Sample Of Normal Galaxies Research Articles

X-ray luminosity-star formation rate scaling relation: Constraints from the eROSITA Final Equatorial Depth Survey (eFEDS)

Aims.We present measurements of the relation between X-ray luminosity and star formation activity for a sample of normal galaxies spanning the redshift range between 0 and 0.25. We use data acquired by SRG/eROSITA for the performance and verification phase program called eROSITA Final Equatorial Depth Survey (eFEDS). The eFEDS galaxies are observed in the 0.2−2.3 keV band.Methods.Making use of a wide range of ancillary data, spanning from the ultraviolet (UV) to mid-infrared wavelengths (MIR), we estimated the star formation rate (SFR) and stellar mass (Mstar) of 888 galaxies, using Code Investigating GALaxy Emission (CIGALE). In order to study sources whose X-ray emission is dominated by X-ray binaries (XRBs), we classified these galaxies into normal galaxies and active galactic nuclei by making use of the observed fluxes in the X-ray, optical, and MIR ranges, as well as the results from the SED fitting. To isolate the contribution of XRBs, which scale with the SFR andMstar, we subtracted the contribution of hot gas, coronally active binaries, and cataclysmic variables to the total X-ray emission. We divided our sample of normal galaxies in star-forming (SFGs) and quiescent galaxies according to their position on the main sequence.Results.We confirm a linear correlation between the X-ray luminosity and the SFR for our sample of SFGs, shown previously in the literature. However, we find this relation to be strongly biased by the completeness limit of the eFEDS survey. Correcting for completeness, we find the fitted relation to be consistent with the literature. We also investigated the relation between X-ray emission from both LMXBs and HMXBs populations withMstarand SFR, respectively. Correcting for completeness, we find our fitted relation to considerably scatter from the literature relation at high specific SFR (SFR/Mstar). We conclude that without accounting for X-ray non-detections, it is not possible to employ eFEDS data to study the redshift evolution of the LMXBs and HMXBs contributions due to completeness issues. Furthermore, we find our sources to largely scatter from the expected Lx/SFR vs. specific SFR relation at high redshift. We discuss the dependence of the scatter on the stellar mass, metallicity, or the globular cluster content of the galaxy.

Revisiting X-Ray-bright Optically Normal Galaxies with the Chandra Source Catalog

X-ray bright optically normal galaxies (XBONGs) are galaxies with X-ray luminosities consistent with those of active galactic nuclei (AGNs) but no evidence of AGN optical emission lines. Crossmatching the Chandra Source Catalog version 2 with the Sloan Digital Sky Survey sample of spectroscopically classified galaxies, we have identified 817 XBONG candidates with L X > 1042 erg s−1 and X-ray to optical flux ratio F XO > 0.1. Comparisons with Wide-field Infrared Survey Explorer colors and near-IR, optical, UV, and radio luminosities show that the loci of XBONGs are in-between those of control samples of normal galaxies and quasars and are consistent with low-luminosity quasars. We find that 43% of the XBONG sample have X-ray colors suggesting N H > 1022 cm−2, double the fraction in the QSO sample, suggesting that a large fraction of XBONG are highly obscured AGNs. However, ∼50% of the XBONGs are not obscured and have X-ray colors harder than those of normal galaxies. Some of these XBONGs have spatially extended X-ray emission. These characteristics suggest that they may be unidentified galaxy groups and clusters. Using the X-ray luminosity functions of QSOs, galaxies, groups and clusters, we estimate the approximate fraction of extended XBONGs to be <20%. We also assess the approximate fraction of XBONGs whose AGN signatures are diluted by stellar light of host galaxies to be ∼30%, based on their redshift and deviation from the extrapolation of the QSO L X–L r relation.

