Companion Galaxy Research Articles

Low-redshift quasars in the SDSS Stripe 82: associated companion galaxies and signature of star formation

We obtained optical spectroscopy of close (< 80 kpc) companion objects of a sample of 12 low redshift quasars (z < 0.3 ) selected from the SDSS Stripe82 area and that are in the subsample of 52 QSOs for which both multicolor host galaxies properties and galaxy environment was recently investigated in detail. We found that for 8 out of 12 sources the companion galaxy is associated to the QSO having a difference of radial velocity that is less than 400 km/s. Many of these associated companions exhibit [OII] $\lambda$3727 \AA~ emission lines suggestive of episodes of (recent) star formation possibly induced by past interactions. The SFR of the companion galaxies as derived from [OII] line luminosity is, however, modest, with a median value of 1.0 +-0.8 M_sun/yr, and the emission lines are barely consistent with expectation from gas ionization by the QSO. The role of the QSO for inducing star formation in close companion galaxies appears meager. For three objects we also detect the starlight spectrum of the QSO host galaxy which is characterized by absorption lines of old stellar population and [OII] emission line.

On the redshifts of the BL Lac 3FGL J0909.0+2310 and its close companion

We report on the redshift of the BL Lac object 3FGL J0909.0+2310 based on observations obtained with the OSIRIS Multi Object Spectrograph (MOS) mounted on the 10.4-m Gran Telescopio Canarias. A redshift of 0.432+/-0.002 was obtained by the identification of three absorption features (CaII K&H and G-band) detected in the spectrum of the BL Lac host galaxy. The closest object to the BL Lac at an angular separation of 3.8" (~21 kpc at this distance) has a similar redshift of 0.431+/-0.002. This companion galaxy could be the most likely cause of the nuclear activity as postulated by studies based on more extended data sets and cosmological models. MOS allows us to study the object's neighbourhood within a field of view of approximately 7'x2' and we find two small groups of galaxies at redshifts 0.28 and 0.39 which are probably not related to the activity of 3FGL~J0909.0+2310.

MOLECULAR GAS ALONG A BRIGHT Hα FILAMENT IN 2A 0335+096 REVEALED BY ALMA

ABSTRACT We present ALMA CO(1–0) and CO(3–2) observations of the brightest cluster galaxy (BCG) in the 2A 0335+096 galaxy cluster (z = 0.0346). The total molecular gas mass of 1.13 ± 0.15 × 109 M ⊙ is divided into two components: a nuclear region and a 7 kpc long dusty filament. The central molecular gas component accounts for 3.2 ± 0.4 × 108 M ⊙ of the total supply of cold gas. Instead of forming a rotationally supported ring or disk, it is composed of two distinct, blueshifted clumps south of the nucleus and a series of low-significance redshifted clumps extending toward a nearby companion galaxy. The velocity of the redshifted clouds increases with radius to a value consistent with the companion galaxy, suggesting that an interaction between these galaxies &lt;20 Myr ago disrupted a pre-existing molecular gas reservoir within the BCG. Most of the molecular gas, 7.8 ± 0.9 × 108 M ⊙, is located in the filament. The CO emission is co-spatial with a 104 K emission-line nebula and soft X-rays from 0.5 keV gas, indicating that the molecular gas has cooled out of the intracluster medium over a period of 25–100 Myr. The filament trails an X-ray cavity, suggesting that the gas has cooled from low-entropy gas that has been lifted out of the cluster core and become thermally unstable. We are unable to distinguish between inflow and outflow along the filament with the present data. Cloud velocities along the filament are consistent with gravitational free-fall near the plane of the sky, although their increasing blueshifts with radius are consistent with outflow.

