The 3.4-μm absorption feature towards three obscured active galactic nuclei

The results of 3–4-μm spectroscopy towards the nuclei of NGC 3094, 7172, and 7479 are reported. In ground-based 8–13-μm spectra, all the sources have strong absorption-like features at ∼10 μm, but they do not have detectable polycyclic aromatic hydrocarbon (PAH) emission features. The 3.4-μm carbonaceous dust absorption features are detected towards all nuclei. NGC 3094 shows a detectable 3.3-μm PAH emission feature, while NGC 7172 and 7479 do not. Nuclear emission whose spectrum shows dust absorption features but no PAH emission features, is thought to be dominated by highly obscured active galactic nuclei (AGNs) activity. For NGC 7172, 7479, and three other such nuclei in the literature, we investigate the optical depth ratios between the 3.4-μm carbonaceous dust and 9.7-μm silicate dust absorption     The     ratios towards three highly obscured AGNs with face-on host galaxies are systematically larger than the ratios in the Galactic diffuse interstellar medium or the ratios for two highly obscured AGNs with edge-on host galaxies. We suggest that the larger ratios can be explained if the obscuring dust is so close to the central AGNs that a temperature gradient occurs in it. If this idea is correct, our results may provide spectroscopic evidence for the presence of the putative 'dusty tori' in the close vicinity of AGNs.     

The 3.4-μm absorption feature towards the nucleus of NGC 5506

We report the results of 3-μm spectroscopy towards the nucleus of a nearby Seyfert 2 galaxy, NGC 5506. A weak absorption feature of carbonaceous dust is detected at ∼3.4 μm. The optical depth ratio of the 3.4-μm carbonaceous dust absorption to the 9.7-μm silicate dust absorption is smaller by more than a factor of 2 than that in the interstellar medium in our Galaxy. The small ratio is consistent with the idea that the obscuration towards the nucleus of NGC 5506 is caused mainly by dust grains in its host galaxy (type S0/a) and that the contribution of carbonaceous dust grains to the visual extinction there is smaller than that in our Galaxy (type Sb/bc).     

The spatial variation of the 3-μm dust features in Circinus

We report spatially resolved variations in the 3.4-μm hydrocarbon absorption feature and the 3.3-μm polycyclic aromatic hydrocarbon (PAH) emission band in the Circinus galaxy over the central few arcsec. The absorption is measured towards warm emitting dust associated with Coronal line regions to the east and west of the nucleus. There is an absorption optical depth  τ_3.4 μm∼ 0.1  in the core which decreases to the west and increases to the east. This is consistent with increased extinction out to ∼40 pc east of the core, supported by the Coronal emission line intensities which are significantly lower to the east than the west. PAH emission is measured to be symmetrically distributed out to ±4 arcsec, outside the differential extinction region. The asymmetry in the 3.4-μm absorption band reflects that seen in the 9.7-μm silicate absorption band reported by Roche et al., and the ratio of the two absorption depths remains approximately constant across the central regions, with  τ_3.4 μm/τ_9.7 μm∼ 0.06 ± 0.01  . This indicates well-mixed hydrocarbon and silicate dust populations, with no evidence for significant changes near the nucleus.     

The distribution of atomic gas and dust in nearby galaxies – I. Presentation of matched-resolution VLA H i and SCUBA 850-μm maps

We present matched-resolution VLA H  i and SCUBA 850-μm maps of 20 IRAS -bright galaxies. Of the galaxies observed, two were not detected in H  i and two were detected in absorption. The H  i distributions of the galaxies have a range of morphologies. Some of the systems appear H  i deficient in the central regions which could be due to a high conversion rate of H  i into molecules or H  i absorption. In contrast to the H  i , the 850-μm emission has a smooth distribution which is concentrated towards the optical centre of each galaxy. We also find evidence for 850-μm emission extending to the periphery of the optical disc in some of the galaxies. Finally, we note that the relative lack of 850-μm emission when compared with H  i does not necessarily mean that the atomic gas and dust do not have similar mass distributions.     

