arXiv subjects

P. Vaisanen

Publications and source records attributed to P. Vaisanen.

At least 37 records · Page 2Linked to original sources

arXiv2009

Discovery of the first symbiotic star in NGC6822

We report the discovery of the first symbiotic star (V=21.6, K_S=15.8 mag) in the Local Group dwarf irregular galaxy NGC6822. This star was identified during a spectral survey of Ha emission-line objects using the Southern African Large Telescope (SALT) during its performance-verification phase. The observed strong emission lines of HI and HeII suggest a high electron density and T* < 130 000 K for the hot companion. The infrared colours allow us to classify this object as an S-type symbiotic star, comprising a red giant losing mass to a compact companion. The red giant is an AGB carbon star, and a semi-regular variable, pulsating in the first overtone with a period of 142 days. Its bolometric magnitude is M_bol=-4.4 mag. We review what is known about the luminosities of extragalactic symbiotic stars, showing that most, possibly all, contain AGB stars. We suggest that a much larger fraction of Galactic symbiotic stars may contain AGB stars than was previously realised.

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arXiv2008

Discovery of a very highly extinguished supernova in a luminous infrared galaxy

We report the discovery of a confirmed supernova (SN) and a supernova-candidate in near-infrared images from the ALTAIR/NIRI adaptive optics system on the Gemini-North Telescope and NICMOS on the Hubble Space Telescope. The Gemini images were obtained as part of a near-infrared K-band search for highly-obscured SNe in the nuclear regions of luminous infrared galaxies. SN 2008cs apparent in the Gemini images is the first SN discovered using laser guide star adaptive optics. It is located at 1500 pc projected distance from the nucleus of the luminous infrared galaxy IRAS 17138-1017. The SN luminosity, JHK colors and light curve are consistent with a core-collapse event suffering from a very high host galaxy extinction of 15.7 +- 0.8 magnitudes in V-band which is to our knowledge the highest yet measured for a SN. The core-collapse nature of SN 2008cs is confirmed by its radio detection at 22.4 GHz using our Very Large Array observations 28 days after the SN discovery, indicating a prominent interaction of the SN ejecta with the circumstellar medium. An unconfirmed SN apparent in the NICMOS images from 2004 is located in the same galaxy at 660 pc projected distance from the nucleus and has a lower extinction.

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arXiv2008

The metallicity extremes of the Sagittarius dSph using SALT spectroscopy of PNe

In this work we present the first spectroscopic results obtained with the Southern African Large Telescope (SALT) telescope during its perfomance-verification phase. We find that the Sagittarius dwarf spheroidal galaxy (Sgr) Sgr contains a youngest stellar population with [O/H] -0.2 and age t>1 Gyr, and an oldest population with [O/H]=-2.0. The values are based on spectra of two planetary nebulae (PNe), using empirical abundance determinations. We calculated abundances for O, N, Ne, Ar, S, Cl, Fe, C and He. We confirm the high abundances of PN StWr2-21 with 12+log(O/H) = 8.57+/-0.02 dex. The other PN studied, BoBn1, is an extraordinary object in that the neon abundance exceeds that of oxygen. The abundances of S, Ar and Cl in BoBn1 yield the original stellar metallicity, corresponding to 12+log(O/H) = 6.72+/-0.16 dex which is 1/110 of the solar value. The actual [O/H] is much higher: third dredge-up enriched the material by a factor of ~12 in oxygen, ~240 in nitrogen and ~70 in neon. Neon as well as nitrogen and oxygen content may have been produced in the intershell of low-mass AGB stars. Well defined broad WR lines are present in the spectrum of StWr2-21 and absent in the spectrum of BoBn1. This puts the fraction of [WR]-type central PNe stars to 67% for dSph galaxies.

