Deep Optical Imaging Research Articles

The Galaxy “Missing Dark Matter” NGC 1052-DF4 is Undergoing Tidal Disruption

Abstract The existence of long-lived galaxies lacking dark matter represents a challenge to our understanding of how galaxies form. Here, we present evidence that explains the lack of dark matter in one such galaxy: NGC 1052-DF4. Deep optical imaging of the system has detected tidal tails in this object caused by its interaction with its neighboring galaxy NGC 1035. As stars are more centrally concentrated than dark matter, tidal stripping will remove a significant percentage of dark matter before affecting the stars of the galaxy. Only of the stellar mass of the galaxy is in the tidal tails, suggesting that the stars of NGC 1052-DF4 are only now starting to be affected by the interaction, while the percentage of remaining dark matter is . This naturally explains the low content of dark matter inferred for this galaxy and reconciles these types of galaxies with our current models of galaxy formation.

Tree species classification using deep learning and RGB optical images obtained by an unmanned aerial vehicle

The diversity of tree species and the complexity of land use in cities create challenging issues for tree species classification. The combination of deep learning methods and RGB optical images obtained by unmanned aerial vehicles (UAVs) provides a new research direction for urban tree species classification. We proposed an RGB optical image dataset with 10 urban tree species, termed TCC10, which is a benchmark for tree canopy classification (TCC). TCC10 dataset contains two types of data: tree canopy images with simple backgrounds and those with complex backgrounds. The objective was to examine the possibility of using deep learning methods (AlexNet, VGG-16, and ResNet-50) for individual tree species classification. The results of convolutional neural networks (CNNs) were compared with those of K-nearest neighbor (KNN) and BP neural network. Our results demonstrated: (1) ResNet-50 achieved an overall accuracy (OA) of 92.6% and a kappa coefficient of 0.91 for tree species classification on TCC10 and outperformed AlexNet and VGG-16. (2) The classification accuracy of KNN and BP neural network was less than 70%, while the accuracy of CNNs was relatively higher. (3) The classification accuracy of tree canopy images with complex backgrounds was lower than that for images with simple backgrounds. For the deciduous tree species in TCC10, the classification accuracy of ResNet-50 was higher in summer than that in autumn. Therefore, the deep learning is effective for urban tree species classification using RGB optical images.

The quiescent fraction of isolated low surface brightness galaxies: observational constraints

ABSTRACT Understanding the formation and evolution of low surface brightness galaxies (LSBGs) is critical for explaining their wide-ranging properties. However, studies of LSBGs in deep photometric surveys are often hindered by a lack of distance estimates. In this work, we present a new catalogue of 479 LSBGs, identified in deep optical imaging data from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). These galaxies are found across a range of environments, from the field to groups. Many are likely to be ultra-diffuse galaxies (UDGs). We see clear evidence for a bimodal population in colour–Sérsic index space, and split our sample into red and blue LSBG populations. We estimate environmental densities for a sub-sample of 215 sources by statistically associating them with nearby spectroscopic galaxies from the overlapping GAMA spectroscopic survey. We find that the blue LSBGs are statistically consistent with being spatially randomized with respect to local spectroscopic galaxies, implying they exist predominantly in low-density environments. However, the red LSBG population is significantly spatially correlated with local structure. We find that $26\pm 5{{\ \rm per\ cent}}$ of isolated, local LSBGs belong to the red population, which we interpret as quiescent. This indicates that high environmental density plays a dominant, but not exclusive, role in producing quiescent LSBGs. Our analysis method may prove to be very useful, given the large samples of LSB galaxies without distance information expected from e.g. the Vera C. Rubin observatory (aka LSST), especially in combination with upcoming comprehensive wide-field spectroscopic surveys.

Subaru/HSC deep optical imaging of infrared sources in the AKARI North Ecliptic Pole-Wide field