Variations in 24-μm morphologies among galaxies in the Spitzer Infrared Nearby Galaxies Survey: new insights into the Hubble sequence

To study the distribution of star formation and dust emission within nearby galaxies, we measured five morphological parameters in the 3.6- and 24-μm wavebands for 65 galaxies in the Spitzer Infrared Nearby Galaxies Survey (SINGS) and eight galaxies that were serendipitously observed by SINGS. The morphological parameters demonstrate strong variations along the Hubble sequence, including statistically significant differences between S0/a-Sab and Sc-Sd galaxies. Early-type galaxies are generally found to be compact, centralized, symmetric sources in the 24-μm band, while late-type galaxies are generally found to be extended, asymmetric sources. These results suggest that the processes that increase the real or apparent sizes of galaxies' bulges also lead to more centralized 24-μm dust emission. Several phenomena, such as strong nuclear star formation, Seyfert activity, or outer ring structures, may cause galaxies to deviate from the general morphological trends observed at 24 μm. We also note that the 24-μm morphologies of Sdm-Im galaxies are quite varied, with some objects appearing very compact and symmetric but others appearing diffuse and asymmetric. These variations reflect the wide variation in star formation in irregular galaxies as observed at other wavelengths. The variations in the 24-μm morphological parameters across the Hubble sequence mirror many of the morphological trends seen in other tracers of the ISM and in stellar emission. However, the 24-μm morphological parameters for the galaxies in this sample do not match the morphological parameters measured in the stellar wavebands. This implies that the distribution of dust emission is related to but not equivalent to the distribution of stellar emission.     

Hyperluminous infrared galaxies

39 galaxies are now known, from follow-up of faint IRAS sources and from submillimetre observations of high-redshift AGN, with far-infrared luminosities >10¹³ L_⊙. 13 of these, which have been found in 60- or 850-μm surveys, form an important unbiased subsample. 12 have been found by comparison of 60-μm surveys with quasar or radio galaxy catalogues, or from infrared surveys with colour selection biased towards AGN, while a further 14 have been found through submillimetre observations of known high-redshift AGN. In this paper I argue, on the basis of detailed modelling of the spectral energy distributions of hyperluminous galaxies with accurate radiative transfer models, and from evidence of high gas mass in several cases, that the bulk of the emission from these galaxies at rest frame wavelengths ≥50 μm is caused by star formation. Even after correction for the effects of lensing, hyperluminous galaxies with emission peaking at rest frame wavelengths ≥50 μm are therefore undergoing star formation at rates >10³ M_⊙ yr⁻¹ and are strong candidates for being primeval galaxies, in the process of a major episode of star formation.     

Extended dust emission in NGC 7465

We present SCUBA 850-μm, JCMT  CO( J =2→1)  , B -band imaging and VLA H  i observations of the NGC 7465/4/3 group of galaxies. The 850-μm emission associated with NGC 7465 extends to at least ∼2 R ₂₅ and is well correlated with the H  i . We investigate a range of possible mechanisms by which dust beyond R ₂₅ may be heated to give the observed extended submillimetre emission. By modelling the dust heating by stars in two extreme geometries, we fail to find any reasonable star formation scenario that is consistent with both the 850-μm and optical data. Furthermore, we do not detect any  CO( J =2→1)  emission coincident with the extended dust and atomic gas as would be expected if significant star formation were occurring. We show that shock-heating of dust via cloud–cloud collisions in the stripped interstellar medium of NGC 7465 could be sufficient to explain the extended 850-μm emission and lack of optical emission in the stripped gas, and suggest that cloud–cloud collisions may be an important dust heating mechanism in gas-rich systems.     

