Progenitors Of Galaxies Research Articles

A comparative high-resolution spectroscopic analysis of in situ and accreted globular clusters

Globular clusters (GCs) are extremely intriguing systems that help in reconstructing the assembly of the Milky Way via the characterisation of their chemo-chrono-dynamical properties. In this study, we use high-resolution spectroscopic archival data from UVES and UVES-FLAMES at the VLT to compare the chemistry of GCs dynamically tagged as either Galactic (NGC 6218, NGC 6522, and NGC 6626) or accreted from distinct merger events (NGC 362 and NGC 1261 from Gaia-Sausage-Enceladus, and Ruprecht 106 from the Helmi Streams) in the metallicity regime where abundance patterns of field stars with different origin effectively separate (−1.3 ≤ [Fe/H] ≤ −1.0 dex). We find remarkable similarities in the abundances of the two Gaia-Sausage-Enceladus GCs across all chemical elements. They both display depletion in the α-elements (Mg, Si and Ca) and statistically significant differences in Zn and Eu compared to in situ GCs. Additionally, we confirm that Ruprecht 106 exhibits a completely different chemical makeup from the other target clusters, being underabundant in all chemical elements. This demonstrates that when high precision is achieved, the abundances of certain chemical elements can not only efficiently separate in situ from accreted GCs, but can also distinguish among GCs born in different progenitor galaxies. In the end, we investigate the possible origin of the chemical peculiarity of Ruprecht 106. Given that its abundances do not match the chemical patterns of the field stars associated with its most likely parent galaxy (i.e. the Helmi Streams), being depleted in the abundances of α-elements in particular, we believe Ruprecht 106 to originate from a less massive galaxy compared to the progenitor of the Helmi Streams.

Gaia-Sausage-Enceladus star formation history as revealed by detailed elemental abundances. An archival study using SAGA data

The Gaia-Sausage-Enceladus merger was a major event in the history of the Milky Way. Debris from this merger has been extensively studied with full kinematic data from the Gaia mission. Understanding the star formation history of the progenitor galaxy aids in our understanding of the evolution of the Milky Way and galaxy formation in general. We aimed to constrain the star formation history of the Gaia-Sausage-Enceladus progenitor galaxy using elemental abundances of member stars. Previous studies on Milky Way satellite dwarf galaxies show that key elemental abundance patterns, which probe different nucleosynthetic channels, reflect the host galaxy's star formation history. We gathered Mg, Fe, Ba, and Eu abundance measurements for Gaia-Sausage-Enceladus stars from the SAGA database. Gaia-Sausage-Enceladus members were selected kinematically. Inspired by previous studies, we used Fe/Mg Ba/Mg Eu/Mg and Eu/Ba as a function of Fe/H to constrain the star formation history of Gaia-Sausage-Enceladus. We used the known star formation histories and elemental abundance patterns of the Sculptor and Fornax dwarf spheroidal galaxies as a comparison. The elemental abundance ratios of Fe/Mg Ba/Mg Eu/Mg and Eu/Ba all increase with Fe/H in Gaia-Sausage-Enceladus. The Eu/Mg begins to increase at Fe/H -2.0$ and continues steadily, contrasting with the trend observed in the Sculptor dSph galaxy. The Eu/Ba increases and remains high across the Fe/H range, unlike the pattern seen in the Sculptor dSph galaxy, and deviates from the Fornax dSph galaxy at high Fe/H . The Ba/Mg is higher than those of the Sculptor dSph galaxy at the lowest Fe/H and gradually increases, similar to the Fornax dSph galaxy. We constrained three main properties of the Gaia-Sausage-Enceladus star formation history: 1) star formation started gradually, 2) it extended for over 2 Gyr, and 3) it was quenched around Fe/H of $-0.5$, likely when it fell into the Milky Way. We show that the elemental abundance ratios Fe/Mg Ba/Mg Eu/Mg and Eu/Ba can be used to trace the star formation history of a disrupted galaxy when these measurements are available over an Fe/H range that is representative of the progenitor galaxy's stellar population.

Linking High-z and Low-z: Are We Observing the Progenitors of the Milky Way with JWST?

