The structure of spiral galaxies — II. Near-infrared properties of spiral arms

We have imaged a sample of 45 face-on spiral galaxies in the K band, to determine the morphology of the old stellar population, which dominates the mass in the disc. The K -band images of the spiral galaxies have been used to calculate different characteristics of the underlying density perturbation such as arm strengths, profiles and cross-sections, and spiral pitch angles. Contrary to expectations, no correlation was found between arm pitch angle and Hubble type, and combined with previous results this leads us to conclude that the morphology of the old stellar population bears little resemblance to the optical morphology used to classify galaxies. The arm properties of our galaxies seem inconsistent with predictions from the simplest density wave theories, and some observations, such as variations in pitch angle within galaxies, seem hard to reconcile even with more complex modal theories. Bars have no detectable effect on arm strengths for the present sample. We have also obtained B -band images of three of the galaxies. For these galaxies we have measured arm cross-sections and strengths, to investigate the effects of disc density perturbations on star formation in spiral discs. We find that B -band arms lead K -band arms and are narrower than K -band arms, apparently supporting predictions made by the large-scale shock scenario, although the effects of dust on B -band images may contribute towards these results.     

The structure of spiral galaxies — I. Near-infrared properties of bulges, discs and bars

We present data for a sample of 45 spiral galaxies over a range of Hubble types, imaged in the near-IR J K bands. Parameters are calculated describing the bulge, disc and bar K -band light distributions, and we look for correlations showing the interrelation between these components. We find that bulge profiles are not well-fitted by the classic de Vaucouleurs profile, and that exponential or R ^1/2 fits are preferred. The bulge-to-disc ratio correlates only weakly with Hubble type. Many of the galaxies show central reddening of their J  −  K colours, which we interpret as due to nuclear starbursts or dusty AGN. We define a new method for measuring the strength of bars, which we call 'equivalent angle'. We stress that this is better than the traditional bar–interbar contrast, as it is not subject to seeing and resolution effects. Bars are found in 40 of the 45 galaxies, nine of which had been previously classified as unbarred. Bar strengths are found not to correlate with disc surface brightness or the presence of near neighbours, but a tendency is found for the most strongly barred galaxies to lie within a restricted, intermediate range of bulge-to-disc ratio. Bar light profiles are found to be either flat or exponentially decreasing along their long axes, with profile type not correlating strongly with Hubble type. Bar short axis profiles are significantly asymmetric, with the steeper profile being generally on the leading edge, assuming trailing arms. In the K band we find bars with higher axial ratios than have been found previously in optical studies.     

A new formula describing the scaffold structure of spiral galaxies

We describe a new formula capable of quantitatively characterizing the Hubble sequence of spiral galaxies including grand design and barred spirals. Special shapes such as ring galaxies with inward and outward arms are also described by the analytic continuation of the same formula. The formula is   r (φ) = A /log [ B tan   (φ/2 N )]  . This function intrinsically generates a bar in a continuous, fixed relationship relative to an arm of arbitrary winding sweep. A is simply a scale parameter while B , together with N , determines the spiral pitch. Roughly, greater N results in tighter winding. Greater B results in greater arm sweep and smaller bar/bulge, while smaller B fits larger bar/bulge with a sharper bar/arm junction. Thus B controls the 'bar/bulge-to-arm' size, while N controls the tightness much like the Hubble scheme. The formula can be recast in a form dependent only on a unique point of turnover angle of pitch – essentially a one-parameter fit, aside from a scalefactor. The recast formula is remarkable and unique in that a single parameter can define a spiral shape with either constant or variable pitch capable of tightly fitting Hubble types from grand design spirals to late-type large barred galaxies. We compare the correlation of our pitch parameter to Hubble type with that of the traditional logarithmic spiral for 21 well-shaped galaxies. The pitch parameter of our formula produces a very tight correlation with ideal Hubble type suggesting it is a good discriminator compared to logarithmic pitch, which shows poor correlation here similar to previous works. Representative examples of fitted galaxies are shown.     

Near-infrared adaptive optics imaging of nuclear spiral structure in the Seyfert galaxy NGC 3227

We present high spatial resolution, near-infrared images in J , H and K of the nucleus of NGC 3227, obtained with the Adaptive Optics Bonnette on the Canada–France–Hawaii Telescope. The ∼0.15 arcsec (17 pc) resolution allows structures to be probed in the core region. Dust obscuration becomes significantly less pronounced at longer wavelengths, revealing the true geometry of the core region. We are able to identify two main features in our maps: (i) a spiralling association of knots with a counterpart in a Hubble Space Telescope F606W image; and (ii) a smaller-scale annulus, orthogonal to the spiral of knots. These features may provide a means to transport material inwards to fuel the active nucleus.     

Dust-penetrated arm classes: insights from rising and falling rotation curves

In the last decade, near-infrared imaging has highlighted the decoupling of gaseous and old stellar discs: the morphologies of optical (Population I) tracers compared to the old stellar disc morphology, can be radically different. Galaxies which appear multi-armed and even flocculent in the optical may show significant grand-design spirals in the near-infrared. Furthermore, the optically determined Hubble classification scheme does not provide a sound way of classifying dust-penetrated stellar discs: spiral arm pitch angles (when measured in the near-infrared) do not correlate with Hubble type. The dust-penetrated classification scheme of Block & Puerari provides an alternative classification based on near-infrared morphology, which is thus more closely linked to the dominant stellar mass component. Here we present near-infrared K -band images of 14 galaxies, on which we have performed a Fourier analysis of the spiral structure in order to determine their near-infrared pitch angles and dust-penetrated arm classes. We have also used the rotation curve data of Mathewson et al. to calculate the rates of shear in the stellar discs of these galaxies. We find a correlation between near-infrared pitch angle and rate of shear: galaxies with wide open arms (the γ class) are found to have rising rotation curves, while those with falling rotation curves belong to the tightly wound α bin. The major determinant of near-infrared spiral arm pitch angle is the distribution of matter within the galaxy concerned. The correlation reported in this study provides the physical basis underpinning spiral arm classes in the dust-penetrated regime and underscores earlier spectroscopic findings by Burstein and Rubin that Hubble type and mass distributions are unrelated.     

