On the evolution of the black hole: spheroid mass ratio

We present the results of a study which uses the 3C RR sample of radio-loud active galactic nuclei to investigate the evolution of the black hole:spheroid mass ratio in the most massive early-type galaxies from  0 < z < 2  . Radio-loud unification is exploited to obtain virial (linewidth) black hole mass estimates from the 3C RR quasars, and stellar mass estimates from the 3C RR radio galaxies, thereby providing black hole and stellar mass estimates for a single population of early-type galaxies. At low redshift  ( z ≲ 1)  , the 3C RR sample is consistent with a black hole:spheroid mass ratio of   M _bh/ M _sph≃ 0.002  , in good agreement with that observed locally for quiescent galaxies of similar stellar mass  ( M _sph≃ 5 × 10¹¹ M_⊙)  . However, over the redshift interval  0 < z < 2  the 3C RR black hole:spheroid mass ratio is found to evolve as   M _bh/ M _sph∝ (1 + z )^2.07±0.76  , reaching   M _bh/ M _sph≃ 0.008  by redshift   z ≃ 2  . This evolution is found to be inconsistent with the local black hole:spheroid mass ratio remaining constant at a moderately significant level (98 per cent). If confirmed, the detection of evolution in the 3C RR black hole:spheroid mass ratio further strengthens the evidence that, at least for massive early-type galaxies, the growth of the central supermassive black hole may be completed before that of the host spheroid.     

The X-ray luminosity function of AGN at z∼ 3

We combine Lyman-break colour selection with ultradeep (≳200 ks) Chandra X-ray imaging over a survey area of ∼0.35 deg² to select high-redshift active galactic nuclei (AGN). Applying careful corrections for both the optical and X-ray selection functions, the data allow us to make the most accurate determination to date of the faint end of the X-ray luminosity function (XLF) at   z ∼ 3  . Our methodology recovers a number density of X-ray sources at this redshift which is at least as high as previous surveys, demonstrating that it is an effective way of selecting high z AGN. Comparing to results at   z = 1  , we find no evidence that the faint slope of the XLF flattens at high z , but we do find significant (factor ∼3.6) negative evolution of the space density of low luminosity AGN. Combining with bright end data from very wide surveys we also see marginal evidence for continued positive evolution of the characteristic break luminosity   L _*  . Our data therefore support models of luminosity-dependent density evolution between   z = 1  and   z = 3  . A sharp upturn in the the XLF is seen at the very lowest luminosities  ( L _X≲ 10^42.5 erg s⁻¹)  , most likely due to the contribution of pure X-ray starburst galaxies at very faint fluxes.     

Galaxy groups at 0.3 ≤z≤ 0.55 – II. Evolution to z∼ 0

We compare deep Magellan spectroscopy of 26 groups at  0.3 ≤ z ≤ 0.55  , selected from the Canadian Network for Observational Cosmology 2 field survey, with a large sample of nearby groups from the 2PIGG catalogue. We find that the fraction of group galaxies with significant [O  ii ]λ3727 emission (≥5 Å) increases strongly with redshift, from ∼29 per cent in 2dFGRS to ∼58 per cent in CNOC2, for all galaxies brighter than  ∼ M _*+ 1.75  . This trend is parallel to the evolution of field galaxies, where the equivalent fraction of emission-line galaxies increases from ∼53 to ∼75 per cent. The fraction of emission-line galaxies in groups is lower than in the field, across the full redshift range, indicating that the history of star formation in groups is influenced by their environment. We show that the evolution required to explain the data is inconsistent with a quiescent model of galaxy evolution; instead, discrete events in which galaxies cease forming stars (truncation events) are required. We constrain the probability of truncation ( P _trunc) and find that a high value is required in a simple evolutionary scenario neglecting galaxy mergers  ( P _trunc≳ 0.3 Gyr⁻¹)  . However, without assuming significant density evolution, P _trunc is not required to be larger in groups than in the field, suggesting that the environmental dependence of star formation was embedded at redshifts   z ≳ 0.45  .     

