Friday, June 1st

arXiv:1205.6815 [pdf, other] A peculiar galaxy appears at redshift 11: properties of a moderate redshift interloper
Matthew Hayes (1), Nicolas Laporte (1), Roser Pello (1), Daniel Schaerer (1 and 2), Jean-Francois Le Borgne (1) ((1) IRAP, Toulouse (2) Observatory of Geneva)
Comments: Accepted by Monthly Notices. 5 pages, 2 figures
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

Laporte et al. (2011) reported a very high redshift galaxy candidate: a lensed J-band dropout (A2667-J1). J1 has a photometric redshift of z=9.6-12, the probability density function for which permits no low or intermediate z solution. We here report new spectroscopic observations of this galaxy with VLT/XShooter, which show clear [OIII]5007AA, Ly-alpha, H-alpha, and H-beta emission and place the galaxy firmly at z=2.082. The oxygen lines contribute only ~25% to the H-band flux, and do not significantly affect the dropout selection of J1. After correcting the broadband fluxes for line emission, we identify two roughly equally plausible natures for A2667-J1: either it is young heavily reddened starburst, or a maximally old system with a very pronounced 4000AA break, upon which a minor secondary burst of star formation is superimposed. Fits show that to make a 3 sigma detection of this object in the B-band (V-band), imaging of depth AB=30.2 (29.5) would be required – despite the relatively bright NIR magnitude, we would need optical data of equivalent depth to the Hubble Ultra Deep Field to rule out the mid-z solution on purely photometric grounds. Assuming that this stellar population can be scaled to the NIR magnitudes of recent HST/WFC3 IR-selected galaxies, we conclude that infeasibly deep optical data AB~32 would be required for the same level of security. There is a population of galaxies at z~2 with continuum colours alone that mimic those of our z=7-12 candidates.

arXiv:1205.6819 [pdf, ps, other] Are passive red spirals truly passive? – The current star formation activity of optically-red disc galaxies
L. Cortese
Comments: 4 pages, 2 figures. Accepted for publication on A&A (Research Note)
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

We use GALEX ultraviolet and WISE 22 micron observations to investigate the current star formation activity of the optically-red spirals recently identified as part of the Galaxy Zoo project. These galaxies were accurately selected from the Sloan Digital Sky Survey in order to be pure discs with low or no current star formation activity, representing one of the best optically-selected samples of candidate passive spirals. However, we show that these galaxies are not only still forming stars at a significant rate >= 1 M_sun/yr but, more importantly, their star formation activity is not different from that of normal star-forming discs of the same stellar mass (M* >= 10^10.2 M_sun). Indeed, these systems lie on the UV-optical blue sequence, even without any corrections for internal dust attenuation, and they follow the same specific star formation rate vs. stellar mass relation of star-forming galaxies. Our findings clearly show that, at high stellar masses, optical colours do not allow to discriminate between actively star-forming and truly quiescent systems.

arXiv:1205.6932 [pdf, ps, other] Near-infrared spectroscopy of stellar populations in nearby spiral galaxies
J. K. Kotilainen, T. Hyvonen, J. Reunanen, V. D. Ivanov
Comments: 13 pages, 5 figures. Accepted to MNRAS. arXiv admin note: text overlap with arXiv:astro-ph/0403133
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

We present high spatial resolution, medium spectral resolution near-infrared (NIR) H- and K-band long-slit spectroscopy for a sample of 29 nearby (z < 0.01) inactive spiral galaxies, to study the composition of their NIR stellar populations. These spectra contain a wealth of diagnostic stellar absorption lines, e.g. MgI 1.575 micron, SiI 1.588 micron, CO (6-3) 1.619 micron, MgI 1.711 micron, NaI 2.207 micron, CaI 2.263 micron and the 12CO and 13CO bandheads longward of 2.29 micron. We use NIR absorption features to study the stellar population and star formation properties of the spiral galaxies along the Hubble sequence, and we produce the first high spatial resolution NIR HK-band template spectra for low redshift spiral galaxies along the Hubble sequence. These templates will find applications in a variety of galaxy studies. The strength of the absorption lines depends on the luminosity and/or temperature of stars and, therefore, spectral indices can be used to trace the stellar population of galaxies. The entire sample testifies that the evolved red stars completely dominate the NIR spectra, and that the hot young star contribution is virtually nonexistent

arXiv:1205.6995 [pdf, ps, other] The X-ray luminous galaxies optically classified as star forming are mostly narrow line Seyfert 1s
N. Castelló-Mor, X. Barcons, L. Ballo, F. J. Carrera, M. J. Ward, C. Jin
Comments: 16 pages, 8 figures, 4 tables
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); Galaxy Astrophysics (astro-ph.GA)

We aim to characterize the nature of galaxies whose optical emission line diagnostics are consistent with star formation, but whose X-ray properties strongly point towards the presence of an AGN. Understanding these sources is of particular importance in assessing the completeness of AGN samples derived from large galaxy surveys, selected solely on the basis of their optical spectral properties.We construct a large sample of 211 NELGs, which have FWHMs Hb emission line <1200 km/s from the SDSS-DR7 galaxy spectroscopic catalogue, for which we are able to construct a classical diagnostic diagram, [OIII]/Hb versus [NII]/Ha (hence z<0.4), and that are also detected in the hard energy band and present in the 2XMM catalogue. This sample offers a large database by which to investigate potential mismatches between optical diagnostics and X-ray emission. Among these 211 objects, which based on our selection criteria are all at z<0.4, we find that 145 galaxies are diagnosed as AGNs, having 2-10 keV X-ray luminosities that span a wide range, from 10^40 erg/s to above 10^44 erg/s. Out of the remaining 66 galaxies, which are instead diagnosed as SF, we find a bimodal distribution in which 28 have X-ray luminosities in excess of 10^42 erg/s, large T (>1), and large X/O ratio (>0.1), while the rest are consistent with being simply SF galaxies. Those 28 galaxies exhibit the broadest Hb line FWHMs, from ~300 to 1200 km/s, and their X-ray spectrum is steeper than average and often displays a soft excess. We therefore conclude that the population of X-ray luminous NELGs with optical lines consistent with those of a starforming galaxy (which represent 19% of our whole sample) is largely dominated by NLS1s. The occurrence of such sources in the overall optically selected sample is small (<2%), hence the contamination of optically selected galaxies by NLS1s is very small.

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