Range Of Galactic Environments Research Articles

AT2023fhn (the Finch): a luminous fast blue optical transient at a large offset from its host galaxy

ABSTRACT Luminous fast blue optical transients (LFBOTs) – the prototypical example being AT 2018cow – are a rare class of events whose origins are poorly understood. They are characterized by rapid evolution, featureless blue spectra at early times, and luminous X-ray and radio emission. LFBOTs thus far have been found exclusively at small projected offsets from star-forming host galaxies. We present Hubble Space Telescope, Gemini, Chandra, and Very Large Array observations of a new LFBOT, AT 2023fhn. The Hubble Space Telescope data reveal a large offset (>3.5 half-light radii) from the two closest galaxies, both at redshift z ∼ 0.24. The location of AT 2023fhn is in stark contrast with previous events, and demonstrates that LFBOTs can occur in a range of galactic environments.

PHANGS: constraining star formation time-scales using the spatial correlations of star clusters and giant molecular clouds

ABSTRACT In the hierarchical view of star formation, giant molecular clouds (GMCs) undergo fragmentation to form small-scale structures made up of stars and star clusters. Here we study the connection between young star clusters and cold gas across a range of extragalactic environments by combining the high resolution (1″) PHANGS–ALMA catalogue of GMCs with the star cluster catalogues from PHANGS–HST. The star clusters are spatially matched with the GMCs across a sample of 11 nearby star-forming galaxies with a range of galactic environments (centres, bars, spiral arms, etc.). We find that after 4 − 6 Myr the star clusters are no longer associated with any gas clouds. Additionally, we measure the autocorrelation of the star clusters and GMCs as well as their cross-correlation to quantify the fractal nature of hierarchical star formation. Young (≤10 Myr) star clusters are more strongly autocorrelated on kpc and smaller spatial scales than the $\gt \, 10$ Myr stellar populations, indicating that the hierarchical structure dissolves over time.

The headlight cloud in NGC 628: An extreme giant molecular cloud in a typical galaxy disk

Context.Cloud-scale surveys of molecular gas reveal the link between giant molecular cloud properties and star formation across a range of galactic environments. Cloud populations in galaxy disks are considered to be representative of the normal star formation process, while galaxy centers tend to harbor denser gas that exhibits more extreme star formation. At high resolution, however, molecular clouds with exceptional gas properties and star formation activity may also be observed in normal disk environments. In this paper we study the brightest cloud traced in CO(2–1) emission in the disk of nearby spiral galaxy NGC 628.Aims.We characterize the properties of the molecular and ionized gas that is spatially coincident with an extremely bright H IIregion in the context of the NGC 628 galactic environment. We investigate how feedback and large-scale processes influence the properties of the molecular gas in this region.Methods.High-resolution ALMA observations of CO(2–1) and CO(1−0) emission were used to characterize the mass and dynamical state of the “headlight” molecular cloud. The characteristics of this cloud are compared to the typical properties of molecular clouds in NGC 628. A simple large velocity gradient (LVG) analysis incorporating additional ALMA observations of13CO(1−0), HCO+(1−0), and HCN(1−0) emission was used to constrain the beam-diluted density and temperature of the molecular gas. We analyzed the MUSE spectrum using Starburst99 to characterize the young stellar population associated with the H IIregion.Results.The unusually bright headlight cloud is massive (1 − 2 × 107 M⊙), with a beam-diluted density ofnH2 = 5 × 104cm−3based on LVG modeling. It has a low virial parameter, suggesting that the CO emission associated with this cloud may be overluminous due to heating by the H IIregion. A young (2 − 4 Myr) stellar population with mass 3 × 105 M⊙is associated.Conclusions.We argue that the headlight cloud is currently being destroyed by feedback from young massive stars. Due to the large mass of the cloud, this phase of the its evolution is long enough for the impact of feedback on the excitation of the gas to be observed. The high mass of the headlight cloud may be related to its location at a spiral co-rotation radius, where gas experiences reduced galactic shear compared to other regions of the disk and receives a sustained inflow of gas that can promote the mass growth of the cloud.

The Fornax Deep Survey (FDS) with VST

Context. Dwarf galaxies are the most common type of galaxies in galaxy clusters. Due to their low mass, they are more vulnerable to environmental effects than massive galaxies, and are thus optimal for studying the effects of the environment on galaxy evolution. By comparing the properties of dwarf galaxies with different masses, morphological types, and cluster-centric distances we can obtain information about the physical processes in clusters that play a role in the evolution of these objects and shape their properties. The Fornax Deep Survey Dwarf galaxy Catalog (FDSDC) includes 564 dwarf galaxies in the Fornax cluster and the in-falling Fornax A subgroup. This sample allows us to perform a robust statistical analysis of the structural and stellar population differences in the range of galactic environments within the Fornax cluster. Aims. By comparing our results with works concerning other clusters and the theoretical knowledge of the environmental processes taking place in galaxy clusters, we aim to understand the main mechanisms transforming galaxies in the Fornax cluster. Methods. We have exploited the FDSDC to study how the number density of galaxies, galaxy colors and structure change as a function of the cluster-centric distance, used as a proxy for the galactic environment and in-fall time. We also used deprojection methods to transform the observed shape and density distributions of the galaxies into the intrinsic physical values. These measurements are then compared with predictions of simple theoretical models of the effects of harassment and ram pressure stripping on galaxy structure. We used stellar population models to estimate the stellar masses, metallicities and ages of the dwarf galaxies. We compared the properties of the dwarf galaxies in Fornax with those in the other galaxy clusters with different masses. Results. We present the standard scaling relations for dwarf galaxies, which are the size-luminosity, Sérsic n-magnitude and color-magnitude relations. New in this paper is that we find a different behavior for the bright dwarfs (−18.5 mag < Mr′ < −16 mag) as compared to the fainter ones (Mr′ > −16 mag): While considering galaxies in the same magnitude-bins, we find that, while for fainter dwarfs the g′−r′ color is redder for lower surface brightness objects (as expected from fading stellar populations), for brighter dwarfs the color is redder for the higher surface brightness and higher Sérsic n objects. The trend of the bright dwarfs might be explained by those galaxies being affected by harassment and by slower quenching of star formation in their inner parts. As the fraction of early-type dwarfs with respect to late-types increases toward the central parts of the cluster, the color-surface brightness trends are also manifested in the cluster-centric trends, confirming that it is indeed the environment that changes the galaxies. We also estimate the strength of the ram-pressure stripping, tidal disruption, and harassment in the Fornax cluster, and find that our observations are consistent with the theoretically expected ranges of galaxy properties where each of those mechanisms dominate. We furthermore find that the luminosity function, color–magnitude relation, and axis-ratio distribution of the dwarfs in the center of the Fornax cluster are similar to those in the center of the Virgo cluster. This indicates that in spite of the fact that the Virgo is six times more massive, their central dwarf galaxy populations appear similar in the relations studied by us.

