Galaxy And Mass Assembly (GAMA): The mechanisms for quiescent galaxy formation at $z<1$ [GA]

One key problem in astrophysics is understanding how and why galaxies switch off their star formation, building the quiescent population that we observe in the local Universe. From the GAMA and VIPERS surveys, we use spectroscopic indices to select quiescent and candidate transition galaxies. We identify potentially rapidly transitioning post-starburst galaxies, and slower transitioning green-valley galaxies. Over the last 8 Gyrs the quiescent population has grown more slowly in number density at high masses (M$*>10^{11}$M$\odot$) than at intermediate masses (M$*>10^{10.6}$M$\odot$). There is evolution in both the post-starburst and green valley stellar mass functions, consistent with higher mass galaxies quenching at earlier cosmic times. At intermediate masses (M$*>10^{10.6}$M$\odot$) we find a green valley transition timescale of 2.6 Gyr. Alternatively, at $z\sim0.7$ the entire growth rate could be explained by fast-quenching post-starburst galaxies, with a visibility timescale of 0.5 Gyr. At lower redshift, the number density of post-starbursts is so low that an unphysically short visibility window would be required for them to contribute significantly to the quiescent population growth. The importance of the fast-quenching route may rapidly diminish at $z<1$. However, at high masses (M$*>10^{11}$M$\odot$), there is tension between the large number of candidate transition galaxies compared to the slow growth of the quiescent population. This could be resolved if not all high mass post-starburst and green-valley galaxies are transitioning from star-forming to quiescent, for example if they rejuvenate out of the quiescent population following the accretion of gas and triggering of star formation, or if they fail to completely quench their star formation.

Read this paper on arXiv…

K. Rowlands, V. Wild, N. Bourne, et. al.
Wed, 26 Jul 17

Comments: 19 pages, 11 figures, 3 tables. Accepted for publication in MNRAS

Small-scale Intensity Mapping: Extended Halos as a Probe of the Ionizing Escape Fraction and Faint Galaxy Populations during Reionization [GA]

We present a new method to quantify the value of the escape fraction of ionizing photons, and the existence of ultra-faint galaxies clustered around brighter objects during the epoch of cosmic reionization, using the diffuse Ly$\alpha$, continuum and H$\alpha$ emission observed around galaxies at $z\sim6$. We model the surface brightness profiles of the diffuse halos considering the fluorescent emission powered by ionizing photons escaping from the central galaxies, and the nebular emission from satellite star-forming sources, by extending the formalisms developed in Mas-Ribas & Dijkstra (2016) and Mas-Ribas et al. (2017). The comparison between our predicted profiles and Ly$\alpha$ observations at $z=5.7$ and $z=6.6$ favors a low ionizing escape fraction, $f_{\rm esc}^{\rm ion}\sim5\%$, for galaxies in the range $-19\gtrsim M_{\rm UV} \gtrsim -21.5$. However, uncertainties and possible systematics in the observations do not allow for firm conclusions. We predict H$\alpha$ and rest-frame visible continuum observations with JWST, and show that JWST will be able to detect extended (a few tens of kpc) fluorescent H$\alpha$ emission powered by ionizing photons escaping from a bright, $L\gtrsim 5L^*$, galaxy. Such observations can differentiate fluorescent emission from nebular emission by satellite sources. We discuss how observations and stacking of several objects may provide unique constraints on the escape fraction for faint galaxies and/or the abundance of ultra-faint radiation sources.

Read this paper on arXiv…

L. Mas-Ribas, J. Hennawi, M. Dijkstra, et. al.
Wed, 26 Jul 17

Comments: 9 pages, 4 figures, re-submitted after referee report to ApJ

Dust properties of the cometary globule Barnard 207 (LDN 1489) [GA]

Barnard 207 (B207, LDN 1489, LBN 777), also known as the Vulture Head nebula, is a cometary globule in the Taurus-Auriga-Perseus molecular cloud region. B207 is known to host a Class I protostar, IRAS 04016+2610, located at a projected distance of ~8,400 au from the dense core centre. Using imaging and photometry over a wide wavelength range, from UV to sub-mm, we study the physical properties of B207 and the dust grains contained within. The core density, temperature, and mass are typical of other globules found in the Milky Way interstellar medium (ISM). The increase in the dust albedo with increasing optical wavelengths, along with the detection of coreshine in the near infrared, indicates the presence of larger dust grains in B207. The measured optical, near-, mid- and far-infrared intensities are in agreement with the CMM+AMM and CMM+AMMI dust grain type of The Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS), suggesting mantle formation on the dust grains throughout the globule. We investigate the possibility of turbulence being responsible for diffusing dust grains from the central core to external outer layers of B207. However, in situ formation of large dust grains cannot be excluded.

