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.Comment: 8 pages, 3 figures. A&A accepted version. Results unchanged but discussion is significantly expande

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).Comment: 19 pages, 13 figures, submitted to MNRAS. Some figures were bitmapped to reduce the size. Full resolution version is available from http://www.saao.ac.za/~akniazev/pub/AM1934_563.pd

[O III]λ5007\lambda 5007 and X-ray Properties of a Complete Sample of Hard X-ray Selected AGNs in the Local Universe

We study the correlation between the [O III]$\lambda 5007$ and X-ray luminosities of local Active Galactic Nuclei (AGNs), using a complete, hard X-ray ($>10$ keV) selected sample in the Swift/BAT 9-month catalog. From our optical spectroscopic observations at the South African Astronomical Observatory and the literature, a catalog of [O III]$\lambda 5007$ line flux for all 103 AGNs at Galactic latitudes of $|b|>15^\circ$ is complied. Significant correlations with intrinsic X-ray luminosity ($L_{\rm X}$) are found both for observed ($L_{\rm [O~III]}$) and extinction-corrected ($L_{\rm [O~III]}^{\rm cor}$) luminosities, separately for X-ray unabsorbed and absorbed AGNs. We obtain the regression form of $L_{\rm [O~III]}$ $\propto L_{\rm 2-10\; keV}^{1.18\pm0.07}$ and $L_{\rm [O~III]}^{\rm cor}$ $\propto L_{\rm 2-10\; keV}^{1.16\pm0.09}$ from the whole sample. The absorbed AGNs with low ($<$0.5\%) scattering fractions in soft X-rays show on average smaller $L_{\rm [O~III]}/L_{\rm X}$ and $L_{\rm [O~III]}^{\rm cor}/L_{\rm X}$ ratios than the other absorbed AGNs, while those in edge-on host galaxies do not. These results suggest that a significant fraction of this population are buried in tori with small opening angles. By using these $L_{\rm [O~III]}$ vs. $L_{\rm X}$ correlations, the X-ray luminosity function of local AGNs (including Compton thick AGNs) in a standard population synthesis model gives much better agreement with the [O III]$\lambda 5007$ luminosity function derived from the Sloan Digital Sky Survey than previously reported. This confirms that hard X-ray observations are a very powerful tool to find AGNs with high completeness.Comment: 14 pages including 11 figures and 3 tables, accepted for publication in ApJ. In this manuscript, the observed 14-195 keV luminosities in Table 1 have been corrected to be exactly the same as in the original Swift/BAT 9-month catalog. Accordingly, Figures 2(a) and 3(a) and a part of Tables 2 and 3 have been updated. The changes from the previous version are small and do not affect the tex

Optical Follow-up of New SMC Wing Be/X-ray Binaries

We investigate the optical counterparts of recently discovered Be/X-ray binaries in the Small Magellanic Cloud. In total four sources, SXP101, SXP700, SXP348 and SXP65.8 were detected during the Chandra Survey of the Wing of the SMC. SXP700 and SXP65.8 were previously unknown. Many optical ground based telescopes have been utilised in the optical follow-up, providing coverage in both the red and blue bands. This has led to the classification of all of the counterparts as Be stars and confirms that three lie within the Galactic spectral distribution of known Be/X-ray binaries. SXP101 lies outside this distribution becoming the latest spectral type known. Monitoring of the Halpha emission line suggests that all the sources bar SXP700 have highly variable circumstellar disks, possibly a result of their comparatively short orbital periods. Phase resolved X-ray spectroscopy has also been performed on SXP65.8, revealing that the emission is indeed harder during the passage of the X-ray beam through the line of sight.Comment: 9 pages, 9 figures, 2 tables, accepted for publication in MNRA

Planetary Nebulae as a Probe of the Local Group Galaxies Evolution

We present the latest results from our study of PNe and HII regions in two Local Group dwarf irregular galaxies IC 10 and NGC 682

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.Comment: 14 pages, 4 figures, accepted to MNRA