Identifying Exoplanets with Deep Learning. II. Two New Super-Earths Uncovered by a Neural Network in K2 Data

Abstract For years, scientists have used data from NASA’s Kepler Space Telescope to look for and discover thousands of transiting exoplanets. In its extended K2 mission, Kepler observed stars in various regions of the sky all across the ecliptic plane, and therefore in different galactic environments. Astronomers want to learn how the populations of exoplanets are different in these different environments. However, this requires an automatic and unbiased way to identify exoplanets in these regions and rule out false-positive signals that mimic transiting planet signals. We present a method for classifying these exoplanet signals using deep learning, a class of machine learning algorithms that have become popular in fields ranging from medical science to linguistics. We modified a neural network previously used to identify exoplanets in the Kepler field to be able to identify exoplanets in different K2 campaigns that exist in a range of galactic environments. We train a convolutional neural network, called AstroNet- K2, to predict whether a given possible exoplanet signal is really caused by an exoplanet or a false positive. AstroNet- K2 is highly successful at classifying exoplanets and false positives, with accuracy of 98% on our test set. It is especially efficient at identifying and culling false positives, but for now, it still needs human supervision to create a complete and reliable planet candidate sample. We use AstroNet- K2 to identify and validate two previously unknown exoplanets. Our method is a step toward automatically identifying new exoplanets in K2 data and learning how exoplanet populations depend on their galactic birthplace.

The Westerbork Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey

We introduce a new, very deep neutral hydrogen (HI) survey being performed with the Westerbork Synthesis Radio Telescope (WSRT). The Westerbork Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) Survey is producing an archive of some of the most sensitive HI observations available, on the angular scales which are most useful for studying faint, diffuse gas in and around nearby galaxies. The survey data are being used to perform careful modeling of the galaxies, characterizing their gas content, morphology, and kinematics, with the primary goal of revealing the global characteristics of cold gas accretion onto spiral galaxies in the local Universe. In this paper, we describe the survey sample selection, the data acquisition, reduction, and analysis, and present the data products obtained during our pilot program, which consists of UGC 2082, NGC 672, NGC 925, and NGC 4565. The observations reveal a first glimpse of the picture that the full HALOGAS project aims to illuminate: the properties of accreting HI in different types of spirals, and across a range of galactic environments. None of the pilot survey galaxies hosts an HI halo of the scale of NGC 891, but all show varying indications of halo gas features. We compare the properties of detected features in the pilot survey galaxies with their global characteristics, and discuss similarities and differences with NGC 891 and NGC 2403.

Oxygen Gas‐Phase Abundance Revisited

We present new measurements of the interstellar gas-phase oxygen abundance along the sight lines toward 19 early-type Galactic stars at an average distance of 2.6 kpc. We derive O i column densities from Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) observations of the weak 1355 Ainter- system transition. We derive total hydrogen column densities (NðH i Þþ 2NðH2Þ) using HST/STIS observa- tions of Lyand Far Ultraviolet Spectroscopic Explorer (FUSE) observations of molecular hydrogen. The molecular hydrogen content of these sight lines ranges from f ðH2 Þ¼ 2NðH2Þ=½NðH i Þþ 2NðH2Þ� ¼ 0:03 to 0.47. The average hHtot=EBV i of 6:3 � 10 21 cm � 2 mag � 1 with a standard deviation of 15% is consistent with previous surveys. The mean oxygen abundance along these sight lines, which probe a wide range of Galactic environments in the distant interstellar medium, is 10 6 (O/H)gas¼ 408 � 13 (1 � in the mean). We see no evi- dence for decreasing gas-phase oxygen abundance with increasing molecular hydrogen fraction, and the rela- tive constancy of (O/H)gas suggests that the component of dust containing the oxygen is not readily destroyed. We estimate that, if 60% of the dust grains are resilient against destruction by shocks, the distant interstellar total oxygen abundance can be reconciliated with the solar value derived from the most recent measurements of 10 6 (O/H)gas� ¼ 517 � 58 (1 � ). We note that the smaller oxygen abundances derived for the interstellar gas within 500 pc or from nearby B star surveys are consistent with a local elemental deficit. Subject headings: dust, extinction — ISM: abundances — ultraviolet: ISM