ABSTRACT We present a five-broad-band (grizy) photometric catalogue of Subaru/Hyper Suprime-Cam (HSC) optical imaging observations at around the North Ecliptic Pole (NEP) where the AKARI infrared (IR) satellite conducted a large survey (NEP-Wide survey). The observations cover almost all the NEP-Wide survey field down to the depth of 28.1, 26.8, 26.3, 25.5, and 25.0 mag (5σ) at grizy, respectively. The five-band HSC catalogue contains about 2.6 million objects, and 70 959 AKARI NEP-Wide counterpart sources are identified in the catalogue. We added existing supplementary catalogues from the u band to the far-IR band, and estimated photo-z for the AKARI-HSC sources. We achieved σΔz/(1 + zs) = 0.06 and an outlier rate of 13.4 per cent at z = 0.2–1.5. Using the spectral energy distribution (SED) template fitting, we classified the AKARI-HSC galaxies into four categories, namely quiescent, star-forming, Type1 active galactic nucleus (AGN), and Type2 AGN, in each redshift bin. At z > 1, the mean SED of star-forming galaxies in mid-IR (3–10 μm) range is significantly different from that of spiral galaxies in the nearby Universe, indicating that many of star-forming galaxies at z > 1 contain a heat source capable of heating dust to temperatures that radiate thermal emission in the mid-IR range. Furthermore, we estimated the number fraction of AGNs (fAGN) in each bin of redshift and IR luminosity (LIR), and examined the dependence of redshift and LIR. In log(LIR/L⊙) = 11.0–14.0, the fAGN shows a significant increase with increasing redshift, regardless of the LIR bins. In contrast, the fAGN shows a slight increase against LIR at z < 1 and no increase with increasing LIR at z > 1.

Deep optical imaging of the dark galaxy candidate AGESVC1 282

The blind H Isurvey Arecibo Galaxy Environment Survey (AGES) detected several unresolved sources in the Virgo cluster, which do not have optical counterparts in the Sloan Digital Sky Survey. The origin of these dark clouds is unknown. They might be crucial objects since they could be the so-called dark galaxies, that is, the dark matter halos without stellar content that are expected from cosmological simulations. In order to reveal the nature of the dark clouds, we took a deep optical image of one them, AGESVC1 282, with the newly-commissioned 1.4 mMilankovićTelescope. After observing it for 10.4 h in theL-filter, the image reached a surface-brightness limit of about 29.1 mag arcsec−2inV. No optical counterpart was detected. We placed an upper limit on theV-band luminosity of the object of 1.1 × 107 L⊙, giving a stellar mass below 1.4 × 107 M⊙and a H I-to-stellar mass ratio above 3.1. By inspecting archival H Iobservations of the surrounding region, we found that none of the standard explanations for optically dark H Iclouds fits the available constraints on this object.

Inverted metallicity gradients in two Virgo cluster star-forming dwarf galaxies: evidence of recent merging?

ABSTRACT We present integral field spectroscopy observations of two star-forming dwarf galaxies in the Virgo cluster (VCC 135 and VCC 324) obtained with Potsdam Multi-Aperture Spectrograph/PPak at the Calar Alto 3.5 m telescope. We derive metallicity maps using the N2 empirical calibrator. The galaxies show positive gas metallicity gradients, contrarily to what is usually found in other dwarfs or spiral galaxies. We measure gradient slopes of 0.20 ± 0.06 and 0.15 ± 0.03 dex/Re for VCC 135 and VCC 324, respectively. Such a trend has been only observed in few, very isolated galaxies or at higher redshifts (z > 1). It is thought to be associated with the accretion of metal-poor gas from the intergalactic medium, a mechanism that would be less likely to occur in a high-density environment like Virgo. We combine emission-line observations with deep optical images to investigate the origin of the peculiar metallicity gradient. The presence of weak underlying substructures in both galaxies and the analysis of morphological diagnostics and ionized gas kinematics suggest that the inflow of metal-poor gas to the central regions of the dwarfs may be related to a recent merging event with a gas-rich companion.

Distortion matrix concept for deep optical imaging in scattering media.

In optical imaging, light propagation is affected by the inhomogeneities of the medium. Sample-induced aberrations and multiple scattering can strongly degrade the image resolution and contrast. On the basis of a dynamic correction of the incident and/or reflected wavefronts, adaptive optics has been used to compensate for those aberrations. However, it only applies to spatially invariant aberrations or to thin aberrating layers. Here, we propose a global and noninvasive approach based on the distortion matrix concept. This matrix basically connects any focusing point of the image with the distorted part of its wavefront in reflection. A singular value decomposition of the distortion matrix allows to correct for high-order aberrations and forward multiple scattering over multiple isoplanatic modes. Proof-of-concept experiments are performed through biological tissues including a turbid cornea. We demonstrate a Strehl ratio enhancement up to 2500 and recover a diffraction-limited resolution until a depth of 10 scattering mean free paths.