Read this paper on arXiv…

A. Togi, A. Witt and D. John
Wed, 26 Jul 17

Comments: 13 pages, 11 figures. Accepted for publication in Astronomy & Astrophysics

Spatial Variations of the Interstellar Polarization and Interstellar Extinction [GA]

For more than 5000 stars with accurate parallaxes from the Hipparcos and Gaia DR1 Tycho-Gaia astrometric solution (TGAS), Tycho-2 photometry, interstellar polarization from eight catalogues and interstellar extinction from eight 3D maps the largest up to date comparison of the polarization and extinction is provided. The extinction maps give different estimations of the extinction and of the polarization efficiency as the polarization divided into extinction $P/A_V$ as well as of the percentage of the stars with the polarization efficiency higher than the limit of Serkowski $P/A_V>0.03$. Using the Hipparcos parallaxes we found about 200 stars (4\%, mainly OB stars) drop higher than the limit when we use any extinction map. However, the usage of more accurate TGAS parallaxes decreases them to only 17 stars (0.3\%). The polarization and extinction are negligible inside the Local Bubble within 80 pc from the Sun. In the vast Bubble’s shell at the distances 80–118 pc from the Sun the polarization and extinction rapidly grow with the distance whereas the position angle of the polarization is oriented predominantly along the shell of the Bubble. Outside the Bubble the polarization and extinction grow with the distance slowly. In addition, within a radius of 80–300 pc of the Sun a disc of some filamentary dust clouds (including well-known Markkanen cloud) is observed as in the polarization map as in the reddening one by Schlegel et al. In this disc the position angle of polarization is preferably oriented along the plane of the disk. For the regions further than 300 pc the position angle of polarization is preferably oriented along the Local spiral arm, i.e. Y coordinate axis. The polarization and its efficiency is lower in the dust layer in the Gould belt than in the equatorial dust layer. It may means different properties of dust in these two layers.

Read this paper on arXiv…

G. Gontcharov
Wed, 26 Jul 17

Comments: 2 pages, 1 figure, conference proceedings – Stars: From Collapse to Collapse, Proceedings of a conference held at Special Astrophysical Observatory, Nizhny Arkhyz, Russia 3-7 October 2016. Edited by Yu. Yu. Balega, D. O. Kudryavtsev, I. I. Romanyuk, and I. A. Yakunin. San Francisco: Astronomical Society of the Pacific, 2017, p.78

ELDAR, a new method to identify AGN in multi-filter surveys: the ALHAMBRA test-case [GA]

We present ELDAR, a new method that exploits the potential of medium- and narrow-band filter surveys to securely identify active galactic nuclei (AGN) and determine their redshifts. Our methodology improves on traditional approaches by looking for AGN emission lines expected to be identified against the continuum, thanks to the width of the filters. To assess its performance, we apply ELDAR to the data of the ALHAMBRA survey, which covered an effective area of $2.38\,{\rm deg}^2$ with 20 contiguous medium-band optical filters down to F814W$\simeq 24.5$. Using two different configurations of ELDAR in which we require the detection of at least 2 and 3 emission lines, respectively, we extract two catalogues of type-I AGN. The first is composed of 585 sources ($79\,\%$ of them spectroscopically-unknown) down to F814W$=22.5$ at $z_{\rm phot}>1$, which corresponds to a surface density of $209\,{\rm deg}^{-2}$. In the second, the 494 selected sources ($83\,\%$ of them spectroscopically-unknown) reach F814W$=23$ at $z_{\rm phot}>1.5$, for a corresponding number density of $176\,{\rm deg}^{-2}$. Then, using samples of spectroscopically-known AGN in the ALHAMBRA fields, for the two catalogues we estimate a completeness of $73\,\%$ and $67\,\%$, and a redshift precision of $1.01\,\%$ and $0.86\,\%$ (with outliers fractions of $8.1\,\%$ and $5.8\,\%$). At $z>2$, where our selection performs best, we reach $85\,\%$ and $77\,\%$ completeness and we find no contamination from galaxies.

