Dwarf Galaxies Research Articles

The Local Group L-band Survey: The First Measurements of Localized Cold Neutral Medium Properties in the Low-metallicity Dwarf Galaxy NGC 6822

Abstract Measuring the properties of the cold neutral medium (CNM) in low-metallicity galaxies provides insights into heating and cooling mechanisms in early Universe-like environments. We report detections of two localized atomic neutral hydrogen (H i) absorption features in NGC 6822, a low-metallicity (0.2 Z ⊙) dwarf galaxy in the Local Group. These are the first unambiguous CNM detections in a low-metallicity dwarf galaxy outside the Magellanic Clouds. The Local Group L-band Survey (LGLBS) enabled these detections, due to its high spatial (15 pc for H i emission) and spectral (0.4 km s−1) resolution. We introduce LGLBS and describe a custom pipeline for searching for H i absorption at high angular resolution and extracting associated H i emission. A detailed Gaussian decomposition and radiative transfer analysis of the NGC 6822 detections reveals five CNM components, with key properties: a mean spin temperature of 32 ± 6 K, a mean CNM column density of 3.1 × 1020 cm−2, and CNM mass fractions of 0.33 and 0.12 for the two sightlines. Stacking nondetections does not reveal low-level signals below our median optical depth sensitivity of 0.05. One detection intercepts a star-forming region, with the H i absorption profile encompassing the CO (2−1) emission, indicating coincident molecular gas and a depression in high-resolution H i emission. We also analyze a nearby sightline with deep, narrow H i self-absorption dips, where the background warm neutral medium is attenuated by intervening CNM. The association of CNM, CO, and Hα emissions suggests a close link between the colder, denser H i phase and star formation in NGC 6822.

(Re)mind the gap: A hiatus in star formation history unveiled by APOGEE DR17

Context. Analysis of several spectroscopic surveys indicates the presence of a bimodality between the disc stars in the abundance ratio space of [α/Fe] versus [Fe/H]. The two stellar groups are commonly referred to as the high-α and low-α sequences. Some models capable of reproducing such a bimodality invoke the presence of a hiatus in the star formation history in our Galaxy, whereas other models explain the two sequences by means of stellar migration. Aims. Our aim is to show that the existence of the gap in the star formation rate between high-α and low-α is evident in the stars of APOGEE DR17, if one plots [Fe/α] versus [α/H], confirming previous suggestions. We then try to interpret the data by means of detailed chemical models. Methods. We compare the APOGEE DR17 red giant stars with the predictions of a detailed chemical evolution model based on the two-infall paradigm, taking into account also the possible accretion of dwarf satellites. Results. The APOGEE DR17 abundance ratios [Fe/α] versus [α/H] exhibit a sharp increase in [Fe/α] at a nearly constant [α/H] (where α elements considered are Mg, Si, O) during the transition between the two disc phases. This observation strongly supports the hypothesis that a hiatus in star formation occurred during this evolutionary phase. Notably, the most pronounced growth in the [Fe/α] versus [α/H] relation is observed for oxygen, as this element is exclusively synthesised in core-collapse supernovae. The revised version of the two-infall chemical evolution model proposed in this study reproduces the APOGEE DR17 abundance ratios better than before. Particularly noteworthy is the model’s ability to predict the hiatus in the star formation between the two infalls of gas, which form the thick and thin disc, respectively, and thus generate abundance ratios compatible with APOGEE DR17 data. Conclusions. We show that the signature of a hiatus in the star formation is imprinted in the APOGEE DR17 abundance ratios. A chemical model predicting a pause in the star formation of a duration of roughly 3.5 Gyr, and in which the high-α disc starts forming from pre-enriched gas by a previous encounter with a dwarf galaxy, could well explain the observations