Discovery of extended structures around two evolved planetary nebulae M2–55 and Abell 2

We report a multi-wavelength study of two evolved planetary nebulae (PNs)M2–55 and Abell 2. Deep optical narrow-band images ([O III], Hα, and [N II]) of M2–55 reveal two pairs of bipolar lobes and a new faint arc-like structure. This arc-shaped filament around M2–55 appears as a well-defined boundary from southwest to southeast, strongly suggesting that this nebula is in interaction with its surrounding interstellar medium. From the imaging data of Wide-field Infrared Survey Explorer (WISE) all-sky survey, we discovered extensive mid-infrared halos around these PNs, which are approximately twice the size of their main nebulae seen in the visible.We also present a mid-resolution optical spectrum of M2–55, which shows that it is a high-excitation evolved PN with a low electron density of 250 cm−3. Furthermore, we investigate the properties of these nebulae from their spectral energy distributions (SEDs) by means of archival data.

BUDHiES IV: Deep 21-cm neutral Hydrogen, optical, and UV imaging data of Abell 963 and Abell 2192 at z ≃ 0.2

ABSTRACT In this paper, we present data from the Blind Ultra-Deep H i Environmental Survey (BUDHiES), which is a blind 21-cm H i spectral line imaging survey undertaken with the Westerbork Synthesis Radio Telescope. Two volumes were surveyed, each with a single pointing and covering a redshift range of 0.164 < z < 0.224. Within these two volumes, this survey targeted the clusters Abell 963 and Abell 2192, which are dynamically different and offer unique environments to study the process of galaxy evolution within clusters. With an integration time of 117 × 12 h on Abell 963 and 72 × 12 h on Abell 2192, a total of 166 galaxies were detected and imaged in H i. While the clusters themselves occupy only 4 per cent of the 73 400 Mpc3 surveyed by BUDHiES, most of the volume consists of large-scale structures in which the clusters are embedded, including foreground and background overdensities and voids. We present the data processing and source detection techniques and counterpart identification based on a wide-field optical imaging survey using the Isaac Newton Telescope and deep ultraviolet (UV) Galaxy Evolution Explorer (GALEX) imaging. Finally, we present H i and optical catalogues of the detected sources as well as atlases of their global H i properties, which include integrated column density maps, position–velocity diagrams, global H i profiles, and optical and UV images of the H i sources.

Optical validation and characterisation of Planck PSZ1 sources at the Canary Islands observatories

We report new galaxy clusters previously unknown included in the first Planck Sunyaev–Zeldovich (SZ) sources catalogue, the PSZ1. The results presented here were achieved during the second year of a two-year observational programme, the ITP13, developed at the Roque de los Muchachos Observatory (La Palma, Spain). Using the 2.5 m Isaac Newton telescope, the 3.5 m Telescopio Nazionale Galileo, the 4.2 m William Herschel telescope and the 10.4 m Gran Telescopio Canarias we characterised 75 SZ sources with low SZ significance, SZ S/N < 5.32. We performed deep optical imaging and spectroscopy in order to associate actual galaxy clusters with the SZ Planck source. We adopted robust criteria, based on the 2D spatial distribution, richness, and velocity dispersions to confirm actual optical counterparts up to z < 0.85. The selected systems are confirmed only if they are well aligned with respect to the PSZ1 coordinate and show high richness and high velocity dispersion. In addition, we also inspected the Compton y-maps and SZ significance in order to identify unrealistic detections. Following this procedure, we identify 26 cluster counterparts associated with the SZ emission, which means that only about 35% of the clusters considered in this low S/N PSZ1 subsample are validated. Forty-nine SZ sources (∼65% of this PSZ1 subset) remain unconfirmed. At the end of the ITP13 observational programme, we have studied 256 SZ sources with Dec ≥ −15° (212 of them completely unknown), finding optical counterparts for 152 SZ sources. The ITP13 validation programme has allowed us to update the PSZ1 purity, which is now more refined, increasing from 72% to 83% in the low SZ S/N regime. Our results are consistent with the predicted purity curve for the full PSZ1 catalogue and with the expected fraction of false detections caused by the non-Gaussian noise of foreground signals. We find a strong correlation between the number of unconfirmed sources and the thermal emission of diffuse galactic dust at 857 GHz, thus increasing the fraction of false Planck SZ detections at low galactic latitudes.