Read this paper on arXiv…

J. Chaves-Montero, S. Bonoli, M. Salvato, et. al.
Wed, 26 Jul 17

Comments: 23 pages, 18 figures, submitted to MNRAS

Impact of seeing and host galaxy into the analysis of photo-polarimetric microvariability in blazars – Case study of the nearby blazars 1ES 1959+650 and HB89 2201+044 [GA]

Blazars, a type of Active Galactic Nuclei, present a particular orientation of their jets close to the line ofsight. Their radiation is thus relativistically beamed, giving rise to extreme behaviors, specially strong variability on very short time-scales (i.e., microvariability). Here we present simultaneous photometric and polarimetric observations of two relatively nearby blazars, 1ES 1959+650 and HB89 2201+044, that were obtained using the Calar Alto Faint Object Spectrograph mounted at the 2.2 m telescope in Calar Alto, Spain. An outstanding characteristic of these two blazars is the presence of well resolved host galaxies. This particular feature allows us to produce a study of their intrinsic polarization, a measurement of the polarization state of the galactic nucleus unaffected by the host galaxy. To carry out this work, we computed photometric fluxes from which we calculated the degree and orientation of the blazars polarization. Then, we analyzed the depolarizing effect introduced by the host galaxy with the main goal to recover the intrinsic polarization of the galactic nucleus, carefully taking into consideration the spurious polarimetric variability introduced by changes in seeing along the observing nights. We find that the two blazars do not present intra-night photo-polarimetric variability, although we do detect a significant inter-night variability. Comparing polarimetric values before and after accounting for the host galaxies, we observe a significant difference in the polarization degree of about 1 % in the case of 1ES 1959+650, and 0.3 % in the case of HB89 2201+044, thus evidencing the non-negligible impact introduced by the host galaxies. We note that this host galaxy effect depends on the weaveband, and varies with changing seeing conditions, so it should be particularly considered when studying frequency-dependent polarization in blazars.

Read this paper on arXiv…

M. Sosa, C. Essen, I. Andruchow, et. al.
Wed, 26 Jul 17

Comments: 11 pages, 8 figures

Origin of low surface brightness galaxies: A dynamical study [GA]

Low Surface Brighness Galaxies (LSBs), inspite of being gas rich, have low star formation rates and are, therefore, low surface brightness in nature. We calculate Q${\rm{RW}}$, the 2-component disc stability parameter as proposed by Romeo \& Wiegert (2011), as a function of galactocentric radius $R$ for a sample of five LSBs, for which mass models, as obtained from HI 21cm radio-sythesis observations and R-band photometry, were available in the literature. We find that the median value of Q${\rm{RW}}^{\rm{min}}$, the minimum of Q${\rm{RW}}$ over $R$, lies between 2.6 and 3.1 for our sample LSBs, which is higher than the median value of 1.8 $\pm$ 0.3 for Q${\rm{RW}}^{\rm{min}}$ for a sampleof high surface brightness galaxies (HSBs) as obtained in earlier studies. This clearly shows that LSBs have more stable discs than HSBs, which could explain their low star formation rates and, possibly, their low surface brightness nature. Interestingly, the calculated values of Q${\rm{RW}}$ decrease only slightly (median Q${\rm{RW}}^{\rm{min}}$ $\sim$ 2.3 – 3) if the discs were taken to respond to the gravitational potential of the dark matter halo only, but reduce by $\sim$ a factor of 2-3 (median Q$_{\rm{RW}}^{\rm{min}}$ $\sim$ 0.7 – 1.5) if they respond to their self-gravity alone. This implies that the dark matter halo is crucial in regulating disc stability in LSBs, which may have important implications for models of galaxy formation and evolution.

Read this paper on arXiv…

P. Garg and A. Banerjee
Wed, 26 Jul 17

Comments: 9 pages, 6 figures, 2 tables (Accepted for publication in the MNRAS)