The recent JWST observation of the Firefly Sparkle at z = 8.3 offers a unique opportunity to link the high- and the low-z Universe. Indeed, the claim of it being a Milky Way (MW) type of assembly at the cosmic dawn opens the possibility of interpreting the observation with locally calibrated galaxy-formation models. Here, we use the a state-of-the-art MW-evolution model to perform forward modeling of our Galaxy's progenitors at high-z. We build a set of mock spectra for the MW building blocks to make predictions for JWST and to interpret the Firefly Sparkle observation. First, we find that the most massive MW progenitor becomes detectable in a deep survey like JADES from z ≈ 8.2, meaning that we could have already observed MW analogs that still need interpretation. Second, we provide predictions for the number of detectable MW progenitors in lensed surveys like the CAnadian NIRISS Unbiased Cluster Survey, and interpret the Firefly Sparkle as a group of MW building blocks. Both the number of detections and the observed NIRCam photometry are consistent with our predictions. By identifying the MW progenitors whose mock photometry best fits the data, we find bursty and extended star formation histories, lasting >150–300 Myr, and estimate their properties: M h ≈ 108−9 M ⊙, M ⋆ ≈ 106.2−7.5 M ⊙, SFR ≈ 0.04–0.20 M ⊙ yr−1, and Z gas ≈ 0.04–0.24 Z ⊙. Uncovering the properties of MW analogs at cosmic dawn by combining JWST observations and locally constrained models will allow us to understand our Galaxy's formation, linking the high- and low-z perspectives.

NOEMA formIng Cluster survEy (NICE): Characterizing eight massive galaxy groups at 1.5 < z < 4 in the COSMOS field

The NOrthern Extended Millimeter Array (NOEMA) formIng Cluster survEy (NICE) is a NOEMA large programme targeting 69 massive galaxy group candidates at z > 2 over six deep fields with a total area of 46 deg2. Here we report the spectroscopic confirmation of eight massive galaxy groups at redshifts 1.65 ≤ z ≤ 3.61 in the Cosmic Evolution Survey (COSMOS) field. Homogeneously selected as significant overdensities of red IRAC sources that have red Herschel colours, four groups in this sample are confirmed by CO and [CI] line detections of multiple sources with NOEMA 3 mm observations, three are confirmed with Atacama Large Millimeter Array (ALMA) observations, and one is confirmed by Hα emission from Subaru/FMOS spectroscopy. Using rich ancillary data in the far-infrared and sub-millimetre, we constructed the integrated far-infrared spectral energy distributions for the eight groups, obtaining a total infrared star formation rate (SFR) of 260–1300 M⊙ yr−1. We adopted six methods for estimating the dark matter masses of the eight groups, including stellar mass to halo mass relations, overdensity with galaxy bias, and NFW profile fitting to radial stellar mass densities. We find that the radial stellar mass densities of the eight groups are consistent with a NFW profile, supporting the idea that they are collapsed structures hosted by a single dark matter halo. The best halo mass estimates are log(Mh/M⊙) = 12.8 − 13.7 with a general uncertainty of 0.3 dex. Based on the halo mass estimates, we derived baryonic accretion rates (BARs) of (1 − 8)×103 M⊙/yr for this sample. Together with massive groups in the literature, we find a quasi-linear correlation between the integrated SFR/BAR ratio and the theoretical halo mass limit for cold streams, Mstream/Mh, with SFR/BAR = 10−0.46 ± 0.22(Mstream/Mh)0.71 ± 0.16 with a scatter of 0.40 dex. Furthermore, we compared the halo masses and the stellar masses with simulations, and find that the halo masses of all structures are consistent with those of progenitors of Mh(z = 0) > 1014 M⊙ galaxy clusters, and that the most massive central galaxies have stellar masses consistent with those of the brightest cluster galaxy progenitors in the TNG300 simulation. Above all, the results strongly suggest that these massive structures are in the process of forming massive galaxy clusters via baryonic and dark matter accretion.

Metallicity distributions of halo stars: do they trace the Galactic accretion history?