Serial optical coherence microscopy for label-free volumetric histopathology

The observation of histopathology using optical microscope is an essential procedure for examination of tissue biopsies or surgically excised specimens in biological and clinical laboratories. However, slide-based microscopic pathology is not suitable for visualizing the large-scale tissue and native 3D organ structure due to its sampling limitation and shallow imaging depth. Here, we demonstrate serial optical coherence microscopy (SOCM) technique that offers label-free, high-throughput, and large-volume imaging of ex vivo mouse organs. A 3D histopathology of whole mouse brain and kidney including blood vessel structure is reconstructed by deep tissue optical imaging in serial sectioning techniques. Our results demonstrate that SOCM has unique advantages as it can visualize both native 3D structures and quantitative regional volume without introduction of any contrast agents.

Stellar-mass Measurements in A133 with Magellan/IMACS

Abstract We present the analysis of deep optical imaging of the galaxy cluster A133 with the IMACS instrument on Magellan. Our multi-band photometry enables stellar-mass measurements in the cluster member galaxies down to a mass limit of (≈0.1 of the Large Magellanic Cloud stellar mass). We observe a clear difference in the spatial distribution of large and dwarf galaxies within the cluster. Modeling these galaxy populations separately, we can confidently track the distribution of stellar mass locked in the galaxies to the cluster’s virial radius. The extended envelope of the cluster’s brightest galaxy can be tracked to ∼200 kpc. The central galaxy contributes ∼1/3 of the total cluster stellar mass within the radius .

The Blue Compact Dwarf Galaxy VCC 848 Formed by Dwarf–Dwarf Merging

Abstract It has long been speculated that many starburst or compact dwarf galaxies are resulted from dwarf–dwarf galaxy merging, but unequivocal evidence for this possibility has rarely been reported in the literature. We present the first study of deep optical broadband images of a gas-dominated blue compact dwarf galaxy (BCD) VCC 848 (M ⋆ ≃ 2 × 108 M ⊙) that hosts extended stellar shells and thus is confirmed to be a dwarf–dwarf merger. VCC 848 is located in the outskirts of the Virgo Cluster. By analyzing the stellar light distribution, we found that VCC 848 is the result of a merging between two dwarf galaxies with a primary-to-secondary mass ratio ≲5 for the stellar components and ≲2 for the presumed dark matter halos. The secondary progenitor galaxy has been almost entirely disrupted. The age–mass distribution of photometrically selected star cluster candidates in VCC 848 implies that the cluster formation rate (CFR, ∝ star formation rate) was enhanced by a factor of ∼7–10 during the past ∼1 Gyr. The merging-induced enhancement of CFR peaked near the galactic center a few hundred Myr ago and has started declining in the last few tens of Myr. The current star formation activities, as traced by the youngest clusters, mainly occur at large galactocentric distances (≳1 kpc). The fact that VCC 848 is still (atomic) gas-dominated after the period of the most violent collision suggests that gas-rich dwarf galaxy merging can result in BCD-like remnants with extended atomic gas distribution surrounding a blue compact center, in general agreement with previous numerical simulations.

Variability and transient search in the SUDARE–VOICE field: a new method to extract the light curves

ABSTRACT The VLT Survey Telescope (VST) Optical Imaging of the CDFS and ES1 Fields Survey, in synergy with the SUDARE survey, is a deep optical ugri imaging of the CDFS and ES1 fields using the VST. The observations for the CDFS field comprise about 4.38 deg2 down to r ∼ 26 mag. The total on-sky time spans over 4 yr in this field, distributed over four adjacent sub-fields. In this paper, we use the multiepoch r-band imaging data to measure the variability of the detected objects and search for transients. We perform careful astrometric and photometric calibrations and point spread function modelling. A new method, referring to as differential running-average photometry, is proposed to measure the light curves of the detected objects. With the method, the difference of PSFs between different epochs can be reduced, and the background fluctuations are also suppressed. Detailed uncertainty analysis and detrending corrections on the light curves are performed. We visually inspect the light curves to select variable objects, and present some objects with interesting light curves. Further investigation of these objects in combination with multiband data will be presented in our forthcoming paper.

