Noah Brosch

The haloes and environments of nearby galaxies (HERON) project is aimed at studying haloes and low surface brightness (LSB) details near galaxies. In this second HERON paper, we consider in detail deep imaging (down to surface brightness of ∼28 mag arcsec−2 in the r band) for 35 galaxies, viewed edge-on. We confirm a range of LSB features previously described in the literature but also report new ones. We classify the observed outer shapes of the galaxies into three main types (and their prototypes): disc/diamond-like (NGC 891), oval (NGC 4302), and boxy (NGC 3628). We show that the shape of the outer disc in galaxies does not often follow the general 3D model of an exponential disc: 17 galaxies in our sample exhibit oval or even boxy isophotes at the periphery. Also, we show that the less flattened the outer disc, the more oval or boxy its structure. Many galaxies in our sample have an asymmetric outer structure. We propose that the observed diversity of the galaxy outer shapes is defined by the merger history and its intensity: if no recent multiple minor or single major merging took place, the outer shape is diamond-like or discy. On the contrary, interacting galaxies show oval outer shapes, whereas recent merging appears to transform the outer shape to boxy.

We report deep imaging of a 15 × 150 kpc tail in the Hickson Compact Group 98 (HCG 98) observed with a 28-inch telescope at the Wise Observatory and confirmed with additional observations with a similar 28-inch at the Polaris Observatory and with archival and public observations. The tail extends from the general region of the two brightest galaxies of the group to the South-East; it is actually not connected to these galaxies but is linked with an extended low surface brightness envelope or halo around the two main galaxies of the group, approximately opposite to the tail, forming a tadpole-like entity which we assume is at the same distance as the galaxy group. The group is approximately centred on this halo, whose dimensions are 170 kpc (major axis) by 69 kpc (minor axis). We used the co-added Sloan Digital Sky Survey images from the IAC Stripe 82 Legacy Project (with the HCG 98 group shown in Román & Trujillo); these are 2--3 mag deeper than the regular Sloan Digital Sky Survey images. We measure the brightness and colours of these features, and study the morphology of the galaxies connected with them and of the diffuse features. We propose that our findings can be understood as a stage of a triple dry merger by the two brightest galaxies of the group with a gas-poor dwarf galaxy (M_r ≃ −16.7 mag; the ‘victim’), with the tail and head nebulous features originating from its disrupted stars. The fate of the group, once all three objects complete their merging, is probably to form a gE or cD galaxy with M_r ≈ −22.1 mag, stressing that this merging process into a giant galaxy need not take place at the centre of a rich cluster of galaxies.

Most of the galaxies in the universe today are in groups, which are key to understanding their evolution. In this work we present a new deep mosaic of 1.2 × 1.0 square degrees of the group of galaxies centered on NGC 5018, acquired at the ESO VLT Survey Telescope. We use u, g, r images to analyze the structure of the group members and to estimate the intragroup light. Taking advantage of deep and multiband photometry and of the large field of view of the VST telescope, we studied the structure of the galaxy members and the faint features leading into the intragroup space, and we give an estimate of the intragroup diffuse light in the NGC 5018 group of galaxies. We found that ∼41% of the total g-band luminosity of the group is in the form of intragroup light (IGL). The IGL has a (g – r) color consistent with those of other galaxies in the group, indicating that the stripping leading to the formation of IGL is ongoing. From the study of this group we can infer that there are at least two different interactions involving the group members: one between NGC 5018 and NGC 5022, which generates the tails and ring-like structures detected in the light, and another between NGC 5022 and MCG-03-34-013, which has produced the H i tail. A minor merging event also happened in the formation history of NGC 5018 that perturbed the inner structure of this galaxy.

The Lunar Ultraviolet Cosmic Imager (LUCI) is an innovative all-spherical mirrors telescope, proposed to fly as a scientific UV imaging payload on a lunar mission in collaboration with Indian Aerospace Company-TeamIndus, Axiom Research Labs Pvt. Ltd. Observations from the Moon provide a unique opportunity to observe the sky from a stable platform far above the Earths atmosphere. LUCI will observe at a fixed elevation angle and will detect stars in the near ultraviolet (200-320 nm) to a limiting magnitude of 12 AB, with a field of view of around 0.5 degrees. The primary science goal is to search for transient sources and flag them for further study. The instrument has been assembled in the class 1000 clean room at the M.G.K Menon Laboratory for Space Sciences. Here we will describe the optomechanical assembly procedures we have carried out during the optical alignment and integration of the payload. Opto-mechanical alignment of the instrument was carried out by using alignment telescope cum autocollimator (for coarse alignment) and ZYGO interferometer (fine alignment). We will also discuss the ground calibration tests performed on the assembled telescope. The results from the ground calibration activities will help in establishing the full calibration matrix of the instrument once operational.

