110 research outputs found
The Nature of LINERs
We present -band (m) spectroscopy of a sample of nine
galaxies showing some degree of LINER activity (classical LINERs, weak-[O {\sc
i}] LINERs and transition objects), together with -band spectroscopy for
some of them. A careful subtraction of the stellar continuum allows us to
obtain reliable [Fe {\sc ii}]m/Pa line ratios. We conclude
that different types of LINERs (i.e., photoionized by a stellar continuum or by
an AGN) cannot be easily distinguished based solely on the [Fe {\sc ii}]m/Pa line ratio.
The emission line properties of many LINERs can be explained in terms of an
aging starburst. The optical line ratios of these LINERs are reproduced by a
model with a metal-rich H {\sc ii} region component photoionized with a single
stellar temperature K, plus a supernova remnant (SNR) component.
The [Fe {\sc ii}] line is predominantly excited by shocks produced by SNRs in
starbursts and starburst-dominated LINERs, while Pa tracks H {\sc ii}
regions ionized by massive young stars. The contribution from SNRs to the
overall emission line spectrum is constrained by the [Fe {\sc ii}]m/Pa line ratio. Although our models for aging starbursts are
constrained only by these infrared lines, they consistently explain the optical
spectra of the galaxies also.
The LINER-starburst connection is tested by predicting the time dependence of
the ratio of the ionizing luminosity () to the supernova rate
(SNr), /(SNr). We predict the relative number of starbursts to
starburst-dominated LINERs (aging starbursts) and show that it is in
approximate agreement with survey findings for nearby galaxies.Comment: Accepted in ApJ (19 pages, 8 figures, uses emulateapj.sty
X-ray spectra of XMM-Newton serendipitous medium flux sources
We report on the results of a detailed analysis of the X-ray spectral
properties of a large sample of sources detected serendipitously with the
XMM-Newton observatory in 25 selected fields. The survey covers a total solid
angle of ~3.5 deg2 and contains 1137 sources with ~10E-15 < S0.5-10 < 10E-12
erg cm-2 s-1. We find evidence for hardening of the average X-ray spectra of
the sources towards fainter fluxes. We interpret this as indicating a higher
degree of photoelectric absorption amongst the fainter population. Absorption
is detected at 95% confidence in 20% of the sources, but it could certainly be
present in many other sources below our detection capabilities. For Broad Line
AGNs (BLAGNs), we detect absorption in ~10% of the sources with column
densities in the range 10E21 - 10E22 cm-2. The fraction of absorbed Narrow
Emission Line galaxies (NELGs, most with intrinsic X-ray luminosities >10E43
erg s-1, and therefore classified as type 2 AGNs) is significantly higher
(40%), with a hint of moderately higher columns. We do not find evidence for a
redshift evolution of the underlying power law index of BLAGNs, which stays
roughly constant at Gamma ~1.9, with intrinsic dispersion of 0.4. A small
fraction (~7%) of BLAGNs and NELGs require the presence of a soft excess, that
we model as a black body with temperature ranging from 0.1 to 0.3 keV.