The fraction of Compton-thick sources in an INTEGRAL complete AGN sample

We study the N _H distribution in a complete sample of 88 active galactic nuclei (AGN) selected in the 20–40 keV band from INTEGRAL /Imager on Board the Integral Satellite (IBIS) observations. We find that the fraction of absorbed  ( N _H≥ 10²² cm²)  sources is 43 per cent while the Compton thick AGN comprise 7 per cent of the sample. While these estimates are fully compatible with previous soft gamma-ray surveys, they would appear to be in contrast with results reported by Risaliti, Maiolino & Salvati using an optically selected sample. This apparent difference can be explained as being due to a selection bias caused by the reduction in high energy flux in Compton thick objects rendering them invisible at our sensitivity limit. Taking this into account, we estimate that the fraction of highly absorbed sources is actually in close agreement with the optically selected sample. Furthermore, we show that the measured fraction of absorbed sources in our sample decreases from 80 to ∼20–30 per cent as a function of redshift with all Compton thick AGN having   z ≤ 0.015  . If we limit our analysis to this distance and compare only the type 2 objects in our sample with the Risaliti et al. objects below this redshift value, we find a perfect match to their N _H distribution. We conclude that in the low-redshift bin we are seeing almost the entire AGN population, from unabsorbed to at least mildly Compton thick objects, while in the total sample we lose the heavily absorbed 'counterparts' of distant and therefore dim sources with little or no absorption. Taking therefore this low z bin as the only one able to provide the 'true' distribution of absorption in types 1 and 2 AGN, we estimate the fraction of Compton thick objects to be ≥24 per cent.     

Deep galaxy counts, extragalactic background light and the stellar baryon budget

We assess the constraints imposed by the observed extragalactic background light (EBL) on the cosmic history of star formation and the stellar-mass density today. The logarithmic slope of the galaxy number–magnitude relation from the Southern Hubble Deep Field imaging survey is flatter than 0.4 in all seven UBVIJHK optical bandpasses, i.e. the light from resolved galaxies has converged from the UV to the near-IR. We find a lower limit to the surface brightness of the optical extragalactic sky of about 15 nW m⁻² sr⁻¹, comparable to the intensity of the far-IR background from COBE data. Assuming a Salpeter initial mass function with a lower cut-off consistent with observations of M subdwarf disc stars, we set a lower limit of Ω_g+s h ²>0.0013  I ₅₀ to the visible (processed gas + stars) mass density required to generate an EBL at a level of 50  I ₅₀ nW m⁻² sr⁻¹; our 'best-guess' value is Ω_g+s h ²≈0.0031  I ₅₀. Motivated by the recent microlensing results of the MACHO collaboration, we consider the possibility that massive dark haloes around spiral galaxies are composed of faint white dwarfs, and show that only a small fraction (≲5 per cent) of the nucleosynthetic baryons can be locked in the remnants of intermediate-mass stars forming at z _F≲5, as the bright early phases of such haloes would otherwise overproduce the observed EBL.     

Compact groups in theory and practice – III. Compact groups of galaxies in the Sixth Data Release of the Sloan Digital Sky Survey

We present the largest publicly available catalogue of compact groups (CGs) of galaxies identified using the original selection criteria of Hickson, selected from the Sixth Data Release of the Sloan Digital Sky Survey (SDSS DR6). We identify 2297 CGs down to a limiting magnitude of   r = 18 (∼0.24 groups  deg⁻²), and 74 791 CGs down to a limiting magnitude of   r = 21 (∼6.7 groups  deg⁻²). This represents 0.9 per cent of all galaxies in the SDSS DR6 at these magnitude levels. Contamination due to gross photometric errors has been removed from the bright sample of groups, and we estimate it is present in the large sample at the 14 per cent level. Spectroscopic information is available for 4131 galaxies in the bright catalogue (43 per cent completeness), and we find that the median redshift of these groups is   z _med= 0.09  . The median line-of-sight (LOS) velocity dispersion within the CGs from the bright catalogue is  σ_LOS≃ 230 km s⁻¹  , and their typical intergalactic separations are of the order of 50–100 kpc. We show that the fraction of groups with interloping galaxies identified as members is in good agreement with the predictions from our previous study of a mock galaxy catalogue, and we demonstrate how to select CGs such that the interloper fraction is well defined and minimized. This observational data set is ideal for large statistical studies of CGs, the role of environment on galaxy evolution and the effect of galaxy interactions in determining galaxy morphology.     

