arXiv subjects

R. Falomo

Publications and source records attributed to R. Falomo.

107 records · Page 6Linked to original sources

arXiv2001

The Luminosity Function of the Host Galaxies of QSOs and BL Lac Objects

A clear insight of the galaxies hosting active galactic nuclei is of fundamental importance for understanding the processes of galaxies and nuclei formation and their cosmic evolution. A good characterization of the host galaxies properties requires images of excellent quality in order to disentangle the light of the galaxy from that of the bright nucleus. To this aim HST has provided a major improvement of data on QSOs (Disney et al. 1995; Bahcall et al. 1996, 1997; Boyce et al. 1998; McLure et al. 1999; Hamilton et al. 2000; Kukula et al. 2001) and BL Lacs (Scarpa et al. 2000, Urry et al. 2000). We present a comparative study of low redshift QSO and BL Lac host galaxy luminosity function (HGLF). To this aim we have considered samples of BL Lacs (Urry et al. 2000) and QSOs (Bahcall et al. 1997; Boyce et al. 1998; McLure et al. 1999) that have been well resolved by images obtained with WFPC2 on board of HST.

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arXiv2001

1RXS J214303.7+065419/RBS 1774: A New Isolated Neutron Star Candidate

We report on the identification of a new possible Isolated Neutron Star candidate in archival ROSAT observations. The source 1RXS J214303.7+065419, listed in the ROSAT Bright Survey as RBS 1774, is very soft, exhibits a thermal spectrum well fitted by a blackbody at ${T}\sim 90$ eV and has a low column density, ${N_H}\sim 5\times 10^{20}$ ${cm}^{-2}$. Catalogue searches revealed no known sources in other energy bands close to the X-ray position of RBS 1774. Follow-up optical observations with NTT showed no peculiar object within the X-ray error circle. The absence of any plausible optical counterpart down to ${m_R}\sim 23$ results in an X-ray to optical flux ratio in excess of 1000.

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arXiv2001

Host galaxies, BH masses and Eddington ratio of radio-loud AGNs

We compare the host galaxies properties of BL Lac objects with those of radio loud quasars (RLQs) imaged by the WFPC2 on board of HST. The considered objects (z<0.5) are always well resolved and their host galaxies satisfactorily modelled by ellipticals. After homogeneous treatment of the data we find RLQs hosts are systematically more luminous (by ~0.7 mag) with respect to the hosts of BL Lacs. Using the M_BH - L_bulge relation, derived for nearby elliptical galaxies, we have evaluated the central black hole masses of our sample of active galaxies. These data are discussed in conjunction with the nuclear luminosity and the Eddington ratio.

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arXiv2000

Near-infrared imaging of the host galaxies of intermediate redshift steep spectrum radio quasars

We present the results of near-infrared H-band (1.65 microns) imaging of 19 steep spectrum radio quasars (SSRQ) in the redshift range 0.5 < z < 1.0. This sample of SSRQs is matched with our previously studied complete sample of 20 flat spectrum radio quasars (FSRQ) with respect to redshift and optical and radio luminosity. We are able to clearly detect the host galaxy in 10 (53 %) SSRQs and marginally in 6 (32 %) others, while the host remains unresolved in 3 (16 %) SSRQs. The galaxies hosting the SSRQs are large (average bulge scale-length R(e) = 9.0+-1.7 kpc) and luminous (average M(H) = -27.2+-1.1). They are, therefore, about 2 mag more luminous than the typical galaxy luminosity L* (M*(H) = -25.0+-0.2), and about 1 mag more luminous than the brightest cluster galaxies (M(H) = -26.3+-0.3). The SSRQ hosts appear to have similar luminosity to those of the FSRQ hosts (M(H) = -27), and they fall between the luminosities of lower redshift (M(H) = -26) and higher redshift (M(H) = -29) radio-loud quasars. The average nucleus-to-galaxy luminosity ratio of SSRQs (LN/LG = 3.8+-3.2) is much smaller than that found for the FSRQs (LN/LG = 21+-11), in good agreement with the current unified models. We confirm for the most luminous SSRQs the trend noted for the FSRQs between the host and nuclear luminosity. This trend supports the suggestion based on studies of lower redshift AGN, that there is a minimum host galaxy luminosity which increases linearly with the quasar luminosity. Finally, FSRQs seem to reside in richer environments than SSRQs, as evidenced by a larger number of close companion galaxies.