Comparing our results on absorption to popular X-ray background synthesis
models, we find absorption in only ~40% of the sources expected. This is due to
a deficiency of heavily absorbed sources (with NH ~10E22 - 10E24 cm-2) in our
sample in comparison with the models. We therefore conclude that the synthesis
models require some revision in their specific parameters.Comment: 20 pages, 30 Postscript figures, A&A in pres
Obscured star formation in the central region of the dwarf galaxy NGC5253
We present HST/NICMOS observations (1.1-2.2micron) and 1.9-4.1micron
spectroscopy of the central region of the dwarf galaxy NGC5253. The HST/NICMOS
observations reveal the presence of a nuclear double star cluster separated by
0.3-0.4arcsec or 6-8pc (for a distance d=4.1Mpc). The double star cluster, also
a bright double source of Pa-alpha emission, appears to be coincident with the
double radio nebula detected at 1.3cm. The eastern near-infrared star cluster
(C1) is identified with the youngest optical cluster, whereas the western star
cluster (C2), although it is almost completely obscured in the optical, becomes
the brightest star cluster in the central region of NGC 5253 at wavelengths
longer than 2micron. Both clusters are extremely young with ages of
approximately 3.5 million years old. C2 is more massive than C1 by a factor of
6 to 20 (M(C2)= 7.7 x 10^5 - 2.6 x 10^6Msun, for a Salpeter IMF in the mass
range 0.1-100Msun). Analysis of the circumnuclear spectrum excluding C1 and C2,
as well as of a number of other near-infrared selected clusters with a range of
(young) ages, suggests that the star formation was triggered across the central
regions of the galaxy. We have also modelled the nuclear UV to mid-infrared
spectral energy distribution (SED) of NGC5253 and found that the infrared part
is well modelled with a highly obscured (A_V= 17mag) young starburst with a
stellar mass consistent with our photometric estimates for C1 and C2. The SED
model predicts a moderately bright polycyclic aromatic hydrocarbon (PAH)
feature at 3.3micron that is not detected in our nuclear L-band spectrum.
NGC5253's low metallicity and a top-heavy IMF likely combine to suppress the
3.3micron PAH emission that is commonly seen in more massive starburst systems.Comment: Accepted for publication in ApJ. High quality versions of Figures 1
and 2 are available upon reques
The 2dF galaxy redshift survey: clustering properties of radio galaxies
The clustering properties of local, S1.4 GHzâ„ 1 mJy, radio sources are investigated for a sample of 820 objects drawn from the joint use of the Faint Images of the Radio Sky at 20 cm (FIRST) and 2dF Galaxy Redshift surveys. To this aim, we present 271 new bJ†19.45 spectroscopic counterparts of FIRST radio sources to be added to those already introduced in our previous paper. The two-point correlation function for the local radio population is found to be entirely consistent with estimates obtained for the whole sample of 2dFGRS galaxies. From measurements of the redshift-space correlation function Ο(s) we derive a redshift-space clustering length s0 = 10.7+0.8 â0.7 Mpc, while from the projected correlation function Î(rT) we estimate the parameters of the real-space correlation function Ο(r) = (r/r0) âÎł, r0 = 6.7+0.9 â1.1 Mpc and Îł= 1.6 ± 0.1, where h = 0.7 is assumed. Different results are instead obtained if we only consider sources that present signatures of active galactic nucleus (AGN) activity in their spectra. These objects are shown to be very strongly correlated, with r0 = 10.9+1.0 â1.2 Mpc and Îł= 2 ± 0.1, a steeper slope than has been claimed in other recent works. No difference is found in the clustering properties of radio-AGNs of different radio luminosity. Comparisons with models for Ο(r) show that AGN-fuelled sources reside in dark matter haloes more massive than âŒ1013.4 Mâ, higher than the corresponding figure for radio-quiet quasi-stellar objects. This value can be converted into a minimum black hole mass associated with radio-loud, AGN-fuelled objects of MminBH⌠109 Mâ. The above results then suggest - at least for relatively faint radio objects - the existence of a threshold black hole mass associated with the onset of significant radio activity such as that of radio-loud AGNs; however, once the activity is triggered, there appears to be no evidence for a connection between black hole mass and level of radio outpu
The 2dF Galaxy Redshift Survey: power-spectrum analysis of the final data set and cosmological implications