From galaxy-scale fueling to nuclear-scale feedback

Powerful radio galaxies are often associated with gas-rich galaxy mergers. These mergers may provide the fuel to trigger starburst and active galactic nuclear (AGN) activity. In this Research Note, we study the host galaxies of three seemingly young or re-started radio sources that drive fast outflows of cool neutral hydrogen (HI) gas, namely 3C 293, 3C 305 and 4C 12.50 (PKS 1345+12). Our aim is to link the feedback processes in the central kpc-scale region with new information on the distribution of stars and gas at scales of the galaxy. For this, we use deep optical V-band imaging of the host galaxies, complemented with HI emission-line observations to study their gaseous environments. We find prominent optical tidal features in all three radio galaxies, which confirm previous claims that 3C 293, 3C 305 and 4C 12.50 have been involved in a recent galaxy merger or interaction. Our data show the complex morphology of the host galaxies, and identify the companion galaxies that are likely involved in the merger or interaction. The radio sources appear to be (re-)triggered at a different stage of the merger; 4C 12.50 is a pre-coalescent and possibly multiple merger, 3C 293 is a post-coalescent merger that is undergoing a minor interaction with a close satellite galaxy, while 3C 305 appears to be shaped by an interaction with a gas-rich companion. For 3C 293 and 3C 305, we do not detect HI beyond the inner ~30-45 kpc region, which shows that the bulk of the cold gas is concentrated within the host-galaxy, rather than along the widespread tidal features.

Gas and stellar spiral arms and their offsets in the grand-design spiral galaxy M51

Theoretical studies on the response of interstellar gas to a gravitational potential disc with a quasi-stationary spiral arm pattern suggest that the gas experiences a sudden compression due to standing shock waves at spiral arms. This mechanism, called a galactic shock wave, predicts that gas spiral arms move from downstream to upstream of stellar arms with increasing radius inside a corotation radius. In order to investigate if this mechanism is at work in the grand-design spiral galaxy M51, we have measured azimuthal offsets between the peaks of stellar mass and gas mass distributions in its two spiral arms. The stellar mass distribution is created by the spatially resolved spectral energy distribution fitting to optical and near infrared images, while the gas mass distribution is obtained by high-resolution CO and HI data. For the inner region (r < 150"), we find that one arm is consistent with the galactic shock while the other is not. For the outer region, results are less certain due to the narrower range of offset values, the weakness of stellar arms, and the smaller number of successful offset measurements. The results suggest that the nature of two inner spiral arms are different, which is likely due to an interaction with the companion galaxy.

NGC 55: a disc galaxy with flat abundance gradients

We present new spectroscopic observations obtained with GMOS@Gemini-S of a sample of 25 hii regions located in NGC 55, a late-type galaxy in the nearby Sculptor group. We derive physical conditions and chemical composition through the te-method for 18 hii regions, and strong-line abundances for 22 hii regions. We provide abundances of He, O, N, Ne, S, Ar, finding a substantially homogenous composition in the ionised gas of the disc of NGC 55, with no trace of radial gradients. The oxygen abundances, both derived with \te- and strong-line methods, have similar mean values and similarly small dispersion: 12+$\log$(O/H)=8.13$\pm$0.18~dex with the former and 12+$\log$(O/H)=8.17$\pm$0.13~dex with the latter. The average metallicities and the flat gradients agree with previous studies of smaller samples of \hii regions and there is a qualitative agreement with the blue supergiant radial gradient as well. We investigate the origin of such flat gradients comparing NGC 55 with NGC 300, its companion galaxy, which is also twin of NGC 55 in terms of mass and luminosity. We suggest that the differences in the metal distributions in the two galaxies might be related to the differences in their K-band surface density profile. The flatter profile of NGC 55 probably causes in this galaxy higher infall/outflow rates than in similar galaxies. This likely provokes a strong mixing of gas and a re-distribution of metals.