HR-GO

Context. Dwarf galaxy streams encode vast amounts of information essential to understanding early galaxy formation and nucleosynthesis channels. Due to the variation in the timescales of star formation history in their progenitors, stellar streams serve as ‘snapshots’ that record different stages of galactic chemical evolution. Aims. This study focusses on the Cetus stream, stripped from a low-mass dwarf galaxy. We aim to uncover its chemical evolution history as well as the different channels of its element production from detailed elemental abundances. Methods. We carried out a comprehensive analysis of the chemical composition of 22 member stars based on their high-resolution spectra. We derived abundances for up to 28 chemical species from C to Dy and, for 20 of them, we account for the departures from local thermodynamic equilibrium (NLTE effects). Results. We confirm that the Cetus stream has a mean metallicity of [Fe/H] = −2.11 ± 0.21. All observed Cetus stars are α enhanced with [α/Fe] ≃ 0.3. The absence of the α-‘knee’ implies that star formation stopped before iron production in type Ia supernovae (SNe Ia) became substantial. Neutron capture element abundances suggest that both the rapid (r-) and the main slow (s-) processes contributed to their origin. The decrease in [Eu/Ba] from a typical r-process value of [Eu/Ba] = 0.7–0.3 with increasing [Ba/H] indicates a distinct contribution of the r- and s-processes to the chemical composition of different Cetus stars. For barium, the r-process contribution varies from 100 to 20% in different sample stars, with an average value of 50%. Conclusions. Our abundance analysis indicates that the star formation in the Cetus progenitor ceased after the onset of the main s-process in low- to intermediate-mass asymptotic giant branch stars but before SNe Ia played an important role. A distinct evolution scenario is revealed by comparing the abundances in the Ursa Minor dwarf spheroidal galaxy, showing the diversity in – and uniqueness of – the chemical evolution of low-mass dwarf galaxies.

Two-process Model and Residual Abundance Analysis of the Milky Way Massive Satellites

The “two-process model” is a promising technique for interpreting stellar chemical abundance data from large-scale surveys (e.g., the Sloan Digital Sky Survey IV/V and the Galactic Archeology with HERMES survey), enabling more quantitative empirical studies of differences in chemical enrichment history between galaxies without relying on detailed yield and evolution models. In this work, we fit two-process model parameters to (1) a luminous giant Milky Way (MW) sample and (2) stars comprising the Sagittarius dwarf galaxy (Sgr). We then use these two sets of model parameters to predict the abundances of 14 elements of stars belonging to the MW and in five of its massive satellite galaxies, analyzing the residuals between the predicted and observed abundances. We find that the model fit to (1) results in large residuals (0.1–0.3 dex) for most metallicity-dependent elements in the metal-rich ([Mg/H] > −0.8) stars of the satellite galaxies. However, the model fit to (2) results in small or no residuals for all elements across all satellite galaxies. Therefore, despite the wide variation in [X/Mg]–[Mg/H] abundance patterns of the satellite galaxies, the two-process framework provides an accurate characterization of their abundance patterns across many elements, but these multielement patterns are systematically different between the dwarf galaxy satellites and the MW disks. We consider a variety of scenarios for the origin of this difference, highlighting the possibility that a large inflow of pristine gas to the MW disk diluted the metallicity of star-forming gas without changing abundance ratios.

Lopsided distribution of MATLAS and ELVES dwarf satellite systems around isolated host galaxies

