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We characterized the kinematics, morphology, and stellar population (SP) properties of a sample of massive compact quiescent galaxies (MCGs, $10 \lesssim \log M_\star$/$M_\odot \lesssim 11$ and $r_{\rm e} \sim 1-3 $kpc) in the MaNGA Survey, with the goal of constraining their formation, assembly history and assessing their relation with non-compact quiescent galaxies. We compared their properties with those of a control sample of median-sized quiescent galaxies ($r_{\rm e} \sim 4-8 $kpc) with similar effective velocity dispersions. MCGs have elevated rotational support, as evidenced by a strong anti-correlation between the Gauss-Hermite moment $h_3$ and $V/\sigma$. In contrast, 30$\%$ of control sample galaxies (CSGs) are slow rotators, and fast-rotating CSGs generally show a weak $h_3-V/\sigma$ anti-correlation. MCGs and CSGs have similar ages, but MCGs are more metal-rich and $\alpha$-enhanced. Both MCGs and CSGs have shallow negative metallicity gradients and flat [$\alpha$/Fe] gradients. On average, MCGs and CSGs have flat age gradients, but CSGs have a significantly larger dispersion of gradient values. The kinematics and SP properties of MCGs suggest that they experienced highly-dissipative gas-rich events, such as mergers, followed by an intense, short, and centrally concentrated burst of star formation, between 4 to 10 Gyr ago ($z\sim0.4-2$), and had a quiet accretion history since then. This sequence of events might be analogous to, although less extreme than, the compaction events which formed compact quiescent galaxies at $z \sim 2$. The small sizes of MCGs, and the high efficiency and short duration of their last star formation episode suggest that they are descendants of compact post-starburst galaxies., Comment: 19 pages, 18 figures (not including appendices). Accepted for publication in the Monthly Notices of the Royal Astronomical Society

In the local Universe, there is a handful of dwarf compact star-forming galaxies with extremely low oxygen abundances. It has been proposed that they are young, having formed a large fraction of their stellar mass during their last few hundred Myr. However, little is known about the fraction of young stellar populations in more massive galaxies. In a previous article, we analyzed 280 000 SDSS spectra to identify a surprisingly large sample of more massive Very Young Galaxies (VYGs), defined to have formed at least $50\%$ of their stellar mass within the last 1 Gyr. Here, we investigate in detail the properties of a subsample of 207 galaxies that are VYGs according to all three of our spectral models. We compare their properties with those of control sample galaxies (CSGs). We find that VYGs tend to have higher surface brightness and to be more compact, dusty, asymmetric, and clumpy than CSGs. Analysis of a subsample with HI detections reveals that VYGs are more gas-rich than CSGs. VYGs tend to reside more in the inner parts of low-mass groups and are twice as likely to be interacting with a neighbour galaxy than CSGs. On the other hand, VYGs and CSGs have similar gas metallicities and large scale environments (relative to filaments and voids). These results suggest that gas-rich interactions and mergers are the main mechanisms responsible for the recent triggering of star formation in low-redshift VYGs, except for the lowest mass VYGs, where the starbursts may arise from a mixture of mergers and gas infall., 33 pages, 24 figures, to be published in MNRAS

The feedback from an active galactic nucleus (AGN) is frequently invoked as a mechanism through which gas can be heated or removed from a galaxy. However, gas fraction measurements in AGN hosts have yielded mixed support for this scenario. Here, we re-visit the assessment of fgas (=MHI/M*) in z10.2 are HI rich by a factor of ~2. However, this gas fraction excess disappears when the control sample is additionally matched in star formation rate (SFR), indicating that these AGN hosts are actually HI normal. At lower stellar mass, the stacking analysis reveals that AGN hosts are HI poor at fixed stellar mass. In the lowest M* regime probed by our sample, 9, Comment: Accepted for publication in MNRAS

