arXiv subjects

Lodovico Coccato

Publications and source records attributed to Lodovico Coccato.

51 records · Page 3Linked to original sources

arXiv2014

Overview of the SDSS-IV MaNGA Survey: Mapping Nearby Galaxies at Apache Point Observatory

We present an overview of a new integral field spectroscopic survey called MaNGA (Mapping Nearby Galaxies at Apache Point Observatory), one of three core programs in the fourth-generation Sloan Digital Sky Survey (SDSS-IV) that began on 2014 July 1. MaNGA will investigate the internal kinematic structure and composition of gas and stars in an unprecedented sample of 10,000 nearby galaxies. We summarize essential characteristics of the instrument and survey design in the context of MaNGA's key science goals and present prototype observations to demonstrate MaNGA's scientific potential. MaNGA employs dithered observations with 17 fiber-bundle integral field units that vary in diameter from 12" (19 fibers) to 32" (127 fibers). Two dual-channel spectrographs provide simultaneous wavelength coverage over 3600-10300 A at R~2000. With a typical integration time of 3 hr, MaNGA reaches a target r-band signal-to-noise ratio of 4-8 (per A, per 2" fiber) at 23 AB mag per sq. arcsec, which is typical for the outskirts of MaNGA galaxies. Targets are selected with stellar mass greater than 1e9 Msun using SDSS-I redshifts and i-band luminosity to achieve uniform radial coverage in terms of the effective radius, an approximately flat distribution in stellar mass, and a sample spanning a wide range of environments. Analysis of our prototype observations demonstrates MaNGA's ability to probe gas ionization, shed light on recent star formation and quenching, enable dynamical modeling, decompose constituent components, and map the composition of stellar populations. MaNGA's spatially resolved spectra will enable an unprecedented study of the astrophysics of nearby galaxies in the coming 6 yr.

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arXiv2014

Understanding the Unique Assembly History of Central Group Galaxies

Central Galaxies (CGs) in massive halos live in unique environments with formation histories closely linked to that of the host halo. In local clusters they have larger sizes ($R_e$) and lower velocity dispersions (sigma) at fixed stellar mass M_star, and much larger R_e at a fixed $σ$ than field and satellite galaxies (non-CGs). Using spectroscopic observations of group galaxies selected from the COSMOS survey, we compare the dynamical scaling relations of early-type CGs and non-CGs at z~0.6, to distinguish possible mechanisms that produce the required evolution. CGs are systematically offset towards larger R_e at fixed $σ$ compared to non-CGs with similar M_star. The CG R_e-M_star relation also shows differences, primarily driven by a sub-population (~15%) of galaxies with large $R_e$, while the M_star-sigma relations are indistinguishable. These results are accentuated when double Sersic profiles, which better fit light in the outer regions of galaxies, are adopted. They suggest that even group-scale CGs can develop extended components by these redshifts that can increase total $R_e$ and M_star estimates by factors of ~2. To probe the evolutionary link between our sample and cluster CGs, we also analyze two cluster samples at z~0.6 and z~0. We find similar results for the more massive halos at comparable z, but much more distinct CG scaling relations at low-z. Thus, the rapid, late-time accretion of outer components, perhaps via the stripping and accretion of satellites, would appear to be a key feature that distinguishes the evolutionary history of CGs.

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arXiv2014

Counter-rotating disks in galaxies: dissecting kinematics and stellar populations with 3D spectroscopy

We present a spectral decomposition technique that separates the contribution of different kinematic components in galaxies from the observed spectrum. This allows to study the kinematics and properties of the stellar populations of the individual components (e.g., bulge, disk, counter-rotating cores, orthogonal structures). Here, we discuss the results of this technique for galaxies that host counter-rotating stellar disks of comparable size. In all the studied cases, the counter-rotating stellar disk is the less massive, the youngest and has different chemical content (metallicity and alpha-elements abundance ratio) than the main galaxy disk. Further applications of the spectral decomposition technique are also discussed.

