Jiwon Chung

We present evidence that the Hector Galaxy Survey galaxy C901005481609968 ( z _cl = 0.0553), which exhibits spatially extended LINER-like emission in optical integral-field spectroscopy (IFS), hosts a low-luminosity active galactic nucleus (LLAGN) that contributes substantially to its ionization budget. Although the galaxy is not selected as an AGN by mid-infrared AGN color criteria, archival Chandra data reveal a compact nuclear X-ray source with log L X ≈ 41.46 erg s ⁻¹ , supporting the presence of an LLAGN. Spatially resolved emission-line diagnostics show LINER-like line ratios across most spaxels with S/N ≥ 3, while spatially resolved τ maps ( τ ≡ Q _pAGB / Q _req ) indicate a widespread photon deficit ( log τ < 0 over most of the mapped region), even under the most optimistic pAGB normalizations, the nuclear region remains at τ < 1. Line-ratio–kinematic tests find no evidence for shock-dominated excitation as the primary driver of the extended emission, although a localized or subdominant shock contribution cannot be ruled out with the present data. We use this galaxy as a pilot case because the combination of Hector IFS and an independent nuclear X-ray constraint provides a stringent validation of the spatially resolved photon-budget framework. Our results indicate that evolved stellar populations alone cannot account for the observed emission, that an additional nuclear ionizing source is required at least in the inner region, and that a weak LLAGN likely contributes to the ionizing budget, particularly in the inner region. Our results demonstrate that extended LINER-like emission can conceal a substantial LLAGN contribution even when traditional optical and infrared AGN indicators are weak and that spatially resolved photon-budget tests combined with X-ray constraints can effectively reveal such hidden activity.

The presence of transitional dwarf galaxies in filaments and cluster outskirts may be closely related to preprocessing in the filament; however, the underlying mechanism is not yet comprehensively understood. We present the spatially resolved chemical and kinematical properties of three blue-cored dwarf early-type galaxies (dE(bc)s) in the Virgo cluster and Virgo-related filaments (Crater and Virgo III) using the Sydney-AAO Multi-object Integral-field spectrograph galaxy Survey. We map the spatial distribution of Hα, oxygen abundance (O/H), nitrogen-to-oxygen abundance ratio (N/O), stellar population age, and gas-stellar internal kinematics. We find irregular shapes of enhanced star-forming regions from the centers to the outlying regions of blue cores in dE(bc)s. These regions are relatively metal poor compared with the surrounding regions, rendering the overall metallicity gradient of each galaxy positive. Furthermore, they exhibit higher N/O ratios at a given O/H relative to their surroundings, implying metal-poor gas infall by external processes. The equivalent width of the Hα emission line in metal-poor regions indicates young age of star formation, 6–8 Myr. The disturbed ionized gas velocity field, one of the most prominent features of galaxy mergers is also discovered in two dE(bc)s. We propose that a moderately dense filament environment is favorable for the formation of blue cores in dEs, in which dE(bc)s in filaments may have already been transformed before they fall into the Virgo cluster. This process may contribute to the composition of galaxy population at the outskirts of the cluster.

The presence of transitional dwarf galaxies in filaments and cluster outskirts may be closely related to pre-processing in the filament; however, the underlying mechanism is not yet comprehensively understood. We present the spatially resolved chemical and kinematical properties of three blue-cored dwarf early-type galaxies (dE(bc)s) in the Virgo cluster and Virgo-related filaments (Crater & Virgo III) using the Sydney-AAO Multi-object Integral-field spectrograph (SAMI) Galaxy Survey. We map the spatial distribution of H-alpha, oxygen abundance (O/H), nitrogen-to-oxygen abundance ratio (N/O), stellar population age, and gas-stellar internal kinematics. We find irregular shapes of enhanced star-forming regions from the centers to the outlying regions of blue cores in dE(bc)s. These regions are relatively metal-poor compared with the surrounding regions, rendering the overall metallicity gradient of each galaxy positive. Furthermore, they exhibit higher N/O ratios at a given O/H relative to their surroundings, implying metal-poor gas infall by external processes. The equivalent width of the H-alpha emission line in metal-poor regions indicates young age of star formation, 6-8 Myr. The disturbed ionized gas velocity field, one of the most prominent features of galaxy mergers is also discovered in two dE(bc)s. We propose that a moderately dense filament environment is favorable for the formation of blue cores in dEs, in which dE(bc)s in filaments may have already been transformed before they fall into the Virgo cluster. This process may contribute to the composition of galaxy population at the outskirts of the cluster.