We present a sample of 66 galaxies belonging to the equatorial part (Dec.= -7$^o$, +7$^o$) of the large so called Eridanus void (after Fairall 1998). The void galaxies are selected as to be separated from the luminous galaxies ($M_{\rm B} < M_{\rm B}^{*} +1$), delineating the void, by more than 2 Mpc. Our main goal is to study systematically the evolutionary parameters of the void sample (metallicity and gas content) and to compare the void galaxy properties with their counterparts residing in denser environments. Besides the general galaxy parameters, compiled mainly from the literature, we present the results of dedicated observations to measure the oxygen abundance O/H in HII-regions of 23 void galaxies obtained with the 11-m SALT telescope (SAAO) and the 6-m BTA telescope (SAO), as well as the O/H estimates derived from the analysis of the SDSS DR12 spectra for 3 objects. We compiled all available data on O/H in 36 these void galaxies, including those for 11 galaxies available in the literature (for one object both SDSS and SALT spectra were used), and analyze this data in relation to galaxy luminosity ($\log$(O/H) versus $M_{\rm B}$). Comparing them with the control sample of similar type galaxies from the Local Volume, we find clear evidence for a substantially lower average metallicity of the Eridanus void galaxies. This result matches well the conclusions of our recent similar study for galaxies in the Lynx-Cancer void., Published in MNRAS; 39 pages, 13 figures, 19 tables

We present new absorption-line analysis and new galaxy survey data obtained for the field around PKS0405-123 at z=0.57. Combining previously known OVI absorbers with new identifications in the higher S/N UV spectra obtained with COS, we have established a sample of 7 OVI absorbers and 12 individual components at z=0.0918-0.495 along the sightline toward PKS0405-123. We complement the available UV absorption spectra with galaxy survey data that reach 100% completeness at projected distances rho0.04L* at rho0.3L* at rho13.6 and no detectable OVI absorption features. The Lya-only absorbers serve as a control sample in seeking the discriminating galactic features that result in the observed OVI absorbing gas at large galactic radii. We find a clear distinction in the radial profiles of mean galaxy surface brightness around different absorbers. Specifically, OVI absorbers are found to reside in regions of higher mean surface brightness at rho, Comment: 16 pages, 7 figures; accepted to Monthly Notices of the Royal Astronomical Society

We identify close companions of Brightest Cluster Galaxies (BCGs) for the purpose of quantifying the rate at which these galaxies grow via mergers. By exploiting deep photometric data from the CFHTLS, we probe the number of companions per BCG (Nc) with luminosity ratios down to those corresponding to potential minor mergers of 20:1. We also measure the average luminosity in companions per galaxy (Lc). We nd that N c and Lc rise steeply with luminosity ratio for both the BCGs, and a control sample of other bright, red, cluster galaxies. The trend for BCGs rises more steeply, resulting in a larger number of close companions. For companions within 50 kpc of a BCG, Nc= 1.38 0.14 and Lc=2.14 0.31 10 10 L and for companions within 50 kpc of a luminosity matched control sample of non-BCGs, Nc=0.87 0.08 and Lc=1.48 0.20 10 10 L . This suggests that the BCGs are likely to undergo more mergers compared to otherwise comparable luminous galaxies. Additionally, compared to a local sample of luminous red galaxies, the more distant sample presented in this study (with redshifts between 0.15-0.39,) shows a higher Nc, suggesting the younger and smaller BCGs are still undergoing hierarchical formation. Using the Millennium Simulations we model and estimate the level of contamination due to unrelated cluster galaxies. The contamination by interloping galaxies is 50% within projected separations of 50 kpc, but within 30 kpc, 60% of identied companions are real physical companions. We conclude that the luminosity of bound merger candidates down to luminosity ratios of 20:1 could be adding as much as 10% to the mass of a typical BCG over 0.5 Gyr at redshifts of z 0.3.

We present the second of two papers concerning the origin and evolution of local early-type galaxies exhibiting dust features. We use optical and radio data to examine the nature of active galactic nucleus (AGN) activity in these objects, and compare these with a carefully constructed control sample. We find that dust lane early-type galaxies are much more likely to host emission-line AGN than the control sample galaxies. Moreover, there is a strong correlation between radio and emission-line AGN activity in dust lane early-types, but not the control sample. Dust lane early-type galaxies show the same distribution of AGN properties in rich and poor environments, suggesting a similar triggering mechanism. By contrast, this is not the case for early-types with no dust features. These findings strongly suggest that dust lane early-type galaxies are starburst systems formed in gas-rich mergers. Further evidence in support of this scenario is provided by enhanced star formation and black hole accretion rates in these objects. Dust lane early-types therefore represent an evolutionary stage between starbursting and quiescent galaxies. In these objects, the AGN has already been triggered but has not as yet completely destroyed the gas reservoir required for star formation.