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arXiv2014

A 3D view of the Hydra I galaxy cluster core - I. Kinematic substructures

We used FORS2 in MXU mode to mimic a coarse 'IFU' in order to measure the 3D large-scale kinematics around the central Hydra I cluster galaxy NGC 3311. Our data show that the velocity dispersion field varies as a function of radius and azimuthal angle and violates point symmetry. Also, the velocity field shows similar dependence, hence the stellar halo of NGC 3311 is a dynamically young structure. The kinematic irregularities coincide in position with a displaced diffuse halo North-East of NGC 3311 and with tidal features of a group of disrupting dwarf galaxies. This suggests that the superposition of different velocity components is responsible for the kinematic substructure in the Hydra I cluster core.

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arXiv2014

A 3D view of the Hydra I cluster core - II. Stellar populations

Several observations of the central region of the Hydra I galaxy cluster point to a multi-epoch assembly history. Using our novel FORS2/VLT spectroscopic data set, we were able to map the luminosity-weighted age, [Fe/H] and [$α$/Fe] distributions for the stellar populations around the cD galaxy NGC 3311. Our results indicate that the stellar populations follow the trends of the photometric substructures, with distinct properties that may aid to constrain the evolutionary scenarios for the formation of the cluster core.

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arXiv2014

The difference in age of the two counter-rotating stellar disks of the spiral galaxy NGC 4138

The Sa spiral NGC 4138 is known to host two counter-rotating stellar disks, with the ionized gas co-rotating with one of them. We measured the kinematics and properties of the two counter-rotating stellar populations to constrain their formation scenario. A spectroscopic decomposition of the observed major-axis spectrum was performed to disentangle the relative contribution of the two counter-rotating stellar and one ionized-gas components. The line-strength indices of the two counter-rotating stellar components were measured and modeled with single stellar population models that account for the alpha/Fe overabundance. The counter-rotating stellar population is younger, marginally more metal poor, and more alpha-enhanced than the main stellar component. The younger stellar component is also associated with a star-forming ring. The different properties of the counter-rotating stellar components of NGC~4138 rule out the idea that they formed due to bar dissolution. Our findings support the results of numerical simulations in which the counter-rotating component assembled from gas accreted on retrograde orbits from the environment or from the retrograde merging with a gas-rich dwarf galaxy.

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arXiv2014

Re-growth of stellar disks in mature galaxies: The two component nature of NGC 7217 revisited with VIRUS-W

Previous studies have reported the existence of two counter-rotating stellar disks in the early-type spiral galaxy NGC7217. We have obtained high-resolution optical spectroscopic data (R ~ 9000) with the new fiber-based Integral Field Unit instrument VIRUS-W at the 2.7m telescope of the McDonald Observatory in Texas. Our analysis confirms the existence of two components. However, we find them to be co-rotating. The first component is the more luminous (~ 77% of the total light), has the higher velocity dispersion (~ 170 km/s) and rotates relatively slowly (projected $v_{max}$ = 50 km/s). The lower luminosity second component, (~ 23% of the total light), has a low velocity dispersion (~ 20 km/s) and rotates quickly (projected $v_{max}$ = 150 km/s). The difference in the kinematics of the two stellar components allows us to perform a kinematic decomposition and to measure the strengths of their Mg and Fe Lick indices separately. The rotational velocities and dispersions of the less luminous and faster component are very similar to those of the interstellar gas as measured from the [OIII] emission. Morphological evidence of active star formation in this component further suggests that NGC7217 may be in the process of (re)growing a disk inside a more massive and higher dispersion stellar halo. The kinematically cold and regular structure of the gas disk in combination with the central almost dust-free morphology allows us to compare the dynamical mass inside of the central 500pc with predictions from a stellar population analysis. We find agreement between the two if a Kroupa stellar initial mass function is assumed.