The presence of blue-cored dwarf early-type galaxies (dE(bc)s) in high-density environments supports the scenario of the transformation of infalling late-type galaxies into quiescent dwarf early-type galaxies by environmental effects. While low-density environments lacking environmental processes could not be relevant to the formation of dE(bc)s, we discovered a large sample of rare dE(bc)s in isolated environments at z < 0.01 using the NASA–Sloan Atlas catalog. Thirty-two isolated dE(bc)s were identified by visual inspection of the Sloan Digital Sky Survey images and g − r color profiles. We found that (1) isolated dE(bc)s exhibit similar structural parameters to dE(bc)s in the Virgo cluster; (2) based on the color–magnitude relation, color gradients, and optical emission lines of dE(bc)s, isolated dE(bc)s show more vigorous, centrally concentrated star formation (SF) compared to their counterparts in the Virgo cluster; (3) at a given stellar mass, isolated dE(bc)s tend to have a larger fraction of gas mass than their Virgo counterparts. We discuss a scenario of episodic SF sustained by gas accretion, suggested by Sánchez Almeida et al., in which the star-bursting blue compact dwarf galaxy (BCD)−quiescent BCD (QBCD) cycle can be repeated during the Hubble time. We suggest that, in this cadence, isolated dE(bc)s might be QBCDs at pre- or post-BCD stages. Our results imply that dE(bc)s comprise a mixture of objects with two types of origins, nature or nurture, depending on their environment.

The presence of blue-cored dwarf early-type galaxies (dE(bc)s) in high-density environments supports the scenario of the transformation of infalling late-type galaxies into quiescent dwarf early-type galaxies by environmental effects. While low-density environments lacking environmental processes could not be relevant to the formation of dE(bc)s, we discovered a large sample of rare dE(bc)s in isolated environments at z < 0.01 using the NASA-Sloan Atlas catalog. Thirty-two isolated dE(bc)s were identified by visual inspection of the Sloan Digital Sky Survey images and g - r color profiles. We found that (1) isolated dE(bc)s exhibit similar structural parameters to dE(bc)s in the Virgo cluster; (2) based on the ultraviolet-r color-magnitude relation, color gradients, and optical emission lines of dE(bc)s, isolated dE(bc)s show more vigorous, centrally concentrated SF compared to their counterparts in the Virgo cluster; (3) at a given stellar mass, isolated dE(bc)s tend to have a larger fraction of gas mass than their Virgo counterparts. We discuss a scenario of episodic SF sustained by gas accretion, suggested by Sanchez Almeida et al., in which the star-bursting blue compact dwarf galaxy (BCD)-quiescent BCD (QBCD) cycle can be repeated during the Hubble time. We suggest that, in this cadence, isolated dE(bc)s might be QBCDs at pre- or post-BCD stages. Our results imply that dE(bc)s comprise a mixture of objects with two types of origins, nature or nurture, depending on their environment.

It has been proposed that the filament environment is closely connected to the pre-processing of galaxies, where their properties may have been changed by environmental effects in the filament before they fell into the galaxy cluster. We present the chemical properties of star-forming dwarf galaxies (SFDGs) in five filamentary structures (Virgo III, Leo Minor, Leo II A, Leo II B, and Canes Venatici) around the Virgo cluster using the Sloan Digital Sky Survey optical spectroscopic data and Galaxy Evolution Explorer ultraviolet photometric data. We investigate the relationship between stellar mass, gas-phase metallicity, and specific star formation rate (sSFR) of the SFDGs in the Virgo filaments in comparison to those in the Virgo cluster and field. We find that, at a given stellar mass, SFDGs in the Virgo filaments show lower metallicity and higher sSFR than those in the Virgo cluster on average. We observe that SFDGs in the Virgo III filament show enhanced metallicities and suppressed star formation activities comparable to those in the Virgo cluster, whereas SFDGs in the other four filaments exhibit similar properties to the field counterparts. Moreover, about half of the galaxies in the Virgo III filament are found to be morphologically transitional dwarf galaxies that are supposed to be on the way to transforming into quiescent dwarf early-type galaxies. Based on the analysis of the galaxy perturbation parameter, we propose that the local environment represented by the galaxy interactions might be responsible for the contrasting features in chemical pre-processing found in the Virgo filaments.

We present the properties of galaxies in filaments around the Virgo cluster with respect to their vertical distance from the filament spine using the NASA–Sloan Atlas catalog. The filaments are mainly composed of low-mass, blue dwarf galaxies. We observe that the g − r color of galaxies becomes blue and stellar mass decreases with increasing vertical filament distance. The galaxies were divided into higher-mass ( ) and lower-mass ( ) subsamples. We also examine the distributions of g − r color, stellar mass, Hα equivalent width (EW(Hα)), near-ultraviolet (NUV) − r color, and H i fraction of the two subsamples with the vertical distance. The lower-mass galaxies exhibit a negative g − r color gradient, whereas higher-mass galaxies have a flat g − r color distribution. We observe a negative EW(Hα) gradient for higher-mass galaxies, whereas lower-mass galaxies show no distinct EW(Hα) variation. In contrast, the NUV − r color distribution of higher-mass galaxies shows no strong trend, whereas the lower-mass galaxies show a negative NUV − r color gradient. We do not see clear gradients of H i fraction in either the higher- or lower-mass subsample. We propose that the negative color and stellar mass gradients of galaxies can be explained by mass assembly from past galaxy mergers at different vertical filament distances. In addition, galaxy interactions might be responsible for the contrasting features of EW(Hα) and NUV − r color distributions between the higher- and lower-mass subsamples. The distributions of H i fraction of the two subsamples suggest that the processes of ram pressure stripping and gas accretion may be ignored in the Virgo filaments.