On the stellar kinematics and mass of the Virgo ultradiffuse galaxy VCC 1287

ABSTRACT Here, we present a kinematical analysis of the Virgo cluster ultradiffuse galaxy (UDG) VCC 1287 based on data taken with the Keck Cosmic Web Imager (KCWI). We confirm VCC 1287’s association both with the Virgo cluster and its globular cluster (GC) system, measuring a recessional velocity of 1116 ± 2 km s−1. We measure a stellar velocity dispersion (19 ± 6 km s−1) and infer both a dynamical mass ($1.11^{+0.81}_{-0.81} \times 10^{9} \ \mathrm{M_{\odot }}$) and mass-to-light ratio (M/L) ($13^{+11}_{-11}$) within the half-light radius (4.4 kpc). This places VCC 1287 slightly above the well-established relation for normal galaxies, with a higher M/L for its dynamical mass than normal galaxies. We use our dynamical mass, and an estimate of GC system richness, to place VCC 1287 on the GC number–dynamical mass relation, finding good agreement with a sample of normal galaxies. Based on a total halo mass derived from GC counts, we then infer that VCC 1287 likely resides in a cored or low-concentration dark matter halo. Based on the comparison of our measurements to predictions from simulations, we find that strong stellar feedback and/or tidal effects are plausibly the dominant mechanisms in the formation of VCC 1287. Finally, we compare our measurement of the dynamical mass with those for other UDGs. These dynamical mass estimates suggest relatively massive haloes and a failed galaxy origin for at least some UDGs.

Galaxy pairs in the SDSS – XIII. The connection between enhanced star formation and molecular gas properties in galaxy mergers

We investigate the connection between star formation and molecular gas properties in galaxy mergers at low redshift (z$\leq$0.06). The study we present is based on IRAM 30-m CO(1-0) observations of 11 galaxies with a close companion selected from the Sloan Digital Sky Survey (SDSS). The pairs have mass ratios $\leq$4, projected separations r$_{\mathrm{p}} \leq$30 kpc and velocity separations $\Delta$V$\leq$300 km s$^{-1}$, and have been selected to exhibit enhanced specific star formation rates (sSFR). We calculate molecular gas (H$_{2}$) masses, assigning to each galaxy a physically motivated conversion factor $\alpha_{\mathrm{CO}}$, and we derive molecular gas fractions and depletion times. We compare these quantities with those of isolated galaxies from the extended CO Legacy Data base for the GALEX Arecibo SDSS Survey sample (xCOLDGASS, Saintonge et al. 2017) with gas quantities computed in an identical way. Ours is the first study which directly compares the gas properties of galaxy pairs and those of a control sample of normal galaxies with rigorous control procedures and for which SFR and H$_{2}$ masses have been estimated using the same method. We find that the galaxy pairs have shorter depletion times and an average molecular gas fraction enhancement of 0.4 dex compared to the mass matched control sample drawn from xCOLDGASS. However, the gas masses (and fractions) in galaxy pairs and their depletion times are consistent with those of non-mergers whose SFRs are similarly elevated. We conclude that both external interactions and internal processes may lead to molecular gas enhancement and decreased depletion times.

ON THE NATURE OF SODIUM EXCESS OBJECTS. I. DATA AND OBSERVED TRENDS

Several studies have reported the presence of sodium excess objects that have neutral atomic absorption lines at 5895A (NaD) and 8190A that are deeper than expected based on stellar population models. van Dokkum & Conroy proposed that low-mass stars are more prevalent in massive early-type galaxies, which may lead to a strong NaI8190 line strength. It is, however, necessary to test this prediction against other prominent line indices in optical wavelengths. We newly identified roughly a thousand NaD excess objects (NEOs) based on the NaD line strength in the redshift range 0.00<z<0.08 from the SDSS DR7. The novelty of this work is that galaxies were carefully identified through direct visual inspection of SDSS images, and we systematically compared the properties of NEOs and those of a control sample of normal galaxies. Note that the majority of galaxies with high velocity dispersion (>250km/s) show NaD excess. Most late-type NEOs have strong Hb line strengths and significant emission lines. This implies that the presence of ISM and/or dust contributes to the increase in NaD line strengths observed for these galaxies. In contrast, the majority of early-type NEOs are predominantly luminous and massive systems. However, we find that models used to reproduce the NaI8190 line strengths that adopt a bottom-heavy IMF are not able to reproduce the observed NaD line strengths. By comparing the observed NaD, Mgb and Fe5270 line strengths with those of the models, we identify a plausible range of parameters. In these models, the majority of early-type NEOs are alpha-enhanced ([a/Fe]~0.3), metal-rich ([Z/H]~0.3) and especially Na-enhanced ([Na/Fe]~0.3). Enhanced Na abundance is a particularly compelling hypothesis for the increase in the strength of the NaD line index in our early-type NEOs that appear devoid of dust, both in their SDSS images and spectra.