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arXiv2013

Removal of systematics in photometric measurements: static and rotating illumination corrections in FORS2@VLT data

Images taken with modern detectors require calibration via flat fielding to obtain the same flux scale across the whole image. One method for obtaining the best possible flat fielding accuracy is to derive a photometric model from dithered stellar observations. A large variety of effects have been taken into account in such modelling. Recently, Moehler et al. (2010) discovered systematic variations in available flat frames for the European Southern Observatory's FORS instrument that change with the orientation of the projected image on the sky. The effect on photometry is large compared to other systematic effects that have already been taken into account. In this paper, we present a correction method for this effect: a generalization of the fitting procedure of Bramich & Freudling (2012) to include a polynomial representation of rotating flat fields. We then applied the method to the specific case of FORS2 photometric observations of a series of standard star fields, and provide parametrised solutions that can be applied by the users. We found polynomial coefficients to describe the static and rotating large-scale systematic flat-field variations across the FORS2 field of view. Applying these coefficients to FORS2 data, the systematic changes in the flux scale across FORS2 images can be improved by ~1% to ~2% of the total flux. This represents a significant improvement in the era of large-scale surveys, which require homogeneous photometry at the 1% level or better.

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arXiv2013

Elliptical galaxies with rapidly decreasing velocity dispersion profiles: NMAGIC models and dark halo parameter estimates for NGC 4494

NGC 4494 is one of several intermediate-luminosity elliptical galaxies inferred to have an unusually diffuse dark matter halo. We use the chi^2-made-to-measure particle code NMAGIC to construct axisymmetric models of NGC 4494 from photometric and various kinematic data. The extended kinematics include light spectra in multiple slitlets out to 3.5 R_e, and hundreds of planetary nebulae velocities out to ~7 R_e, thus allowing us to probe the dark matter content and orbital structure in the halo. We use Monte Carlo simulations to estimate confidence boundaries for the halo parameters, given our data and modelling set-up. We find that the true potential of the dark matter halo is recovered within Delta G (merit function)<26 (Delta chi^2<59) at 70% confidence level (C.L.), and within Delta G<32 (Delta chi^2<70) at 90% C.L.. These numbers are much larger than the usually assumed Delta chi^2=2.3 (4.6) for 70% (90%) C.L. for two free parameters, perhaps case-dependent, but calling into question the general validity of the standard assumptions used for halo and black hole mass determinations. The best-fitting models for NGC 4494 have a dark matter fraction of about 0.6\pm0.1 at 5R_e (70% C.L.), and are embedded in a dark matter halo with circular velocity ~200 km/s. The total circular velocity curve (CVC) is approximately flat at v_c=220 km/s outside ~0.5R_e. The orbital anisotropy of the stars is moderately radial. These results are independent of the assumed inclination of the galaxy, and edge-on models are preferred. Comparing with the halos of NGC 3379 and NGC 4697, whose velocity dispersion profiles also decrease rapidly from the center outwards, the outer CVCs and dark matter halos are quite similar. NGC 4494 shows a particularly high dark matter fraction inside ~3R_e, and a strong concentration of baryons in the center.

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arXiv2012

Radially Extended Kinematics in the S0 Galaxy NGC 2768 from Planetary Nebulae, Globular Clusters and Starlight

There are only a few tracers available to probe the kinematics of individual early-type galaxies beyond one effective radius. Here we directly compare a sample of planetary nebulae (PNe), globular clusters (GCs) and galaxy starlight velocities out to ~4 effective radii, in the S0 galaxy NGC 2768. Using a bulge-to-disk decomposition of a K-band image we assign PNe and starlight to either the disk or the bulge. We show that the bulge PNe and bulge starlight follow the same radial density distribution as the red subpopulation of GCs, whereas the disk PNe and disk starlight are distinct components. We find good kinematic agreement between the three tracers to several effective radii (and with stellar data in the inner regions). Further support for the distinct nature of the two galaxy components come from our kinematic analysis. After separating the tracers into bulge and disk components we find the bulge to be a slowly rotating pressure-supported system, whereas the disk reveals a rapidly rising rotation curve with a declining velocity dispersion profile. The resulting V/sigma ratio for the disk resembles that of a spiral galaxy and hints at an origin for NGC 2768 as a transformed late-type galaxy. A two-component kinematic analysis for a sample of S0s will help to elucidate the nature of this class of galaxy.