The properties of satellite dwarf galaxies provide important empirical insights for verifying cosmological models on galaxy scales. Their phase-space correlations, in particular, offer interesting insights into a broad range of models, since they are dominated by gravity and are largely independent of the specific dark matter flavor or baryonic processes that are considered. Next to the much-debated planes-of-satellites phenomenon, the lopsided distribution of satellites relative to their host galaxy has been studied observationally and in cosmological simulations. The degree to which observed lopsidedness is consistent with expectations from simulations is still unclear. We quantified the level of lopsidedness in isolated observed satellite systems under six different metrics. We studied 47 systems from the MATLAS survey beyond the local volume (LV) as well as 21 LV satellite systems from the ELVES survey. The satellite systems are complete to an estimated absolute magnitude of M ∼ −9. We find that the so-called wedge metric, counting the number of dwarfs in wedges with varying opening angles, is best suited to capture a system’s overall lopsidedness. Under this metric, our analysis reveals that ∼16 percent of the tested satellite systems exhibit a statistically significant degree of lopsidedness when compared to systems with randomly generated satellite position angles. This presents a notable excess over the expected 5% (2σ level) of significantly lopsided systems in a sample with no overall inherent lopsidedness. To gain the most rounded picture, however, a combination of metrics that are sensitive to different features of lopsidedness should be used. Combining all tested metrics, the number of significantly lopsided systems increases to ∼21 percent. Contrary to recent results from the literature, we find more lopsided systems among the red early-type galaxies in the MATLAS survey compared to the mostly blue late-type hosts in ELVES. We further find that satellite galaxies at larger distances from the host, potentially recently accreted, are likely the primary contributors to the reported excess of lopsidedness. Our results set the groundwork that allows a comparison with similar systems in cosmological simulations to assess the consistency with the standard model.

Lyα Profile Shape as an Escape-fraction Diagnostic at High Redshift

While the shape of the Lyα profile is viewed as one of the best tracers of ionizing-photon escape fraction (f esc) within low-redshift (z ∼ 0.3) surveys of the Lyman continuum, this connection remains untested at high redshift. Here, we combine deep, rest-UV Keck/LRIS spectra of 80 objects from the Keck Lyman Continuum Spectroscopic Survey with rest-optical Keck/MOSFIRE spectroscopy in order to examine potential correlations between Lyα profile shape and the escape of ionizing radiation within z ∼ 3 star-forming galaxies. We measure the velocity separation between double-peaked Lyα emission structure (v sep), between red-side Lyα emission peaks and systemic (v Lyα,red), and between red-side emission peaks and low-ionization interstellar absorption lines (v Lyα−LIS). We find that the IGM-corrected ratio of ionizing to nonionizing flux density is significantly higher in KLCS objects with lower v Lyα,red. We find no significant trend between measures of ionizing-photon escape and v Lyα−LIS. We compare our results to measurements of z ∼ 0.3 “Green Peas” from the literature and find that KLCS objects have larger v sep at fixed v Lyα,red, larger f esc at fixed v Lyα,red, and higher v Lyα,red overall than z ∼ 0.3 analogs. We conclude that the Lyα profile shapes of our high-redshift sources are fundamentally different, and that measurements of profile shape such as v Lyα,red map on to f esc in different ways. We caution against building reionization-era f esc diagnostics based purely on Lyα profiles of low-redshift dwarf galaxies. Tracing v sep, v Lyα,red, and f esc in a larger sample of z ∼ 3 galaxies will reveal how these variables may be connected for galaxies at the epoch of reionization.

Strong He i Emission Lines in High N/O Galaxies at z ∼ 6 Identified in JWST Spectra: High He/H Abundance Ratios or High Electron Densities?

We present He i/Hβ flux and He/H abundance ratios in three James Webb Space Telescope galaxies with significant constraints on N/O abundance ratios, GS-NDG-9422, RXCJ2248-ID, and GLASS150008 at z ∼ 6 mostly with the spectroscopic coverage from He i λ4471 and He ii λ4686 to He i λ7065, and comparing with 68 local dwarf galaxies. We find that these high-z galaxies present strong He i emission with He i/Hβ flux ratios generally larger than those of local dwarf galaxies. We derive He/H with all of the detected Hei, He ii, and 2−3 hydrogen Balmer lines in the same manner as the local He/H determination conducted for cosmology studies. These high-z galaxies show He overabundance He/H ≳0.10 or high electron density of n e ∼ 103−4 cm−3 much larger than local values at low O/H, 12+log(O/H)=7−8 . In contrast, we obtain low He/H and n e values for our local dwarf galaxies by the same technique with the same helium and hydrogen lines, and confirm that the difference between the high-z and local dwarf galaxies is not mimicked by systematics. While two scenarios of (1) He overabundance and (2) high electron density are not clearly concluded, we find that there is a positive correlation between the He/H–N/O or n e–N/O plane by the comparison of the high-z and local dwarf galaxies. Scenario (1) suggests that the overabundant helium and nitrogen are not explained by the standard chemical enrichment of core-collapse supernovae, but by the CNO-cycle products and equilibrium ratios, respectively. Scenario (2) indicates that the strong helium lines originated from the central dense clouds of the high-z galaxies by excessive collisional excitation.

