Ido Finkelman

Ionized gas in E/S0 galaxies with dust lanes

We report the results of multicolour observations of 30 E/S0 galaxies with dust lanes. For each galaxy we obtained broad-band images and narrow-band images using interference filters isolating the H�+[NII] emission lines to derive the amount and morphology of dust and ionized gas. To improve the wavelength coverage we retrieved data from the SDSS and 2MASS and combined these with our data. Ionized gas is detected in 25 galaxies and shows in most cases a smooth morphology, although knots and filamentary structure are also observed in some objects. The extended gas distribution closely follows the dust structure, with a clear correlation between the mass of both components. An extinction law by the extragalactic dust in the dark lanes is derived and is used to estimate the dust content of the galaxies. The derived extinction law is used to correct the measured colours for intrinsic dust extinction and the data are �

Determining the extragalactic extinction law with SALT

We present CCD imaging observations of early-type galaxies with dark lanes obtained with the Southern African Large Telescope (SALT) during its performance-verification phase. The observations were performed in six spectral bands that span the spectral range from the near-ultraviolet atmospheric cutoff to the near-infrared. We derive the extinction law by the extragalactic dust in the dark lanes in the spectral range 1.11�m −1 < λ −1 < 2.94�m −1 by fitting model galaxies to the unextinguished parts of the image, and subtracting from these the actual images. This procedure allows the derivation, with reasonably high signal-to-noise, of the extinction in each spectral band we used for each resolution element of the image. We also introduce an alternative method to derive the extinction values by comparing various colour-indices maps under the assumption of negligible intrinsic colour gradients in these galaxies. We than compare the results obtained using these two methods. We compare the total-to-selective extinction derived for these galaxies with previously obtained results and with similar extinction values of Milky Way dust to derive conclusions about the properties of extragalactic dust in different objects and conditions. We find that the extinction curves run parallel to the Galactic extinction curve, which implies that the properties of dust in the extragalactic enviroment are similar to those of the Milky Way, despite our original expectations. The ratio of the total V band extinction to the selective extinction between the V and B bands is derived for each galaxy with an average of 2.82±0.38, compared to a canonical value of 3.1 for the Milky Way. The similar values imply that galaxies with well-defined dark lanes have characteristic dust grain sizes similar to those of Galactic dust. We use total optical extinction values to estimate the dust mass for each galaxy, compare these with dust masses derived from IRAS measurements, and find them in the range 10 4 to 10 7 M⊙.

Determining the extragalactic extinction law with SALT – II. Additional sample

We present new results from an ongoing programme to study the dust extragalactic extinction law in E/S0 galaxies with dust lanes with the Southern African Large Telescope (SALT) during its performance verification phase. The wavelength dependence of the dust extinction for seven galaxies is derived in six spectral bands ranging from the near-ultraviolet atmospheric cut-off to the near-infrared. The derivation of an extinction law is performed by fitting model galaxies to the unextinguished parts of the image in each spectral band, and subtracting from these the actual images. We compare our results with the derived extinction law in the Galaxy and find them to run parallel to the Galactic extinction curve with a mean total-to-selective extinction value of R V = 2.71 ± 0.43. We use total optical extinction values to estimate the dust mass for each galaxy, compare these with dust masses derived from IRAS measurements, and find them to range from 10 4 to 10 7 M ⊙ . We study the case of the well-known dust-lane galaxy NGC 2685 for which Hubble Space Telescope/Wide Field Planetary Camera 2 (HST/WFPC2) data are available to test the dust distribution on different scales. Our results imply a scale-free dust distribution across the dust lanes, at least within ~ 1 arcsec (~60 pc) regions.

