arXiv subjects

M. Arnaboldi

Publications and source records attributed to M. Arnaboldi.

At least 55 records · Page 3Linked to original sources

arXiv2014

Spectral decomposition of the stellar kinematics in the polar disk galaxy NGC 4650A

Context. The prototype of Polar Ring Galaxies NGC 4650A contains two main structural components, a central spheroid, which is the host galaxy, and an extended polar disk. Both photometric and kinematic studies revealed that these two components co-exist on two different planes within the central regions of the galaxy. Aims. The aim of this work is to study the spectroscopic and kinematic properties of the host galaxy and the polar disk in the central regions of NGC 4650A by disentangling their contributions to the observed galaxy spectrum. Methods. We applied the spectral decomposition technique introduced in previous works to long-slit spectroscopic observations in the CaII triplet region. We focused the analysis along the PA = 152 that corresponds to the photometric minor axis of the host galaxy, where the superimposition of the two components is more relevant and the spectral decomposition is necessary. We aim to disentangle the stars that move in the equatorial plane of the host galaxy from those that move in the meridan plane, which is along the polar disk. Results. We successfully disentangled the spectra of the two structural components of NGC 4650A and measured their line-of-sight velocity and velocity dispersion profiles, and the stellar content along PA = 152. The host galaxy shows significant rotation along its photometric minor axis, indicating that the gravitational potential is not axisymmetric. The polar disk shows a kinematic decoupling: the inner regions counter-rotating with respect the outer-regions and the host spheroid. This suggests a complex formation history for the polar disk, characterised by mass accretion with decoupled angular momenta.

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arXiv2014

A census of H$α$ emitters in the intergalactic medium of the NGC 2865 system

Tidal debris which are rich in HI gas, formed in interacting and merging systems, are suitable laboratories to study star formation outside galaxies. Recently, several such systems were observed, which contained many young star forming regions outside the galaxies. In previous works, we have studied young star forming regions outside galaxies in different systems with optical and/or gaseous tidal debris, all of them with available archive GALEX/UV images, in order to understand how often they occur and in which type of environments. In this paper we searched for star forming regions around the galaxy NGC2865, a shell galaxy which is circled by a ring of HI, with a total mass of 1.2 x 10$^9$ M$_\odot$. Using the Multi-Slit Imaging Spectroscopy Technique with the Gemini telescope, we detected all H$α$ emitting sources in the surroundings of the galaxy NGC2865, down to a flux limit of 10$^{-18}$ erg cm$^{-2}$ s$^{-1}$ Å$^{-1}$. Together with Near and Far-Ultraviolet flux information we characterize the star formation rates, masses, ages, and metallicities for these HII regions. In total, we found 26 emission-line sources in a 60 $\times$ 60 Kpc field centered over the southeastern tail of the HI gas present around the galaxy NGC2865. Out of the 26 H$α$ emitters, 19 are in the satellite galaxy FGCE 0745 and seven are intergalactic HII regions scattered over the south tail of the HI gas around NGC2865. We found that the intergalactic HII regions are young ($<$200 Myr) with stellar masses in the range 4 X 10$^3$M$_\odot$ to 17x10$^6$ M$_\odot$. These are found in a region of low HI gas density, where the probability of forming stars is expected to be low. For one of the intergalactic HII regions we estimated a solar oxygen abundance of 12 + log(O/H) $\sim$ 8.7. We also were able to estimate the metallicity for the satellite galaxy FGCE0745 to be 12 + log(O/H) ~ 8.0.