THE FAR-INFRARED, UV, AND MOLECULAR GAS RELATION IN GALAXIES UP TOz= 2.5

We use the infrared excess (IRX) FIR/UV luminosity ratio to study the relation between the effective UV attenuation (A_IRX) and the UV spectral slope (beta) in a sample of 450 1<z<2.5 galaxies. The FIR data is from very deep Herschel observations in the GOODS fields that allow us to detect galaxies with SFRs typical of galaxies with log(M)>9.3. Thus, we are able to study galaxies on and even below the main SFR-stellar mass relation (main sequence). We find that main sequence galaxies form a tight sequence in the IRX--beta plane, which has a flatter slope than commonly used relations. This slope favors a SMC-like UV extinction curve, though the interpretation is model dependent. The scatter in the IRX-beta plane, correlates with the position of the galaxies in the SFR-M plane. Using a smaller sample of galaxies with CO gas masses, we study the relation between the UV attenuation and the molecular gas content. We find a very tight relation between the scatter in the IRX-beta plane and the specific attenuation (S_A), a quantity that represents the attenuation contributed by the molecular gas mass per young star. S_A is sensitive to both the geometrical arrangement of stars and dust, and to the compactness of the star forming regions. We use this empirical relation to derive a method for estimating molecular gas masses using only widely available integrated rest-frame UV and FIR photometry. The method produces gas masses with an accuracy between 0.12-0.16 dex in samples of normal galaxies between z~0 and z~1.5. Major mergers and sub-millimeter galaxies follow a different S_A relation.

INFRARED PROPERTIES OF A COMPLETE SAMPLE OF STAR-FORMING DWARF GALAXIES

We present a study of a large, statistically complete sample of star-forming dwarf galaxies using mid-infrared observations from the {\it Spitzer Space Telescope}. The relationships between metallicity, star formation rate (SFR) and mid-infrared color in these systems show that the galaxies span a wide range of properties. However, the galaxies do show a deficit of 8.0 \um\ polycyclic aromatic hydrocarbon emission as is apparent from the median 8.0 \um\ luminosity which is only 0.004 \lstarf\ while the median $B$-band luminosity is 0.05 \lstarb. Despite many of the galaxies being 8.0 \um\ deficient, there is about a factor of 4 more extremely red galaxies in the [3.6] $-$ [8.0] color than for a sample of normal galaxies with similar optical colors. We show correlations between the [3.6] $-$ [8.0] color and luminosity, metallicity, and to a lesser extent SFRs that were not evident in the original, smaller sample studied previously. The luminosity--metallicity relation has a flatter slope for dwarf galaxies as has been indicated by previous work. We also show a relationship between the 8.0 \um\ luminosity and the metallicity of the galaxy which is not expected given the competing effects (stellar mass, stellar population age, and the hardness of the radiation field) that influence the 8.0 \um\ emission. This larger sample plus a well-defined selection function also allows us to compute the 8.0 \um\ luminosity function and compare it with the one for the local galaxy population. Our results show that below 10$^{9}$ $L$\solar, nearly all the 8.0 \um\ luminosity density of the local universe arises from dwarf galaxies that exhibit strong \ha\ emission -- i.e., 8.0 \um\ and \ha\ selection identify similar galaxy populations despite the deficit of 8.0 \um\ emission observed in these dwarfs.

CARS: The CFHTLS-Archive-Research Survey

Weak gravitational lensing (WL) has been established as one of the most promising probes of cosmology. So far, most studies have exploited the shear effect of WL leading to coherent distortions of galaxy shapes. But WL also introduces coherent magnifications. We want to detect this cosmic magnification effect in large samples of high-redshift galaxies selected from the Deep part of the CFHTLS. Lyman-break galaxies (LBGs) at z=2.5-5, are used as a background sample and are cross-correlated to foreground lens galaxies selected by photo-z's. The signals of LBGs in different magnitude bins are compared to predictions from WL theory. An optimally weighted correlation function is estimated by taking into account the slope of external LBG luminosity functions. For the first time, we detect cosmic magnification in a sample of normal galaxies. These background sources are also the ones with the highest redshifts so far used for WL measurements. The amplitude and angular dependence of the cross-correlation functions agree well with theoretical expectations and the lensing signal is detected with high significance. In particular, we detect an anti-correlation between faint LBGs and foreground galaxies which cannot be caused by redshift overlap. (abridged)