The standard cosmological scenario predicts a hierarchical formation for galaxies. Many substructures have been found in the Galactic halo, usually identified as clumps in kinematic spaces, like the energy-angular momentum space ($E-L_z$), under the hypothesis that these quantities should be conserved during the interaction. If these clumps also feature different chemical abundances, such as the metallicity distribution function (MDF), these two arguments together (different kinematic and chemical properties) are often used to motivate their association with distinct and independent merger debris. The aim of this study is to explore to what extent we can couple kinematic characteristics and metallicities of stars in the Galactic halo to reconstruct the accretion history of the Milky Way (MW). In particular, we want to understand whether different clumps in the $E-L_z$ space with different MDFs should be associated with distinct merger debris. We analysed dissipationless, self-consistent, high-resolution N-body simulations of a MW-type galaxy accreting a satellite with a mass ratio of 1:10, with different orbital parameters and different metallicity gradients, which were assigned a posteriori. We confirm that accreted stars from a sim 1:10 mass ratio merger event redistribute in a wide range of $E$ and $L_z$, due to the dynamical friction process, and are thus not associated with a single region. Because satellite stars with different metallicities can be deposited in different regions of the $E-L_z$ space (on average the more metal-rich ones end up more gravitationally bound to the MW), this implies that a single accretion of sim 1:10 can manifest with different MDFs, in different regions of the $E-L_z$ space. Groups of stars with different $E$, $L_z$, and metallicities may be interpreted as originating from different satellite galaxies, but our analysis shows that these interpretations are not physically motivated. In fact, as we show, the coupling of kinematic information with MDFs to reconstruct the accretion history of the MW can bias the reconstructed merger tree towards increasing the number of past accretions and decreasing the masses of the progenitor galaxies.

A detailed chemical study of the extreme velocity stars in the galaxy

ABSTRACT Two decades on, the study of hypervelocity stars is still in its infancy. These stars can provide novel constraints on the total mass of the Galaxy and its dark matter distribution. However how these stars are accelerated to such high velocities is unclear. Various proposed production mechanisms for these stars can be distinguished using chemo-dynamic tagging. The advent of Gaia and other large surveys have provided hundreds of candidate hyper velocity objects to target for ground-based high-resolution follow-up observations. We conduct high-resolution spectroscopic follow-up observations of 16 candidate late-type hyper velocity stars using the Apache Point Observatory and the McDonald Observatory. We derive atmospheric parameters and chemical abundances for these stars. We measure up to 22 elements, including the following nucleosynthetic families: $\alpha$ (Mg, Si, Ca, and Ti), light/odd-Z (Na, Al, V, Cu, and Sc), Fe-peak (Fe, Cr, Mn, Co, Ni, and Zn), and neutron capture (Sr, Y, Zr, Ba, La, Nd, and Eu). Our kinematic analysis shows one candidate is unbound, two are marginally bound, and the remainder are bound to the Galaxy. Finally, for the three unbound or marginally bound stars, we perform orbit integration to locate possible globular cluster or dwarf galaxy progenitors. We do not find any likely candidate systems for these stars and conclude that the unbound stars are likely from the the stellar halo, in agreement with the chemical results. The remaining bound stars are all chemically consistent with the stellar halo as well.

What drives the corpulence of galaxies?

Nearby dwarf galaxies display a variety of effective radii (sizes) at a given stellar mass, suggesting different evolution scenarios according to their final “stellar” size. The TNG hydrodynamical simulations present a bimodality in the z = 0 size–mass relation (SMRz0) of dwarf galaxies, at r1/2, ⋆ ∼ 450 pc. Using the TNG50 simulation, we explored the evolution of the most massive progenitors of dwarf galaxies (z = 0 log(M⋆/M⊙) between 8.4 and 9.2) that end up as central galaxies of their groups. We split these dwarfs into three classes of the SMRz0: “Normals” from the central spine of the main branch, and “Compacts” from the secondary branch as well as the lower envelope of the main branch. Both classes of Compacts see their stellar sizes decrease from z ∼ 1 onwards in contrast to Normals, while the sizes of the gas and dark matter (DM) components continue to increase (as for Normals). A detailed analysis reveals that Compacts live in poorer environments, and thus suffer fewer major mergers from z = 0.8 onwards, which otherwise would pump angular momentum into the gas, allowing strong gas inflows, producing inner star formation, and thus leading to the buildup of a stellar core. Compacts are predicted to be rounder and to have bluer cores. Compact dwarfs of similar sizes are observed in the GAMA survey, but the bimodality in size is less evident and the most compact dwarfs tend to be passive rather than star forming, as in TNG50. Our conclusions should therefore be confirmed with future cosmological hydrodynamical simulations.