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arXiv2014

The NIR structure of the barred galaxy NGC253 from VISTA

[abridged] We used J and Ks band images acquired with the VISTA telescope as part of the science verification to quantify the structures in the stellar disk of the barred Sc galaxy NGC253. Moving outward from the galaxy center, we find a nuclear ring within the bright 1 kpc diameter nucleus, then a bar, a ring with 2.9 kpc radius. From the Ks image we obtain a new measure of the deprojected length of the bar of 2.5 kpc. The bar's strength, as derived from the curvature of the dust lanes in the J-Ks image, is typical of weak bars. From the deprojected length of the bar, we establish the corotation radius (R_CR=3 kpc) and bar pattern speed (Omega_b = 61.3 km /s kpc), which provides the connection between the high-frequency structures in the disk and the orbital resonances induced by the bar. The nuclear ring is located at the inner Lindblad resonance. The second ring does not have a resonant origin, but it could be a merger remnant or a transient structure formed during an intermediate stage of the bar formation. The inferred bar pattern speed places the outer Lindblad resonance within the optical disk at 4.9 kpc, in the same radial range as the peak in the HI surface density. The disk of NGC253 has a down-bending profile with a break at R~9.3 kpc, which corresponds to about 3 times the scale length of the inner disk. We discuss the evidence for a threshold in star formation efficiency as a possible explanation of the steep gradient in the surface brightness profile at large radii. The NIR photometry unveils the dynamical response of the NGC253 stellar disk to its central bar. The formation of the bar may be related to the merger event that determined the truncation of stars and gas at large radii and the perturbation of the disk's outer edge.

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arXiv2013

Signatures of accretion events in the halos of early-type galaxies from comparing PNe and GCs kinematics

We have compared the halo kinematics traced by globular clusters (GCs) and planetary nebulae (PNe) for two elliptical galaxies in the Fornax and Virgo clusters NGC 1399 and NGC 4649, and for the merger remnant NGC 5128 (Centaurus A). We find differences in the rotational properties of the PN, red GC, and blue GC systems in all these three galaxies. NGC 1399 PNe and GCs show line of sight velocity distributions in specific regions that are significantly different, based on Kolmogorov-Smirnov tests. The PN system shows multi-spin components, with nearly opposite direction of rotation in the inner and the outer parts. The GCs velocity field is not point-symmetric in the outer regions of the galaxy, indicating that the system has not reached dynamical equilibrium yet. In NGC 4649 PNe, red and blue GCs have different rotation axes and rotational velocities. Finally, in NGC 5128 both PNe and GCs deviate from equilibrium in the outer regions of the galaxy, and in the inner regions the PN system is rotationally supported, whereas the GC system is dominated by velocity dispersion. The observed different kinematic properties, including deviations from point-symmetry, between PNe and GCs suggest that these systems are accreted at different times by the host galaxy, and the most recent accretion took place only few Gyr ago.We discuss two scenarios which may explain some of these differences: i) tidal stripping of loosely-bound GCs, and ii) multiple accretion of low luminosity and dwarf galaxies. Because these two mechanisms affect mostly the GC system, differences with the PNe kinematics can be expected.

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arXiv2013

The planetary nebula population in the halo of M87

We investigate the diffuse light in the outer regions of the nearby elliptical galaxy M87 in the Virgo cluster, using planetary nebulas (PNs) as tracers. The surveyed areas (0.43 squared degrees) cover M87 up to a radial distance of 150 kpc, in the ransition region between galaxy halo and intracluster light (ICL). All PNs are identified through the on-off band technique using automatic selection criteria based on the distribution of the detected sources in the colour-magnitude diagram and the properties of their point-spread function. We extract a catalogue of 688 objects down to m_5007=28.4, with an estimated residual contamination from foreground stars and background Lyalpha galaxies, which amounts to ~35% of the sample. This is one of the largest extragalactic PN samples in number of candidates, magnitude depth, and radial extent, which allows us to carry out an unprecedented photometric study of the PN population in the outer regions of M87. We find that the logarithmic density profile of the PN distribution is shallower than the surface brightness profile at large radii. This behaviour is consistent with the superposition of two components associated with the halo of M87 and with the ICL, which have different luminosity specific PN numbers, the ICL contributing three times more PNs per unit light. Because of the depth of this survey we are also able to study the shape of the PN luminosity function (PNLF) in the outer regions of M87. We find a slope for the PNLF that is steeper at fainter magnitudes than the standard analytical PNLF formula and adopt a generalised model that treats the slope as a free parameter. Comparing the PNLF of M87 and the M31 bulge, both normalised by the sampled luminosity, the M87 PNLF contains fewer bright PNs and has a steeper slope towards fainter magnitudes.