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arXiv2000

The HST Survey of BL Lacertae Objects: Morphological Properties of Low redshift Host Galaxies

We report on the optical properties of a sample of 30 BL Lac host galaxies in the redshift range $0.03<z<0.2$, as derived from HST observations. All galaxies are fully resolved in the WFPC2 (F702W filter) images, allowing a quantitative analysis in two dimensions. Most and possibly all these galaxies have characteristics very similar to those of ``normal'' giant ellipticals. The luminosity, ellipticity, isophote twisting and amount of disky or boxy isophotes are consistent with those found in non-active ellipticals and in radio galaxies. In all cases the BL Lac nucleus is well centered in the main body of its host galaxy, a result that argues strongly against the microlensing hypothesis for any significant fraction of the population. A search for faint sub-structures in the host galaxies has not revealed notable signatures of tidal distortions or sub-components (faint disks, bars, X features, etc.), and with only one exception, there are no prominent dusty features in the central regions. Instead, the BL Lac host galaxies are smooth and unperturbed, suggesting that strong external gravitational interactions are not important to ongoing activity. A careful examination of the environment around the nucleus, however, shows a high incidence of close companion objects, whose nature remains unclear pending spectroscopic observations.

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arXiv2000

Optical surface photometry of radio galaxies - II. Observations and data analysis

Optical imaging observations for 50 radio galaxies are presented. For each object isophotal contours, photometric profiles, structural parameters (position angle, ellipticity, Fourier coefficients), and total magnitudes are given. These observations, obtained in the Cousins R band, complement the data presented in a previous paper and are part of a larger project aimed at studying the optical properties of low redshift (z<0.12) radio galaxies (Govoni et al. 1999). Comments for each individual source are reported.

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arXiv1999

The optical properties of low redshift radio galaxies

We present morphological and photometric properties of 79 low redshift (z<0.12) radio galaxies extracted from two radio flux limited samples of radio sources. All objects are imaged in the R band and for a subsample we have also obtained B band images. The sample includes sources of both FRI and FRII radio morphological type. Through the decomposition of the luminosity profiles and the analysis of the structural profiles (ellipticity, PA, c4) of the galaxies we are able to characterize in detail the optical properties of the radio galaxies. The average absolute magnitude is M_HOST(tot)=-24.0 with a clear trend for FRI sources to be 0.5 mag brighter than FRII galaxies. In about 40% of the objects observed we find an excess of light in the nucleus that is attributed to the presence of a nuclear point source whose luminosity is on average 1-2% of the total flux of the host galaxy. The luminosity of these nuclear point sources appears correlated with the core radio power of the galaxies. Radio galaxies follow the same mu_e - R_e relationship as normal elliptical galaxies. The distribution of ellipticity, the amount of twisting and shape of isophotes (boxy,disky) do not differ significantly from other ellipticals. The evidence for recent interactions is therefore rather modest. Finally on average radio galaxies are bluer and have a color dispersion larger than normal elliptical galaxies, and the average color gradient in radio galaxies appears slightly steeper than in normal ellipticals. These results support a scenario where radio emission is weakly related with the overall properties and/or the activity have negligible effects on the global characteristics of the host galaxy.

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arXiv1999

Optical imaging of the host galaxies of X-ray selected BL Lacertae objects

We investigate the properties of the host galaxies of X-ray selected (high frequency peaked) BL Lac objects using a large and homogeneous data set of high spatial resolution R-band observations of 52 BL Lacs in the EMSS and Slew samples. The redshift distribution of the BL Lacs ranges from z = 0.04 to z>0.7, with average and median redshifts z = 0.26 and z = 0.24, respectively. Eight objects are at unknown redshift. We are able to resolve 45 objects out of the 52 BL Lacs. For all the well-resolved sources, we find the host to be a luminous elliptical galaxy. In a few cases a disk is not ruled out but an elliptical model is still preferred. The average absolute magnitude of the host galaxies is = -23.9+-0.6, while the average scale length of the host is = 9+-5 kpc. There is no difference in the host properties between the EMSS and Slew samples. We find a good agreement between the results derived by the surveys of Wurtz et al. (ground-based data) and Urry et al. (HST data), and by our new deeper imaging. The average luminosity of the BL Lac hosts is between those of F-R I and F-R II radio galaxies in Govoni et al., supporting the idea that both radio galaxy types could contribute to the parent population. The BL Lac hosts follow the F-P relation for giant ellipticals and exhibit a modest luminosity evolution with redshift. Finally, we find a slight correlation between the nuclear and host luminosity and a bimodal distribution in the nuclear/host luminosity ratio.

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arXiv1999

HST Observations of BL Lacertae Environments

We analyze images of BL Lacertae objects obtained with the {\it Hubble Space Telescope} WFPC2 and the F814W filter. The nine objects cover a redshift range of 0.19 to 0.997. The relatively deep images are sufficient to detect galaxies at least one magnitude below M$^{*}_{I}$ (--21.4) and in most cases to three magnitudes below M$^{*}$. Galaxy enhancement over the average background is found around four out of the nine objects. Results for some cases are confirmed by ground-based imaging. In the other cases, the redshifts of the target BL Lac objects may be incorrect or they are truly isolated. These findings reinforce the idea that on average, BL Lac objects are found in regions of above average galaxy density. However, isolated objects apparently can host BL Lac nuclei too, a result that has implications for the processes that trigger/fuel the nuclear activity.

