velocity with increasing radius. The effect of both factors appears to increase with redshift. Qualitatively, the observations suggest that baryons in the early (high-redshift) Universe efficiently condensed at the centres of dark-matter haloes when gas fractions were high and dark matter was less concentrated.

Hα3: an Hα imaging survey of HI selected galaxies from ALFALFA - VI. The role of bars in quenching star formation from z = 3 to the present epoch

A growing body of evidence indicates that the star formation rate per unit stellar mass (sSFR) decreases with increasing mass in normal main-sequence star-forming galaxies. Many process es have been advocated as being responsible for this trend (a lso known as mass quenching), e.g., feedback from active galactic nuc lei (AGNs), and the formation of classical bulges. In order t o improve our insight into the mechanisms regulating the star formati on in normal star-forming galaxies across cosmic epochs, we det rmine a refined star formation versus stellar mass relation in the l ocal Universe. To this end we use the H α narrow-band imaging followup survey (Hα3) of field galaxies selected from the HI Arecibo Legacy Fast A LF Survey (ALFALFA) in the Coma and Local superclusters. By complementing this local determination with high-redshift measurements from the literature, we re construct the star formation history of main-sequence galaxies as a funct ion of stellar mass from the present epoch up to z = 3. In agreement with previous studies, our analysis shows that quenching mechan isms occur above a threshold stellar mass Mknee that evolves with redshift as∝ (1 + z)2. Moreover, visual morphological classification of individ ual objects in our local sample reveals a sharp increase in th e fraction of visually classified strong bars with mass, hinti ng that strong bars may contribute to the observed downturn i n the sSFR aboveMknee. We test this hypothesis using a simple but physically motiv ated numerical model for bar formation, finding that strong bars can rapidly quench star formation in the central few kpc of field galaxies. We conclude that strong bars contribute si gnificantly to the red colors observed in the inner parts of massive galax ies, although additional mechanisms are likely required to quench the star formation in the outer regions of massive spiral galaxi es. Intriguingly, when we extrapolate our model to higher re dshifts, we successfully recover the observed redshift evolution for Mknee.A growing body of evidence indicates that the star formation rate per unit stellar mass (sSFR) decreases with increasing mass in normal main-sequence star-forming galaxies. Many processes have been advocated as being responsible for this trend (also known as mass quenching), e.g., feedback from active galactic nuclei (AGNs), and the formation of classical bulges. In order to improve our insight into the mechanisms regulating the star formation in normal star-forming galaxies across cosmic epochs, we determine a refined star formation versus stellar mass relation in the local Universe. To this end we use the Hα narrow-band imaging followup survey (Hα3) of field galaxies selected from the HI Arecibo Legacy Fast ALFA Survey (ALFALFA) in the Coma and Local superclusters. By complementing this local determination with high-redshift measurements from the literature, we reconstruct the star formation history of main-sequence galaxies as a function of stellar mass from the present epoch up to z = 3. In agreement with previous studies, our analysis shows that quenching mechanisms occur above a threshold stellar mass Mknee that evolves with redshift as ∝(1 + z) 2 . Moreover, visual morphological classification of individual objects in our local sample reveals a sharp increase in the fraction of visually classified strong bars with mass, hinting that strong bars may contribute to the observed downturn in the sSFR above Mknee. We test this hypothesis using a simple but physically motivated numerical model for bar formation, finding that strong bars can rapidly quench star formation in the central few kpc of field galaxies. We conclude that strong bars contribute significantly to the red colors observed in the inner parts of massive galaxies, although additional mechanisms are likely required to quench the star formation in the outer regions of massive spiral galaxies. Intriguingly, when we extrapolate our model to higher redshifts, we successfully recover the observed redshift evolution for Mknee. Our study highlights how the formation of strong bars in massive galaxies is an important mechanism in regulating the redshift evolution of the sSFR for field main-sequence galaxies.

Hα3: an Hα imaging survey of HI selected galaxies from ALFALFA - II. Star formation properties of galaxies in the Virgo cluster and surroundings