Observations of Extremely Metal-poor O Stars: Weak Winds and Constraints for Evolution Models

Metal-poor massive stars drive the evolution of low-mass galaxies, both locally and at high redshift. However, quantifying the feedback they impart to their local surroundings remains uncertain because models of stellar evolution, mass loss, and ionizing spectra are unconstrained by observations below 20% solar metallicity (Z ⊙). We present new Keck Cosmic Web Imager optical spectroscopy of three O-type stars in the nearby dwarf galaxies Leo P, Sextans A, and WLM, which have gas-phase oxygen abundances of 3%–14% Z ⊙. To characterize their fundamental stellar properties and radiation-driven winds, we fit PoWR atmosphere models to the optical spectra simultaneously with Hubble Space Telescope far-ultraviolet (FUV) spectra and multiwavelength photometry. We find that all three stars have effective temperatures consistent with their spectral types and surface gravities typical of main-sequence dwarf stars. Yet, the combination of those inferred parameters and luminosity for the two lower-Z stars is not reproduced by stellar evolution models, even those that include rotation or binary interactions. The scenario of multiple-star systems is difficult to reconcile with all available data, suggesting that these observations pose a challenge to current evolution models. We highlight the importance of validating the relationship between stellar mass, temperature, and luminosity at very low Z for accurate estimates of ionizing photon production and spectral hardness. Finally, all three stars’ FUV wind profiles reveal low mass-loss rates and terminal wind velocities in tension with expectations from widely adopted radiation-driven wind models. These results provide empirical benchmarks for future development of mass-loss and evolution models for metal-poor stellar populations.

Dwarf galaxies in the MATLAS Survey: Hubble Space Telescope observations of the globular cluster systems of 74 ultra-diffuse galaxies

Ultra-diffuse galaxies (UDGs), characterised by their low surface brightness and large physical size, constitute a subclass of dwarf galaxies that challenge our current understanding of galaxy formation and evolution. In this paper, we probe the properties of 74 UDGs, identified in the MATLAS survey, based on a comprehensive study of their globular cluster (GC) populations. We obtained high-resolution HST imaging of these galaxies using the ACS F606W and F814W filters, which allowed us to select GCs on the basis of the colour and concentration index. After a background subtraction and completeness correction, we calculated an overall total of 387 GCs. The number of GCs per galaxy ranges from 0 to 38, with the majority (64%) having low counts (0 − 2 GCs). On average, the more massive UDGs tend to host a larger number of GCs. We find that our UDGs have specific frequencies (SN) ranging from 0 to 91, with a small population (9%) having SN > 30. The median SN of our sample is similar to the one for the Perseus cluster UDGs, despite the fact that our UDGs are found in lower density environments. The SN measurements for individual galaxies can extend beyond those found in Perseus, but remain below the values found for UDGs in the Virgo and Coma cluster. Based on a trending analysis of the SN values with the host galaxy properties, we find trends with host galaxy size, roundness, colour, and local density. For the UDGs with sufficiently high statistics, we studied 2D density maps of the GC distributions, which display a variety of appearances: symmetric, asymmetric, off-centre, and elongated. The UDGs with disturbed density maps also show disturbed stellar light morphologies. We further quantified the distribution by modelling it with a Sérsic profile, finding Re, GC/Re, gal ∼ 1.0, which indicates that the GCs follow the stellar light of the host galaxy.