Strong gravitational lensing by spiral galaxies

We investigate gravitational lensing using a realistic model of disc galaxies. Most of the mass is contained in a large spherical isothermal dark matter halo, but the potential is modified significantly in the core by a gravitationally dominant exponential disc. The method used is adapted from a very general multilens ray-tracing technique developed by Mo¨ller. We investigate the effects of the disc-to-halo mass ratio, the disc scalelength, the disc inclination to the line of sight and the lens redshift on two strong-lensing cross-sections: the cross-section for multiple imaging and the cross-section for large magnifications, in excess of a factor of 10. We find that the multiple-imaging cross-section can be enhanced significantly by an almost edge-on Milky Way disc compared with a singular isothermal sphere (SIS) in individual cases; however, when averaged over all disc inclinations, the cross-section is only increased by about 50 per cent. These results are consistent with other recent work. The presence of a disc, however, increases the inclination-averaged high-magnification cross-section by an order of magnitude compared with a SIS. This result has important implications for magnification bias in future lens surveys, particularly those in the submillimetre waveband, where dust extinction in the lensing galaxy has no effect on the brightness of the images.     

ROSAT PSPC observations of nearby spiral galaxies – II. Statistical properties

We present a statistical analysis of the largest X-ray survey of nearby spiral galaxies in which diffuse emission has been separated from discrete source contributions. Regression and rank-order correlation analyses are used to compare X-ray properties, such as total, source and diffuse luminosities and diffuse emission temperature, with a variety of physical and multiwavelength properties, such as galaxy mass, type and activity, and optical and infrared luminosity.
The results are discussed in terms of the way in which hot gas and discrete X-ray sources scale with the mass and activity of galaxies, and with the star formation rate. We find that the X-ray properties of starburst galaxies are dependent primarily on their star-forming activity, whilst for more quiescent galaxies, galaxy mass is the more important parameter. One of the most intriguing results is the tight linear scaling between far-infrared and diffuse X-ray luminosity across the sample, even though the hot gas changes from a hydrostatic corona to a free wind across the activity range sampled here.     

New estimates of scale heights and spiral structures for non-edge-on spiral galaxies

On the basis of Poisson＇s equation for the logarithmic perturbation of matter density, we provide improved estimates of scale heights and spiral structures for non-edge-on spiral galaxies by subtracting the surface brightness distributions from observed images. As examples, the non-edge-on spiral galaxies PGC 24996, which is face-on, and M31, which is inclined, are studied. The scale height, pitch angle and inclination angle of M31, our nearest neighbor, that are presented in this work, agree well with previous research.     

Spectroscopic constraints on the stellar population of elliptical galaxies in the Coma cluster

Near-infrared spectra for a sample of 31 elliptical galaxies in the Coma cluster are obtained. The galaxies are selected to be ellipticals (no lenticulars), with a large spatial distribution, covering both the core and outskirts of the cluster (i.e. corresponding to regions with large density contrasts). CO_sp (2.3-μm) absorption indices, measuring the contribution from intermediate-age red giant and supergiant stars to the near-infrared light of the ellipticals, are then estimated.
It is found that the strength of CO_sp features in elliptical galaxies increases from the core ( r <02) to the outskirts ( r >02) of the Coma cluster. Using the Mg₂ strengths, it is shown that the observed effect is not caused by metallicity and is mostly caused by the presence of a younger population (giant and supergiant stars) in ellipticals in the outskirts (low-density region) of the cluster.
Using the CO_sp features, the origin of the scatter on the near-infrared Fundamental Plane (the relation between the effective diameter, effective surface brightness and velocity dispersion) of elliptical galaxies is studied. Correcting this relation for contributions from the red giant and supergiant stars, the rms scatter reduces from 0.077 to 0.073 dex. Although measurable, the contribution from these intermediate-age stars to the scatter on the near-infrared Fundamental Plane of ellipticals is only marginal.
A relation is found between the CO_sp and V − K colours of ellipticals, corresponding to a slope of 0.036±0.016, significantly shallower than that from the Mg₂–( V − K ) relation. This is studied using stellar synthesis models.     

The scatter in the near-infrared colour–magnitude relation in spiral galaxies

We have determined a dust-free colour–magnitude (CM) relation for spiral galaxies, by using I  −  K colours in edge-on galaxies above the plane. We find that the scatter in this relation is small and approximately as large as can be explained by observational uncertainties. The slope of the near-IR CM relation is steeper for spirals than for elliptical galaxies. We suggest two possible explanations. First, the difference could be caused by vertical colour gradients in spiral galaxies. In that case these gradients should be similar for all galaxies, on average ∼0.15 dex in [Fe/H] per scaleheight, and should increase for later galaxy types. The most likely explanation, however, is that spirals and ellipticals have intrinsically different CM relations. This means that the stars in spirals are younger than those in ellipticals. The age, however, or the fraction of young stars in spiral galaxies would be determined solely by the luminosity of the galaxy, and not by its environment.     

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.