The star formation rates (SFRs) and metallicities of a sample of 294 galaxies with visually classified, strong, large-scale bars are compared to a control sample of unbarred disc galaxies selected from the Sloan Digital Sky Survey Data Release 4. The fibre (inner few kpc) metallicities of barred galaxies are uniformly higher (at a given mass) than the unbarred sample by ∼0.06 dex. However, the fibre SFRs of the visually classified barred galaxies are higher by about 60 per cent only in the galaxies with total stellar mass M★ > 1010 M⊙. The metal enhancement at M★ 1010 M⊙. However, due to the much lower fraction of pairs than bars, bars account for ∼3.5 times more triggered central star formation than interactions.

With an adaptive optics imaging survey of 148 B6-A7 stars, we have tested the hypothesis that unresolved lower-mass companions are the source of the unexpected X-ray detections of stars in this spectral type range. The sample is composed of 63 stars detected in X-rays within the ROSAT All-Sky Survey and 85 stars that form a control sample; both subsets have the same restricted distribution of spectral type, age, X-ray sensi tivity and separation coverage. A total of 68 companion candidates are resolved with separations ranging from 0: 00 3 to 26: 00 2, with 23 new detections. The multiple star frequency of the X-ray sample based on companions resolved within the ROSAT error ellipse is found to be 43 +66 %. The corresponding control sample multiple star frequency is three times lower at 12 +43% ‐ a difference of 31 ± 7%. These results are presented in the first of a series of papers b ased on our Volume-limited A-Star (VAST) survey ‐ a comprehensive study of the multiplicity of A-type stars.

This paper is part of a series devoted to the investigation of a large sample of brightest cluster galaxies (BCGs), their properties and the relationships between these and the properties of the host clusters. In this paper, we compare the stellar population properties derived from high signal-to-noise, optical long-slit spectra with the GALEX ultraviolet (UV) colour measurements for 36 nearby BCGs to understand the diversity in the most rapidly evolving feature in old stellar systems, the UV-upturn. We investigate: (1) the possible differences between the UV-upturn of BCGs and those of a control sample of ordinary ellipticals in the same mass range, as well as possible correlations between the UV-upturn and other general properties of the galaxies; (2) possible correlations between the UV-upturn and the properties of the host clusters; (3) recently proposed scenarios where helium-sedimentation in the cluster centre can produce an enhanced UV-upturn. We find systematic differences between the UV-colours of BCGs and ordinary ellipticals, but we do not find correlations between these colours and the properties of the host clusters. Furthermore, the observations do not support the predictions made by the helium-sedimentation model as an enhancer of the UV-upturn.

We describe the effect of AGN light on host galaxy optical and UV-optical colours, as determined from X-ray-selected AGN host galaxies at z~1, and compare the AGN host galaxy colours to those of a control sample matched to the AGN sample in both redshift and stellar mass. We identify as X-ray-selected AGNs 8.7 +4/-3 per cent of the red-sequence control galaxies, 9.8 +/-3 per cent of the blue-cloud control galaxies, and 14.7 +4/-3 per cent of the green-valley control galaxies. The nuclear colours of AGN hosts are generally bluer than their outer colours, while the control galaxies exhibit redder nuclei. AGNs in blue-cloud host galaxies experience less X-ray obscuration, while AGNs in red-sequence hosts have more, which is the reverse of what is expected from general considerations of the interstellar medium. Outer and integrated colours of AGN hosts generally agree with the control galaxies, regardless of X-ray obscuration, but the nuclear colours of unobscured AGNs are typically much bluer, especially for X-ray luminous objects. Visible point sources are seen in many of these, indicating that the nuclear colours have been contaminated by AGN light and that obscuration of the X-ray radiation and visible light are therefore highly correlated. Red AGN hosts are typically slightly bluer than red-sequence control galaxies, which suggests that their stellar populations are slightly younger. We compare these colour data to current models of AGN formation. The unexpected trend of less X-ray obscuration in blue-cloud galaxies and more in red-sequence galaxies is problematic for all AGN feedback models, in which gas and dust is thought to be removed as star formation shuts down. [See paper for full abstract.]