The Lower Limit of Dynamical Black Hole Masses Detectable in Virgo Compact Stellar Systems Using the JWST/NIRSpec IFU

Due to observational challenges, the mass function of black holes (BH) at lower masses is poorly constrained in the local universe. Understanding the occupation fraction of BHs in low-mass galaxies is crucial for constraining the origins of supermassive BH seeds. Compact stellar systems (CSSs), including ultracompact dwarf galaxies (UCDs) and compact elliptical galaxies (cEs), are potential intermediate-mass BH hosts. Despite the difficulties posed by their limited spheres of influence, stellar dynamical modeling has been effective in estimating central BH masses in CSSs. Some CSSs may harbor a BH constituting up to 20% of their host stellar mass, while others might not have a central BH. In support of our ongoing efforts to determine the BH masses in select CSSs in the Virgo cluster using JWST/NIRSpec IFU observations and orbit-superposition dynamical models, we create mock kinematic data mimicking the characteristics of observed cEs/UCDs in the Virgo cluster with different BH masses. We then construct a series of dynamical models using the orbit-superposition code FORSTAND and explore the accuracy of recovering the BH mass. We find that the mass of BHs comprising 1% or more of the total host stellar mass can be accurately determined through kinematic maps featuring higher-order velocity moments. We also assess how BH mass measurement is affected by deprojection methods, regularization factors, anisotropy parameters, orbit initial conditions, the absence of higher-order velocity moments, the spatial resolution, and the signal-to-noise ratio.

Mapping Multiphase Metals in Star-forming Galaxies: A Spatially Resolved UV+Optical Study of NGC 5253

We present a pioneering, spatially resolved, multiphase gas abundance study on the blue compact dwarf galaxy NGC 5253, targeting 10 star-forming (SF) clusters inside six far-UV (FUV) Hubble Space Telescope/Cosmic Origins Spectrograph (COS) pointings with cospatial optical Very Large Telescope/MUSE observations throughout the galaxy. The SF regions span a wide range of ages (1–15 Myr) and are distributed at different radii (50–230 pc). We performed a robust absorption-line profile fitting on the COS spectra, covering 1065–1430 Å in the FUV, allowing an accurate computation of neutral-gas abundances for 13 different ions sampling eight elements. These values were then compared with the ionized-gas abundances, measured using the direct method on MUSE integrated spectra inside analog COS apertures. Our multiphase, spatially resolved comparisons find abundances, which are lower in the neutral gas than the ionized gas by 0.22, 0.80, and 0.58 dex for log(O/H), log(N/H), and log(N/O), respectively. We modeled the chemical abundance distributions and evaluated correlations as a function of the radius and age. It was found that, while N, O, and N/O abundances decrease as a function of age in the ionized gas, they increase with age in the neutral gas. No strong correlations for N, O, or N/O were observed as a function of the radius. The N/O and N/H offsets between the phases were found to decrease with age, providing evidence that chemical enrichment happens differentially, first in the ionized-gas phase around 2–5 Myrs (due to N-rich Wolf–Rayet stars) and then mixing out into the cold neutral gas on longer timescales of 10–15 Myr.