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arXiv2007

Adaptive optics imaging and optical spectroscopy of a multiple merger in a luminous infrared galaxy

(abridged) We present near-infrared (NIR) adaptive optics imaging obtained with VLT/NACO and optical spectroscopy from the Southern African Large Telescope (SALT) of a luminous infrared galaxy (LIRG) IRAS 19115-2124. These data are combined with archival HST imaging and Spitzer imaging and spectroscopy, allowing us to study this disturbed interacting/merging galaxy, dubbed the Bird, in extraordinary detail. In particular, the data reveal a triple system where the LIRG phenomenon is dominated by the smallest of the components. One nucleus is a regular barred spiral with significant rotation, while another is highly disturbed with a surface brightness distribution intermediate to that of disk and bulge systems, and hints of remaining arm/bar structure. We derive dynamical masses in the range 3-7x10^10 M_solar for both. The third component appears to be a 1-2x10^10 M_solar mass irregular galaxy. The total system exhibits HII galaxy-like optical line ratios and strengths, and no evidence for AGN activity is found from optical or mid-infrared data. The star formation rate is estimated to be 190 M_solar/yr. We search for SNe, super star clusters, and detect 100-300 km/s outflowing gas from the Bird. Overall, the Bird shows kinematic, dynamical, and emission line properties typical for cool ultra luminous IR galaxies. However, the interesting features setting it apart for future studies are its triple merger nature, and the location of its star formation peak - the strongest star formation does not come from the two major K-band nuclei, but from the third irregular component. Aided by simulations, we discuss scenarios where the irregular component is on its first high-speed encounter with the more massive components.

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arXiv2007

The XMM large scale structure survey: optical vs. X-ray classifications of active galactic nuclei and the unified scheme

Our goal is to characterize AGN populations by comparing their X-ray and optical classifications. We present a sample of 99 spectroscopically identified X-ray point sources in the XMM-LSS survey which are significantly detected in the [2-10] keV band, and with more than 80 counts. We performed an X-ray spectral analysis for all of these 99 X-ray sources. Introducing the fourfold point correlation coefficient, we find only a mild correlation between the X-ray and the optical classifications, as up to 30% of the sources have differing X-ray and optical classifications: on one hand, 10% of the type 1 sources present broad emission lines in their optical spectra and strong absorption in the X-rays. These objects are highly luminous AGN lying at high redshift and thus dilution effects are totally ruled out, their discrepant nature being an intrinsic property. Their X-ray luminosities and redshifts distributions are consistent with those of the unabsorbed X-ray sources with broad emission lines. On the other hand, 25/32 are moderate luminosity AGN, which are both unabsorbed in the X-rays and only present narrow emission lines in their optical spectra. The majority of them have an optical spectrum which is representative of the host galaxy. We finally infer that dilution of the AGN by the host galaxy seems to account for their nature. 5/25 have been defined as Seyfert 2. In conclusion, most of these 32 discrepant cases can be accounted for by the standard AGN unified scheme, as its predictions are not met for only 12% of the 99 X-ray sources. ABRIDGED

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arXiv2007

The Sloan Digital Sky Survey-II Supernova Survey: Technical Summary

The Sloan Digital Sky Survey-II (SDSS-II) has embarked on a multi-year project to identify and measure light curves for intermediate-redshift (0.05 < z < 0.35) Type Ia supernovae (SNe Ia) using repeated five-band (ugriz) imaging over an area of 300 sq. deg. The survey region is a stripe 2.5 degrees wide centered on the celestial equator in the Southern Galactic Cap that has been imaged numerous times in earlier years, enabling construction of a deep reference image for discovery of new objects. Supernova imaging observations are being acquired between 1 September and 30 November of 2005-7. During the first two seasons, each region was imaged on average every five nights. Spectroscopic follow-up observations to determine supernova type and redshift are carried out on a large number of telescopes. In its first two three-month seasons, the survey has discovered and measured light curves for 327 spectroscopically confirmed SNe Ia, 30 probable SNe Ia, 14 confirmed SNe Ib/c, 32 confirmed SNe II, plus a large number of photometrically identified SNe Ia, 94 of which have host-galaxy spectra taken so far. This paper provides an overview of the project and briefly describes the observations completed during the first two seasons of operation.

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arXiv2007

The polar ring galaxy AM1934-563 revisited

We report long-slit spectroscopic observations of the dust-lane polar-ring galaxy AM1934-563 obtained with the Southern African Large Telescope (SALT) during its performance-verification phase. The observations target the spectral region of the Ha, [NII] and [SII] emission-lines, but show also deep NaI stellar absorption lines that we interpret as produced by stars in the galaxy. We derive rotation curves along the major axis of the galaxy that extend out to about 8 kpc from the center for both the gaseous and the stellar components, using the emission and absorption lines. We derive similar rotation curves along the major axis of the polar ring and point out differences between these and the ones of the main galaxy. We identify a small diffuse object visible only in Ha emission and with a low velocity dispersion as a dwarf HII galaxy and argue that it is probably metal-poor. Its velocity indicates that it is a fourth member of the galaxy group in which AM1934-563 belongs. We discuss the observations in the context of the proposal that the object is the result of a major merger and point out some observational discrepancies from this explanation. We argue that an alternative scenario that could better fit the observations may be the slow accretion of cold intergalactic gas, focused by a dense filament of galaxies in which this object is embedded (abridged).