We explore properties of close galaxy pairs and merging systems selected from the Sloan Digital Sky Survey Data Release 4 in different environments with the aim to assess the relative importance of the role of interactions over global environmental processes. For this purpose, we perform a comparative study of galaxies with and without close companions as a function of local density and host halo mass, carefully removing sources of possible biases. We find that at low- and high-local-density environments, colours and concentration indices of close galaxy pairs are very similar to those of isolated galaxies. At intermediate densities, we detect significant differences, indicating that close pairs could have experienced a more rapid transition on to the red sequence than isolated galaxies. The presence of a correlation between concentration index and colours indicates that the physical mechanism responsible for the colour transformation also operates in the transformation of the luminous matter distribution. At fixed local densities, we find a dependence of the red galaxy fraction on dark matter halo mass for galaxies with or without a close companion. This suggests the action of host halo mass related effects. Regardless of dark matter halo mass, we show that the percentage of red galaxies in close pairs and in the control sample are comparable at low- and high-local-density environments. However, at intermediate local densities, the gap in the red fraction between close pairs and the control galaxies increases from ∼10 per cent in low-mass haloes up to ∼50 per cent in the most massive ones. Interestingly, we also detect that 50 per cent of merging systems populate the intermediate local environments, with a large fraction of them being extremely red and bulge dominated. Our findings suggest that in intermediate-density environments galaxies are efficiently pre-processed by close encounters and mergers before entering higher local density regions., Facultad de Ciencias Astronómicas y Geofísicas

We present the properties of a large sample (12,282) of nearly face-on low surface brightness (LSB) disk galaxies selected from the main galaxy sample of SDSS-DR4. These properties include B-band central surface brightness mu_0(B), scale lengths h, integrated magnitudes, colors, and distances D. This sample has mu_0(B) values from 22 to 24.5 mag arcsec^{-2} with a median value of 22.42 mag arcsec^{-2}, and disk scale lengths ranging from 2 to 19 kpc. They are quite bright with M_B taking values from -18 to -23 mag with a median value of -20.08 mag. There exist clear correlations between logh and M_B, logh and logD, logD and M_B. However, no obvious correlations are found between mu_0(B) and logh, colors etc. The correlation between colors and logh is weak even though it exists. Both the optical-optical and optical-NIR color-color diagrams indicate that most of them have a mixture of young and old stellar populations. They also satisfy color-magnitude relations, which indicate that brighter galaxies tend generally to be redder. The comparison between the LSBGs and a control sample of nearly face-on disk galaxies with higher surface brightness (HSB) with mu_0(B) from 18.5 to 22 mag arcsec^{-2} show that, at a given luminosity or distance, the observed LSB galaxies tend to have larger scale lengths. These trends could be seen gradually by dividing both the LSBGs and HSBGs into two sub-groups according to surface brightness. A volume-limited sub-sample was extracted to check the incompleteness of surface brightness. The only one of the property relations having an obvious change is the relation of logh versus mu_0(B), which shows a correlation in this sub-sample., 14 pages, 18 figures, accepted for publication in MNRAS

We present a multiwavelength radio study of a sample of nearby Fanaroff-Riley class II (FRII) radio galaxies, matched with the sample of known X-shaped radio sources in size, morphological properties and redshift, using new Giant Metrewave Radio Telescope (GMRT) data and archival data from the Very Large Array (VLA). Our principal aim in this paper is to provide a control sample for earlier studies of samples of `X-shaped' radio sources, which have similar luminosities and small-scale radio structures to our targets but exhibit large-scale extensions to their lobes that more typical FRII sources lack; earlier spectral work with the GMRT has suggested that these `wings' sometimes have flat spectral indices at low frequencies, in contrast to expectations from models in which the wings are formed hydrodynamically or by jet reorientation. In our new observations we find that almost all of our target FRII radio galaxies show standard spectral steepening as a function of distance from the hotspot at the low frequencies (610 MHz and 240 MHz) provided by the GMRT data, even when transverse extensions to the lobes are present. However, one source, 3C321, has a low-surface-brightness extension to one lobe that shows a flatter spectral index than the high-surface-brightness hotspots/lobes, as found in X-shaped sources.