Spectroscopic study of the peculiar galaxy IC 883

We analyze new optical spectroscopic observations obtained at the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences with the SCORPIO focal reducer (in the modes of a Fabry–Perot interferometer (FPI) and long-slit spectroscopy) and the Multi-Pupil Fiber Spectrograph for the galaxy IC 883. We have confirmed that the main body of the galaxy rotates around its minor axis. The positions of the dynamical axes of the stellar and gaseous components have been found to differ by ~10°. The velocities in the SE tail do not correspond to the circular rotation around the galaxy’s minor axis. This structure is probably a fragment of an unwound curved spiral arm. Regions with high velocity dispersions and peculiarities in the velocity fields have been found along the minor axis. Our study of the age and metallicity of the galaxy’s stellar population has shown that the mean values of these parameters in the stellar disk, except for the central region (r ≤ 5ʺ), are ≈1 Gyr and ≈−0.4 dex, respectively. Both young (2−5 × 108 yr) and old (5−10 × 109 yr) stellar populations are present in the circumnuclear region. Our analysis of the spectroscopic data for the bright feature 8ʺ south of the nucleus coincident in position with a compact X-ray source has shown that this is apparently a dwarf galaxy or a remnant of a companion galaxy. Our FPI observations in the Hα emission line and direct images have revealed a region of ionized gas that together with the already known structures along the minor axis forms a clumpy tidal structure of ionized gas pulled from the companion galaxy. The results of our study confirm the previously proposed hypothesis that the observed peculiar structures were formed by the merger of two galaxies. However, it can be said that IC 883 does not belong to the class of polar-ring galaxies.

A MASSIVE MOLECULAR GAS RESERVOIR IN THE z = 2.221 TYPE-2 QUASAR HOST GALAXY SMM J0939+8315 LENSED BY THE RADIO GALAXY 3C220.3

ABSTRACT We report the detection of CO(J = 3 &rarr; ?> 2) line emission in the strongly lensed submillimeter galaxy (SMG) SMM J0939+8315 at z = 2.221, using the Combined Array for Research in Millimeter-wave Astronomy. SMM J0939+8315 hosts a type-2 quasar, and is gravitationally lensed by the radio galaxy 3C220.3 and its companion galaxy at z = 0.685. The 104 GHz continuum emission underlying the CO line is detected toward 3C220.3 with an integrated flux density of S cont = 7.4 ± 1.4 mJy. Using the CO(J = 3 &rarr; ?> 2) line intensity of I CO(3-2) = (12.6 ± 2.0) Jy km s−1, we derive a lensing- and excitation-corrected CO line luminosity of L CO(1-0) &prime; ?> = (3.4 ± 0.7) × 1010 (10.1/μ L) K km s−1 pc2 for the SMG, where μ L is the lensing magnification factor inferred from our lens modeling. This translates to a molecular gas mass of M gas = (2.7 ± 0.6) × 1010 (10.1/μ L) M ⊙. Fitting spectral energy distribution models to the (sub)-millimeter data of this SMG yields a dust temperature of T = 63.1 &minus; 1.3 &plus; 1.1 ?> K, a dust mass of M dust = (5.2 ± 2.1) × 108 (10.1/μ L) M ⊙, and a total infrared luminosity of L IR = (9.1 ± 1.2) ×1012 (10.1/μ L) L ⊙. We find that the properties of the interstellar medium of SMM J0939+8315 overlap with both SMGs and type-2 quasars. Hence, SMM J0939+8315 may be transitioning from a starbursting phase to an unobscured quasar phase as described by the “evolutionary link” model, according to which this system may represent an intermediate stage in the evolution of present-day galaxies at an earlier epoch.

KECK/ESI LONG-SLIT SPECTROSCOPY OF SBS 1421+511: A RECOILING QUASAR NUCLEUS IN AN ACTIVE GALAXY PAIR?

ABSTRACT We present Keck/Echellette Spectrograph and Imager long-slit spectroscopy of SBS 1421+511, a system consisting of a quasar at z = 0.276 and an extended source 3″ north of the quasar. The quasar shows a blue-skewed profile of Balmer broad emission lines, which can be well modeled as emissions from a circular disk with a blueshift velocity of ∼1400 km s−1. The blueshift is better interpreted as resulting from a recoiling active black hole than from a super-massive black hole binary, since the line profile almost kept steady for over one decade in the quasar rest frame. Alternative interpretations are possible as well, such as emissions from a bipolar outflow or a circular disk with spiral emissivity perturbations. The extended source shows Seyfert-like narrow-line ratios and a [O iii] luminosity of &gt; 1.4 &times; 10 8 L &CircleDot; ?> , with almost the same redshift as the quasar and a projected distance of 12.5 kpc at the redshift. SBS 1421+511 is thus likely to be an interacting galaxy pair with a dual active galactic nucleus. Alternatively, the quasar companion only appears to be active but not necessarily so: the gas before/in/behind the companion galaxy is illuminated by the quasar as an extended emission-line region is detected at a similar distance in the opposite direction southern to the quasar, which may be generated either by tidal interactions between the galaxy pair or large-scale outflows from the quasar.