Deep optical imaging within complex scattering media

Optical imaging has had a central role in elucidating the underlying biological and physiological mechanisms in living specimens owing to its high spatial resolution, molecular specificity and minimal invasiveness. However, its working depth for in vivo imaging is extremely shallow, and thus reactions occurring deep inside living specimens remain out of reach. This problem originates primarily from multiple light scattering caused by the inhomogeneity of tissue obscuring the desired image information. Adaptive optical microscopy, which minimizes the effect of sample-induced aberrations, has to date been the most effective approach to addressing this problem, but its performance has plateaued because it can suppress only lower-order perturbations. To achieve an imaging depth beyond this conventional limit, there is increasing interest in exploiting the physics governing multiple light scattering. New approaches have emerged based on the deterministic measurement and/or control of multiple-scattered waves, rather than their stochastic and statistical treatment. In this Review, we provide an overview of recent developments in this area, with a focus on approaches that achieve a microscopic spatial resolution while remaining useful for in vivo imaging, and discuss their present limitations and future prospects. Optical microscopy is limited to shallow in vivo imaging depths owing to the exponential extinction of single-scattered waves by multiple light scattering. In this Review, we survey methodologies for deep optical imaging that maintain microscopic resolution by making deterministic use of multiple-scattered waves.

Hubble Space Telescope Imaging of Antlia B: Star Formation History and a New Tip of the Red Giant Branch Distance

Abstract A census of the satellite population around dwarf galaxy primary hosts in environments outside the Local Group is essential to understanding Λ cold dark matter galaxy formation and evolution on the smallest scales. We present deep optical Hubble Space Telescope imaging of the gas-rich, faint dwarf galaxy Antlia B (M V = −9.4)—a likely satellite of NGC 3109 (D = 1.3 Mpc)—discovered as part of our ongoing survey of primary host galaxies similar to the Magellanic Clouds. We derive a new tip of the red giant branch distance of D = 1.35 ± 0.06 Mpc (m − M = 25.65 ± 0.10), consistent with membership in the nearby NGC 3109 dwarf association. The color–magnitude diagram (CMD) shows both a prominent old, metal-poor stellar component and confirms a small population of young, blue stars with ages ≲1 Gyr. We use the CMD fitting algorithm MATCH to derive the star formation history (SFH) and find that it is consistent with the typical dwarf irregular or transitional dwarf galaxy (dTrans) in the Local Group. Antlia B shows relatively constant stellar mass growth for the first ∼10–11 Gyr and almost no growth in the last ∼2–3 Gyr. Despite being gas-rich, Antlia B shows no evidence of active star formation (i.e., no Hα emission) and should therefore be classified as a dTrans dwarf. Both Antlia B and the Antlia dwarf (dTrans) are likely satellites of NGC 3109, suggesting that the cessation of ongoing star formation in these galaxies may be environmentally driven. Future work studying the gas kinematics and distribution in Antlia B will explore this scenario in greater detail. Our work highlights the fact that detailed studies of nearby dwarf galaxies in a variety of environments may continue to shed light on the processes that drive the SFH and evolution of dwarf galaxies more generally.

Comparative Analysis of the Effects of Upconversion Nanoparticles on Normal and Tumor Brain Cells.

Glioma is the most aggressive type of brain tumors encountered in medical practice. The high frequency of diagnosed cases and risk of metastasis, the low efficiency of traditional therapy, and the usually unfavorable prognosis for patients dictate the need to develop alternative or combined approaches for an early diagnosis and treatment of this pathology. High expectations are placed on the use of upconversion nanoparticles (UCNPs). In this study, we have produced and characterized UCNPs doped with the rare-earth elements ytterbium and thulium. Our UCNPs had photoluminescence emission maxima in the visible and infrared spectral regions, which allow for deep optical imaging of tumor cells in the brain. Moreover, we evaluated the toxicity effects of our UCNPs on a normal brain and glioma cells. It was revealed that our UCNPs are non-toxic to glioma cells but have a moderate cytotoxic effect on primary neuronal cultures at high concentrations, a condition that is characterized by a decreased cellular viability and changes in the functional metabolic activity of neuron-glial networks. Despite the great potential associated with the use of these UCNPs as fluorescent markers, there is a need for further studies on the rate of the UCNPs accumulation and excretion in normal and tumor brain cells, and the use of their surface modifications in order to reduce their cytotoxic effects.