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arXiv2013

Planetary Nebula Spectrograph survey of S0 galaxy kinematics. II. Clues to the origins of S0 galaxies

The stellar kinematics of the spheroids and discs of S0 galaxies contain clues to their formation histories. Unfortunately, it is difficult to disentangle the two components and to recover their stellar kinematics in the faint outer parts of the galaxies using conventional absorption line spectroscopy. This paper therefore presents the stellar kinematics of six S0 galaxies derived from observations of planetary nebulae (PNe), obtained using the Planetary Nebula Spectrograph. To separate the kinematics of the two components, we use a maximum-likelihood method that combines the discrete kinematic data with a photometric component decomposition. The results of this analysis reveal that: the discs of S0 galaxies are rotationally supported; however, the amount of random motion in these discs is systematically higher than in comparable spiral galaxies; and the S0s lie around one magnitude below the Tully--Fisher relation for spiral galaxies, while their spheroids lie nearly one magnitude above the Faber--Jackson relation for ellipticals. All of these findings are consistent with a scenario in which spirals are converted into S0s through a process of mild harassment or "pestering," with their discs somewhat heated and their spheroid somewhat enhanced by the conversion process. In such a scenario, one might expect the properties of S0s to depend on environment. We do not see such an effect in this fairly small sample, although any differences would be diluted by the fact that the current location does not necessarily reflect the environment in which the transformation occurred. Similar observations of larger samples probing a broader range of environments, coupled with more detailed modelling of the transformation process to match the wide range of parameters that we have shown can now be measured, should take us from these first steps to the definitive answer as to how S0 galaxies form.

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arXiv2013

The Planetary Nebula Spectrograph survey of S0 galaxy kinematics. Data and Overview

The origins of S0 galaxies remain obscure, with various mechanisms proposed for their formation, likely depending on environment. These mechanisms would imprint different signatures in the galaxies' stellar kinematics out to large radii, offering a method for distinguishing between them. We aim to study a sample of six S0 galaxies from a range of environments, and use planetary nebulae (PNe) as tracers of their stellar populations out to very large radii, to determine their kinematics in order to understand their origins. Using a special-purpose instrument, the Planetary Nebula Spectrograph, we observe and extract PNe catalogues for these six systems*. We show that the PNe have the same spatial distribution as the starlight, that the numbers of them are consistent with what would be expected in a comparable old stellar population in elliptical galaxies, and that their kinematics join smoothly onto those derived at smaller radii from conventional spectroscopy. The high-quality kinematic observations presented here form an excellent set for studying the detailed kinematics of S0 galaxies, in order to unravel their formation histories. We find that PNe are good tracers of stellar kinematics in these systems. We show that the recovered kinematics are largely dominated by rotational motion, although with significant random velocities in most cases.

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arXiv2011

Stellar population and the origin of intra-cluster stars around brightest cluster galaxies: the case of NGC 3311

Context. We investigate the stellar population and the origin of diffuse light around brightest cluster galaxies. Aims. We study the stellar population of the dynamically hot stellar halo of NGC 3311, the brightest galaxy in the Hydra I cluster, and that of photometric substructures in the diffuse light to constrain the origin of these components. Methods. We analyze absorption lines in medium-resolution, long-slit spectra in the wavelength range 4800-5800 angstrom obtained with FORS2 at the Very Large Telescope. We measure the equivalent width of Lick indices out to 20 kpc from the center of NGC 3311 and fit them with stellar population models that account for the [alpha/Fe] overabundance. Results. Stars in the dynamically hot halo of NGC 3311 are old (age > 13 Gyr), metal-poor ([Z/H] ~ -0.35), and alpha-enhanced ([alpha/Fe] ~ 0.48). Together with the high velocity dispersion, these measurements indicate that the stars in the halo were accreted from the outskirts of other early-type galaxies, with a possible contribution from dwarf galaxies. We identify a region in the halo of NGC 3311 associated with a photometric substructure where the stellar population is even more metal-poor ([Z/H] ~ -0.73). In this region, our measurements are consistent with a composite stellar population superposed along the line of sight, consisting of stars from the dynamically hot halo of NGC 3311 and stars stripped from dwarf galaxies. The latter component contributes < 28% to the local surface brightness. Conclusions. The build-up of diffuse light around NGC 3311 is on-going. Based on the observed stellar population properties, the dominant part of these stars may have come from the outskirts of bright early-type galaxies, while stars from stripped dwarf galaxies are presently being added.

astro-ph.CO↗

arXiv2011

Chemical abundances of the PRGs UGC7576 and UGC9796. I. Testing the formation scenario