The SLUGGS survey: chromodynamical modelling of the lenticular galaxy NGC 1023

Globular clusters (GCs) can be considered discrete, long-lived, dynamical tracers that retain crucial information about the assembly history of their parent galaxy. In this paper, we present a new catalogue of GC velocities and colours for the lenticular galaxy NGC 1023, we study their kinematics and spatial distribution, in comparison with the underlying stellar kinematics and surface brightness profile, and we test a new method for studying GC properties. Specifically, we decompose the galaxy light into its spheroid (assumed to represent the bulge + halo components) and disk components and use it to assign to each GC a probability of belonging to one of the two components. Then we model the galaxy kinematics, assuming a disk and spheroidal component, using planetary nebulae (PNe) and integrated stellar light. We use this kinematic model and the probability previously obtained from the photometry to recalculate for each GC its likelihood of being associated with the disk, the spheroid, or neither. We find that the reddest GCs are likely to be associated with the disk, as found for faint fuzzies in this same galaxy, suggesting that the disk of this S0 galaxy originated at z ~ 2. The majority of blue GCs are found likely to be associated with the spheroidal (hot) component. The method also allows us to identify objects that are unlikely to be in equilibrium with the system. In NGC1023 some of the rejected GCs form a substructure in phase space that is connected with NGC 1023 companion galaxy.

Revealing the nature of star forming blue early-type galaxies at low redshift

Context: Star forming early-type galaxies with blue optical colours at low redshift can be used to test our current understanding of galaxy formation and evolution. Aims: We want to reveal the fuel and triggering mechanism for star formation in these otherwise passively evolving red and dead stellar systems. Methods: We undertook an optical and ultraviolet study of 55 star forming blue early-type galaxies, searching for signatures of recent interactions that could be driving the molecular gas into the galaxy and potentially triggering the star formation. Results: We report here our results on star forming blue early-type galaxies with tidal trails and in close proximity to neighbouring galaxies that are evidence of ongoing or recent interactions between galaxies. There are 12 galaxies with close companions with similar redshifts, among which two galaxies are having ongoing interactions that potentially trigger the star formation. Two galaxies show a jet feature that could be due to the complete tidal disruption of the companion galaxy. The interacting galaxies have high star formation rates and very blue optical colours. Galaxies with no companion could have undergone a minor merger in the recent past. Conclusions: The recent or ongoing interaction with a gas-rich neighbouring galaxy could be responsible for bringing cold gas to an otherwise passively evolving early-type galaxy. The sudden gas supply could trigger the star formation, eventually creating a blue early-type galaxy. The galaxies with ongoing tidal interaction are blue and star forming, thereby implying that blue early-type galaxies can exist even when the companion is on flyby so does not end up in a merger.

Low-redshift quasars in the SDSS Stripe 82. Host galaxy colours and close environment