Normal galaxies in the all-sky survey by the eROSITA X-ray telescope of the Spectrum-X-Gamma observatory

We analyze the statistical properties of normal galaxies to be detected in the all-sky survey by the eROSITA X-ray telescope of the Spectrum-X-Gamma observatory. With the current configuration and parameters of the eROSITA telescope, the sensitivity of a 4-year-long all-sky survey will be ≈10−14 erg s−1 in the 0.5–2 keV band. This will allow ∼(1.5–2) × 104 normal galaxies with approximately the same contribution of star-forming and elliptical galaxies to be detected. All galaxies of the X-ray survey are expected to enter into the existing far-infrared (IRAS) or near-infrared (2MASS) catalogs; the sample of star-forming galaxies will be approximately equivalent in sensitivity to the sample of star-forming galaxies in the IRAS catalog of infrared sources. Thus, a large homogeneous sample of normal galaxies with measured X-ray, near-infrared, and far-infrared fluxes will be formed. About 90% of the galaxies in the survey are located within ∼200–400 Mpc. A typical (most probable) galaxy will have a luminosity log L X ∼ 40.5–41.0, will be located at a distance of ∼70–90 Mpc, and will be either a star-forming galaxy with a star formation rate of ∼20M ⊙ yr−1 whose X-ray emission is produced by ultraluminous X-ray sources (ULXs) or an elliptical galaxy with amass log M * ∼ 11.3 emitting through to a hot interstellar gas. The galaxies within 35 Mpc will collectively contain ∼102 ULXs with luminosities log L X > 40, ∼80% of whichwill be the only luminous source in the galaxy. Thus, although the angular resolution of the eROSITA telescope is too low for the luminosity function of compact sources in galaxies to be studied in detail, the survey data will allow one to investigate its bright end and, possibly, to impose constraints on the maximum luminosity of ULXs.

E+A and companion galaxies - I. A catalogue and statistics

Based on our intensive spectroscopic campaign with the GoldCam spectrograph on the Kitt Peak National Observatory (KPNO) 2.1-m telescope, we have constructed the first catalogue of E+A galaxies with spectroscopic companion galaxies, and investigated a probability that an E+A galaxy has close companion galaxies. We selected 660 E+A galaxies with 4.0 A < Hδ EW at a redshift of <0.167 from the Data Release 5 of the Sloan Digital Sky Survey (SDSS). We selected their companion candidates from the SDSS imaging data, and classified them into true companions, fore/background galaxies and companion candidates using the SDSS and our KPNO spectra. We observed 26 companion candidates of E+A galaxies at the KPNO to measure their redshifts. Their spectra showed that 17 targets are true companion galaxies. The number of spectroscopically confirmed E+A's companions is now 34. This becomes the first catalogue of E+A galaxies with spectroscopic companion systems. We found that E+A galaxies have 54 per cent larger probability of having companion galaxies (7.88 per cent) as compared to the comparison sample of normal galaxies (5.12 per cent). A statistical test shows that the probabilities are different with 99.7 per cent significance. Our results based on spectroscopy tighten the connection between the dynamical merger/interaction and the origin of E+A galaxies.

The North Galactic Pole +30° Zone Galaxies. I. A Comparative Study of Galaxies with Different Nuclear Activity