Confusion limit resulting from galaxies: using the Infrared Array Camera on board SIRTF

Recent ISO data have allowed, for the first time, observationally based estimates for source confusion in mid-infrared surveys. We use the extragalactic source counts from ISOCAM in conjunction with K -band counts to predict the confusion resulting from galaxies in deep mid-infrared observations. We specifically concentrate on the near-future Space Infrared Telescope Facility ( SIRTF ) mission, and calculate expected confusion for the Infrared Array Camera (IRAC) on board SIRTF . A defining scientific goal of the IRAC instrument will be the study of high-redshift galaxies using a deep, confusion-limited wide-field survey at 3–10 μm . A deep survey can reach 3-μJy sources with reasonable confidence in the shorter wavelength IRAC bands. Truly confusion-limited images with the 8 μm will be difficult to obtain because of practical time constraints, unless infrared galaxies exhibit very strong evolution beyond the deepest current observations. We find L * galaxies to be detectable to z =3–3.5 at 8 μm, which is slightly more pessimistic than found in 1999 by Simpson & Eisenhardt.     

The X-ray spectra of optically selected Seyfert 2 galaxies: are there any Seyfert 2 galaxies with no absorption?

We present an X-ray spectral analysis of a sample of eight bona fide Seyfert 2 galaxies, selected on the basis of their high [O  iii ] λ 5007 flux, from the Ho et al. spectroscopic sample of nearby galaxies. We find that, in general, the X-ray spectra of our Seyfert 2 galaxies are complex, with some of our objects having spectra different from the 'typical' spectrum of X-ray selected Seyfert 2 galaxies. Two (NGC 3147 and 4698) show no evidence for intrinsic absorption. We suggest that this is a result of the fact that when the torus suppresses the intrinsic medium and hard energy flux, underlying emission from the host galaxy, originating in circumnuclear starbursts, and scattering from warm absorbers contributes in these energy bands more significantly. Our ASCA data alone cannot discriminate whether low-absorption objects are Compton-thick active galactic nuclei (AGNs) with a strong scattered component or lack an obscuring torus. The most striking example of our low absorption Seyfert 2 is NGC 4698. Its spectrum could be explained by either a dusty warm absorber or a lack of broad-line clouds so that its appearance as a Seyfert 2 is intrinsic and not a result of absorption.     

The SCUBA Local Universe Galaxy Survey – II. 450-μm data: evidence for cold dust in bright IRAS galaxies

This is the second in a series of papers presenting results from the SCUBA Local Universe Galaxy Survey. In our first paper we provided 850-μm flux densities for 104 galaxies selected from the IRAS Bright Galaxy Sample and we found that the 60-, 100-μm ( IRAS ) and 850-μm (SCUBA) fluxes could be adequately fitted by emission from dust at a single temperature. In this paper we present 450-μm data for the galaxies. With the new data, the spectral energy distributions of the galaxies can no longer be fitted with an isothermal dust model – two temperature components are now required. Using our 450-μm data and fluxes from the literature, we find that the 450/850-μm flux ratio for the galaxies is remarkably constant, and this holds from objects in which the star formation rate is similar to our own Galaxy, to ultraluminous infrared galaxies (ULIRGs) such as Arp 220. The only possible explanation for this is if the dust emissivity index for all of the galaxies is ∼2 and the cold dust component has a similar temperature in all galaxies     . The 60-μm luminosities of the galaxies were found to depend on both the dust mass and the relative amount of energy in the warm component, with a tendency for the temperature effects to dominate at the highest L ₆₀. The dust masses estimated using the new temperatures are higher by a factor of ∼2 than those determined previously using a single temperature. This brings the gas-to-dust ratios of the IRAS galaxies into agreement with those of the Milky Way and other spiral galaxies which have been intensively studied in the submm.     

Starburst galaxies and structure in the submillimetre background towards the Hubble Deep Field