Detecting galaxy tidal features using self-supervised representation learning

ABSTRACT Low surface brightness substructures around galaxies, known as tidal features, are a valuable tool in the detection of past or ongoing galaxy mergers, and their properties can answer questions about the progenitor galaxies involved in the interactions. The assembly of current tidal feature samples is primarily achieved using visual classification, making it difficult to construct large samples and draw accurate and statistically robust conclusions about the galaxy evolution process. With upcoming large optical imaging surveys such as the Vera C. Rubin Observatory’s Legacy Survey of Space and Time, predicted to observe billions of galaxies, it is imperative that we refine our methods of detecting and classifying samples of merging galaxies. This paper presents promising results from a self-supervised machine learning model, trained on data from the Ultradeep layer of the Hyper Suprime-Cam Subaru Strategic Program optical imaging survey, designed to automate the detection of tidal features. We find that self-supervised models are capable of detecting tidal features, and that our model outperforms previous automated tidal feature detection methods, including a fully supervised model. An earlier method applied to real galaxy images achieved 76 per cent completeness for 22 per cent contamination, while our model achieves considerably higher (96 per cent) completeness for the same level of contamination. We emphasize a number of advantages of self-supervised models over fully supervised models including maintaining excellent performance when using only 50 labelled examples for training, and the ability to perform similarity searches using a single example of a galaxy with tidal features.

Dark No More: The Low-luminosity Stellar Counterpart of a Dark Cloud in the Virgo Cluster* * Based on observations obtained with the Hobby–Eberly Telescope (HET), which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximillians-Universitaet Muenchen, and Georg-August Universitaet Goettingen. The HET is named in honor of its principal benefactors, William P. Hobby

We have discovered the stellar counterpart to the ALFALFA Virgo 7 cloud complex, which has been thought to be optically dark and nearly star-free since its discovery in 2007. This ∼190 kpc long chain of enormous atomic gas clouds (M H i ∼ 109 M ⊙) is embedded in the hot intracluster medium of the Virgo galaxy cluster but is isolated from any galaxy. Its faint, blue stellar counterpart, BC6, was identified in a visual search of archival optical and UV imaging. Follow-up observations with the Green Bank Telescope, Hobby–Eberly Telescope, and Hubble Space Telescope demonstrate that this faint counterpart is at the same velocity as the atomic gas, actively forming stars, and metal-rich (12 + (O/H) = 8.58 ± 0.25). We estimate its stellar mass to be only log(M*/M⊙)∼4.4 , making it one of the most gas-rich stellar systems known. Aside from its extraordinary gas content, the properties of BC6 are entirely consistent with those of a recently identified class of young, low-mass, isolated, and star-forming clouds in Virgo that appear to have formed via extreme ram pressure stripping events. We expand the existing discussion of the origin of this structure and suggest NGC 4522 as a likely candidate; however, the current evidence is not fully consistent with any of our proposed progenitor galaxies. We anticipate that other “dark” gas clouds in Virgo may have similarly faint, star-forming counterparts. We aim to identify these through the help of a citizen science search of the entire cluster.

Euclid view of the dusty star-forming galaxies at ≳ detected in wide area submillimetre surveys