We present results derived from four stellar occultations by the plutino object (208996) 2003 AZ₈₄, detected on 2011 January 8 (single-chord event), 2012 February 3 (multi-chord), 2013 December 2 (single-chord), and 2014 November 15 (multi-chord). Our observations rule out an oblate spheroid solution for 2003 AZ₈₄'s shape. Instead, assuming hydrostatic equilibrium, we find that a Jacobi triaxial solution with semiaxes km can better account for all our occultation observations. Combining these dimensions with the rotation period of the body (6.75 hr) and the amplitude of its rotation light curve, we derive a density g cm⁻³, a geometric albedo . A grazing chord observed during the 2014 occultation reveals a topographic feature along 2003 AZ₈₄'s limb, which can be interpreted as an abrupt chasm of width ∼23 km and depth km, or a smooth depression of width ∼80 km and depth ∼13 km (or an intermediate feature between those two extremes).

We have selected a sample of 41 extremely isolated galaxies (EIGs) from the local Universe using both optical and H i Arecibo Legacy Fast ALFA survey redshifts. Narrow-band H α and wide-band imaging along with public data were used to derive star formation rates (SFRs), star formation histories and morphological classifications for the EIGs. We have found that the extreme isolation of the EIGs does not affect considerably their star formation compared to field galaxies. EIGs are typically ‘blue cloud’ galaxies that fit the ‘main sequence of star-forming galaxies’ and may show asymmetric star formation and strong compact star-forming regions. We discovered surprising environmental dependences of the H i content, |${\it M} _{\rm H\,\small {I}}$|⁠, and of the morphological type of EIGs; the most isolated galaxies (of subsample EIG-1) have lower |${\it M} _{\rm H\,\small {I}}$| on average (with 2.5σ confidence) and a higher tendency to be early types (with 0.94 confidence) compared to the less isolated galaxies of subsample EIG-2. To the best of our knowledge, this is the first study that finds an effect in which an isolated sample shows a higher fraction of early types compared to a less isolated sample. Both early-type and late-type EIGs follow the same colour-to-M_*, SFR-to-M_* (main sequence) and |${\it M} _{\rm H\,\small {I}}$|-to-M_* relations. This indicates that the mechanisms and factors governing star formation, colour and the |${\it M} _{\rm H\,\small {I}}$|-to-M_* relation are similar in early-type and late-type EIGs, and that the morphological type of EIGs is not governed by their |${\it M} _{\rm H\,\small {I}}$| content, colour or SFR.

The 2175 Å UV extinction feature was discovered in the mid-1960s, yet its physical origin remains poorly understood. One suggestion is absorption by polycyclic aromatic hydrocarbon (PAH) molecules, which is supported by theoretical molecular structure computations and by laboratory experiments. PAHs are positively detected by their 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7 μm IR emission bands, which are specified by their modes of vibration. A definitive empirical link between the 2175 Å UV extinction and the IR PAH emission bands, however, is still missing. We present a new sample of hot stars that have both 2175 Å absorption and IR PAH emission. We find significant shifts of the central wavelength of the UV absorption feature, up to 2350 Å, but predominantly in stars that also have IR PAH emission. These UV shifts depend on stellar temperature in a fashion that is similar to the shifts of the 6.2 and 7.7 μm IR PAH bands, that is, the features are increasingly more redshifted as the stellar temperature decreases, but only below ∼15 kK. Above 15 kK both UV and IR features retain their nominal values. Moreover, we find a suggestive correlation between the UV and IR shifts. We hypothesize that these similar dependences of both the UV and IR features on stellar temperature hint at a common origin of the two in PAH molecules and may establish the missing link between the UV and IR observations. We further suggest that the shifts depend on molecular size, and that the critical temperature of ∼15 kK above which no shifts are observed is related to the onset of UV-driven hot-star winds and their associated shocks.