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arXiv2012

A tale of two tails and an off-centered envelope: diffuse light around the cD galaxy NGC 3311 in the Hydra I cluster

The formation of intracluster light and of the extended halos around brightest cluster galaxies is closely related to morphological transformation, tidal stripping, and disruption of galaxies in clusters. We analyze Ks- and V-band surface photometry as well as deep long-slit spectra, and establish a link between the structures in the light distribution, the absorption line kinematics, and the LOS velocity distributions of nearby galaxies and planetary nebulae (PNs). The central galaxy NGC 3311 is surrounded by an extended symmetric outer halo with n=10 and an additional, off-centered envelope ~ 50" to the North-East. Its luminosity L_V= 1.2x10^{10} +/- 6.0 x 10^8 L_sun corresponds to ~50 % of the luminosity of the symmetric halo in the same region. Based on measured PN velocities, at least part of the off-centered envelope consists of high-velocity accreted stars. We have also discovered two tidal streams in the cluster center, emerging from the dwarf galaxy HCC 026 and from the S0 galaxy HCC 007. The HCC 026 stream is redshifted by ~1200 km/s with respect to NGC 3311, similarly as HCC 026 itself, a fraction of PNs in the off-centered envelope, and several other dwarf galaxies nearby. The stars in one of the HCC 026 tails are known to be consistent with the low-metallicity population of HCC 026, and our photometry shows that this galaxy is already mostly dissolved in the tidal field. The tidal stream around HCC 007 extends over at least 110 kpc. It is fairly thick and is brighter on the side of the asymmetric outer halo of NGC 3311, which it may join. Its luminosity is several 10^9 L_sun, similar to the luminosity of the stripped-down galaxy HCC 007. The redshift of the stream is determined from a few PN velocities and is similar to that for HCC 007 and HCC 026.

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arXiv2011

Galaxies in X-ray Groups I: Robust Membership Assignment and the Impact of Group Environments on Quenching

Understanding the mechanisms that lead dense environments to host galaxies with redder colors, more spheroidal morphologies, and lower star formation rates than field populations remains an important problem. As most candidate processes ultimately depend on host halo mass, accurate characterizations of the local environment, ideally tied to halo mass estimates and spanning a range in halo mass and redshift are needed. In this work, we present and test a rigorous, probabalistic method for assigning galaxies to groups based on precise photometric redshifts and X-ray selected groups drawn from the COSMOS field. The groups have masses in the range 10^13 < M_200c/M_sun < 10^14 and span redshifts 0<z<1. We characterize our selection algorithm via tests on spectroscopic subsamples, including new data obtained at the VLT, and by applying our method to detailed mock catalogs. We find that our group member galaxy sample has a purity of 84% and completeness of 92% within 0.5 R200c. We measure the impact of uncertainties in redshifts and group centering on the quality of the member selection with simulations based on current data as well as future imaging and spectroscopic surveys. As a first application of our new group member catalog which will be made publicly available, we show that member galaxies exhibit a higher quenched fraction compared to the field at fixed stellar mass out to z~1, indicating a significant relationship between star formation and environment at group scales. We also address the suggestion that dusty star forming galaxies in such groups may impact the high-l power spectrum of the cosmic microwave background and find that such a population cannot explain the low power seen in recent SZ measurements.