On the Origin of SN 2016hil—A Type II Supernova in the Remote Outskirts of an Elliptical Host

Abstract Type II supernovae (SNe) stem from the core collapse of massive (>8 M ⊙) stars. Due to their short lifespan, we expect a very low rate of such events in elliptical hosts, where the star formation rate is low, and which are mostly comprised of an old stellar population. SN 2016hil (iPTF16hil) is an SN II located in the extreme outskirts of an elliptical galaxy at z = 0.0608 (projected distance 27.2 kpc). It was detected near peak (M r ∼ −17 mag) 9 days after the last non-detection. The event has some potentially peculiar properties: it presented an apparently double-peaked light curve, and its spectra suggest low metallicity content (Z < 0.4 Z ⊙). We place a tentative upper limit on the mass of a potential faint host at using deep optical imaging from Keck/LRIS. In light of this, we discuss the possibility of the progenitor forming locally and other more exotic formation scenarios such as a merger or common-envelope evolution causing a time-delayed explosion. Further observations of the explosion site in the UV are needed in order to distinguish between the cases. Regardless of the origin of the transient, observing a population of such seemingly hostless SNe II could have many uses, including an estimate the amount of faint galaxies in a given volume, and tests of the prediction of a time-delayed population of core-collapse SNe in locations otherwise unfavorable for the detection of such events.

Stellar Velocity Dispersion of a Massive Quenching Galaxy at z = 4.01

Abstract We present the first stellar velocity dispersion measurement of a massive quenching galaxy at z = 4. The galaxy is first identified as a massive z ≥ 4 galaxy with suppressed star formation from photometric redshifts based on deep multiband data. A follow-up spectroscopic observation with MOSFIRE on Keck revealed strong multiple absorption features, which are identified as Balmer lines, giving a secure redshift of z = 4.01. This is the most distant quiescent galaxy known to date. Thanks to the high S/N of the spectrum, we are able to estimate the stellar velocity dispersion, , making a significant leap from the previous highest redshift measurement at z = 2.8. Interestingly, we find that the velocity dispersion is consistent with that of massive galaxies today, implying no significant evolution in velocity dispersion over the last 12 Gyr. Based on a stringent upper limit on its physical size from deep optical images (r eff < 1.3 kpc), we find that its dynamical mass is consistent with the stellar mass inferred from photometry. Furthermore, the galaxy is located on the mass fundamental plane extrapolated from lower redshift galaxies. The observed no strong evolution in σ suggests that the mass in the core of massive galaxies does not evolve significantly, while most of the mass growth occurs in the outskirts of the galaxies, which also increases the size. This picture is consistent with a two-phase formation scenario in which mass and size growth is due to accretion in the outskirts of galaxies via mergers. Our results imply that the first phase may be completed as early as z ∼ 4.

Optical validation and characterization of Planck PSZ2 sources at the Canary Islands observatories

Context. The second legacy catalog of Planck Sunyaev–Zeldovich (SZ) sources, hereafter PSZ2, provides the largest galaxy cluster sample selected by means of the SZ signature of the clusters in a full sky survey. In order to fully characterize this PSZ2 sample for cosmological studies, all the members should be validated and the physical properties of the clusters, including mass and redshift, should be derived. However, at the time of its publication, roughly 21% of the 1653 PSZ2 members had no known counterpart at other wavelengths. Aims. Here, we present the second and last year of observations of our optical follow-up program 128-MULTIPLE-16/15B (hereafter LP15), which has been developed with the aim of validating all the unidentified PSZ2 sources in the northern sky with declinations higher than −15° that have no correspondence in the first Planck catalog PSZ1. The description of the program and the first year of observations have been presented previously. Methods. The LP15 program was awarded 44 observing nights that were spread over two years with the Isaac Newton Telescope (INT), the Telescopio Nazionale Galileo (TNG), and the Gran Telescopio Canarias (GTC), all at Roque de los Muchachos Observatory (La Palma). Following the same method as described previously, we performed deep optical imaging for more than 200 sources with the INT and spectroscopy for almost 100 sources with the TNG and GTC at the end of the LP15 program. We adopted robust confirmation criteria based on velocity dispersion and richness estimates for the final classification of the new galaxy clusters as the optical counterparts of the PSZ2 detections. Results. Here, we present the observations of the second year of LP15, as well as the final results of the program. The full LP15 sample comprises 190 previously unidentified PSZ2 sources. Of these, 106 objects were studied before, while the remaining sample (except for 6 candidates) has been completed in the second year and is discussed here. In addition to the LP15 sample, we here study 42 additional PSZ2 objects that were originally validated as real clusters because they matched a WISE or PSZ1 counterpart, but they had no measured spectroscopic redshift. In total, we confirm the optical counterparts for 81 PSZ2 sources after the full LP15 program, 55 of them with new spectroscopic information. Forty of these 81 clusters are presented in this paper. After the LP15 observational program the purity of the PSZ2 catalog has increased from 76.7% originally to 86.2%. In addition, we study the possible reasons for false detection, and we report a clear correlation between the number of unconfirmed sources and galactic thermal dust emission.