arXiv subjects

S. Pellegrini

Publications and source records attributed to S. Pellegrini.

At least 55 records · Page 3Linked to original sources

arXiv2007

The HI content of Early-Type Galaxies from the ALFALFA survey I. Catalogued HI sources in the Virgo cluster

Aims: We are using the Arecibo Legacy Fast ALFA survey (ALFALFA), which is covering 17% of the sky at 21 cm, to study the HI content of Early-Type galaxies (ETG) in an unbiased way. The aim is to get an overall picture of the hot, warm and cold ISM of ETG, as a function of galaxy mass and environment, to understand its origin and fate, and to relate it to the formation and evolution history of these objects. Methods: This paper deals with the first part of our study, which is devoted to the 8-16 deg. declination strip in the Virgo cluster. In this sky region, using the Virgo Cluster Catalogue (VCC), we have defined an optical sample of 939 ETG, 457 of which are brighter than the VCC completeness limit at B_T=18.0. We have correlated this optical sample with the catalogue of detected HI sources from ALFALFA. Results: Out of the 389 ETG from the VCC with B_T<=18.0, outside the 1 deg. region of poor HI detection around M87, and corrected for background contamination of VCC galaxies without a known radial velocity, only 9 galaxies (2.3%) are detected in HI with a completeness limit of 3.5 and 7.6 x 10^7 Mo of HI for dwarf and giant ETG, respectively. In addition 4 VCC ETG with fainter magnitudes are also detected. Our HI detection rate is lower than previously claimed. The majority of the detected ETG appear to have peculiar morphology and to be located near the edges of the Virgo cluster. Conclusions: Our preliminary conclusion is that cluster ETG contain very little neutral gas, with the exceptions of a few peculiar dwarf galaxies at the edge of the ETG classification and of very few larger ETG, where the cold gas could have a recent external origin.

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arXiv2007

A deep Chandra look at the low L_B elliptical NGC821: X-ray binaries, a galactic wind and emission at the nucleus

The relatively nearby (distance=24.1 Mpc) elliptical galaxy NGC821, hosting a central massive black hole but inactive at all wavelengths, was observed with Chandra for a total exposure of 230 ksec, to search for nuclear emission and gas available for accretion. Within its optical image, 41 sources were detected, with spectral properties typical of low mass X-ray binaries (LMXBs). The fractions of LMXBs in the field and in globular clusters were determined, together with their X-ray luminosity function (XLF) down to L(0.3-8 keV)=2\times 10^{37} erg/s. At the galactic center a source of L(0.3-8 keV)=6\times 10^{38} erg/s was detected for the first time, slightly extended. Its spectral shape is quite hard (Γ=1.49^{+0.14}_{-0.13}), without intrinsic absorption. It is surrounded by three sources with spectral shape typical of LMXBs and luminosities on the brightest end of the XLF. One is consistent with being pointlike; the others could be the superposition of few point sources and/or truly diffuse emission, with one resembling a jet-like feature. Diffuse emission was detected out to R\sim 30'', and comes mostly from unresolved LMXBs, with a minor contribution from other types of stellar sources. Different lines of investigation consistently provide no evidence for hot gas. Hydrodynamical simulations show that stellar mass losses are driven out of NGC821 in a wind sustained by type Ia supernovae, but also hot accreting gas within a very small inner region. A companion paper presents further observational results from $Spitzer$ and the VLA, and possible accretion modalities for this central massive black hole.

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arXiv2007

The X-ray and Radio Connection in Low-luminosity Active Nuclei

We present the results of the correlation between the nuclear 2-10 keV X-ray and radio (at 2cm, 6cm and 20cm) luminosities for a well defined sample of local Seyfert galaxies. We use a sample of low luminosity radio galaxies (LLRGs) for comparison. In both Seyfert and LLRGs samples, X-ray and radio luminosities are significantly correlated over 8 orders of magnitude, indicating that the X-ray and radio emission sources are strongly coupled. Moreover, both samples show a similar regression slope, L(X)$\propto$ L(R)^(0.97), but Seyfert galaxies are three orders of magnitude less luminous in the radio band than LLRGs. This suggests that either similar physical mechanisms are responsible for the observed emission or a combination of different mechanisms ends up producing a similar correlation slope. Indeed, the common belief for LLRG is that both the X-ray and radio emission are likely dominated by a relativistic jet component, while in Seyfert galaxies the X-ray emission probably arises from a disk-corona system and the radio emission is attributed to a jet/outflow component. We investigate the radio loudness issue in the two samples and find that the Seyfert galaxies and the LLRGs show a different distribution of the radio loudness parameters. No correlation is found between the luminosity and the radio loudness, however the latter is related to the black hole mass and anti-correlated with the Eddington ratio. The dichotomy in the radio loudness between Seyfert and LLRG observed down to low Eddington ratios, L(2-10)/L(Edd) $\sim$ 10^(-8), does not support the idea that the origin of the radio loudness is due to a switch in the accretion mode.