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arXiv2007

Adaptive Optics Discovery of Supernova 2004ip in the Nuclear Regions of the Luminous Infrared Galaxy IRAS 18293-3413

We report a supernova discovery in Ks-band images from the NAOS CONICA adaptive optics (AO) system on the ESO Very Large Telescope (VLT). The images were obtained as part of a near-infrared search for highly-obscured supernovae in the nuclear regions of luminous and ultraluminous infrared galaxies. SN 2004ip is located within a circumnuclear starburst at 1.4 arcsec (or 500 pc) projected distance from the K-band nucleus of the luminous infrared galaxy IRAS 18293-3413. The supernova luminosity and light curve are consistent with a core-collapse event suffering from a host galaxy extinction of up to about 40 magnitudes in V-band which is as expected for a circumnuclear starburst environment. This is the first supernova to be discovered making use of AO correction and demonstrates the potential of the current 8-meter class telescopes equipped with AO in discovering supernovae from the innermost nuclear regions of luminous and ultraluminous infrared galaxies.

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arXiv2006

A comparison of density structures of a star forming and a non-star-forming globule. DCld303.8-14.2 and Thumbprint Nebula

We present a study of radial density structure of the star forming globule, DCld303.8-14.2 (DC303), and a non-star forming globule, Thumbprint Nebula (TPN), using near-infrared data taken with the ISAAC instrument on the Very Large Telescope. We derive the extinction through the globules using the color excess technique and examine the radial density distribution using Bonnor-Ebert and power-law models. The two globules have significantly different density structures. The extinction profile of DC303 is best fitted with a single power-law with an exponent p = 2.29 \pm 0.08. An unstable Bonnor-Ebert model with a dimensionless parameter ξ_max = 23 \pm 3 provides equally good fit to data. The extinction profile of TPN flattens at small radii, making the profile significantly different from the profile of DC303. We are unable to fit the Bonnor-Ebert model for TPN in a robust manner, but derive the lower limit ξ_max > 8 for the dimensionless outer edge. The density profile derived for TPN is typical compared to recently observed pre-protostellar globules, with high ξ_max value which could be interpreted as the presence of significant additional support or very slow contraction.

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arXiv2006

Faint far-infrared sources: Galaxies, clusters, or cluster galaxies

We present results from an on-going follow-up campaign of far-infrared sources detected as part of our ISOPHOT Cosmic IR Background project. Fields have been imaged in the optical and near-infrared, and we find at least a third of the FIR targets areas to contain a bright and nearby star-forming galaxy. We also explore the largely neglected possibility that instead of individual galaxies some of the fainter FIR sources are confused sums of several sources - or even whole cores of galaxy clusters at redshifts of z \sim 0.4-0.8. We look for correlations in the FIR positions with extremely red objects (EROs) and significant peaks in the galaxy surface density and peaks in cluster red sequence signal. Several matches are found and we have set out to study cluster candidates spectroscopically. The campaign is producing an interesting base to study IR-luminous, strongly star-forming galaxies in potential cluster environments.

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arXiv2006

First science with SALT: peering at the accreting polar caps of the eclipsing polar SDSS J015543.40+002807.2

We describe briefly the properties of the recently completed Southern African Large Telescope (SALT), along with its first light imager SALTICAM. Using this instrument, we present 4.3 hr of high speed unfiltered photometric observations of the eclipsing polar SDSSJ015543.40+002807.2 with time resolution as short as 112 ms, the highest quality observations of this kind of any polar to date. The system was observed during its high luminosity state. Two accreting poles are clearly seen in the eclipse light curve. The binary system parameters have been constrained: the white dwarf mass is at the low end of the range expected for cataclysmic variables. Correlations between the positions of the accretion regions on or near the surface of the white dwarf and the binary system parameters were established. The sizes of the accretion regions and their relative movement from eclipse to eclipse were estimated: they are typically 4-7 deg depending on the mass of the white dwarf. The potential of these observations will only fully be realised when low state data of the same kind are obtained and the contact phases of the eclipse of the white dwarf are measured.