ABSTRACT We investigate the constraints provided by the Euclid space observatory on the physical properties of dusty star-forming galaxies (DSFGs) at $z\gtrsim 1.5$ detected in wide area submillimetre surveys with Herschel. We adopt a physical model for the high-z progenitors of spheroidal galaxies, which form the bulk of DSFGs at $z\gtrsim 1.5$. We improve the model by combining the output of the equations of the model with a formalism for the spectral energy distribution (SED). After optimizing the SED parameters to reproduce the measured infrared luminosity function and number counts of DFSGs, we simulated a sample of DSFGs over 100 $\hbox{deg}^2$ and then applied a $5\, \sigma$ detection limit of $37\,$ mJy at 250 $\mu$m. We estimated the redshifts from the Euclid data and then fitted the Euclid+Herschel photometry with the code CIGALE to extract the physical parameters. We found that 100 per cent of the Herschel galaxies are detected in all 4 Euclid bands above $3\, \sigma$. For 87 per cent of these sources the accuracy on $1 + z$ is better than 15 per cent. The sample comprises mostly massive, i.e. $\log (M_{\star }/{\rm M}_{\odot })\sim 10.5{\!-\!}12.9$, highly star forming, i.e. $\log (\hbox{SFR}/{\rm M}_{\odot }\hbox{yr}^{-1})\sim 1.5{\!-\!}4$, dusty, i.e. $\log (M_{\rm dust}/{\rm M}_{\odot })\sim 7.5{\!-\!}9.9$, galaxies. The measured stellar masses have a dispersion of 0.19 dex around the true value, thus showing that Euclid will provide reliable stellar mass estimates for the majority of the bright DSFGs at $z\gtrsim 1.5$ detected by Herschel. We also explored the effect of complementing the Euclid photometry with that from the Vera C. Rubin Observatory/LSST.

Teaming up Radio and Sub-mm/FIR Observations to Probe Dusty Star-Forming Galaxies

In this paper, we investigate the benefits of teaming up data from the radio to the far-infrared (FIR) regime for the characterization of dusty star-forming galaxies (DSFGs). These galaxies are thought to be the star-forming progenitors of local massive quiescent galaxies and to play a pivotal role in the reconstruction of the cosmic star formation rate density up to high redshift. Due to their dust-enshrouded nature, DSFGs are often invisible in the near-infrared/optical/UV bands. Therefore, they necessitate observations at longer wavelengths, primarily the FIR band, where dust emission occurs, and the radio band, which is not affected by dust absorption. Combining data from these two spectral windows makes it possible to characterize even the dustiest objects, enabling the retrieval of information about their age, dust temperature, and star-formation status, and facilitates the differentiation between various galaxy populations that evolve throughout cosmic history. Despite the detection of faint radio sources being a challenging task, this study demonstrates that an effective strategy to build statistically relevant samples of DSFGs would be reaching deep sensitivities in the radio band, even restricted to smaller areas, and then combining these radio observations with FIR/submm data. Additionally, this paper quantifies the improvement in the spectral energy distribution (SED) reconstruction of DSFGs by incorporating ALMA band measurements, in particular, in its upgraded status thanks to the anticipated Wideband Sensitivity Upgrade.

Differences in the properties of disrupted and surviving satellites of Milky-Way-mass galaxies in relation to their host accretion histories

ABSTRACT From the chemodynamical properties of tidal debris in the Milky Way, it has been inferred that the dwarf satellites that have been disrupted had different chemical abundances from their present-day counterparts of similar mass that survive today, specifically, they had lower [Fe/H] and higher [Mg/Fe]. Here we use the Artemis simulations to study the relation between the chemical abundances of disrupted progenitors of MW-mass galaxies and their stellar mass, and the evolution of the stellar mass–metallicity relations (MZR) of this population with redshift. We find that these relations have significant scatter, which correlates with the accretion redshifts (zacc) of satellites, and with their cold gas fractions. We investigate the MZRs of dwarf populations accreted at different redshifts and find that they have similar slopes, and also similar with the slope of the MZR of the surviving population (≈0.32). However, the entire population of disrupted dwarfs displays a steeper MZR, with a slope of ≈0.48, which can be explained by the changes in the mass spectrum of accreted dwarf galaxies with redshift. We find strong relations between the (mass-weighted) 〈zacc〉 of the disrupted populations and their global chemical abundances (〈[Fe/H]〉 and 〈[Mg/Fe]〉), which suggests that chemical diagnostics of disrupted dwarfs can be used to infer the types of merger histories of their hosts. For the case of the MW, our simulations predict that the bulk of the disrupted population was accreted at 〈zacc〉 ≈ 2, in agreement with other findings. We also find that disrupted satellites form and evolve in denser environments, closer to their hosts, than their present-day counterparts.