Dwarf galaxies as a probe of a primordially magnetized Universe

Aims. The true nature of primordial magnetic fields (PMFs) and their role in the formation of galaxies remains elusive. To shed light on these unknowns, we investigated their impact by varying two sets of properties: (i) accounting for the effect of PMFs on the initial matter power spectrum and (ii) accounting for their magneto-hydrodynamical effects on the formation of galaxies. By comparing both, we can determine the dominant agent in shaping galaxy evolution. Methods. We used the magneto-hydrodynamics code RAMSES to generate multiple new zoom-in simulations for eight different host halos of dwarf galaxies across a wide luminosity range of 103 − 106 L⊙. These halos were selected from a ΛCDM cosmological box, tracking their evolution down to redshift z = 0. We explored a variety of primordial magnetic field (comoving) strengths of Bλ ranging from 0.05 to 0.50 nG. Results. We find that magnetic fields in the interstellar medium not only modify star formation processes in dwarf spheroidal galaxies, but these fields also entirely prevent the formation of stars in less compact, ultra-faint galaxies with halo masses and stellar masses below, respectively, ∼2.5 × 109 and 3 × 106 M⊙. At high redshifts, the impact of PMFs on host halos of dwarf galaxies through the modification of the matter power spectrum is more dominant than the influence of magneto-hydrodynamics in shaping their gaseous structure. Through the amplification of small perturbations ranging in mass from 107 to 109 M⊙ in the ΛCDM+PMFs matter power spectrum, primordial fields expedite the formation of the first dark matter halos, leading to an earlier onset and a higher star formation rate at redshifts of z > 9. We investigated the evolution of various energy components and demonstrated that magnetic fields with an initial strength of Bλ ≥ 0.05 nG exhibit a strong growth of magnetic energy, accompanied by a saturation phase that begins soon after the growth phase. These trends persist consistently, regardless of the initial conditions or whether it is the classical ΛCDM model or ΛCDM modified by PMFs. Lastly, we investigated the impact of PMFs on the present-time observable properties of dwarf galaxies, namely: the half light radius, V-band luminosity, mean metallicity, and velocity dispersion profile. We find that PMFs with moderate strengths of Bλ ≤ 0.10 nG show an impressive agreement with the scaling relations of the observed Local Group dwarfs. However, stronger fields lead to larger sizes and higher velocity dispersions.

A Radio Study of Persistent Radio Sources in Nearby Dwarf Galaxies: Implications for Fast Radio Bursts

We present 1–12 GHz Karl G. Jansky Very Large Array observations of nine off-nuclear persistent radio sources (PRSs) in nearby (z ≲ 0.055) dwarf galaxies, along with high-resolution European VLBI Network observations for one of them at 1.7 GHz. We explore the plausibility that these PRSs are associated with fast radio burst (FRB) sources by examining their properties—physical sizes, host-normalized offsets, spectral energy distributions (SEDs), radio luminosities, and light curves—and compare them to those of the PRSs associated with FRB 20121102A and FRB 20190520B, two known active galactic nuclei (AGN), and one likely AGN in our sample with comparable data, as well as other radio transients exhibiting characteristics analogous to FRB-PRSs. We identify a single source in our sample, J1136+2643, as the most promising FRB-PRS, based on its compact physical size and host-normalized offset. We further identify two sources, J0019+1507 and J0909+5655, with physical sizes comparable to FRB-PRSs, but which exhibit large offsets and flat spectral indices potentially indicative of a background AGN origin. We test the viability of neutron star wind nebula and hypernebula models for J1136+2643 and find that the physical size, luminosity, and SED of J1136+2643 are broadly consistent with these models. Finally, we discuss the alternative interpretation that the radio sources are instead powered by accreting massive black holes, and we outline future prospects and follow-up observations for differentiating between these scenarios.

Euclid Early Release Observations -- Globular clusters in the Fornax galaxy cluster, from dwarf galaxies to the intracluster field

Euclid Early Release Observations -- Globular clusters in the Fornax galaxy cluster, from dwarf galaxies to the intracluster field

Explaining the oblate morphology of dwarf spheroidals with wave dark matter perturbations

ABSTRACT We investigate whether the oblate, spheroidal morphology of common dwarf spheroidal galaxies (dSph) may result from the slow relaxation of stellar orbits within a halo of wave dark matter ($\psi$DM) when starting from an initial disc of stars. Stellar orbits randomly walk over a Hubble time, perturbed by the pervasive ‘granular’ interference pattern of $\psi$DM, that fully modulates the dark matter density on the de Broglie scale. Our simulations quantify the level of stellar disc thickening over the Hubble time, showing that distribution of stars is predicted to become an oblate spheroid of increasing radius, that plausibly accounts for the morphology of dSph galaxies. We predict a low level of residual rotation remains after a Hubble time at the 1–3 km/s level, depending on orientation, that compares with recent claims of rotation for some well-studied local dSph galaxies. This steady internal dynamical evolution may be witnessed directly with JWST for well-resolved dwarf galaxies, appearing more oblate with look back time and tending to small discs of young stars at high redshift.