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arXiv2006

Multi-wavelength and black hole mass properties of Low Luminosity Active Nuclei

We investigate the relation between the X-ray nuclear emission, optical emission line, radio luminosity and black hole mass for a sample of nearby Seyfert galaxies. Strong linear correlations between the 2-10 keV and [OIII], radio luminosities have been found, showing the same slopes found in quasars and luminous Seyfert galaxies, thus implying independence from the level of nuclear activity displayed by the sources. Moreover, despite the wide range of Eddington ratios (L/L(Edd)) tested here (six orders of magnitude, from 0.1 down to 10^(-7), no correlation is found between the X-ray, optical emission lines, radio luminosities and the black hole mass. These results suggest that low luminosity Seyfert galaxies are a scaled down version of luminous AGN and probably are powered by the same physical processes.

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arXiv2006

Probing the Low-Luminosity XLF in Normal Elliptical Galaxies

We present the first low luminosity (LX > 5 - 10 1036 erg s-1) X-ray luminosity functions (XLFs) of low-mass X-ray binaries (LMXBs) determined for two typical old elliptical galaxies, NGC 3379 and NGC 4278. Because both galaxies contain little diffuse emission from hot ISM and no recent significant star formation (hence no high-mass X-ray binary contamination), they provide two of the best homogeneous sample of LMXBs. With 110 and 140 ks Chandra ACIS S3 exposures, we detect 59 and 112 LMXBs within the D25 ellipse of NGC 3379 and NGC 4278, respectively. The resulting XLFs are well represented by a single power-law with a slope (in a differential form) of 1.9 0.1. In NGC 4278, we can exclude the break at LX ~ 5 x 1037 erg s-1 that was recently suggested to be a general feature of LMXB XLFs. In NGC 3379 instead we find a localized excess over the power law XLF at ~4 x 1037 erg s-1, but with a marginal significance of ~1.6s. Because of the small number of luminous sources, we cannot constrain the high luminosity break (at 5 x 1038 erg s-1) found in a large sample of early type galaxies. While the optical luminosities of the two galaxies are similar, their integrated LMXB X-ray luminosities differ by a factor of 4, consistent with the relation between the X-ray to optical luminosity ratio and the globular cluster specific frequency.

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arXiv2006

Reconciling optical and X-ray mass estimates: the case of the elliptical galaxy NGC3379

NGC3379 is a well studied nearby elliptical for which optical investigations have claimed a little dark matter content, or even no dark matter. Recently, its total mass profile M(r) has been derived by exploiting Chandra observations of its extended and X-ray emitting interstellar medium, based on the hypothesis of hydrostatic equilibrium for the hot gas. The resulting total mass within the effective radius Re has been claimed to be a few times larger than found by optical studies. Here we show that part of the discrepancy can be due to an underestimate of the optically derived mass, and the remaining discrepancy of a factor of ~2 can be explained by deviations from hydrostatic equilibrium of the hot gas. By using hydrodynamical simulations tailored to reproduce the observed hot gas properties of NGC3379, and by assuming as input for the simulations the total mass profile derived optically, we show that i) the hot gas at the present time has X-ray properties consistent with those observed only if it is outflowing over most of the galactic body, and ii) an overestimate of M of the same size found in the recent X-ray analysis is recovered when assuming hydrostatic equilibrium. We also show that the hot gas is outflowing even for a dark matter fraction within Re as large as derived with the standard X-ray procedure based on the hydrostatic equilibrium assumption, which shows the unapplicability of the method for this galaxy. Finally, we find that the whole range of dark mass amount and distribution allowed for by optical studies is compatible with a hot gas flow with the observed X-ray properties.