Context. We present the analysis of H alpha 3, an H alpha narrow-band imaging follow-up survey of 409 galaxies selected from the HI Arecibo Legacy Fast ALFA Survey (ALFALFA) in the Local Supercluster, including the Virgo cluster, in the region 11(h) \textless RA \textless 16(h); 4(o) \textless Dec \textless 16(o) 350 \textless cz \textless 2000 km s(-1). Aims. Taking advantage of H alpha 3, which provides the complete census of the recent massive star formation rate (SFR) in HI-rich galaxies in the local Universe and of ancillary optical data from SDSS we explore the relations between the stellar mass, the HI mass, and the current, massive SFR of nearby galaxies in the Virgo cluster. We compare these with those of isolated galaxies in the Local Supercluster, and we investigate the role of the environment in shaping the star formation properties of galaxies at the present cosmological epoch. Methods. By using the H alpha hydrogen recombination line as a tracer of recent star formation, we investigated the relationships between atomic neutral gas and newly formed stars in different environments (cluster and field), for many morphological types (spirals and dwarfs), and over a wide range of stellar masses (10(7.5) to 10(11.5) M-circle dot). To quantify the degree of environmental perturbation, we adopted an updated calibration of the HI deficiency parameter which we used to divide the sample into three classes: unperturbed galaxies (Def(HI) \textless= 0.3), perturbed galaxies (0.3 \textless Def(HI) \textless 0.9), and highly perturbed galaxies (Def(HI) \textgreater= 0.9). Results. Studying the mean properties of late-type galaxies in the Local Supercluster, we find that galaxies in increasing dense local galaxy conditions (or decreasing projected angular separation from M 87) show a significant decrease in the HI content and in the mean specific SFR, along with a progressive reddening of their stellar populations. The gradual quenching of the star formation occurs outside-in, consistently with the predictions of the ram pressure model. Once considered as a whole, the Virgo cluster is effective in removing neutral hydrogen from galaxies, and this perturbation is strong enough to appreciably reduce the SFR of its entire galaxy population. Conclusions. An estimate of the present infall rate of 300-400 galaxies per Gyr in the Virgo cluster is obtained from the number of existing HI-rich late-type systems, assuming 200-300 Myr as the time scale for HI ablation. If the infall process has been acting at a constant rate, this would imply that the Virgo cluster has formed approximately 2 Gyr ago, consistently with the idea that Virgo is in a young state of dynamical evolution.

MUSE sneaks a peek at extreme ram-pressure stripping events – II. The physical properties of the gas tail of ESO137−001

We present a study of the physical properties of the disc and tail of ESO137-001, a galaxy suffering from extreme ram-pressure stripping during its infall into the Norma cluster. With sensitive and spatially resolved MUSE (Multi Unit Spectroscopic Explorer) spectroscopy, we analyse the emission line diagnostics in the tail of ESO137-001, finding high values of [N II]/H alpha and [O I]/H alpha that are suggestive of the presence of shocks in turbulent gas. However, the observed line ratios are not as strong as commonly seen in pure shock heating models, suggesting that other emission mechanisms may contribute to the observed emission. Indeed, part of the observed emission, particularly at close separations from the galaxy disc, may originate from recombination of photoionized gas stripped from the main body of ESO137-001. We also identify a large number of bright compact knots within in the tail, with line ratios characteristic of H II regions. These H II regions, despite residing in a stripped gas tail, have quite typical line ratios, densities, temperatures, and metallicity (similar to 0.7 solar). The majority of these H II regions are embedded within diffuse gas from the tail, which is dynamically cool (sigma similar to 25-50 km s(-1)). This fact, together with a lack of appreciable gradients in age and metallicity, suggests that these H II regions formed in situ. While our analysis represents a first attempt to characterize the rich physics of the ESO137-001 tail, future work is needed to address the importance of other mechanisms, such as thermal conduction and magnetohydrodynamic waves, in powering the emission in the tail.

65 kpc of ionized gas trailing behind NGC 4848 during its first crossing of the Coma cluster

In a 5 hour Halpha exposure of the N-W region of the Coma cluster with the 2.1m telescope at SPM (Mx) we discovered a 65 kpc cometary emission of ionized gas trailing behind the SBab galaxy NGC 4848. The tail points in the opposite direction of the cluster center, in the same direction where stripped HI has been detected in previous observations. The galaxy shows bright HII regions in an inner ring-like pattern, where the star formation takes place at the prodigious rate of 8.9 Msun/yr. From the morphology of the galaxy and of the trailing material, we infer that the galaxy is suffering from ram pressure due to its high velocity motion through the cluster IGM. We estimate that 4 x 10^9 Msun of gas is swept out from the galaxy forming the tail. Given the ambient conditions in the Coma cluster (rho = 6.3 x 10^-27 g/cm^3; sigma_vel = 940 km/s) simulations predict that the ram pressure mechanism is able to remove such an amount of gas in less than 200 Myr. This, combined with the geometry of the interaction, indicating radial infall into the cluster, leads to the conclusion that NGC 4848 is caught in its first passage through the dense cluster environment.