A database containing 618 and star-forming (A/SF) galaxies and 564 galaxies in a 120° × 6° wide strip crossing the north Galactic pole was constructed in order to compare the global properties of active galaxies against a control sample of normal galaxies. This database combines a literature and catalog search with new optical measurements from the Fpg (red) and Jpg (blue) band images of the STScI Digitized Sky Survey (DSS). We provide alternative names, accurate coordinates, morphological type, activity classes, red and blue apparent magnitudes, 2MASS near-infrared J–H and H–K colors, apparent diameters, axial ratios, position angles, and number counts of neighboring objects in a circle of radius 50 kpc. We also present an atlas of 103 interacting and merging systems among these galaxies. The integrated properties of A/SF and galaxies in this sample are compared using a multivariate factor analysis, which reveals that A/SF galaxies are objects with relatively late morphological types, and are more inclined and have bluer optical colors than galaxies. In this sample, all merging and interacting galaxies are A/SF objects. Star-forming galaxies are objects with relatively late morphological types, lower absolute luminosities and linear sizes, bluer colors, and higher inclination than sample X-ray or radio sources, as well as Seyfert galaxies. The near-infrared colors of the sample galaxies are independent parameters and do not correlate with activity level or any other parameter of the galaxies.

Active and Star-forming Galaxies and Their Supernovae

To investigate the extent to which nuclear starbursts or other nuclear activity may be connected with enhanced star formation activity in the host galaxy, we perform a statistical investigation of supernovae (SNe) discovered in host galaxies from four samples: the Markarian galaxies sample, the Second Byurakan Survey (SBS) sample, the north Galactic pole (NGP) sample of active or star-forming galaxies, and the NGP sample of normal galaxies. Forty-seven SNe in 41 Mrk galaxies, 10 SNe in six SBS galaxies, 29 SNe in 26 NGP active or star-forming galaxies, and 29 SNe in 26 NGP normal galaxies have been studied. We find that the rate of SNe, particularly core-collapse (Types Ib/c and II) SNe, is higher in active or star-forming galaxies in comparison with normal galaxies. Active or star-forming host galaxies of SNe are generally of later morphological type and have lower luminosity and smaller linear size than normal host galaxies of SNe. The radial distribution of SNe in active and star-forming galaxies shows a higher concentration toward the center of the active host galaxy than is the case for normal host galaxies, and this effect is more pronounced for core-collapse SNe. Ib/c-type SNe have been discovered only in active and star-forming galaxies of our samples. About 78% of these SNe are associated with H II regions or are located very close to the nuclear regions of these active galaxies, which are in turn hosting AGNs or starburst nuclei. Besides these new results, our study also supports the conclusions of several other earlier papers. We find that Type Ia SNe occur in all galaxy types, whereas core-collapse SNe of Types Ib/c and II are found only in spiral and irregular galaxies. The radial distribution of Type Ib SNe in their host galaxies is more centrally concentrated than that of Type II and Ia SNe. The radial distances of Types Ib/c and II SNe, from the nuclei of their host galaxies, is larger for barred spiral hosts. Core-collapse SNe are concentrated in spiral arms and are often close to or in the H II regions, whereas Type Ia SNe show only a loose association with spiral arms and no clear association with H II regions.

Chandra-SDSS Normal and Star-Forming Galaxies. I. X-Ray Source Properties of Galaxies Detected by theChandra X-Ray Observatoryin SDSS DR2

We have cross-correlated X-ray catalogs derived from archival Chandra X-Ray Observatory ACIS observations with a Sloan Digital Sky Survey Data Release 2 (DR2) galaxy catalog to form a sample of 42 serendipitously X-ray-detected galaxies over the redshift interval 0.03 < z < 0.25. This pilot study will help fill in the redshift gap between local X-ray-studied samples of normal galaxies and those in the deepest X-ray surveys. Our chief purpose is to compare optical spectroscopic diagnostics of activity (both star formation and accretion) with X-ray properties of galaxies. Our work supports a normalization value of the X-ray-star formation rate correlation consistent with the lower values published in the literature. The difference is in the allocation of X-ray emission to high-mass X-ray binaries relative to other components, such as hot gas, low-mass X-ray binaries, and/or active galactic nuclei (AGNs). We are able to quantify a few pitfalls in the use of lower resolution, lower signal-to-noise ratio optical spectroscopy to identify X-ray sources (as has necessarily been employed for many X-ray surveys). Notably, we find a few AGNs that likely would have been misidentified as non-AGN sources in higher redshift studies. However, we do not find any X-ray-hard, highly X-ray-luminous galaxies lacking optical spectroscopic diagnostics of AGN activity. Such sources are members of the X-ray-bright, optically normal galaxy (XBONG) class of AGNs.