We present a photometrical and morphological multicolor study of the properties of low redshift (z<0.3) quasar hosts based on a large and homogeneous dataset of quasars derived from the Sloan Digital Sky Survey (DR7). We used quasars that were imaged in the SDSS Stripe82 that is up to 2 mag deeper than standard Sloan images. This sample is part of a larger dataset of ~400 quasars at z<0.5 for which both the host galaxies and their galaxy environments were studied (Falomo et al. 2014,Karhunen et al. 2014). For 52 quasars we undertake a study of the color of the host galaxies and of their close environments in u,g,r,i and z bands. We are able to resolve almost all the quasars in the sample in the filters g,r,i and z and also in $u$ for about 50% of the targets. We found that the mean colors of the QSO host galaxy (g-i=0.82+-0.26; r-i=0.26+-0.16 and u-g=1.32+-0.25) are very similar to the values of a sample of inactive galaxies matched in terms of redshift and galaxy luminosity with the quasar sample. There is a suggestion that the most massive QSO hosts have bluer colors.Both quasar hosts and the comparison sample of inactive galaxies have candidates of close ($<$ 50 kpc) companion galaxies for ~30% of the sources with no significant difference between active and inactive galaxies. We do not find significant correlation between the central black hole (BH) mass and the quasar host luminosity that appears to be extra luminous at a given BH mass with respect to the local relation (M_BH -- M_host) for inactive galaxies. This confirms previous suggestion that a substantial disc component, not correlated to the BH mass, is present in the galaxies hosting low z quasars. These results support a scenario where the activation of the nucleus has negligible effects on the global structural and photometrical properties of the hosting galaxies.

TESTING DENSITY WAVE THEORY WITH RESOLVED STELLAR POPULATIONS AROUND SPIRAL ARMS IN M81

Stationary density waves rotating at a constant pattern speed $\Omega_{\rm P}$ would produce age gradients across spiral arms. We test whether such age gradients are present in M81 by deriving the recent star formation histories (SFHs) of 20 regions around one of M81's grand-design spiral arms. For each region, we use resolved stellar populations to determine the SFH by modeling the observed color-magnitude diagram (CMD) constructed from archival Hubble Space Telescope (HST) F435W and F606W imaging. Although we should be able to detect systematic time delays in our spatially-resolved SFHs, we find no evidence of star formation propagation across the spiral arm. Our data therefore provide no convincing evidence for a stationary density wave with a single pattern speed in M81, and instead favor the scenario of kinematic spiral patterns that are likely driven by tidal interactions with the companion galaxies M82 and NGC 3077.

ULTRAVIOLET MORPHOLOGY AND UNOBSCURED UV STAR FORMATION RATES OF CLASH BRIGHTEST CLUSTER GALAXIES

Brightest cluster galaxies (BCGs) are usually quiescent, but many exhibit star formation. Here we exploit the opportunity provided by rest-frame UV imaging of galaxy clusters in the CLASH (Cluster Lensing and Supernovae with Hubble) Multi-Cycle Treasury Project to reveal the diversity of UV morphologies in BCGs and to compare them with recent simulations of the cool, star-forming gas structures produced by precipitation-driven feedback. All of the CLASH BCGs are detected in the rest-frame UV (280 nm), regardless of their star-formation activity, because evolved stellar populations produce a modest amount of UV light that traces the relatively smooth, symmetric, and centrally peaked stellar distribution seen in the near infrared. Ultraviolet morphologies among the BCGs with strong UV excesses exhibit distinctive knots, multiple elongated clumps, and extended filaments of emission that distinctly differ from the smooth profiles of the UV-quiet BCGs. These structures, which are similar to those seen in the few star-forming BCGs observed in the UV at low redshift, are suggestive of bi-polar streams of clumpy star formation, but not of spiral arms or large, kpc-scale disks. Based on the number of streams and lack of culprit companion galaxies, these streams are unlikely to have arisen from multiple collisions with gas-rich galaxies. These star-forming UV structures are morphologically similar to the cold-gas structures produced in simulations of precipitation-driven AGN feedback in which jets uplift low-entropy gas to greater altitudes, causing it to condense. Unobscured star-formation rates estimated from CLASH UV images using the Kennicutt relation range up to 80 solar masses per year in the most extended and highly structured systems. The circumgalactic gas-entropy threshold for star formation in CLASH BCGs at z~0.2-0.5 is indistinguishable from that for clusters at z < 0.2.