We use an 850-μm SCUBA map of the Hubble Deep Field (HDF) to study the dust properties of optically-selected starburst galaxies at high redshift. The optical/infrared (IR) data in the HDF allow a photometric redshift to be estimated for each galaxy, together with an estimate of the visible star-formation rate. The 850-μm flux density of each source provides the complementary information: the amount of hidden, dust-enshrouded star formation activity. Although the 850-μm map does not allow detection of the majority of individual sources, we show that the galaxies with the highest UV star-formation rates are detected statistically, with a flux density of about S ₈₅₀=0.2 mJy for an apparent UV star-formation rate of 1  h ⁻² M_⊙ yr⁻¹. This level of submillimetre output indicates that the total star-forming activity is on average a factor of approximately 6 times larger than the rate inferred from the UV output of these galaxies. The general population of optical starbursts is then predicted to contribute at least 25 per cent of the 850-μm background. We carry out a power-spectrum analysis of the map, which yields some evidence for angular clustering of the background source population, but at a level lower than that seen in Lyman-break galaxies. Together with other lines of argument, particularly from the NICMOS HDF data, this suggests that the 850-μm background originates over an extremely wide range of redshifts – perhaps 1≲ z ≲6.     

The K-band Hubble diagram of submillimetre galaxies and hyperluminous galaxies

We present the K -band Hubble diagrams ( K – z relations) of submillimetre-selected galaxies and hyperluminous galaxies (HLIRGs). We report the discovery of a remarkably tight K – z relation of HLIRGs, indistinguishable from that of the most luminous radio galaxies. Like radio galaxies, the HLIRG K – z relation at   z ≲ 3  is consistent with a passively evolving ∼3 L _* instantaneous starburst starting from a redshift of   z ∼ 10  . In contrast, many submillimetre-selected galaxies are ≳2 mag fainter, and the population has a much larger dispersion. We argue that dust obscuration and/or a larger mass range may be responsible for this scatter. The galaxies so far proved to be hyperluminous may have been biased towards higher AGN bolometric contributions than submillimetre-selected galaxies due to the 60-μm selection of some, so the location on the K – z relation may be related to the presence of the most massive active galactic nucleus. Alternatively, a particular host galaxy mass range may be responsible for both extreme star formation and the most massive active nuclei.     

Star-forming regions in blue compact dwarf galaxies

We present near-infrared 2.1-μm continuum and Brγ and 1–0S(1) line images of three blue compact dwarf galaxies: II Zw 40, NGC 5253 and He 2-10. Comparison of the morphologies of the emission-line regions and continuum together with the line ratios shows that in these cases the starbursts are typified by strongly peaked Brγ and weak diffuse H₂ emission, indicating the presence of one or more compact nuclei, and also tidal tails. We consider the possible origins of the H₂ emission, and conclude that shock excitation in cloud collisions, and hence dynamical processes such as interactions or mergers, is important. By comparing models of the equivalent width of Brγ to other age constraints from the literature we show that the star formation must have occurred not only recently but in a short-duration burst. The current centres of star formation in all three galaxies are of a similar age but, whereas the hotspots in He 2-10 are coeval, we find phase differences between those in NGC 5253. We compare the masses and sizes of the star formation sites to those of present-day globular clusters.     

IRAC photometric analysis and the mid-IR photometric properties of Lyman-break galaxies

We present photometric analysis of deep mid-infrared (mid-IR) observations obtained by Spitzer /IRAC covering the fields Q1422+2309, Q2233+1341, DSF2237a,b, HDFN, SSA22a,b and B20902+34, giving the number counts and the depths for each field. In a sample of 751 Lyman-break galaxies (LBGs) lying in those fields, 443, 448, 137 and 152 are identified at 3.6-, 4.5-, 5.8-, 8.0-μm IRAC bands, respectively, expanding their spectral energy distribution to rest-near-IR and revealing that LBGs display a variety of colours. Their rest-near-IR properties are rather inhomogeneous, ranging from those that are bright in IRAC bands and exhibit  [ R ]−[3.6] > 1.5  colours to those that are faint or not detected at all in IRAC bands with  [ R ]−[3.6] < 1.5  colours and these two groups of LBGs are investigated. We compare the mid-IR colours of the LBGs with the colours of star-forming galaxies and we find that LBGs have colours consistent with star-forming galaxies at   z ∼ 3  . The properties of the LBGs detected in the 8-μm IRAC band (rest-frame K band) are examined separately, showing that they exhibit redder  [ R ]−[3.6]  colours than the rest of the population and that although in general, a multiwavelength study is needed to reach more secure results, IRAC 8-μm band can be used as a diagnostic tool, to separate high z , luminous AGN-dominated objects from normal star-forming galaxies at   z ∼ 3  .     