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arXiv2006

The Eclipsing ULX in NGC 3379

We report recent Chandra observations of the ULX in the elliptical galaxy NGC3379 that clearly detect two flux variability cycles. Comparing these data with the Chandra observation of ~5yr ago, we measure a flux modulation with a period of ~12.6hr. Moreover, we find that the emission undergoes a correlated spectral modulation, becoming softer at low flux. We argue that our results establish this source as a ULX binary in NGC3379. Given the old stellar population of this galaxy, the ULX is likely to be a soft transient, however historical X-ray sampling suggests that the current on phase has lasted ~10yr. We discuss our results in terms of ADC and wind-feedback models. We constrain the donor mass and orbital period at the onset of mass transfer within 1.15-1.4M and 12.5-16hr, respectively. The duration of the mass-transfer phase so far is probably ~1Gyr and the binary has been a soft X-ray transient throughout this time. These constraints are insensitive to the mass of the accretor.

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arXiv2005

The X-ray emission properties and the dicothomy in the central stellar cusp shapes of early-type galaxies

Hubble Space Telescope revealed a dicothomy in the central surface brightness profiles of early-type galaxies, that subsequently have been grouped into two families: core, boxy, anisotropic systems and cuspy ("power law"), disky, rotating ones. Here we investigate whether a dicothomy is present also in the X-ray properties of the two families. We consider both their total soft emission (Lsx,tot), that is a measure of the galactic hot gas content, and their nuclear hard emission (Lhx,nuc), mostly coming from Chandra observations, that is a measure of the nuclear activity. At any optical luminosity, the highest Lsx,tot values are reached by core galaxies; this is explained with their being central dominant galaxies of groups, subclusters or clusters, in many of the logLsx,tot(erg/s)>~41.5 cases. The highest Lhx,nuc values, similar to those of classical AGNs, in this sample are hosted only by core galaxies; at low luminosity AGN levels, Lhx,nuc is independent of the central stellar profile shape. The presence of optical nuclei (also found by HST) is unrelated with the level of Lhx,nuc, even though the highest Lhx,nuc are all associated with optical nuclei. The implications of these findings for the galaxy evolution and the accretion modalities at the present epoch are discussed.

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arXiv2005

The nature of Composite Seyfert/Star-forming galaxies revealed by X-ray observations

This paper presents new Chandra and BeppoSAX observations aimed at investigating the optical/X-ray mismatch in the enigmatic class of the Composite galaxies, discovered by a cross-correlation of IRAS and ROSAT all sky survey catalogues. These galaxies have been classified as star-forming objects on the basis of their optical spectra, while the detection of weak broad wings in the H(alpha) emission in a few of them and their high X-ray luminosity in the ROSAT band indicated the presence of an active nucleus. The analysis of Chandra observations for 4 Composites has revealed nuclear point-like sources, with a typical AGN spectrum (Gamma = 1.7-1.9) and little intrinsic absorption. A strong flux variability has been observed on different time scales, in particular most of the sources were brighter at the ROSAT epoch. Although of relative low luminosity for the AGN class (L(2-10 keV) = 3-60 x 10^{41} erg/s), the active nucleus is nevertheless dominant in the X-ray domain. At other wavelengths it appears to be overwhelmed by the starburst and/or host galaxy light, yielding the Composite classification for these objects.

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arXiv2005

Nuclear Accretion in Galaxies of the Local Universe: Clues from Chandra Observations

In order to find an explanation for the radiative quiescence of supermassive black holes in the local Universe, for a sample of nearby galaxies the most accurate estimates are collected for the mass of a central black hole M_BH, the nuclear X-ray luminosity L_X,nuc and the circumnuclear hot gas density and temperature, by using Chandra data. L_X,nuc varies by \sim 3 orders of magnitude and does not show a relationship with M_BH or with the Bondi mass accretion rate \dotM_B. L_X,nuc is always much lower than expected if \dotM_B ends in a standard accretion disc with high radiative efficiency (this instead can be the case of the active nucleus of Cen A). Radiatively inefficient accretion as in the standard ADAF modeling may explain the low luminosities of a few cases; for others, the predicted luminosity is still too high and, in terms of Eddington-scaled quantities, it is increasingly higher than observed, for increasing \dotM_B. Variants of the simple radiatively inefficient scenario including outflow and convection may reproduce the low emission levels observed, since the amount of matter actually accreted is reduced considerably. However, the most promising scenario includes feedback from accretion on the surrounding gas: this has the important advantages of naturally explaining the observed lack of relationship between L_X,nuc, M_BH and \dotM_B, and of evading the problem of the fate of the material accumulating in the central galactic regions over cosmological times.