The study of the chemical abundances of HII regions in polar ring galaxies and their implications for the evolutionary scenario of these systems has been a step forward both in tracing the formation history of the galaxy and giving hints on the mechanisms at work during the building of disk by cold accretion process. It's now important to establish whether such results are typical for the class of polar disk galaxies as whole. The present work aims at checking the cold accretion of gas through a "cosmic filament" as a possible scenario for the formation of the polar structures in UGC7576 and UGC9796. If these form by cold accretion, we expect the HII regions abundances and metallicities to be lower than those of same-luminosity spiral disks, with values of the order of Z ~ 1/10 Zsun, as predicted by cosmological simulations. We have used deep long-slit spectra, obtained with DOLORES@TNG in the optical wavelengths, of the brightest HII regions associated with the polar structures to derive their chemical abundances and star formation rate. We used the "Empirical methods", based on the intensities of easily observable lines, to derive the oxygen abundance 12+log(O/H) of both galaxies. Such values are compared with those typical for different morphological galaxy types of comparable luminosity. The average metallicity values for UGC7576 and UGC9796 are Z = 0.4 Zsun and Z = 0.1 Zsun respectively. Both values are lower than those measured for ordinary spirals of similar luminosity and UGC7576 presents no metallicity gradient along the polar structure. These data, toghether with other observed features, available for the two PRGs in previous works, are compared with the predictions of simulations of tidal accretion, cold accretion and merging, to disentangle between these scenarios.

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arXiv2011

Unravelling the origins of S0 galaxies using maximum likelihood analysis of planetary nebulae kinematics

To investigate the origins of S0 galaxies, we present a new method of analyzing their stellar kinematics from discrete tracers such as planetary nebulae. This method involves binning the data in the radial direction so as to extract the most general possible non-parametric kinematic profiles, and using a maximum likelihood fit within each bin in order to make full use of the information in the discrete kinematic tracers. Both disk and spheroid kinematic components are fitted, with a two-dimensional decomposition of imaging data used to attribute to each tracer a probability of membership in the separate components. Likelihood clipping also allows us to identify objects whose properties are not consistent with the adopted model, rendering the technique robust against contaminants and able to identify additional kinematic features. The method is first tested on an N-body simulated galaxy to assess possible sources of systematic error associated with the structural and kinematic decomposition, which are found to be small. It is then applied to the S0 system NGC~1023, for which a planetary nebula catalogue has already been released and analyzed by (Noordermeer et al., 2008). The correct inclusion of the spheroidal component allows us to show that, contrary to previous claims, the stellar kinematics of this galaxy are indistinguishable from those of a normal spiral galaxy, indicating that it may have evolved directly from such a system via gas stripping or secular evolution. The method also successfully identifies a population of outliers whose kinematics are different from those of the main galaxy; these objects can be identified with a stellar stream associated with the companion galaxy NGC~1023A.

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arXiv2011

The unmixed kinematics and origins of diffuse stellar light in the core of the Hydra I cluster (Abell 1060)

Diffuse intracluster light (ICL) and cD galaxy halos are believed to originate from galaxy evolution and disruption. We present a kinematic study of the ICL in the Hydra I cluster core, using planetary nebulas (PNs) as tracers. We used multi-slit imaging spectroscopy with FORS2 on VLT-UT1 to detect 56 PNs associated with diffuse light in the central 100 x 100 kpc^2 of Hydra I. We measured their [OIII] magnitudes, sky positions, and velocity distribution (LOSVD), and compared with the phase-space distribution of nearby galaxies. The luminosity function of the detected PNs is consistent with that expected at a distance of ~50 Mpc. Their number density is ~4 times lower for the light seen than expected, and we discuss ram pressure stripping of the PNs by the hot ICM as one of the possible explanations. The LOSVD histogram of the PNs is highly non-Gaussian and multipeaked: it is dominated by a broad central component with sigma~500 km/s at around the average velocity of the cluster, and shows two additional narrower peaks at 1800 km/s and 5000 km/s. The main component is broadly consistent with the outward continuation of the intracluster halo of NGC 3311, which was earlier shown to have a velocity dispersion of ~470 km/s at radii of >50". Galaxies with velocities in this range are absent in the central 100 x 100 kpc^2 and may have been disrupted earlier to build this component. The PNs in the second peak in the LOSVD at 5000 km/s are coincident spatially and in velocities with a group of dwarf galaxies in the MSIS field. They may trace the debris from the ongoing tidal disruption of these galaxies. Most of the diffuse light in the core of Hydra is still not phase-mixed. The build-up of ICL and the dynamically hot cD halo around NGC 3311 are ongoing, through the accretion of material from galaxies falling into the cluster core and tidally interacting with its potential well.