Kinematics, abundances and origin of brightest cluster galaxies

We present kinematic parameters and absorption line strengths for three brightest cluster galaxies, NGC 6166, 6173 and 6086. We find that NGC 6166 has a velocity dispersion profile which rises beyond 20 arcsec from the nucleus, with a halo velocity dispersion in excess of 400 km s⁻¹. All three galaxies show a positive and constant h ₄ Hermite moment. The rising velocity dispersion profile in NGC 6166 thus indicates an increasing mass-to-light ratio. Rotation is low in all three galaxies, and NGC 6173 and 6086 show possible kinematically decoupled cores. All three galaxies have Mg₂ gradients similar to those found in normal bright ellipticals, which are not steep enough to support simple dissipative collapse models, but these could be accompanied by dissipationless mergers which would tend to dilute the abundance gradients. The [Mg/Fe] ratios in NGC 6166 and 6086 are higher than that found in NGC 6173, and if NGC 6173 is typical of normal bright ellipticals, this suggests that cDs cannot form from late mergers of normal galaxies.     

A far-infrared view of the Lockman hole from ISO 95-μm observations – II. Optical identifications and insights into the nature of the far-infrared sources

We present the optical identifications of a 95-μm ISOPHOT sample in the Lockman hole over an area of approximately half a deg². The Rodighiero et al. catalogue includes 36 sources, making up a complete flux-limited sample for   S _95 μm≥ 100 mJy  . Reliable sources were detected, with decreasing but well-controlled completeness, down to   S _95 μm≃ 20 mJy  . We have combined mid-infrared (IR) and radio catalogues in this area to identify the potential optical counterparts of the far-IR sources. We found 14 radio and 13 15-μm associations, 10 of which have both associations. For the 11 sources with spectroscopic redshift, we have performed a spectrophotometric analysis of the observed spectral energy distributions (SEDs). Four of these 95-μm sources have been classified as faint IR (FIR) galaxies  ( L _FIR < 1. e 11 L_⊙)  , six as luminous IR galaxies (LIRGs) and only one as an ultraluminous IR galaxy (ULIRG). We have discussed the redshift distribution of these objects, comparing our results with evolutionary model predictions 95 and 175 μm. Given their moderate distances (the bulk of the closest spectroscopically identified objects lying at   z < 0.2  ), their luminosities and star formation rates (SFR; median value  ∼ 10 M_⊙ yr⁻¹  ), the sources unveiled by ISOPHOT at 95 μm seem to correspond to the low redshift  ( z < 0.3)  FIRBACK 175-μm population, composed of dusty, star-forming galaxies with moderate SFRs. We computed and compared different SFR estimators, and found that the SF derived from the bolometric IR luminosity is well correlated with that computed from the radio and mid-IR fluxes.     

Overtones of silicate and aluminate minerals and the 5–8 μm ice bands of deeply embedded objects

The distinct patterns, relatively low intensities and peak positions of overtone-combination bands of silicates and oxides suggest that the 5–8 μm spectral region can provide clues for the dust composition when near optically thick conditions exist for the 10-μm silicate feature. We present 1000–2500 cm⁻¹ room-temperature laboratory spectra obtained from powders of silicate, aluminate and nitride minerals and silicate glasses. The spectra exhibit overtone absorption bands with mass absorption coefficients ∼100 times weaker than the fundamentals. These data are compared with the 5–8 μm spectra of deeply embedded young stellar objects observed with the Short Wavelength Spectrometer on the Infrared Space Observatory . Fits of the laboratory data to the observations, after subtraction of the 6.0-μm H₂O ice feature and the 6.0-μm feature identified with organic refractory material, indicate that crystalline melilite (a silicate) or metamict hibonite (a radiation-damaged crystalline aluminate) may be responsible for much of the 6.9-μm absorption feature in the observations, with melilite providing the best match. A weaker 6.2-μm absorption in the young stellar object spectra is well matched by the spectra of hydrous crystalline amphibole silicates (actinolite and tremolite). Relative abundances of Si–O in room-temperature amphiboles to low-temperature H₂O ice are in the range 0.46–3.9 and in melilite are in the range 2.5–8.6. No astronomical feature was matched by the overtones of amorphous silicates because these bands are too broad and peak at the wrong wavelength. Hence, this analysis is consistent with the 10-μm features of these objects being due to a mixture of crystalline and amorphous silicates, rather than only amorphous silicates.     