Principal component analysis ofInternational Ultraviolet Explorergalaxy spectra

We analyse the UV spectral energy distribution of a sample of normal galaxies listed in the International Ultraviolet Explorer (IUE) Newly Extracted Spectra (INES) Guide No. 2 – Normal Galaxies using a principal component analysis. The sample consists of the IUE short-wavelength (SW) spectra of the central regions of 118 galaxies, where the IUE aperture included more than 1 per cent of the galaxy size. The principal components are associated with the main components observed in the ultraviolet (UV) spectra of galaxies. The first component, accounting for the largest source of diversity, may be associated with the UV continuum emission. The second component represents the UV contribution of an underlying evolved stellar population. The third component is sensitive to the amount of activity in the central regions of galaxies and measures the strength of star-formation events. In all the samples analysed here, the principal component representative of star-forming activity accounts for a significant percentage of the variance. The fractional contribution to the spectral energy distribution (SED) by the evolved stars and by the young population are similar. Projecting the SEDs on to their eigenspectra, we find that none of the coefficients of the principal components can outline an internal correlation or can correlate with the optical morphological types. In a subsample of 43 galaxies, consisting of almost only compact and BCD galaxies, the third principal component defines a sequence related to the degree of starburst activity of the galaxy.

Structure and Evolution of Starburst and Normal Galaxies

A comparative study of the rest-frame morphology and structural properties of optically selected starburst galaxies at redshift z 1 is carried out using multi-wave-band (BViz) high-resolution images taken by the Advanced Camera for Surveys as part of the Great Observatories Origins Deep Survey. We classify galaxies into starburst, early, and late types by comparing their observed spectral energy distributions with local templates. We find that early-type systems have significantly higher rest-frame B-band concentration indices and active galacic nucleus (AGN) fraction (>25 %) than late-type spirals and optically selected starbursts. These results are consistent with the scenario that early-epoch (z 1) gas-rich dissipative processes (e.g., major mergers) have played an important role in developing large central concentrations in early-type E/Sa galaxies and that a concurrent growth of central black holes and bulges occurs in some of these early merger events. The lower AGN fraction and concentration indices in the majority of the optically selected starbursts at z 1 suggest that either the starbursts and early types are different in nature (being, respectively, disk and bulge dominated) and/or they are in different evolutionary phases, such that some of the starbursts in major mergers evolve into early types as the dynamical phase of the merger evolves and the spectral signature of the starburst fades out. The starbursts have, on average, larger asymmetries than our control sample of normal galaxies in both rest-frame B and R bands, suggesting that a significant fraction of the starburst activity is tidally triggered.

The Globular Cluster Systems of the Early-Type Galaxies NGC 3379, NGC 4406, and NGC 4594 and Implications for Galaxy Formation

We have investigated the global properties of the globular cluster (GC) systems of three early-type galaxies: the Virgo Cluster elliptical NGC 4406, the field elliptical NGC 3379, and the field S0 galaxy NGC 4594. These galaxies were observed as part of a wide-field CCD survey of the GC populations of a large sample of normal galaxies beyond the Local Group. Images obtained with the Mosaic detector on the Kitt Peak 4 m telescope provide radial coverage to at least 24', or ~70–100 kpc. We use BVR photometry and image classification to select GC candidates and thereby reduce contamination from non-GCs, as well as Hubble Space Telescope WFPC2 data to help quantify the contamination that remains. The GC systems of all three galaxies have color distributions with at least two peaks and show modest negative color gradients. The proportions of blue GCs range from 60% to 70% of the total populations. The GC specific frequency (SN) of NGC 4406 is 3.5 ± 0.5, ~20% lower than past estimates and nearly identical to SN for the other Virgo Cluster elliptical included in our survey, NGC 4472. SN for NGC 3379 and NGC 4594 are 1.2 ± 0.3 and 2.1 ± 0.3, respectively; these are similar to past values, but the errors have been reduced by a factor of 2–3. We compare our results for the early-type sample (including NGC 4472) to models for the formation of massive galaxies and their GC systems. Of the scenarios we consider, a hierarchical merging picture, in which metal-poor GCs form at high redshift in protogalactic building blocks and metal-rich GC populations are built up over time during subsequent gas-rich mergers, appears most consistent with the data.