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arXiv2010

The PN.S Elliptical Galaxy Survey: a standard LCDM halo around NGC 4374?

As part of our current programme to test LCDM predictions for dark matter (DM) haloes using extended kinematical observations of early-type galaxies, we present a dynamical analysis of the bright elliptical galaxy NGC 4374 (M84) based on ~450 Planetary Nebulae (PNe) velocities from the PN.Spectrograph, along with extended long-slit stellar kinematics. This is the first such analysis of a galaxy from our survey with a radially constant velocity dispersion profile. We find that the spatial and kinematical distributions of the PNe agree with the field stars in the region of overlap. The velocity kurtosis is consistent with zero at almost all radii. We construct a series of Jeans models, fitting both velocity dispersion and kurtosis to help break the mass-anisotropy degeneracy. Our mass models include DM halos either with shallow cores or with central cusps as predicted by cosmological simulations - along with the novel introduction in this context of adiabatic halo contraction from baryon infall. Both classes of models confirm a very massive dark halo around NGC 4374, demonstrating that PN kinematics data are well able to detect such haloes when present. Considering the default cosmological mass model, we confirm earlier suggestions that bright galaxies tend to have halo concentrations higher than LCDM predictions, but this is found to be solved if either a Salpeter IMF or adiabatic contraction with a Kroupa IMF is assumed. Thus for the first time a case is found where the PN dynamics may well be consistent with a standard dark matter halo. A cored halo can also fit the data, and prefers a stellar mass consistent with a Salpeter IMF. The less dramatic dark matter content found in lower-luminosity "ordinary" ellipticals suggests a bimodality in the halo properties which may be produced by divergent baryonic effects during their assembly histories.

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arXiv2010

A survey for Ly alpha galaxies at z~3.1

We describe the results of a deep survey for Ly alpha emission line galaxies at z~3.1, carried out with the multislit imaging spectroscopy (MSIS) technique, with the FORS2 spectrograph on VLT-UT1. We discuss the criteria used to select the emission line galaxies and present the main physical characteristics of the sample: redshift, observed flux and equivalent width distributions.

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arXiv2010

The dynamically hot stellar halo around NGC 3311: a small cluster-dominated central galaxy

An important open question is the relation between intracluster light and the halos of central galaxies in galaxy clusters. Here we report results from an on going project with the aim to characterize the dynamical state in the core of the Hydra I (Abell 1060) cluster around NGC 3311. Methods: We analyze deep long-slit absorption line spectra reaching out to ~25 kpc in the halo of NGC 3311. Results: We find a very steep increase in the velocity dispersion profile from a central sigma_0=150 km/s to sigma_out ~450 km/s at R ~ 12 kpc. Farther out, to ~25 kpc, sigma appears to be constant at this value, which is ~60% of the velocity dispersion of the Hydra I galaxies. With its dynamically hot halo kinematics, NGC 3311 is unlike other normal early-type galaxies. Conclusions: These results and the large amount of dark matter inferred from X-rays around NGC 3311 suggest that the stellar halo of this galaxy is dominated by the central intracluster stars of the cluster, and that the transition from predominantly galaxy-bound stars to cluster stars occurs in the radial range 4 to 12 kpc from the center of NGC 3311. We comment on the wide range of halo kinematics observed in cluster central galaxies, depending on the evolutionary state of their host clusters.