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arXiv2006

SDSS J160043.6+074802.9: a very rapid sdO pulsator

We report the serendipitous discovery of the Sloan Digital Sky Survey star, SDSS J160043.6+074802.9 to be a very rapid pulsator. The variation is dominated by a frequency near 8380 microHz (period = 119.33 s) with a large amplitude (0.04 mag) and its first harmonic at 16760 microHz (59.66 s; 0.005 mag). In between these frequencies, we find at least another 8 variations with periods between 62 and 118 seconds and amplitudes between about 0.007 and 0.003 mag; weaker oscillations might also be present. Preliminary spectrograms from the performance verification phase of the Southern African Large Telescope indicate that SDSS J160043.6+074802.9 is a spectroscopic binary consisting of an sdO star and a late-type main-sequence companion. This makes it the first unambiguous detection of such an sdO star to pulsate, and certainly the first found to exhibit multi-frequency variations.

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arXiv2005

Final Analysis of ELAIS 15 $μ$m Observations : Method, Reduction and Catalogue

We present the Final Analysis of the European Large Area ISO Survey (ELAIS) 15 $μ$m observations, carried out with the ISOCAM instrument on board the Infrared Space Observatory (ISO). The data reduction method, known as LARI method, is based on a mathematical model of the detector's behaviour and was specifically designed for the detection of faint sources in ISO-CAM/PHOT data. The method is fully interactive and leads to very reliable and complete source lists. The resulting catalogue includes 1923 sources detected with $S/N > 5$ in the {0.5 -- 100 mJy} flux range and over an area of 10.85 {deg$^2$} split into four fields, making it the largest non-serendipitous extragalactic source catalogue obtained to date from ISO data. This paper presents the concepts underlying the data reduction method together with its latest enhancements. The data reduction process, the production and basic properties of the resulting catalogue are discussed. The catalogue quality is assessed by means of detailed simulations, optical identifications and comparison with previous analyses.

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arXiv2004

A population of extreme mid-to-near-infrared sources: obscured AGN and dusty starbursts

We present a sample of mid-infrared detected sources from the European Large Area ISO Survey (ELAIS) regions characterised by strong mid-IR radiation with faint near-IR and optical counterparts. These extreme mid-to-near-IR objects (EMNOs) are defined here by a flux ratio of f_15um / f_2.2um > 25. This population is not obvious in deeper small area ISO surveys, though it produces more than 20% of the observed cosmic IR background radiation (CIRB) at 15um above 1 mJy. Near-future large area deep mid-IR surveys with the Spitzer Space Telescope, however, are bound to uncover large amounts of these objects, which we argue to most likely be obscured AGN, based on SED shapes and X-ray data. Very strong dusty starbursts at z>1 may also have high mid-to-near-IR flux ratios, but using the MIR/NIR and FIR/MIR ratios these may be separated. Most of our EMNOs appear to be ULIRGs, half are also extremely red objects (ERO). A curious case of a low redshift, less luminous object with a very young stellar population is also found. We predict that the simple broad band selection method makes EMNOs a useful window into high-redshift obscured nuclear activity and its sought after relation to star-formation, in a similar way that EROs have been used to define samples of high-redshift early type galaxies.

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arXiv2004

An Overdensity of Extremely Red Objects Around Faint Mid-IR galaxies

We have searched for Extremely Red Objects (EROs) around faint mid-IR selected galaxies in ELAIS fields. We find a significant overdensity, by factors of 2 to 5, of these EROs compared to field EROs in the same region and literature random field ERO counts. The excess is similar to that found previously in the fields of known high redshift quasars and AGN. While with the present data it cannot be definitely shown whether the overdensity is physically connected to the mid-IR source, a derived radial distribution does suggest this. The fraction of EROs among K-selected galaxies is high in the mid-IR fields in agreement with the picture that the EROs responsible for the overdensity are members of high redshift clusters of galaxies. We find R-K>5 selected EROs to be more clustered around the mid-IR galaxies than I-K>4 EROs, though statistics are weak because of small numbers. However, this would be consistent with a cluster/galaxy group scenario if, as we argue, the former colour selection finds preferentially more strongly clustered early type galaxies, whereas the latter selection includes a larger fraction of dusty EROs. Finally, using the mid-IR data, we are able to limit the fraction of ULIRG type very dusty EROs at K<18 magnitude to less than 10% of the total ERO population.