ITAL]ISO[/ITAL] LWS Observations of the Two Nearby Spiral Galaxies NGC 6946 and NGC 1313

We present the analysis of Infrared Space Observatory long-wavelength spectrometer (LWS) observations of the two nearby late-type galaxies NGC 1313 and NGC 6946. Both galaxies have been fully mapped in the [C II] far-infrared fine-structure line at 158 μm, and some regions have been observed also in the [O I] 63 μm and [N II] 122 μm lines. We use these observations to derive the physical properties of the atomic interstellar medium, to establish how they relate to other interstellar medium components (gas and dust), and to establish how they vary with different galaxy components such as nucleus, spiral arms, and disk. The [C II] line is the main cooling line of the atomic medium. In NGC 6946 and NGC 1313, its emission represents 0.8% of the infrared emission. Moreover, the [C II] emission can be spatially associated with three components: the nucleus, the star-forming regions in spiral arms, and the diffuse galaxy disk. This last component contributes 40% in NGC 6946 and ~30% in NGC 1313 to the total emission. We apply the photodissociation region (PDR) model by Kaufman et al. to derive PDR physical parameters responsible for the neutral atomic gas emission (G0, n, and Ts). The results do not significantly differ from what Malhotra et al. recently found by modeling the integrated emission of a sample of 60 normal galaxies. This suggests that the emission in each region under the LWS beam in NGC 6946 and NGC 1313 (corresponding to a linear size of ~1.5 kpc) is likely to arise from a mixture of components similar to the mixture producing the integrated emission of normal galaxies. However, some regions in NGC 6946 have a G0/n ratio ~2–3 times smaller than the mean value found for the normal galaxy sample (1.3), suggesting that the beam-averaged contribution of a less active component in these regions is higher than its contribution in the integrated emission of normal galaxies or, conversely, that the bulk of the integrated emission of the normal galaxies is dominated by a few active regions probably located in their nuclei. CO(1–0) and [C II] in NGC 6946 are well correlated, and the mean [C II]/CO ratio agrees with the mean integrated ratios of the normal galaxies sample. This value (~500) is a factor of ~2 less than the mean ratio found for a sample of normal galaxies observed with the Kupier Airborne Observatory (KAO) by Stacey et al. This difference is probably due to the fact that the KAO beam (55'') is smaller than the LWS beam (75''), such that the Stacey et al. KAO observations are likely to be more biased toward the nucleus of the galaxies and therefore toward more active regions. In NGC 1313 only four LWS regions have been observed in CO(1–0), and three of them were detected. The [C II]/CO(1–0) seems to systematically increase from the northeast to the south, along the S-shaped spiral arm, indicating that the interstellar medium conditions in NGC 1313 are much more inhomogeneous than the conditions in NGC 6946. H I and [C II] in NGC 6946 are completely uncorrelated, probably because they arise from different gas components: [C II] arises principally in dense and warm PDR and H I from diffuse (n 3 × 103 cm-3) gas. On the other hand, in NGC 1313 we successfully detect two distinct gas components: a cirrus-like component, for which H I and [C II] are weakly correlated as observed in our Galaxy, and a component associated with dense PDRs completely uncorrelated from H I as observed in NGC 6946. Finally, we find that the H I residing in dense gas surrounding the star-forming regions and presumably recently photodissociated constitutes a few percent of the total H I. In turn, this dense gas component produces most of the [C II] emission emitted by the atomic neutral medium, even if its contribution is lower in NGC 1313 than in NGC 6946. On the other hand, the [C II] emission arising from ionized gas is higher in NGC 1313 than in NGC 6946.

A morphological comparison between the central region in AGN and normal galaxies using HST data