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arXiv2004

Chandra discovery of activity in the quiescent nuclear black hole of NGC821

We report the results of the Chandra ACIS-S observations of the elliptical galaxy NGC 821, which harbors a supermassive nuclear black hole (of 3.5x10^7 solar masses), but does not show sign of AGN activity. A small, 8.5" long (~1 kpc at the galaxy's distance of 23 Mpc), S-shaped, jet-like feature centered on the nucleus is detected in the 38 ksec ACIS-S integrated exposure of this region. The luminosity of this feature is L_X ~ 2.6 x 10^39 erg/s (0.3-10 keV), and its spectrum is hard (described by a power-law of Gamma = 1.8^{+0.7}_{-0.6}; or by thermal emission with kT > 2 keV). We discuss two possibilities for the origin of this feature: (1) a low-luminosity X-ray jet, or (2) a hot shocked gas. In either case, it is a clear indication of nuclear activity, detectable only in the X-ray band. Steady spherical accretion of the mass losses from the central stellar cusp within the accretion radius, when coupled to a high radiative efficiency, already provides a power source exceeding the observed radiative losses from the nuclear region.

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arXiv2004

X-ray properties of the "Composite" Seyfert/Star-forming galaxies

An enigmatic, small class of IR and X-ray luminous sources, named ``Composite'' starburst/Seyfert galaxies, has been defined from IRAS and RASS data. The objects have optical spectra dominated by the features of HII galaxies (plus, in some cases, weak Seyfert signatures) but X-ray luminosities higher than expected from starbursts and more typical of Seyfert nuclei. The true nature of this class of objects is still unknown. We present Chandra data of four of these galaxies that were obtained to investigate the nature of the X-ray source. The X-ray spectrum, the lack of any significant extended component, and the observed variability indicate that the AGN is the dominant component in the X-ray domain.

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arXiv2004

Chandra's Discovery of Activity in the Quiescent Nuclear Black Hole of NGC 821

We report the results of the {\it Chandra} ACIS-S observations of the elliptical galaxy NGC 821, which harbors a supermassive nuclear black hole (of $3.5 \times 10^7 M_{\odot}$), but does not show sign of AGN activity. A small, 8.5$^{\prime\prime}$ long ($\sim 1$ kpc at the galaxy's distance of 23 Mpc), S-shaped, jet-like feature centered on the nucleus is detected in the 38 ksec ACIS-S integrated exposure of this region. The luminosity of this feature is $L_X \sim 2.6 \times 10^{39} \rm ergs s^{-1}$ (0.3-10 keV), and its spectrum is hard (described by a power-law of $Γ= 1.8^{+0.7}_{-0.6}$; or by thermal emission with $kT >2$ keV). We discuss two possibilities for the origin of this feature: (1) a low-luminosity X-ray jet, or (2) a hot shocked gas. In either case, it is a clear indication of nuclear activity, detectable only in the X-ray band. Steady spherical accretion of the mass losses from the central stellar cusp within the accretion radius, when coupled to a high radiative efficiency, already provides a power source exceeding the observed radiative losses from the nuclear region.

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arXiv2004

On the use of X-rays to determine dynamical properties of elliptical galaxies

The X-ray emitting interstellar medium of early type galaxies is often used as a tool to determine their total mass M and stellar orbital anisotropy beta profiles, based on the hypothesis of hydrostatic equilibrium for the hot gas. Here we investigate the effects that deviations from equilibrium have on M and beta estimates, by using simple analytical calculations and hydrodynamical simulations representative of gas rich galaxies. We show that the deviations of the X-ray determined beta^{est} and M^{est} from the true values are linked by a remarkably simple relation; in particular, M is underestimated if beta is overestimated, and more radially anisotropic orbital distributions than true are deduced in presence of gas infall velocities of the order of the local stellar velocity dispersion (as are likely in the central regions of galactic cooling flows). The results of this analysis are applied to the most thoroughly investigated bright elliptical, NGC4472. We show that beta^{est} recently derived from X-rays corresponds to a galaxy that is unstable by radial orbit instability. Then, assuming as true beta and M the optically derived values, we show that the differences beta^{est}-beta and M^{est}-M agree with the predictions found here in the case of lack of hydrostatic equilibrium, which points to the latter as a possible explanation for the discrepancies (abridged).