Low surface brightness dwarf galaxies and their globular cluster populations around the low-density environment of our closest S0 NGC 3115

Abstract Understanding faint dwarf galaxies is fundamental to the development of a robust theory of galaxy formation on small scales. Since the discovery of a population of ultra diffuse galaxies (UDGs) rich in globular clusters (GCs) in Coma, an increasing number of studies on low surface brightness dwarf galaxies (LSBds) have been published in recent years. The most massive LSBds have been observed predominantly in groups and clusters, with properties displaying dependence on the environment. In this work, we use deep DECam imaging to systematically identify LSBds and their GC populations around the low-density environment of NGC 3115. We carefully analyse the structure and morphology of 24 candidates, 18 of which are reported for the first time. Most candidates exhibit red colours suggesting a connection between their colour and distance to NGC 3115. We followed up with Gemini GMOS imaging 9 LSBds to properly identify their GC populations. We derive lower limits for the number of GCs associated with each galaxy. Our analysis reveals that they occur around of the same loci of Fornax LSB dwarf GC systems. The relationship between the number of GCs and total mass provides a tool in which, by counting the GCs in these galaxies, we estimate an upper limit for the total mass of these LSB dwarfs, obtaining the mean value of ∼3.3 × 1010 M⊙. Our results align with expectations for dwarf-sized galaxies, particularly regarding the distribution and specific frequency of their GC systems.

Understanding elliptical galaxies with warm dark matter

We fit the solution of hydrostatic equations to the observed total and baryonic mass densities ρtot(r) and ρb(r) of 23 massive elliptical galaxies. Our purpose is to investigate how well these measurements constrain the cusp or core of elliptical galaxies, and the dark matter comoving temperature-to-mass ratio, or equivalently, the adiabatic invariant vhrms(1), of dark matter. These studies reinforce the view that the lower bound of the measured distribution of the comoving thermal velocity vhrms(1) is of cosmological origin, in agreement with studies of spiral and dwarf galaxies.

Understanding the origin of early-type dwarfs: the spectrophotometric study of CGCG014−074

ABSTRACT Early-type dwarf galaxies constitute a prevalent population in the central regions of rich groups and clusters in the local Universe. These low-luminosity and low-mass stellar systems play a fundamental role in the assembly of the luminous galaxies observed today, according to the Lambda cold dark matter hierarchical theory. The origin of early-type dwarfs has been linked to the transformation of disc galaxies interacting with the intracluster medium, especially in dense environments. However, the existence of low-luminosity early-type galaxies in low-density environments presents a challenge to this scenario. This study presents a comprehensive photometric and spectroscopic analysis of the early-type dwarf galaxy CGCG014−074 using deep Gemini GMOS (Gemini Multi-Object Spectrograph) data, focusing on its peculiarities and evolutionary implications. CGCG014−074 exhibits distinct features, including a rotating inner disc, an extended stellar formation with a quiescent phase since about 2 Gyr ago, and the presence of boxy isophotes. From the kinematic analysis, we confirm CGCG014−074 as a nucleated early-type dwarf galaxy with embedded disc. The study of its stellar population parameters using different methods provides significant insights into the galaxy’s evolutionary history. These results show an old and metal-poor nucleus (${\sim}9.3$ Gyr and $\mathrm{[Z/H]}\sim -0.84$ dex), while the stellar disc is younger (${\sim}4.4$ Gyr) with a higher metallicity ($\mathrm{[Z/H]}\sim -0.40$ dex). These distinctive features collectively position CGCG014−074 as a likely building block galaxy that has evolved passively throughout its history.