The exceptional X-ray variability of the dwarf Seyfert nucleus NGC 4395

An analysis of the X-ray variability of the low-luminosity Seyfert nucleus NGC 4395, based on a long XMM–Newton observation, is presented. The power spectrum shows a clear break from a flat spectrum  (α≈ 1)  to a steeper spectrum  (α≈ 2)  at a frequency   f _br= 0.5–3.0 × 10⁻³ Hz  , comparable to the highest characteristic frequency found previously in a Seyfert galaxy. This extends the measured   M _BH− f _br  values to lower M _BH than previous studies of Seyfert galaxies, and is consistent with an inverse scaling of variability frequency with black hole mass. The variations observed are among the most violent seen in an active galactic nuclei to date, with the fractional rms amplitude  ( F _var)  exceeding 100 per cent in the softest band. The amplitude of the variations seems intrinsically higher in NGC 4395 than most other Seyfert galaxies, even after accounting for the differences in characteristic frequencies. The origin of this difference is not clear, but it is unlikely to be a high accretion rate (   L / L _Edd≲ 20  per cent for NGC 4395). The variations clearly follow the linear rms–flux relation, further supporting the idea that this is a ubiquitous characteristics of accreting black holes. The variations are highly coherent between different energy bands with any frequency-dependent time delay limited to ≲1 per cent.     

Clustering of X-ray selected active galactic nuclei

A total of 235 active galactic nuclei (AGN) from two different soft X-ray surveys [the ROSAT Deep Survey (DRS) and the ROSAT International X-ray Optical Survey (RIXOS)] with redshifts between 0 and 3.5 are used to study the clustering of X-ray selected AGN and its evolution. A 2σ significant detection of clustering of such objects is found on scales < 40–80 h ⁻¹ Mpc in the RIXOS sample, while no clustering is detected on any scales in the DRS sample. Assuming a single power-law model for the spatial correlation function (SCF), quantitative limits on the AGN clustering have been obtained: a comoving correlation length 1.5 ≲  r ₀ ≲ 3.3  h ⁻¹ Mpc is implied for comoving evolution, while 1.9 ≲  r ₀ ≲ 4.8 for stable clustering and 2.2 ≲  r ₀ ≲ 5.5 for linear evolution. These values are consistent with the correlation lengths and evolutions obtained for galaxy samples, but imply smaller amplitude or faster evolution than recent ultraviolet and optically selected AGN samples. We also constrain the ratio of bias parameters between X-ray selected AGN and IRAS galaxies to be ≲ 1.7 on scales ≲ 10  h ⁻¹ Mpc, a somewhat smaller value than is inferred from local large-scale dynamical studies.     

Primordial magnetic fields and formation of molecular hydrogen

We study the implications of primordial magnetic fields for the thermal and ionization history of the post-recombination era. In particular, we compute the effects of dissipation of primordial magnetic fields owing to ambipolar diffusion and decaying turbulence in the intergalactic medium (IGM) and the collapsing haloes, and compute the effects of the altered thermal and ionization history on the formation of molecular hydrogen. We show that, for magnetic field strengths in the range  2 × 10⁻¹⁰≲ B ₀≲ 2 × 10⁻⁹ G  , the molecular hydrogen fraction in IGM and collapsing halo can increase by a factor of 5 to 1000 over the case with no magnetic fields. We discuss the implication of the increased molecular hydrogen fraction on the radiative transfer of ultraviolet photons and the formation of first structures in the universe.     

How big were the first cosmological objects?