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arXiv2011

Dating the formation of the counter-rotating stellar disc in the spiral galaxy NGC 5719 by disentangling its stellar populations

We present the results of the VLT/VIMOS integral-field spectroscopic observations of the inner 28"x28" (3.1 kpc x 3.1 kpc) of the interacting spiral NGC 5719, which is known to host two co-spatial counter-rotating stellar discs. At each position in the field of view, the observed galaxy spectrum is decomposed into the contributions of the spectra of two stellar and one ionised-gas components. We measure the kinematics and the line strengths of the Lick indices of the two stellar counter-rotating components. We model the data of each stellar component with single stellar population models that account for the alpha/Fe overabundance. We also derive the distribution and kinematics of the ionised-gas disc, that is associated with the younger, less rich in metals, more alpha-enhanced, and less luminous stellar component. They are both counter-rotating with respect the main stellar body of the galaxy. These findings prove the scenario where gas was accreted first by NGC 5719 onto a retrograde orbit from the large reservoir available in its neighbourhoods as the result of the interaction with its companion NGC 5713, and subsequently fuelled the in situ formation of the counter-rotating stellar disc.

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arXiv2010

Counter-dispersed slitless-spectroscopy technique: planetary nebula velocities in the halo of NGC 1399

Using a counter-dispersed slitless spectroscopy technique, we detect and measure the line-of-sight velocities of 187 planetary nebulae (PNe) around one of the nearest cD galaxies, NGC 1399, with FORS1 on the VLT. We describe the method for identifying and classifying the emission-line sources and the procedure for computing their J2000 coordinates and velocities. The number of PN detections and the errors in the velocity measurements (37 km/s indicate that this technique is comparable to other methods, such as that described by Teodorescu et al. (2005). We present the spatial distribution of the PNe and a basic analysis of their velocities. The PN two-dimensional velocity field shows marginal rotation consistent with other studies. We also find a low-velocity substructure in the halo and a flatter velocity-dispersion profile compared to previous observations that extends to ~400 arcsec. The detection of a low-velocity subcomponent underscores the importance of discrete velocity tracers for the detection of un-mixed components. The new velocity-dispersion profile is in good agreement with revised velocity dispersions for the red globular clusters in NGC 1399, using the data of Schuberth et al. (2009). The outer parts of this profile are consistent with one of the dynamical models of Kronawitter et al. (2000), which corresponds to a circular velocity of ~340 km/s and a rescaled B-band mass-to-light ratio of ~20 at 7' radius. These measurements trace the kinematics of the outer halo and disentangle the heterogenous populations in the Fornax Cluster core. The new data set the stage for a revised dynamical model of the outer halo of NGC 1399.

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arXiv2010

Distinct core and halo stellar populations and the formation history of the bright Coma cluster early-type galaxy NGC 4889

We study the stellar population far into the halo of one of the two brightest galaxies in the Coma cluster, NGC 4889, based on deep medium resolution spectroscopy with FOCAS at the Subaru 8.2m telescope. We fit single stellar population models to the measured line-strength (Lick) indices (Hbeta, Mgb, [MgFe]' and ). Combining with literature data, we construct radial profiles of metallicity, [alpha/Fe] element abundance ratio and age for NGC 4889, from the center out to ~60 kpc (~4Re). We find evidence for different chemical and star formation histories for stars inside and outside 1.2Re = 18 kpc radius. The inner regions are characterized by a steep [Z/H] gradient and high [alpha/Fe] at ~2.5 times solar value. In the halo, between 18 and 60 kpc, the [Z/H] is near-solar with a shallow gradient, while [alpha/Fe] shows a strong negative gradient, reaching solar values at ~60 kpc. We interpret these data in terms of different formation histories for both components. The data for the inner galaxy are consistent with a rapid, quasi-monolithic, dissipative merger origin at early redshifts, followed by one or at most a few dry mergers. Those for the halo argue for later accretion of stars from old systems with more extended star formation histories. The half-light radius of the inner component alone is estimated as ~6 kpc, suggesting a significantly smaller size of this galaxy in the past. This may be the local stellar population signature of the size evolution found for early-type galaxies from high-redshift observations.

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