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arXiv2004

Number Counts of Bright Extremely Red Objects: Evolved Massive Galaxies at z~1

We present results on number counts of Extremely Red Objects (EROs) in a 2850 arcmin^2 near-infrared survey performed in European Large Area ISO Survey (ELAIS) fields at K<17.5. Counts of EROs are extended to brighter levels than available previously, giving 0.002 +/- 0.001 arcmin^-2 at K<16.5 and consistent numbers with literature values at fainter magnitudes. Photometric redshifts from HYPERZ as well as GRASIL model SEDs of galaxies imply that our EROs are located in the range z=0.7-1.5, with the bulk of the population at z~1. Taking advantage of the ISO data in the fields, we use mid-IR detections to constrain the number of dusty EROs, and also discuss the superior capabilities of Spitzer Space Telescope to detect dusty EROs. Both the mid-IR data and the use of colour-colour diagrammes indicate that at most 10-20% of the EROs in this bright regime are dusty starbursting systems. The space density of our EROs, interpreted to be counterparts of local >2-3L^star massive galaxies at around z~1, is estimated to be approx. 2x10^-5 Mpc^-3, which is consistent with local values. Furthermore, the cumulative number counts at our bright magnitudes are remarkably well fitted by pure luminosity evolution models.

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arXiv2004

The European Large Area ISO Survey (ELAIS): The Final Band-merged Catalogue

We present the final band-merged ELAIS catalogue at 6.7, 15, 90, and 175 $μ$m, and the associated data at u,g,r,i,z,J,H,K, and 20cm. The origin of the survey, infrared and radio observations, data-reduction and optical identifications are briefly reviewed, and a summary of the area covered, and completeness limit for each infrared band is given. A detailed discussion of the band-merging and optical association strategy is given. The total catalogues consists of 2860 sources. For extragalactic sources observed in 3 or more infrared bands, colour-colour diagrams are presented and discussed in terms of the contributing infrared populations. Spectral energy distributions are shown for selected sources and compared with cirrus, M82 and Arp220 starburst, and AGN dust torus models. Spectroscopic redshifts are tabulated, where available. For the N1 and N2 areas, the INT ugriz Wide Field Survey, permits photometric redshifts to be estimated for galaxies and quasars. These agree well with the spectroscopic redshifts, within the uncertainty of the photometric method. The redshift distribution is given for selected ELAIS bands and colour-redshift diagrams are discussed. There is a high proportion of ultraluminous infrared galaxies in the ELAIS Catalogue ($> 10 %$ of 15 $μ$m sources), many with Arp220-like colours. 10 hyperluminous infrared galaxies and 10 EROs are found in the survey. The large numbers of ultraluminous galaxies imply very strong evolution in the star-formation rate between z = 0 and 1.

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arXiv2002

Near and mid-infrared colours of star-forming galaxies in ELAIS fields

We present J and K-band near-infrared photometry of a sample of mid-infrared sources detected by the Infrared Space Observatory (ISO) as part of the European Large Area ISO-Survey (ELAIS) and study their classification and star-forming properties. We have used the Preliminary ELAIS Catalogue for the 6.7 micron (LW2) and 15 micron (LW3) fluxes. All of the high-reliability LW2 sources and 80 per cent of the LW3 sources are identified in the near-IR survey reaching K = 17.5 mag. The near- to mid-IR flux ratios can effectively be used to separate stars from galaxies in mid-IR surveys. The stars detected in our survey region are used to derive a new accurate calibration for the ELAIS ISOCAM data in both the LW2 and LW3 filters. We show that near to mid-IR colour-colour diagrams can be used to further classify galaxies, as well as study star-formation. The ISOCAM ELAIS survey is found to mostly detect strongly star-forming late-type galaxies, possibly starburst powered galaxies, and it also picks out obscured AGN. The ELAIS galaxies yield an average mid-IR flux ratio LW2/LW3 = 0.67 +/- 0.27. We discuss this [6.7/15] ratio as a star formation tracer using ISO and IRAS data of a local comparison sample. We find that the [K/15] ratio is also agood indicator of activity level in galaxies and conclude that the drop in the [6.7/15] ratio seen in strongly star-forming galaxies is a result of both an increase of 15 mic emission and an apparent depletion of 6.7 mic emission. Near-IR data together with the mid-IR give the possibility to estimate the relative amount of interstellar matter in the galaxies.

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