Faint mm NIKA2 dusty star-forming galaxies: Finding the high-redshift population

Aims. High-redshift dusty star-forming galaxies (DSFGs) are proposed to be the progenitors of massive quiescent galaxies arising at cosmic noon, providing a crucial insight into the formation, assembly, and early quenching of massive galaxies in the early Universe. However, their high redshift combined with high dust obscuration adds significant difficulties to their redshift measurement, which is mandatory for detailed studies of their physical properties. Blind mm spectral scans are the most unbiased way in prinicple for obtaining accurate spectroscopic redshifts for these sources, but identifying faint molecular and atomic lines within limited telescope time for faint DSFGs is also difficult with these scans. Methods. We developed a new framework to constrain the source redshift. The method jointly accounts for the detection and/or nondetection of spectral lines and the prior information from the photometric redshift and total infrared luminosity from spectral energy distribution analysis. The method uses the estimated total infrared luminosity to predict the line fluxes at given redshifts and generates model spectra. The redshift-dependent spectral models were then compared with the observed spectra to determine the redshift. Results. We applied this joint redshift analysis method to four high-z dusty star-forming galaxy candidates selected from the NIKA2 observations of the HLSJ091828.6+514223 (HLS) field that were further observed by NOEMA with blind spectral scans. These sources only have Herschel SPIRE photometry as ancillary data. They were selected because SPIRE counterparts are faint or entirely lacking and thus favor to select the highest-redshift candidates. The method finds a spectroscopic redshift of 4 in the five NOEMA-counterpart detected sources, with z > 3. Based on these measurements, we derived the CO and [CI] lines and mm continuum fluxes from the NOEMA data and studied the properties of their interstellar medium and star formation. We find cold dust temperatures in some of the HLS sources compared to the general population of submm galaxies, which might be related to the bias introduced by the SPIRE-dropout selection. All sources except for one have a short gas-depletion time of a few hundred million years, which is typical of high-z submm galaxies. The only exception shows a longer gas-depletion time of up to a few billion years. This is comparable to the gas-depletion times of main-sequence galaxies at the same redshift. Furthermore, we identify a possible overdensity of dusty star-forming galaxies at z = 5.2 that is traced by two sources in our sample, as well as a lensed galaxy HLSJ091828.6+514223. Conclusions. We demonstrate that our method when applied to mm-selected DSFGs is able to determine the redshift accurately. This accuracy with only multiple emission lines with a low signal-to-noise ratio shows promising potential for the blind redshift search in large samples of high-z DSFGs, even in the absence of optical to near infrared photometric redshifts.

Chemical Cartography of the Sagittarius Stream with Gaia

The stellar stream connected to the Sagittarius (Sgr) dwarf galaxy is the most massive tidal stream that has been mapped in the Galaxy, and is the dominant contributor to the outer stellar halo of the Milky Way (MW). We present metallicity maps of the Sgr stream, using 34,240 red giant branch stars with inferred metallicities from Gaia BP/RP spectra. This sample is larger than previous samples of Sgr stream members with chemical abundances by an order of magnitude. We measure metallicity gradients with respect to Sgr stream coordinates (Λ, B), and highlight the gradient in metallicity with respect to stream latitude coordinate B, which has not been observed before. Including the core, we find ∇[M/H] = −2.48 ± 0.08 × 10−2 dex deg−1 above the stream track (B > B 0, where B 0 = 1.5° is the latitude of the Sgr remnant) and ∇[M/H] = −2.02 ± 0.08 × 10−2 dex deg−1 below the stream track (B < B 0). By painting metallicity gradients onto a tailored N-body simulation of the Sgr stream, we find that the observed metallicities in the stream are consistent with an initial radial metallicity gradient in the Sgr dwarf galaxy of ∼−0.1 to −0.2 dex kpc−1, well within the range of observed metallicity gradients in Local Group dwarf galaxies. Our results provide novel observational constraints for the internal structure of the dwarf galaxy progenitor of the Sgr stream. Leveraging new large data sets in conjunction with tailored simulations, we can connect the present-day properties of disrupted dwarfs in the MW to their initial conditions.