We study the morphology of the central region of a sample of Active Galactic Nuclei (AGN) and a ``control'' sample of normal galaxies using archival observations of the WFPC2 instrument onboard the Hubble Space Telescope (HST). We use the ellipse fitting technique in order to get a good description of the inner ``smooth'' light distribution of the galaxy. We then divide the observed galaxy image by the artificial image from the fitted ellipses in order to detect morphological signatures in the central region around the nucleus of the galaxy. We perform quantitative comparisons of different subgroups of our sample of galaxies (according to the Hubble type and the nuclear activity of the galaxies) by calculating the average amplitude of the structures that are revealed with the ellipse fitting technique. Our main conclusions are as follows: 1) All AGNs show significant structure in their inner 100 pc and 1 kpc regions whose amplitude is similar in all of them, independent of the Hubble type of the host galaxy. 2) When considering early-type galaxies, non-AGN galaxies show no structure at all, contrary to what we find for AGN. 3) When considering late-type galaxies, both AGN and non-AGN galaxies show significant structure in their central region. Our results are consistent with the hypothesis that all early-type galaxies host a supermassive black hole, but only those that have enough material in the central regions to fuel it show an active nucleus. The situation is more complicated in late-type galaxies. Either not all of them host a central black hole, or, in some of them, the material inside the innermost 100 pc region is not transported to the scales of the central engine for some reason, or the large amount of gas and dust hides the active nucleus from our sight.

The gas content of peculiar galaxies: Counterrotators and polar rings

This paper studies the global ISM content in a sample of 104 accreting galaxies, including counterrotators and polar rings, which spans the entire Hubble sequence. The molecular, atomic and hot gas content of accretors is compared to a newly compiled sample of normal galaxies. We present results of a small survey of the line of 12CO with the 15 m SEST telescope on a sample of 11 accretors (10 counterrotators and 1 polar ring). The SEST sample is enlarged with published data from 48 galaxies, for which observational evidence of counterrotation in the gas and/or the stars has been found. Furthermore, the available data on a sample of 46 polar ring galaxies has been compiled. In order to explore the existence of an evolutionary path linking the two families of accretors, the gas content of counterrotators and polar rings is compared. It was found that the normalized content of cold gas () in polar rings is ~1 order of magnitude higher than the reference value derived for normal galaxies. The inferred gas masses are sufficient to stabilize polar rings through self-gravity. In contrast, it was found that the cold gas content of counterrotators is close to normal for all galaxy types. Although counterrotators and polar rings probably share a common origin, the gas masses estimated here confirm that light gas rings accreted by future counterrotators may have evolved faster than the self-gravitating structures of polar rings. In this scenario, the transformation of atomic into molecular gas could be enhanced near the transition region between the prograde and the retrograde disks, especially in late-type accretors characterized by a high content of primordial gas. This is tentatively confirmed in this work: the measured H2/HI ratio seems larger in counterrotators than in normal or polar ring galaxies for types later than S0s.

BVRI imaging of M51-type interacting galaxy pairs -- III. Analysis of the photometric parameters

Photometric bulge and disc properties of a sample of 21 M51-type pairs observed in BVRI bands are analysed, and the derived parameters obtained by the R1/4-law and exponential bulge models are compared with the sample of normal galaxies reported by de Jong & van der Kruit in 1994. Some characteristic properties of the intensity profiles are then interpreted by 3D N-body simulations, in which both components of the pair are described by an exponential star + gas disc embedded in a live bulge and halo. The disc central surface brightness μ0 was found to be similar for the M51-type and for normal galaxies, being 21.5 ± 0.8B mag arcsec−2, when the exponential bulge model was used. The main galaxies had generally almost normal photometric properties, whereas for the companions secular evolution must have played an important role. Even though the mean scalelengths for the M51-type galaxies were smaller than for the comparison galaxies, their bulges were larger: especially the companions had extremely large bulge sizes relative to their disc scalelengths having 〈re(bulge)/h〉(B) = 0.37 ± 0.10 in comparison with 0.15 ± 0.09 for normal galaxies. Consequently, the bulge-to-disc luminosity ratios for the companions were also generally larger than known for any of the Hubble types of normal galaxy. N-body simulations indicated that the inner disc can be steepened during the interaction, and that this steepened disc can be erroneously interpreted as belonging to the bulge. Simultaneously material in the outer disc is redistributed, producing shallow outer profiles similar to those observed for typical M51-type galaxies. These processes were demonstrated by applying photometric bulge-to-disc decompositions for some of the simulated intensity profiles. The decompositions also showed that one must be cautious when applying the R1/4-law bulge model, especially for interacting galaxies. A nearly parabolic experiment showed that the redistribution of matter in the disc during the close passage can be long-lived, suggesting that galaxy interactions even with small companions may play an important role in secular evolution for galaxies in clusters.