We calculate the cooling times at constant density for haloes with virial temperatures from 100 K to  1×10⁵ K  that originate from a 3 σ fluctuation of a CDM power spectrum in three different cosmologies. Our intention is to determine the first objects that can cool to low temperatures, but not to follow their dynamical evolution. We identify two generations of haloes: those with low virial temperatures,   T _vir≲9000 K  that remain largely neutral, and those with larger virial temperatures that become ionized. The lower temperature, lower mass haloes are the first to cool to 75 per cent of their virial temperature. The precise temperature and mass of the first objects are dependent upon the molecular hydrogen (H₂) cooling function and the cosmological model. The higher mass haloes collapse later but, in this paradigm, cool much more efficiently once they have done so, first via electronic transitions and then via molecular cooling: in fact, a greater residual ionization once the haloes cool below 9000 K results in an enhanced H₂ production and hence a higher cooling rate at low temperatures than for the lower mass haloes, so that within our constant-density model it is the former that are the first to cool to really low temperatures. We discuss the possible significance of this result in the context of CDM models in which the shallow slope of the initial fluctuation spectrum on small scales leads to a wide range of halo masses (of differing overdensities) collapsing over a small redshift interval. This 'crosstalk' is sufficiently important that both high- and low-mass haloes collapse during the lifetimes of the massive stars which may be formed at these epochs. Further investigation is thus required to determine which generation of haloes plays the dominant role in early structure formation.     

The contribution of unresolved radio-loud AGN to the extragalactic diffuse gamma-ray background

We present a calculation of the blazar contribution to the extragalactic diffuse γ -ray background (EGRB) in the EGRET energy range. Our model is based on inverse-Compton scattering as the dominant γ -ray production process in the jets of flat spectrum radio quasars (FSRQs) and BL Lac objects, and on the unification scheme of radio-loud AGN. According to this picture, blazars represent the beamed fraction of the Fanaroff–Riley radio galaxies (FR galaxies).
The observed log  N –log  S distribution and redshift distribution of both FSRQs and BL Lacs constrain our model. Depending slightly on the evolutionary behaviour of blazars, we find that unresolved AGN underproduce the intensity of the extragalactic background radiation. With our model only 20–40 per cent of the extragalactic background emission can be explained by unresolved blazars if we integrate to a maximum redshift of Z _max=3. For Z _max=5, blazars could account for 40–80 per cent of the EGRB. Roughly 70–90 per cent of the AGN contribution to the EGRB would result from BL Lacs. While the systematic uncertainties in our estimate for the FSRQ contribution appear small, in the case of BL Lacs our model parameters are not consistent with the results from studies in other wavelength regimes, and therefore may have larger systematic uncertainties. Thus we end up with two possibilities, depending on whether we underpredict or overpredict the BL Lac contribution: either unresolved AGN cannot account for the entire EGRB, or unresolved BL Lacs produce the observed background.
We predict a significant flattening of the γ -ray log  N –log  S function in the next two decades of flux below the EGRET threshold.     

The Gunn–Peterson effect and the Lyman alpha forest

We show that spatial correlations in a stochastic large-scale velocity field in an otherwise smooth intergalactic medium (homogeneous comoving density) superposed on the general Hubble flow may cause a 'line-like' structure in QSO spectra similar to the population of unsaturated Lyα forest lines which usually are attributed to individual clouds with 10¹¹ ≲ N _{H i}  5 × 10¹³ cm⁻². Therefore there is no clear observational distinction between a diffuse intergalactic medium and discrete intergalactic clouds. It follows that the H  i density in the diffuse intergalactic medium might be substantially underestimated if it is determined from the observed intensity distribution near the apparent continuum in high-resolution spectra of QSOs. Our tentative estimate implies a diffuse neutral hydrogen opacity τ_GP ∼ 0.3 at z  ∼ 3 and a current baryon density Ω_IGM ≃ 0.08, assuming a Hubble constant H ₀ = 70 km s⁻¹ Mpc⁻¹.     