Introducing the Condor array telescope II – deep imaging observations of the edge-on spiral galaxy NGC 5907 and the NGC 5866 Group: yet another view of the iconic stellar stream

ABSTRACT We used the Condor array telescope to obtain deep imaging observations through the luminance filter of the entirety of the NGC 5866 Group, including a very extended region surrounding the galaxy NGC 5907 and its stellar stream. We find that the stellar stream consists of a single curved structure that stretches 220 kpc from a brighter eastern stream to a fainter western stream that bends to the north and then curls back toward the galaxy. This result runs contrary to a previous claim of a second loop of the stellar stream but is consistent with another previous description of the overall morphology of the stream. We further find that: (1) an extension of the western stream appears to bifurcate near its apex, (2) there is an apparent gap of ≈6 kpc in the western stream due east of the galaxy, (3) contrary to a previous claim, there is no evidence of the remnant of a progenitor galaxy within the eastern stream, although (4) there are many other possible progenitor galaxies, (5) there is another structure that, if it is at the distance of the galaxy, stretches 240 kpc and contains two very large, very low-surface-brightness ‘patches’ of emission, one of which was noted previously and another of which was not. We note the number and variety of stellar streams in the vicinity of NGC 5907 and the apparent gap in the western stream, which may be indicative of a dark sub-halo or satellite in the vicinity of the galaxy.

Hints of a disrupted binary dwarf galaxy in the Sagittarius stream

ABSTRACT In this work, we look for evidence of a non-unity mass ratio binary dwarf galaxy merger in the Sagittarius stream. Simulations of such a merger show that, upon merging with a host, particles from the less massive galaxy will often mostly be found in the extended stream and less so in the central remnant. Motivated by these simulations, we use the Apache Point Observatory Galactic Evolution Experiment Data Release 17 chemical data from approximately 1100 stars in both the Sagittarius remnant and stream to look for evidence of contamination from a second dwarf galaxy. We separate the Sagittarius data into its remnant and stream and compare the [Mg/Fe] content of the two populations. In particular, we select [Mg/Fe] to search for hints of unique star formation histories among our sample stars. Comparing the stream and remnant populations, we find regions that have distinct [Mg/Fe] distributions for fixed [Fe/H], in addition to distinct chemical tracks in [Mg/Fe]–[Fe/H] abundance space. We show that there are large regions of the tracks for which the probability of the two samples being drawn from the same distribution is very low (p &amp;lt; 0.05). Furthermore, we show that the two tracks can be fit with unique star formation histories using simple, one-zone galactic chemical evolution models. While more work must be done to discern whether the hypothesis presented here is true, our work hints at the possibility that Sagittarius may consist of two dwarf galaxy progenitors.

Massive red spiral galaxies in SDSS-IV MaNGA survey

ABSTRACT Massive red spiral galaxies (MRSGs) are supposed to be the possible progenitors of lenticular galaxies (S0s). We select a large sample of MRSGs ($M_*\gt 10^{10.5}\rm {\rm M}_{\odot }$) from Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) DR17 using the g − r colour versus stellar mass diagram, along with control samples of blue spirals and S0s. Our main results are as follows: (1) After comparing the Sérsic index, concentration parameter, asymmetry parameter distribution, size–mass relation, and Σ1 (stellar mass surface density within the central 1 kpc)−mass relation, we find MRSGs are similar to S0s and have more compact and symmetric structures than blue spirals. MRSGs also resemble S0s in Dn4000, metallicity, Mgb/$\rm \left\langle Fe \right\rangle$, and V/σ radial profile. (2) By using MaNGA 2D spectra data, we separate the spatial regions into inner (R &amp;lt; 0.8Re) and outer (0.8 &amp;lt; R &amp;lt; 1.5Re) regions, and detect residual star formation in the outer regions of MRSGs. (3) When we select a sub-sample of MRSGs with NUV − r &amp;gt; 5, we find that they are completely star formation quenched in both inner and outer regions. Compared to optically selected MRSGs, NUV − r selected MRSGs appear to be more concentrated and have more massive dark matter haloes. The similarities between S0s and MRSGs suggest the possible evolutionary trend between MRSGs and S0s.