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arXiv2010

Kinematics and line strength indices in the halos of the Coma Brightest Cluster Galaxies NGC 4874 and NGC 4889

We investigate the stellar kinematics and line strength indices in the outer halos of brightest cluster galaxies (BCGs) in the Coma cluster to obtain the outer halo V_rot and sigma profiles and to derive constraints on the formation history of these objects. Methods: We analyzed absorption lines in deep, medium-resolution, long-slit spectra in the wavelength range ~ 4500 - 5900 Angstrom, out to ~50 kpc for NGC 4874 and ~65 kpc for NGC 4889, probing regions with a surface brightness down to mu_R ~24 mag/arcsec^2. Results: These data provide stellar velocity and velocity dispersion profiles along the major axes of both BCGs, and also along the minor axis of NGC 4889. The kinematic properties of NGC 4874 and NGC 4889 halos extend the previous relations of early-type galaxy halos to bright luminosities and indicate that the stars in the outer regions are still bound to these galaxies. For NGC 4889 we also determine Hbeta, Mg and Fe line strength indices, finding strong radial gradients for Mg and Fe. The current dataset for NGC 4889 is one of the most extended in radius, including both stellar kinematics AND line strength index measurements.

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arXiv2010

Chemical abundances in the polar disk of NGC4650A: implications for cold accretion scenario

The aim of the present study is to test whether the cold accretion of gas through a "cosmic filament" Macciò et al. 2006 is a possible formation scenario for the polar disk galaxy NGC 4650A. If polar disks form from cold accretion of gas, the abundances of the HII regions may be similar to those of very late-type spiral galaxies, regardless of the presence of a bright central stellar spheroid, with total luminosity of few 10^9 Lsun. We use deep long slit spectra obtained with the FORS2 spectrograph at the VLT in the optical and near-infrared wavelength ranges for the brightest HII regions in the disk polar disk of NGC 4650A. The strongest emission lines ([OII] Hbeta, [OIII], Halpha) were used to derived oxygen abundances, metallicities and the global star formation rates for the disk. The deep spectra available allowed us to measure the Oxygen abundances (12 + log (O/H)) using the "Empirical method" based on intensities of the strongest emission lines, and the "Direct method", based on the determination of the electron temperature from the detection of weak auroral lines, as the [OIII] at 4363 Angstrom. The Oxygen abundance measured for the polar disk is then compared with those measured for different galaxy types of similar total luminosities, and then compared against the predictions of different polar ring formation scenarios. The average metallicity values for the polar disk in NGC 4650A is Z=0.2 Zsun, and it is lower that the values measured for ordinary spirals of similar luminosity. Moreover the gradient of the metallicity is flat along the polar disk major axis, which implies none or negligible metal enrichment from the stars in the older central spheroid. The low metallicity value in the polar disk NGC 4650A and the flat metallicity gradient are both consistent with a later infall of metal-poor gas, as expected in the cold accretion processes.

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arXiv2009

Metallicity of the polar disk in NGC4650A: constraints for cold accretion scenario

We used high resolution spectra in the optical and near-infrared wavelength range to study the abundance ratios and metallicities of the HII regions associated with the polar disk in NGC4650A, in order to put constraints on the formation of the polar disk through cold gas accretion along a filament; this might be the most realistic way by which galaxies get their gas. We have compared the measured metallicities for the polar structure in NGC4650A with those of different morphological types and we have found that they are similar to those of late-type galaxies: such results is consistent with a polar disk formed by accretion from cosmic web filaments of external cold gas.

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arXiv2009

Self-gravitating warped discs around supermassive black holes

We consider warped equilibrium configurations for stellar and gaseous disks in the Keplerian force-field of a supermassive black hole, assuming that the self-gravity of the disk provides the only acting torques. Modeling the disk as a collection of concentric circular rings, and computing the torques in the non-linear regime, we show that stable, strongly warped precessing equilibria are possible. These solutions exist for a wide range of disk-to-black hole mass ratios $M_d/M_{bh}$, can span large warp angles of up to $\pm\sim 120°$, have inner and outer boundaries, and extend over a radial range of a factor of typically two to four. These equilibrium configurations obey a scaling relation such that in good approximation $\phidot/Ω\propto M_d/M_{bh}$ where $\phidot$ is the (retrograde) precession frequency and $Ω$ is a characteristic orbital frequency in the disk. Stability was determined using linear perturbation theory and, in a few cases, confirmed by numerical integration of the equations of motion. Most of the precessing equilibria are found to be stable, but some are unstable. The main result of this study is that highly warped disks near black holes can persist for long times without any persistent forcing other than by their self-gravity. The possible relevance of this to galactic nuclei is briefly discussed.

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