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arXiv1999

HST Observations of the Optical Jets of PKS 0521-365, 3C371, and PKS 2201+044

HST observations have led to the discovery of the optical counterpart of the radio jet of PKS 2201+044, and to a detailed analysis of the optical jets of PKS 0521-365 and 3C371. At HST spatial resolution these jets are well resolved, displaying knotty morphologies. When compared with radio maps of appropriate resolution, a clear one-to-one correspondence between optical and radio structures is found, showing that all detected optical structures are indeed related to the radio synchrotron emission. Photometry of the brightest knots shows that the radio-to-optical spectral index and the derived intensity of the equipartition magnetic field are approximately constant along the jet. Thus, present observations suggest that the electron energy distribution does not change significantly all along the jet.

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arXiv1999

The discovery of polarization in the afterglow of GRB 990510 with the ESO Very Large Telescope

Following a BeppoSAX alert (Piro 1999a) and the discovery of the OT at SAAO (Vreeswijk et al. 1999a), we observed GRB 990510 with the FORS instrument on ESO's VLT Unit 1 (`Antu'). The burst is unremarkable in gamma rays, but in optical is the first one to show good evidence for jet-like outflow (Stanek et al. 1999, Harrison et al. 1999). We report the detection of significant linear polarization in the afterglow: it is (1.6 +/- 0.2)% 0.86 days after trigger, and after 1.81 days is consistent with that same value, but much more uncertain. The polarization angle is constant on a time scale of hours, and may be constant over one day. We conclude that the polarization is intrinsic to the source and due to the synchrotron nature of the emission, and discuss the random and ordered field geometries that may be responsible for it.

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arXiv1999

The HST Survey of BL Lac Objects: Gravitational Lens Candidates and Other Unusual Sources

We present HST observations of seven unusual objects from the HST ``snapshot survey'' of BL Lac objects, of which four are gravitational lens candidates. In three cases a double point sources is observed: 0033+595, with 1.58 arcsec separation, and 0502+675 and 1440+122, each with $\sim 0.3$ arcsec separation. The last two also show one or more galaxies, which could be either host or lensing galaxies. If any are confirmed as lenses, these BL Lac objects are excellent candidates for measuring H$_0$ via gravitational time delay because of their characteristic rapid, high amplitude variability. An additional advantage is that, like other blazars, they are likely superluminal radio sources, in which case the source plane is mapped out over a period of years, providing strong additional constraints on the lensing mass distribution. The fourth gravitational lens candidate is 1517+656, which is surrounded by three arclets forming an almost perfect ring of radius 2.4 arcsec. If this is indeed an Einstein ring, it is most likely a background source gravitationally lensed by the BL Lac object host galaxy and possibly a surrounding group or cluster. In the extreme case that all four candidates are true lenses, the derived frequency of gravitational lensing in this BL Lac sample would be an order of magnitude higher than in comparable quasar samples. We also report on three other remarkable BL Lac objects: 0138-097, which is surrounded by a large number of close companion galaxies; 0806+524, whose host galaxy contains an uncommon arc-like structure; and 1959+650, which is hosted by a gas rich elliptical galaxy with a prominent dust lane of $\sim 5\times 10^5 M_\odot$.

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arXiv1998

HST snapshot imaging of BL Lac objects

Snapshot images of about 100 BL Lac objects were obtained with WFPC2 on HST. Sources from various samples, in the redshift range 0.05 to 1.2, were observed and 61 resolved (51 with known z). The high resolution and homogeneity of the images allow us to address the properties of the immediate environments of BL Lacs with unprecedented capability. Host galaxies of BL Lacs are luminous ellipticals (on average 1 mag brighter than L*) with no or little disturbed morphology. The nucleus, that is always well centered onto the galaxy, contributes in the optical (R band) to about half of the total luminosity of the object (range of the contribution from 0.1 to 10). The undisturbed morphology suggests that the nuclear activity has marginal effect on the overall properties of the hosts. Nonetheless several examples of close companions have been detected. The luminosity distribution of host galaxies is compared with that of a large sample of FR-I radio galaxies.