We have produced sensitive, high-resolution radio maps of 12 SMGs in the Lockman Hole using combined MERLIN and VLA data at a frequency of 1.4 GHz. Integrating for 350hr yielded an r.m.s. noise of 6.0 uJy/beam and a resolution of 0.2-0.5". For the first time, wide-field data from the two arrays have been combined in the (u,v) plane and the bandwidth smearing response of the VLA data has been removed. All of the SMGs are detected in our maps as well as sources comprising a non-submm luminous control sample. We find evidence that SMGs are more extended than the general uJy radio population and that therefore, unlike in local ULIRGs, the starburst component of the radio emission is extended and not confined to the galactic nucleus. For the eight sources with redshifts we measure linear sizes between 1 and 8 kpc with a median of 5 kpc. Therefore, they are in general larger than local ULIRGs which may support an early-stage merger scenario for the starburst trigger. X-rays betray AGN in six of the 33 sources in the combined sample. All but one of these are in the control sample, suggesting a lower incidence of AGN amongst the submm-luminous galaxies which is, in turn, consistent with increased X-ray absorption in these dust-obscured starbursts. Only one of our sources is resolved into multiple, distinct components with our high-resolution data. Finally, compared to a previous study of faint radio sources in the GOODS-N field we find systematically smaller source sizes and no evidence for a tail extending to ~4". Possible reasons for this are discussed., In press at MNRAS. 12 pages. High-resolution PDF version available at http://www.roe.ac.uk/~rji/merlin.pdf

We use Sloan Digital Sky Survey Data Release 2 QSO spectra to constrain the dust-reddening caused by intervening damped Lyman-alpha systems (DLAs). Comparing the spectral index distribution of a 70 sight-line DLA sample with that of a large control sample reveals no evidence for dust-reddening at z~3. Our limit on the shift in spectral index, |Delta(alpha)| < 0.19 (3-sigma), corresponds to a limit on the colour excess due to SMC-like dust-reddening, E(B-V) < 0.02 mag (3-sigma). This is inconsistent with the early studies of Fall, Pei and collaborators who used the small QSO and DLA samples available. Comparison of the DLA and control magnitude distributions also reveals >2-sigma evidence for an excess of bright and/or a deficit of faint QSOs with foreground DLAs. Higher equivalent width DLAs give a stronger signal. We interpret this as the signature of gravitational magnification due to the intervening DLAs., 6 pages, 6 figures (8 EPS files), 2 tables. Accepted by MNRAS Letters. v2: S/N threshold for fiducial DLA sample somewhat higher and so limits on dust-reddening slightly weaker. Now includes discussion of DLA incompleteness at low N(HI). Qualitative results and conclusions unchanged. Added table of QSOs and DLAs; complete table available at http://www.ast.cam.ac.uk/~mim/pub.html

We present XMM data for the supercluster A901/2, at z ~ 0.17, which is combined with deep imaging and 17-band photometric redshifts (from the COMBO-17 survey), 2dF spectra and Spitzer 24um data, to identify AGN in the supercluster. The 90ksec XMM image contains 139 point sources, of which 11 are identified as supercluster AGN with L_X(0.5-7.5keV) > 1.7x10^41 erg/cm2/s. The host galaxies have M_R 98 per cent significance. The AGN host galaxies lie predominantly in areas of moderate projected galaxy density and with more local blue galaxies than the control sample, with the exception of one very bright Type I AGN very near the centre of a cluster. These environments are similar to, but not limited to, cluster outskirts and blue groups. Despite the large number of potential host galaxies, no AGN are found in regions with the highest galaxy density (excluding some cluster cores where emission from the ICM obscures moderate luminosity AGN). AGN are also absent from the areas with lowest galaxy density. We conclude that the prevalence of cluster AGN is linked to their environment.