Continuum removal in Hα extragalactic measurements

We point out an important source of error in measurements of extragalactic Hα emission and suggest ways to reduce it. The Hα line, used for estimating star formation rates, is commonly measured by imaging in a narrow band and a wide band, both of which include the line. The image analysis relies on the accurate removal of the underlying continuum. We discuss in detail the derivation of the emission-line’s equivalent width and flux for extragalactic extended sources, and the required photometric calibrations. We describe commonly used continuum-subtraction procedures, and discuss the uncertainties that they introduce. Specifically, we analyse errors introduced by colour effects. We show that the errors in the measured Hα equivalent width induced by colour effects can lead to underestimates as large as 40 per cent and overestimates as large as 10 per cent, depending on the underlying galaxy’s stellar population and the continuum-subtraction procedure used. We also show that these errors may lead to biases in results of surveys, and to the underestimation of the cosmic star formation rate at low redshifts (the low z points in the Madau plot). We suggest a method to significantly reduce these errors using a single colour measurement.

Dust and ionized gas association in E/S0 galaxies with dust lanes: clues to their origin

We present results from an on-going programme to study the dust and ionized gas in E/S0 galaxies with dust lanes. Our data, together with results from previous studies of E/S0 galaxies, are used to demonstrate the tight relationship between these two components. This relationship is discussed in light of our current understanding of the nature and origin of the interstellar medium (ISM), and in particular in the context of the interplay between the different multi-temperature components. We show that focusing on dust obscured regions as tracers of the ISM, and on their properties, serves as independent evidence for the external origin of the dust and ionized gas.

Polar ring galaxies in the Galaxy Zoo

We report observations of 16 candidate polar-ring galaxies (PRGs) identified by the Galaxy Zoo project in the Sloan Digital Sky Survey (SDSS) data base. Deep images of five galaxies are available in the SDSS Stripe82 data base, while to reach similar depth we observed the remaining galaxies with the 1.8-m Vatican Advanced Technology Telescope. We derive integrated magnitudes and u − r colours for the host and ring components and show continuum-subtracted Hα+[N II] images for seven objects. We present a basic morphological and environmental analysis of the galaxies and discuss their properties in comparison with other types of early-type galaxies. Follow-up photometric and spectroscopic observations will allow a kinematic confirmation of the nature of these systems and a more detailed analysis of their stellar populations.

A candidate polar-ring galaxy in the Subaru Deep Field

We discuss the properties of an object in the Subaru Deep Field (SDF) classified as a galaxy in on-line data bases and revealed on the Subaru images as a genuine polarring galaxy (PRG) candidate. We analyse available photometric data and conclude that this object consists of a & 5 Gyr old early-type central body surrounded by a faint, narrow inner ring tilted at a � 25 � angle relative to the polar axis of the host galaxy. The halo surrounding the main stellar body exhibits a diversity of spatially extended stellar features of low surface brightness, including a faint asymmetric stellar cloud and two prominent loops. These faint features, together with the unperturbed morphology of the central host, are clear signs of a recent coalescence of two highly unequal mass galaxies, most likely a pre-existing early-type galaxy and a close-by gasrich dwarf galaxy. The presumed stellar remnants observed near the edges of the ring, including possibly the surviving captured companion itself, indicate that the merger is still taking place.

H i in HO: Hoag's Object revisited

We present new HI observations of Hoags Object (HO) obtained with the Westerbork Synthesis Radio Telescope. The data show that the luminous optical ring around the elliptical body has a bright HI counterpart that shares the kinematical properties of the optical ring. The entire HI structure is twice as large as the optical ring and shows a mild warp in its outer regions relative to the inner ring. We detect two additional HI sources close in redshift to that of HO, and report on a newly identified SDSS optical companion galaxy. The HI sources are similar to 0.3 and similar to 1 Mpc away in projected distance, and the companion galaxy is also similar to 1 Mpc away. Our main conclusion is that the HI detected in HO shows no indication that this galaxy has experienced a recent (less than similar to 1 Gyr ago) accretion event. At least one of the two additional HI detected objects does not have an optical counterpart. One possibility is that this object is an HI filament left over from an interaction shaping HO, in which case this interaction must also have occurred at least 1-2 Gyr ago.

Hoag's object: the quintessential ring galaxy

Abstract. We present new observations of Hoag’s Object, known as “the most perfect ring galaxy,” that show that a preferred explanation for this object is (a) the formation of a triaxial elliptical galaxy some 10 Gyr ago, (b) the accretion of a large disk of neutral hydrogen at about the same time, (c) low-level star formation in the HI disk for all the time since that event triggered by the triaxial potential of the core.