Companions around the nearest luminous galaxies: Segregation and selection effects

AbstractUsing the “Updated Nearby Galaxy Catalog”, we consider different properties of companion galaxies around luminous hosts in the Local Volume. The data on stellar masses, linear diameters, surface brightnesses, HI‐richness, specific star formation rate (sSFR), and morphological types are discussed for members of the nearest groups, including the Milky Way and M 31 groups, as a function of their separation from the hosts. Companion galaxies in groups tend to have lower stellar masses, smaller linear diameters, and fainter mean surface brightnesses as the distance to their host decreases. The hydrogen‐to‐stellar mass ratio of the companions increases with their linear projected separation from the dominant luminous galaxy. This tendency is more expressed around the bulge‐dominated hosts. While linear separation of the companions decreases, their mean sSFR becomes lower, accompanied with the increasing sSFR scatter. the typical linear projected separation of dSphs around the bulge‐dominated hosts, 350 kpc, is substantially larger than that around the disk‐dominated ones, 130 kpc. This difference probably indicates the presence of larger hot/warm gas haloes around the early‐type host galaxies. The mean fraction of dSph (quenched) companions in the 11 nearest groups as a function of their projected separation Rp can be expressed as ƒ(E) = (0.55–0.69)×Rp. The fraction of dSphs around the Milky Way and M 31 looks much higher than in other nearby groups because the quenching efficiency dramatically increases towards the ultra‐low mass companions. We emphasize that the observed properties of the Local Group are not typical for other groups in the Local Volume due to the role of selection effects caused by our location inside the Local Group. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The accretion histories of brightest cluster galaxies from their stellar population gradients

We analyse the spatially-resolved stellar populations of 9 local ($z<0.1$) Brightest Cluster Galaxies (BCGs) observed with VIMOS in IFU mode. Our sample is composed of 7 slow-rotating and 2 fast-rotating BCGs. We do not find a connection between stellar kinematics and stellar populations in this small sample. The BCGs have shallow metallicity gradients (median $\Delta$[Fe/H] $= -0.11\pm0.1$), high central metallicities (median $[$Fe/H]$_{[\alpha/Fe]=0} = 0.13\pm0.07$), and a wide range of central ages (from 5 to 15 Gyr). We propose that the reason for this is diverse evolutionary paths in BCGs. 67 per cent of the sample (6/9) show $\sim 7$ Gyr old central ages, which reflects an active accretion history, and 33 per cent of the sample (3/9) have central ages older than 11 Gyr, which suggest no star formation since $z=2$. The BCGs show similar central stellar populations and stellar population gradients to early-type galaxies of similar mass (M$_{dyn}> 10^{11.3}$M$_{\odot}$) from the ATLAS$^{3D}$ survey (median [Z/H] $= 0.04\pm0.07$, $\Delta$[Z/H] $= -0.19\pm0.1$). However, massive early-type galaxies from ATLAS$^{3D}$ have consistently old ages (median Age $=12.0\pm3.8$Gyr). We also analyse the close massive companion galaxies of two of the BCGs. These galaxies have similar stellar populations to their respective BCGs.

EXTREMELY METAL-POOR GALAXIES: THE ENVIRONMENT

We have analyzed bibliographical observational data and theoretical predictions, in order to probe the environment in which extremely metal-poor dwarf galaxies (XMPs) reside. We have assessed the HI component and its relation to the optical galaxy, the cosmic web type (voids, sheets, filaments and knots), the overdensity parameter and analyzed the nearest galaxy neighbours. The aim is to understand the role of interactions and cosmological accretion flows in the XMP observational properties, particularly the triggering and feeding of the star formation. We find that XMPs behave similarly to Blue Compact Dwarfs; they preferably populate low-density environments in the local Universe: ~60% occupy underdense regions, and ~75% reside in voids and sheets. This is more extreme than the distribution of irregular galaxies, and in contrast to those regions preferred by elliptical galaxies (knots and filaments). We further find results consistent with previous observations; while the environment does determine the fraction of a certain galaxy type, it does not determine the overall observational properties. With the exception of five documented cases (four sources with companions and one recent merger), XMPs do not generally show signatures of major mergers and interactions; we find only one XMP with a companion galaxy within a distance of 100 kpc, and the HI gas in XMPs is typically well-behaved, demonstrating asymmetries mostly in the outskirts. We conclude that metal-poor accretion flows may be driving the XMP evolution. Such cosmological accretion could explain all the major XMP observational properties: isolation, lack of interaction/merger signatures, asymmetric optical morphology, large amounts of unsettled, metal-poor HI gas, metallicity inhomogeneities, and large specific star formation.