The IPHAS-POSS-I proper motion survey of the Galactic plane

We present millimetre observations of a sample of 12 high-redshift ultraluminous infrared galaxies (ULIRGs) in the extended growth strip (EGS). These objects were initially selected on the basis of their observed mid-IR colours (  0.0 < [3.6]−[4.5] < 0.4  and  −0.7 < [3.6]−[8.0] < 0.5  ) to lie at high redshift  1.5 ≲ z ≲ 3  , and subsequent 20–38 μm mid-IR spectroscopy confirms that they lie in a narrow redshift window centred on   z ≈ 2  . We detect 9/12 of the objects in our sample at high significance  (>3σ)  with a mean 1200 μm flux of  〈 F _1200 μm〉= 1.6 ± 0.1  mJy. Our millimetre photometry, combined with existing far-IR photometry from the Far-IR Deep Extragalactic Legacy Survey (FIDELS) and accurate spectroscopic redshifts, places constraints both sides of the thermal dust peak. This allows us to estimate the dust properties, including the far-IR luminosity, dust temperature and dust mass. We find that our sample is similar to other high- z and intermediate- z ULIRGs, and local systems, but has a different dust selection function than submillimeter-selected galaxies. Finally, we use existing 20-cm radio continuum imaging to test the far-IR/radio correlation at high redshift. We find that our sample is consistent with the local relation, implying little evolution. Furthermore, this suggests that our sample selection method is efficient at identifying ultraluminous, starburst-dominated systems within a very narrow redshift range centred at   z ∼ 2  .     

Dust and hydrogen molecules in the extremely metal-poor dwarf galaxy SBS 0335–052

During the early stages of galaxy evolution, the metallicity is generally low and nearby metal-poor star-forming galaxies may provide templates for primordial star formation. In particular, the dust content of such objects is of great importance, because early molecular formation can take place on grains. To gain insight into primeval galaxies at high redshift, we examine the dust content of the nearby extremely low-metallicity galaxy SBS  0335–052  which hosts a very young starburst (≲10⁷ yr). In young galaxies, the dust formation rate in Type II supernovae governs the amount of dust, and by incorporating recent results on dust production in Type II supernovae we model the evolution of dust content. If the star-forming region is compact (≲100 pc), as suggested by observations of SBS  0335–052  , our models consistently explain the quantity of dust, far-infrared luminosity, and dust temperature in this low-metallicity object. We also discuss the H₂ abundance. The compactness of the region is important to H₂ formation, because the optical depth of dust for UV photons becomes large and H₂ dissociation is suppressed. We finally focus on implications for damped Ly α systems.     

The redshift distribution of FIRST radio sources at 1 mJy

We present spectra for a sample of radio sources from the FIRST survey, and use them to define the form of the redshift distribution of radio sources at mJy levels. We targeted 365 sources and obtained 46 redshifts (13 per cent of the sample). We find that our sample is complete in redshift measurement to R ∼18.6, corresponding to z ∼0.2. Galaxies were assigned spectral types based on emission-line strengths. Early-type galaxies represent the largest subset (45 per cent) of the sample and have redshifts 0.15≲ z ≲0.5; late-type galaxies make up 15 per cent of the sample and have redshifts 0.05≲ z ≲0.2; starbursting galaxies are a small fraction (∼6 per cent), and are very nearby ( z ≲0.05). Some 9 per cent of the population have Seyfert 1/quasar-type spectra, all at z ≳0.8, and 4 per cent are Seyfert 2 type galaxies at intermediate redshifts ( z ∼0.2).
Using our measurements and data from the Phoenix survey (Hopkins et al.), we obtain an estimate for N ( z ) at S _1.4 GHz≥1 mJy and compare this with model predictions. At variance with previous conclusions, we find that the population of starbursting objects makes up ≲5 per cent of the radio population at S ∼1 mJy.     

Constraints on the mass and abundance of black holes in the Galactic halo: the high-mass limit

We establish constraints on the mass and abundance of black holes in the Galactic halo by determining their impact on globular clusters, which are conventionally considered to be little evolved. Using detailed Monte Carlo simulations and simple evolutionary models, we argue that black holes with masses M _bh≳(1–3)×10⁶ M_⊙ can comprise no more than a fraction f _bh≈0.17 of the total halo density at Galactocentric radius R ≈8 kpc. This bound arises from requiring stability of the cluster mass function. A more restrictive bound may be derived if we demand that the probability of destruction of any given, low-mass M _c≈(2.5–7.5)×10⁴ M_⊙] globular cluster not exceed 50 per cent; this bound is f _bh≲0.025–0.05 at R ≈8 kpc. This constraint improves those based on disc heating and dynamical friction arguments as well as current lensing results. At smaller radius the constraint on f _bh strengthens, while at larger radius an increased fraction of black holes is allowed.     