We present a power-spectrum analysis of the final 2dF Galaxy Redshift Survey (2dFGRS), employing a direct Fourier method. The sample used comprises 221 414 galaxies with measured redshifts. We investigate in detail the modelling of the sample selection, improving on previous treatments in a number of respects. A new angular mask is derived, based on revisions to the photometric calibration. The redshift selection function is determined by dividing the survey according to rest-frame colour, and deducing a self-consistent treatment of k-corrections and evolution for each population. The covariance matrix for the power-spectrum estimates is determined using two different approaches to the construction of mock surveys, which are used to demonstrate that the input cosmological model can be correctly recovered. We discuss in detail the possible differences between the galaxy and mass power spectra, and treat these using simulations, analytic models and a hybrid empirical approach. Based on these investigations, we are confident that the 2dFGRS power spectrum can be used to infer the matter content of the universe. On large scales, our estimated power spectrum shows evidence for the âbaryon oscillations' that are predicted in cold dark matter (CDM) models. Fitting to a CDM model, assuming a primordial ns= 1 spectrum, h= 0.72 and negligible neutrino mass, the preferred parameters are Ωmh= 0.168 ± 0.016 and a baryon fraction Ωb/Ωm= 0.185 ± 0.046 (1Ï errors). The value of Ωmh is 1Ï lower than the 0.20 ± 0.03 in our 2001 analysis of the partially complete 2dFGRS. This shift is largely due to the signal from the newly sampled regions of space, rather than the refinements in the treatment of observational selection. This analysis therefore implies a density significantly below the standard Ωm= 0.3: in combination with cosmic microwave background (CMB) data from the Wilkinson Microwave Anisotropy Probe (WMAP), we infer Ωm= 0.231 ± 0.02
The 2dF Galaxy Redshift Survey: correlation with the ROSAT-ESO flux-limited X-ray galaxy cluster survey
The ROSAT-European Southern Observatory (ESO) flux-limited X-ray (REFLEX) galaxy cluster survey and the Two-degree Field Galaxy Redshift Survey (2dFGRS), respectively, comprise the largest, homogeneous X-ray selected cluster catalogue and completed galaxy redshift survey. In this work, we combine these two outstanding data sets in order to study the effect of the large-scale cluster environment, as traced by X-ray luminosity, on the properties of the cluster member galaxies. We measure the LXâÏr relation from the correlated data set and find it to be consistent with recent results found in the literature. Using a sample of 19 clusters with LXâ„ 0.36 Ă 1044 erg sâ1 in the 0.1-2.4 keV band, and 49 clusters with lower X-ray luminosity, we find that the fraction of early spectral type (η=â1.4), passively evolving galaxies is significantly higher in the high-LX sample within R200. We extend the investigation to include composite bJ cluster luminosity functions, and find that the characteristic magnitude of the Schechter-function fit to the early-type luminosity function is fainter for the high-LX sample compared to the low-LX sample (ÎM*= 0.58 ± 0.14). This seems to be driven by a deficit of such galaxies with MbJâŒâ21. In contrast, we find no significant differences between the luminosity functions of star-forming, late-type galaxies. We believe these results are consistent with a scenario in which the high-LX clusters are more dynamically evolved systems than the low-LX cluster
The 2dF Galaxy Redshift Survey: correlation with the ROSAT-ESO flux-limited X-ray galaxy cluster survey
The ROSAT-European Southern Observatory (ESO) flux-limited X-ray (REFLEX) galaxy cluster survey and the Two-degree Field Galaxy Redshift Survey (2dFGRS), respectively, comprise the largest, homogeneous X-ray selected cluster catalogue and completed galaxy redshift survey. In this work, we combine these two outstanding data sets in order to study the effect of the large-scale cluster environment, as traced by X-ray luminosity, on the properties of the cluster member galaxies. We measure the LââÏᔣ relation from the correlated data set and find it to be consistent with recent results found in the literature. Using a sample of 19 clusters with Lâ â„ 0.36 Ă 10âŽâŽ erg sâ»Âč in the 0.1â2.4 keV band, and 49 clusters with lower X-ray luminosity, we find that the fraction of early spectral type (η=â1.4), passively evolving galaxies is significantly higher in the high-Lâ sample within Râââ. We extend the investigation to include composite bⱌ cluster luminosity functions, and find that the characteristic magnitude of the Schechter-function fit to the early-type luminosity function is fainter for the high-Lâ sample compared to the low-Lâ sample (ÎM*= 0.58 ± 0.14). This seems to be driven by a deficit of such galaxies with MbⱌâŒâ21. In contrast, we find no significant differences between the luminosity functions of star-forming, late-type galaxies. We believe these results are consistent with a scenario in which the high-Lâ clusters are more dynamically evolved systems than the low-Lâ clusters
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