The internal dynamics and environments of Relics and compact massive ETGs with TNG50

ABSTRACT Relic galaxies are massive, compact, and quiescent objects observed in the local Universe that have not experienced any significant interaction episodes or merger events since about z = 2, remaining relatively unaltered since their formation. On the other hand, massive and compact early-type galaxies (cETGs) in the local Universe appear to show similar properties to Relic galaxies, despite having substantial accretion history. Relic galaxies, with frozen history, can provide important clues about the intrinsic processes related to the evolutionary pathways of ETGs and the role that mergers play in their evolution. Using the high-resolution cosmological simulation TNG50-1 from IllustrisTNG project, we investigate the assembly history of a sample of massive, compact, old, and quiescent subhaloes split by satellite accretion fraction. We compare the evolutionary pathways at three cosmic epochs: z = 2, z = 1.5, and z = 0, using the orbital decomposition numerical method to investigate the stellar dynamics of each galactic kinematical component and their environmental correlations. Our results point to a steady pathway across time that is not strongly dependent on the mergers or the environment. Relics and cETGs do not show a clear preference for high- or low-density environments within the volume explored at z = 0, as they are found in both scenarios. However, the progenitors of Relic galaxies have consistently resided in high-density environments since z = 2, while cETGs were shifted to such environments at a later stage. The merger history can be recovered from the stellar kinematics imprints in the local Universe. Relics and cETGs show consistently dynamical similarities at z = 2 and differences at z = 0 to disc, bulge, and hot inner stellar halo. In the current scenario, the mergers that drive the growth of cETGs do not give rise to a new and distinct evolutionary pathway when compared to Relics.

Prospective dark matter annihilation signals from the Sagittarius Dwarf Spheroidal

ABSTRACT The Sagittarius Dwarf Spheroidal galaxy (Sgr) is investigated as a target for dark matter (DM) annihilation searches utilizing J-factor distributions calculated directly from a high-resolution hydrodynamic simulation of the infall and tidal disruption of Sgr around the Milky Way. In contrast to past studies, the simulation incorporates DM, stellar and gaseous components for both the Milky Way and the Sgr progenitor galaxy. The simulated distributions account for significant tidal disruption affecting the DM density profile. Our estimate of the J-factor value for Sgr, JSgr = 1.48 × 1010 M$_\odot ^2$ kpc−5 (6.46 × 1016 GeV cm−5), is significantly lower than found in prior studies. This value, while formally a lower limit, is likely close to the true J-factor value for Sgr. It implies a DM cross-section incompatibly large in comparison with existing constraints would be required to attribute recently observed gamma-ray emission from Sgr to DM annihilation. We also calculate a J-factor value using a NFW profile fitted to the simulated DM density distribution to facilitate comparison with past studies. This NFW J-factor value supports the conclusion that most past studies have overestimated the dark matter density of Sgr on small scales. This, together with the fact that the Sgr has recently been shown to emit gamma-rays of astrophysical origin, complicate the use of Sgr in indirect DM detection searches.

Formation of dense filaments induced by runaway supermassive black holes

ABSTRACT A narrow linear object extending ∼60 kpc from the centre of a galaxy at redshift z ∼ 1 has recently been discovered and interpreted as shocked gas filament forming stars. The host galaxy presents an irregular morphology, implying recent merger events. Supposing that each of the progenitor galaxies has a central supermassive black hole (SMBH) and the SMBHs are accumulated at the centre of the merger remnant, a fraction of them can be ejected from the galaxy with a high velocity due to interactions between SMBHs. When such a runaway SMBH (RSMBH) passes through the circumgalactic medium (CGM), converging flows are induced along the RSMBH path, and star formation could eventually be ignited. We show that the CGM temperature prior to the RSMBH perturbation should be below the peak temperature in the cooling function to trigger filament formation. While the gas is temporarily heated due to compression, the cooling efficiency increases, and gas accumulation becomes allowed along the path. When the CGM density is sufficiently high, the gas can cool down and develop a dense filament by z = 1. The mass and velocity of the RSMBH determine the scale of filament formation. Hydrodynamical simulations validate the analytical expectations. Therefore, we conclude that the perturbation by RSMBHs is a viable channel to form the observed linear object. Using the analytical model validated by simulations, we show that the CGM around the linear object to be warm ($T \lesssim 2 \times 10^5$ K) and dense ($n \gtrsim 2 \times 10^{-5} (T/2 \times 10^5 \, K)^{-1} \, {\rm cm^{-3}}$).