Detection and characterization of detached tidal dwarf galaxies

Tidal interactions between galaxies often give rise to tidal tails, which can harbor concentrations of stars and interstellar gas resembling dwarf galaxies. Some of these tidal dwarf galaxies (TDGs) have the potential to detach from their parent galaxies and become independent entities, but their long-term survival is uncertain. In this study, we conducted a search for detached TDGs associated with a sample of 39 interacting galaxy pairs in the local Universe using infrared, ultraviolet, and optical images. We employed IR colors and UV/optical/IR spectral energy distributions to identify potential interlopers, such as foreground stars or background quasars. Through spectroscopic observations using the Boller and Chivens spectrograph at San Pedro Mártir Observatory, we confirmed that six candidate TDGs are at the same redshift as their putative parent galaxy pairs. We identified and measured emission lines in the optical spectra and calculated nebular oxygen abundances, which range from log(O/H) = 8.10 ± 0.01 to 8.51 ± 0.02. We have serendipitously discovered an additional detached TDG candidate in Arp72 using available spectra from SDSS. Utilizing the photometric data and the CIGALE code for stellar population and dust emission fitting, we derived the stellar masses, stellar population ages, and stellar metallicities for these detached TDGs. Compared to standard mass-metallicity relations for dwarf galaxies, five of the seven candidates have higher than expected metallicities, confirming their tidal origins. One of the seven candidates remains unclear due to large uncertainties in metallicity, and another has stellar and nebular metallicities compatible with those of a preexisting dwarf galaxy. The latter object is relatively compact in the optical relative to its stellar mass, in contrast to the other candidate TDGs, which have large diameters for their stellar masses compared to most dwarf galaxies. The derived stellar population ages range from 100 Myr to 900 Myr, while the inferred stellar masses are between 2 × 106 M⊙ and 8 × 107 M⊙. Four of the six TDGs are associated with the gas-rich M51-like pair Arp 72, one TDG is associated with a second M51-like pair Arp 86, and another is associated with Arp 65, an approximately equal mass pair. In spite of the relatively low stellar masses of these TDGs, they have survived for at least 100–900 Myrs, suggesting that they are stable and in dynamical equilibrium. We conclude that encounters with a relatively low-mass companion (1/10th–1/4th of the mass of the primary) can also produce long-lasting TDGs.

From gas to stars: MUSEings on the internal evolution of IC 1613

Context. The kinematics and chemical composition of stellar populations of different ages provide crucial information on the evolution of the various components of a galaxy. Aim. Our aim is to determine the kinematics of individual stars as a function of age in IC 1613, a star-forming, gas-rich, and isolated dwarf galaxy of the Local Group (LG). Methods. We present results of a new spectroscopic survey of IC 1613 conducted with MUSE, an integral field spectrograph mounted on the Very Large Telescope. We extracted ∼2000 sources, from which we separated stellar objects for their subsequent spectral analysis. The quality of the dataset allowed us to obtain accurate classifications (Teff to better than 500 K) and line-of-sight velocities (with average δv ∼ 7 km s−1) for about 800 stars. Our sample includes not only red giant branch (RGB) and main sequence (MS) stars, but also a number of probable Be and C stars. We also obtained reliable metallicities (δ[Fe/H] ∼ 0.25 dex) for about 300 RGB stars. Results. The kinematic analysis of IC 1613 revealed for the first time the presence of stellar rotation with high significance. We found general agreement with the rotation velocity of the neutral gas component. Examining the kinematics of stars as a function of broad age ranges, we find that the velocity dispersion increases as a function of age, with the behaviour being very clear in the outermost pointings, while the rotation-to-velocity dispersion support decreases. On timescales of < 1 Gyr, the stellar kinematics still follow very closely that of the neutral gas, while the two components decouple on longer timescales. The chemical analysis of the RGB stars revealed average properties comparable to other Local Group dwarf galaxies. We also provide a new estimation of the inclination angle using only independent stellar tracers. Conclusions. Our work provides the largest spectroscopic sample of an isolated LG dwarf galaxy. The results obtained seem to support the scenario in which the stars of a dwarf galaxy are born from a less turbulent gas over time.