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arXiv2003

The role of Compton heating in cluster cooling flows

Recent observations by Chandra and XMM-Newton demonstrate that the central gas in "cooling flow" galaxy clusters has a mass cooling rate that decreases rapidly with decreasing temperature. This contrasts the predictions of a steady state cooling flow model. On the basis of these observational results, the gas can be in a steady state only if a steady temperature dependent heating mechanism is present; alternatively the gas could be in an unsteady state, i.e., heated intermittently. Intermittent heating can be produced by accretion on the supermassive black hole residing in the central cluster galaxy, via Compton heating. This mechanism can be effective provided that the radiation temperature of the emitted spectrum is higher than the gas temperature. Here we explore whether this heating mechanism can be at the origin of the enigmatic behavior of the hot gas in the central regions of ``cooling flow'' clusters. Although several characteristics of Compton heating appear attractive in this respect, we find that the fraction of absorbed heating for realistic gas and radiation temperatures falls short by two orders of magnitude of the required heating.

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arXiv2003

The complex FeK line of the Narrow-Line Seyfert 1 galaxy IRAS13349+2438

The observation of the Narrow-Line Seyfert 1 galaxy IRAS13349+2438 performed by XMM-Newton European Photon Imaging Camera between 0.3-10 keV, is presented here. The broadband spectrum of the source is dominated at low energies (E < 2 keV) by a strong excess of emission and by complex emission/absorption features between 5.5-8.0 keV. The soft X-ray spectrum is consistent with ionized absorption found by the Reflection Grating Spectrometer (Sako et al. (2001)). We focus on the 2-10 keV spectrum which shows clear evidence for a broad, complex FeK alpha line, previously unseen by ASCA, and for an Fe K-shell edge detected at ~7.3-7.4 keV (rest-frame). The presence of this edge could be explained by either partial covering or ionized/relativistic reflection models, with the latter being preferred and with a resulting power law slope of Gamma \~2.2. The line profile is complex, with a broad bump between ~ 5.5-6.5 keV and a narrow emission line at ~ 7 keV, separated by a sharp drop at ~ 6.8 keV. This profile is compatible with two possible scenarios: i) a broad, ionized and gravitationally redshifted Laor diskline plus a narrow and ionized emission line; ii) a broad, ionized and single Schwarzschild (double-peaked) diskline with a superimposed narrow absorption line.

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arXiv2003

An XMM-Newton and Chandra investigation of the nuclear accretion in the Sombrero Galaxy (NGC4594)

We present an analysis of the XMM-Newton and Chandra ACIS-S observations of the LINER nucleus of the Sombrero galaxy and we discuss possible explanations for its very sub-Eddington luminosity by complementing the X-ray results with high angular resolution observations in other bands. The X-ray investigation shows a hard (Gamma=1.89) and moderately absorbed (N_H=1.8 10^21 cm^-2) nuclear source of 1.5 10^40 erg s^-1 in the 2-10 keV band, surrounded by hot gas at a temperature of \sim 0.6 keV. The bolometric nuclear luminosity is at least \sim 200 times lower than expected if mass accreted on the supermassive black hole, that HST shows to reside at the center of this galaxy, at the rate predicted by the spherical and adiabatic Bondi accretion theory and with the high radiative efficiency of a standard accretion disc. The low luminosity, coupled to the observed absence of Fe-K emission in the nuclear spectrum, indicates that such a disc is not present. This nucleus differs from bright unobscured AGNs also for the lack of high flux variability and of prominent broad Halpha emission. However, it is also too faint for the predictions of simple radiatively inefficient accretion taking place at the Bondi rate; it could be too radio bright, instead, for radiatively inefficient accretion that includes strong mass outflows or convection. This discrepancy could be solved by the possible presence of nuclear radio jets. An alternative explanation of the low luminosity, in place of radiative inefficiency, could be unsteady accretion.

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arXiv2003

The nature of the "Composite" Seyfert/Star-forming Galaxies

We present the results obtained with BeppoSAX observations of the three Composite Seyfert/star-forming galaxies: IRAS 20051-1117, IRAS 04392-0123 and IRAS 01072+4954. These sources belong to an enigmatic class of X-ray sources detected in the ROSAT All-Sky Survey (Moran et al. 1996) which is composed of 6 low redshift galaxies. Their optical spectra are dominated by the features of HII galaxies while their X-ray luminosities (< 10^{42} ergs/s) are typical of Seyfert galaxies. IRAS 20051-1117 shows a 2-10 keV spectrum well described by a power-law with photon index = 1.9 and low intrinsic absorption. This result, the ratio Flux(2-10 keV)/Flux([OIII]) and the significant X-ray variability detected, clearly rule out a Compton thick nature of this source. IRAS 04392-0123 and IRAS 01072+4954, instead, have a BeppoSAX flux a factor of ~ 50-80 smaller than in the previous ROSAT observations, resulting in poor statistics, that prevents detailed modeling.

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