Dust formation in primordial Type II supernovae

We have investigated the formation of dust in the ejecta of Type II supernovae (SNe), mostly of primordial composition, to answer the question of where the first solid particles are formed in the Universe. However, we have also considered non-zero progenitor metallicity values up to Z = Z_⊙ . The calculations are based on standard nucleation theory, and the scheme has been tested for the first time on the well-studied case of SN1987A, yielding results that are in agreement with the available data. We find that: (i) the first dust grains are predominantly made of silicates, amorphous carbon (AC), magnetite and corundum; and (ii) the largest grains are the AC ones, with sizes around 300 Å, whereas the other grain types have smaller radii, around 10–20 Å . The grain size distribution depends somewhat on the thermodynamics of the ejecta expansion, and variations in the results by a factor ≈2 might occur within reasonable estimates of the relevant parameters. Also, and for the same reason, the grain size distribution is essentially unaffected by metallicity changes. The predictions on the amount of dust formed are very robust: for Z =0 , we find that SNe with masses in the range (12–35) M_⊙ produce about 0.08 M_⊙≲ M _d≲0.3 M_⊙ of dust per supernova. The above range increases by roughly three times as the metallicity is increased to solar values. We discuss the implications and the cosmological consequences of the results.     

Constraining the cosmological baryon density with X-ray clusters

We study the properties of X-ray galaxy clusters in four cold dark matter models with different baryon fractions Ω_BM, ranging from 5 to 20 per cent. By using an original three-dimensional hydrodynamic code based on the piecewise parabolic method, we run simulations on a box with a size of 64  h ⁻¹ Mpc and we identify the clusters by selecting the peaks in the X-ray luminosity field. We analyse these mock catalogues by computing the mass function, the luminosity function, the temperature distribution and the luminosity–temperature relation. By comparing the predictions of the different models to a series of recent observational results, we find that only the models with low baryonic content agree with the data, while models with larger baryon fraction are well outside the 1σ error bars. In particular, the analysis of the luminosity functions, both bolometric and in the energy band [0.5–2] keV, requires Ω_BM ≲ 0.05 when we fix the values h  = 0.5 and n  = 0.8 for the Hubble parameter and the primordial spectral index, respectively. Moreover we find that, independently of the cosmological scenario, all the considered quantities have very little redshift evolution, particularly between z  = 0.5 and 0.     

Molecular hydrogen in damped Lyα systems: clues to interstellar physics at high redshift

In order to interpret H₂ quasar absorption-line observations of damped Lyα systems (DLAs) and subDLAs, we model their H₂ abundance as a function of dust-to-gas ratio, including H₂ self-shielding and dust extinction against dissociating photons. Then, we constrain the physical state of the gas by using H₂ data. Using H₂ excitation data for DLAs with H₂ detections, we derive a gas density  1.5 ≲ log n (cm⁻³) ≲ 2.5  , temperature  1.5 ≲ log T (K) ≲ 3  , and an internal ultraviolet (UV) radiation field (in units of the Galactic value)  0.5 ≲ log χ≲ 1.5  . We then find that the observed relation between the molecular fraction and the dust-to-gas ratio of the sample is naturally explained by the above conditions. However, it is still possible that H₂ deficient DLAs and subDLAs with H₂ fractions less than  ∼10⁻⁶  are in a more diffuse and warmer state. The efficient photodissociation by the internal UV radiation field explains the extremely small H₂ fraction  (≲10⁻⁶)  observed for  κ≲ 1/30  (κ is the dust-to-gas ratio in units of the Galactic value); H₂ self-shielding causes a rapid increase in, and large variations of, H₂ abundance for  κ≳ 1/30  . We finally propose an independent method to estimate the star formation rates of DLAs from H₂ abundances; such rates are then critically compared with those derived from other proposed methods. The implications for the contribution of DLAs to the cosmic star formation history are briefly discussed.