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Galaxy interactions and mergers can enhance or reduce star formation, but a complete understanding of the involved processes is still lacking. The effect of dwarf galaxy mergers is even less clear than their massive counterpart. We present a study on a dwarf merger remnant in the Virgo cluster, VCC322, which might form a triple system with VCC334 and VCC319. We identify a prominent long and straight tail-like substructure that has a size comparable to its host galaxy VCC322. By comparing the color–color (g − r versus r − H) distribution with simple stellar population models, we infer that the metallicity and stellar age of this tail are Z ⋆ ∼ 0.02 Z ⊙ and t ⋆ ∼ 10 Gyr, respectively. In VCC319, we find a sign of isophotal twisting. This suggests that VCC319 may be subject to tidal interaction. An analysis of the Sloan Digital Sky Survey optical spectra of VCC322 indicates mass- and light-weighted ages of about 109.8 yr and 107.5 yr, respectively, indicating an ongoing star formation activity. However, the star formation in VCC322 seems suppressed when compared to other star-forming dwarfs of comparable stellar masses. Our finding of shock excitation of optical emission lines indicates that interaction-induced shock may contribute to the heating of cold gas and suppression of star formation. [ABSTRACT FROM AUTHOR]

In this paper we present our studies on the stellar populations and star formation histories (SFHs) for the Reines et al. sample of 136 dwarf galaxies that host active galactic nuclei (AGNs), selected from the Sloan Digital Sky Survey Data Release 8. We derive stellar populations and reconstruct SFHs for these AGN-host dwarfs using the stellar population synthesis code STARLIGHT. Our results suggest that these AGN-host dwarfs have assembled their stellar masses within a narrow period of time with the stellar mass-weighted ages in the range of 109–1010 yr, but show a wide diversity of SFHs with the luminosity-weighted stellar ages in the range of 107–1010 yr. The old population (t > 109 yr) contributes most to the galaxy light for the majority of the sample; the young population (t < 108 yr) also appears in significant but widely varying fractions, while the intermediate-age population (108 < t < 109 yr) in general contributes less to the optical continuum at 4020 Å. We also find that these dwarfs follow a similar mass–metallicity relation to normal star-forming galaxies, indicating that AGNs have little effect on the chemical evolution of the host galaxy. We further investigate the relation between the derived SFHs and morphology of the host galaxy, and find no correlation. Comparing the SFHs with the luminosity of the [O iii ] λ5007 line (), we find a mild correlation exists when erg s−1, indicating a physical connection between star formation and AGN activities in these dwarf galaxies. [ABSTRACT FROM AUTHOR]

We selected an active galactic nuclei (AGNs) sample (0 < |$z$| ≤ 0.35) from Sloan Digital Sky Survey Data Release 7, and measured the broad- (⁠|$\rm H\alpha ^{b}/H\beta ^{b}$|⁠) and narrow-line Balmer decrements (⁠|$\rm H\alpha ^{n}/H\beta ^{n}$|⁠) of 554 selected AGNs. We found that the distributions of Balmer decrements can be fitted by a Gaussian function and give the best estimates of |$\rm H\alpha ^{b}/H\beta ^{b}= 3.16$| with a standard deviation 0.07 dex, and |$\rm H\alpha ^{n}/H\beta ^{n}= 4.37$| with a standard deviation 0.10 dex. We inspected the distributions of |$\rm H\alpha ^{b}/H\beta ^{b}$| and |$\rm H\alpha ^{n}/H\beta ^{n}$| in the Baldwin−Phillips−Terlevich (BPT) diagram and found that only narrow-line Balmer decrements depend on the physical conditions of the narrow-line region (NLR). We tested the relationship between |$\rm H\alpha ^{b}/H\beta ^{b}$| and |$\rm H\alpha ^{n}/H\beta ^{n}$|⁠, and found that |$\rm H\alpha ^{b}/H\beta ^{b}$| does not correlate with |$\rm H\alpha ^{n}/H\beta ^{n}$|⁠. We investigated the relationship between Balmer decrements and Seyfert sub-type, and found that only broad-line Balmer decrements correlate with Seyfert sub-type, We also examined the dependency of Balmer decrements on AGN properties, and found that Balmer decrements have no correlation with optical luminosity, but show some dependence on accretion rate. These results indicate that the NLR is subject to more reddening by dust than the broad-line region. [ABSTRACT FROM AUTHOR]

In this paper we present the stellar population synthesis results for a sample of 75 bulges in isolated spiral Sb-Sc galaxies, using the spectroscopic data from the Sloan Digital Sky Survey and the STARLIGHT code. We find that both pseudo-bulges and classical bulges in our sample are predominantly composed of old stellar populations, with mean mass-weighted stellar age around 10 Gyr. While the stellar population of pseudo-bulges is, in general, younger than that of classical bulges, the difference is not significant, which indicates that it is hard to distinguish pseudo-bulges from classical bulges, at least for these isolated galaxies, only based on their stellar populations. Pseudo-bulges have star formation activities with relatively longer timescale than classical bulges, indicating that secular evolution is more important in this kind of systems. Our results also show that pseudo-bulges have a lower stellar velocity dispersion than their classical counterparts, which suggests that classical bulges are more dispersion-supported than pseudo-bulges. [ABSTRACT FROM AUTHOR]