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arXiv1998

The host galaxies of BL Lac objects in the near--infrared

We present the results of near-infrared H band (1.65 microns) imaging of 11 BL Lac objects with redshifts ranging from z = 0.05 to 0.9. We are able to clearly detect the host galaxy in seven low redshift (z<=0.24) BL Lacs, while the four unresolved BL Lacs have either high or unknown redshift. The galaxies hosting the low redshift BL Lacs are large (average bulge scale length R(e) = 8.8+-9.9 kpc) and luminous (average M(H) = -25.8+-0.5), i.e. slightly brighter than the typical galaxy luminosity L* (M*(H) = -25.0+-0.2), and of similar luminosity to or slightly fainter than brightest cluster galaxies (M(H) = -26.3+-0.3). The average optical/near-infrared colour and colour gradient of the BL Lac hosts (R-H = 2.2+-0.5; d(R-H)/d(log r) = -0.09$+-0.04) are consistent with the hosts being normal ellipticals, indicating that the nuclear activity has only a marginal effect on the star formation history and other properties of the hosts. The BL Lac hosts appear slightly less luminous than those of higher redshift flat spectrum radio quasars. The nucleus-to-galaxy luminosity ratio of the BL Lacs is similar to that of low redshift radio galaxies and consistent with what found in previous optical studies of BL Lacs. However, it is smaller that that found for flat spectrum radio quasars, suggesting there is a difference in the intrinsic brightness of the nuclear source or in the Doppler beaming factor between the two types of blazars.

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arXiv1997

Near-infrared imaging of the host galaxies of flat spectrum radio quasars

We present the results of a high resolution (0.27 arcsec/px) near-infrared H band (1.65 micron) imaging survey of a complete sample of 20 flat spectrum radio quasars (FSRQ) extracted from the 2Jy catalogue of radio sources. The observed objects are intrinsically luminous with median M(B) = -25.5. The median redshift of the objects in the sample is z = 0.65. At this redshift, the H band observations probe the old stellar population of the hosts at rest frame wavelength of 1 micron. We are able to detect the host galaxy clearly for six (30 %) FSRQs and marginally for six (30 %) other FSRQs, while the object remains unresolved for eight (40 %) cases. We find the galaxies hosting FSRQs to be very luminous (M(H) = -27). Compared with the typical galaxy luminosity L* (M*(H) = -25) they appear 2 mag brighter, although the undetected hosts may reduce this difference. They are also at least as bright as, and probably by 1 mag brighter than, the brightest cluster galaxies (M(H) = -26). The luminosities of the FSRQ hosts are intermediate between host galaxies of low redshift radio-loud quasars and BL Lac objects (M(H) = -26), and the hosts of high redshift radio-loud quasars (M(H) = -29), in good agreement with current unified models for radio-loud AGN, taking into account stellar evolution in the elliptical host galaxies. Finally, we find an indicative trend between the host and nuclear luminosity for the most luminous FSRQs, supporting the suggestion based on studies of lower redshift AGN, that there is a minimum host galaxy luminosity which increases linearly with the quasar luminosity.

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arXiv1997

The $z=0.558$ absorption system towards PKS 0118-272: A candidate Damped Ly $α$ system at low redshift

We present a spectroscopic and imaging study of the z=0.558 MgII absorption system towards the BL Lac PKS 0118-272. At the absorber redshift we detect lines of the low ionization species MgI, MgII, CaII, TiII, MnII, and FeII. The column densities of these species are typical of the interstellar gas in the disk of the Galaxy. By assuming dust-free gas with solar abundances we infer N(HI) > 2.5 x 10^19 cm^-2. The high value of this conservative lower limit indicates that the absorber is a Damped Ly alpha system. We find [Ti/Fe] = +0.3, in agreement with [Ti/Fe] measurements in DLA absorbers, but [Mn/Fe] = +0.4, at variance with the values [Mn/Fe]<0 common to DLA systems. The measured [Ti/Fe] and [Mn/Fe] ratios match remarkably well the differential depletion pattern of low-density interstellar clouds in the Galaxy. Unlike high-redshift DLA systems (z>2), the z=0.558 absorber seems to originate in a galaxy that has already attained the abundances and dust content of present-day disk galaxies. The analysis of our imaging data lends support to the presence of an intervening galaxy. After careful subtraction of the BL Lac image, an object at 1.6 arcsec from PKS 0118-272 is detected. At the absorber redshift the projected distance (14 h_50^-1 kpc) and the absolute magnitude (M_R ~ -22.3) of this companion are consistent with those found for galaxies associated with low-redshift DLA systems.

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arXiv1994

The Near-IR-Optical-UV Emission of BL Lacertae Objects

Near--infrared, optical and ultraviolet quasi--simultaneous observations of 11 BL Lacertae objects are reported. For all but one source the dereddened spectral flux distribution in the $8\cdot10^{13}-2\cdot10^{15}$ Hz frequency range can be described by a single power law f$_ν\propto ν^{-α}$ with average spectral index $<α>$ = 0.88 $\pm$ 0.42 (standard deviation) plus, where relevant, the contribution of the host galaxy. In most cases the non simultaneous soft X--ray fluxes obtained by the {\it Einstein Observatory} lie on or below the extrapolation of the power law. The results are compared with the average spectral properties of other samples of BL Lacs studied separately in the IR--optical and in the UV bands. The implications for existing models of the objects are shortly discussed.

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