The SCUBA HAlf Degree Extragalactic Survey – VI. 350-μm mapping of submillimetre galaxies

A follow-up survey using the Submillimetre High-Angular Resolution Camera (SHARC-II) at 350 μm has been carried out to map the regions around several 850-μm-selected sources from the Submillimetre HAlf Degree Extragalactic Survey (SHADES). These observations probe the infrared (IR) luminosities and hence star formation rates in the largest existing, most robust sample of submillimetre galaxies (SMGs). We measure 350-μm flux densities for 24 850-μm sources, seven of which are detected at ≥2.5σ within a 10 arcsec search radius of the 850-μm positions. When results from the literature are included the total number of 350-μm flux density constraints of SHADES SMGs is 31, with 15 detections. We fit a modified blackbody to the far-IR (FIR) photometry of each SMG, and confirm that typical SMGs are dust-rich  ( M _dust≃ 9 × 10⁸ M_⊙)  , luminous  ( L _FIR≃ 2 × 10¹² L_⊙)  star-forming galaxies with intrinsic dust temperatures of ≃35 K and star formation rates of  ≃400 M_⊙ yr⁻¹  . We have measured the temperature distribution of SMGs and find that the underlying distribution is slightly broader than implied by the error bars, and that most SMGs are at 28 K with a few hotter. We also place new constraints on the 350-μm source counts, N ₃₅₀(>25 mJy) ∼ 200–500 deg⁻².     

Detection of an H92α recombination line in the starburst galaxy NGC 660

We have used the Very Large Array (VLA) to search for the H92α radio recombination line (RRL) in four starburst galaxies. In NGC 660, the line was detected over a 17Å‐8 arcsec² region near its starburst nucleus. The line and continuum emission indicate that the RRL-emitting gas is most likely in the form of a cluster of H ii regions with a small filling factor. Using a simple model we find that the total ionized mass in the nuclear region is in the range 2–8Å‐10⁴ M⊙ and the rate of production of UV photons N _Lyc∼1–3Å‐10⁵³ s⁻¹. The ratio of H92α and Brγ line intensities in NGC 660 indicates that extinction is significant even at λ=2 μm. The velocity field of the ionized gas is consistent with a rotating disc with an average velocity gradient of ∼15 km s⁻¹ arcsec⁻¹. The dynamical mass within the central 500 pc is ∼4Å‐10⁸ M⊙ and may be about ∼6Å‐10⁷ M⊙ within the central 120 pc. No line was detected in the other galaxies (NGC 520, NGC 1614 and NGC 6946) to a 3σ limit of 300 μJy. In the starburst galaxies in which RRLs have been detected, we find that there is a rough correlation between the integrated H92α line flux density and both the total far-infrared flux density and the radio continuum emission from the central region.     

The European Large Area ISO Survey – III. 90-μm extragalactic source counts

We present results and source counts at 90 μm extracted from the preliminary analysis of the European Large Area ISO Survey (ELAIS). The survey covered about 12 deg² of the sky in four main areas and was carried out with the ISOPHOT instrument onboard the Infrared Space Observatory ( ISO ). The survey is at least an order of magnitude deeper than the IRAS 100-μm survey and is expected to provide constraints on the formation and evolution of galaxies. The majority of the detected sources are associated with galaxies on optical images. In some cases the optical associations are interacting pairs or small groups of galaxies, suggesting that the sample may include a significant fraction of luminous infrared galaxies. The source counts extracted from a reliable subset of the detected sources are in agreement with strongly evolving models of the starburst galaxy population.