Towards DIB mapping in galaxies beyond 100 Mpc

Diffuse Interstellar Bands (DIBs) are non-stellar weak absorption features of unknown origin found in the spectra of stars viewed through one or several clouds of Interstellar Medium (ISM). Research of DIBs outside the Milky Way is currently very limited. Specifically spatially resolved investigations of DIBs outside of the Local Group is, to our knowledge, inexistent. Here, we explore the capability of the high sensitivity Integral Field Spectrograph, MUSE, as a tool to map diffuse interstellar bands at distances larger than 100 Mpc. We use MUSE commissioning data for AM 1353-272 B, the member with highest extinction of the "The Dentist's Chair", an interacting system of two spiral galaxies. High signal-to-noise spectra were created by co-adding the signal of many spatial elements distributed in a geometry of concentric elliptical half-rings. We derived decreasing radial profiles for the equivalent width of the $\lambda$5780.5 DIB both in the receding and approaching side of the companion galaxy up to distances of $\sim$4.6 kpc from the center of the galaxy. Likewise, interstellar extinction, as derived from the Halpha/Hbeta line ratio displays a similar trend, with decreasing values towards the external parts. This translates into an intrinsic correlation between the strength of the DIB and the extinction within AM 1353-272 B consistent with the current existing global trend between these quantities when using measurements for both Galactic and extragalactic sight lines. Mapping of DIB strength in the Local Universe as up to now only done for the Milky Way seems feasible. This offers a new approach to study the relationship between DIBs and other characteristics and species of the ISM in different conditions as those found in our Galaxy to the use of galaxies in the Local Group and/or single sightlines towards supernovae, quasars and galaxies outside the Local Group.

SPIRAL DENSITY WAVES IN M81. II. HYDRODYNAMIC SIMULATIONS OF THE GAS RESPONSE TO STELLAR SPIRAL DENSITY WAVES

Gas response to the underlying stellar spirals is explored for M81 using unmagnetized hydrodynamic simulations. Constrained within the uncertainty of observations, 18 simulations are carried out to study the effects of selfgravity and to cover the parameter space comprising three different sound speeds and three different arm strengths. The results are confronted with those data observed at wavelengths of 8 $\mu$m and 21 cm. In the outer disk, the ring-like structure observed in 8 $\mu$m image is consistent with the response of cold neutral medium with an effective sound speed 7 km s$^{-1}$, while for the inner disk, the presence of spiral shocks can be understood as a result of 4:1 resonances associated with the warm neutral medium with an effective sound speed 19 km s$^{-1}$. Simulations with single effective sound speed alone cannot simultaneously explain the structures in the outer and inner disks. This justifies the coexistence of cold and warm neutral media in M81. The anomalously high streaming motions observed in the northeast arm and the outward shifted turning points in the iso-velocity contours seen along the southwest arm are interpreted as signatures of interactions with companion galaxies. The level of simulated streaming motions narrows down the uncertainty of observed arm strength toward larger amplitudes.

A forming wide polar-ring galaxy at z ~ 0.05 in the VST Deep Field of the Fornax cluster

We present the first deep photometry of a good candidate for a forming polar ring galaxy at redshift z~0.05. This object, named FCSS J033710.0-354727, is a background galaxy in the VST deep field of the Fornax cluster. The deep exposures combined with the high angular resolution of the OmegaCAM at VST allow us to carry out the first detailed photometric analysis for this system in the g and i bands to derive the galaxy structure and colors. Results show that the central object resembles a disk galaxy, surrounded by a ring-like structure 2 times more extended than the central disk. The warped geometry and the presence of bright knots observed along the polar direction, as well as the several debris detected on the NW side with colors comparable to those of the galaxy, suggest that the polar structure is still forming. We argue that the wide polar ring/disk is the result of the ongoing disruption of a companion galaxy in the potential of the central object, which is 2-3 times more massive than the accreting galaxy.