[en] We use a sample of 47 homogeneous and high-sensitivity CO images taken from the Nobeyama and BIMA surveys to demonstrate that, contrary to common belief, a significant number (∼40%) of H I-deficient nearby spiral galaxies are also depleted in molecular hydrogen. While H I deficiency by itself is not a sufficient condition for molecular gas depletion, we find that H₂ reduction is associated with the removal of H I inside the galaxy optical disk. Those H I-deficient galaxies with normal H₂ content have lost H I mainly from outside their optical disks, where the H₂ content is low in all galaxies. This finding is consistent with theoretical models in which the molecular fraction in a galaxy is determined primarily by its gas column density. Our result is supported by indirect evidence that molecular deficient galaxies form stars at a lower rate or have dimmer far infrared fluxes than gas rich galaxies, as expected if the star formation rate is determined by the molecular hydrogen content. Our result is consistent with a scenario in which, when the atomic gas column density is lowered inside the optical disk below the critical value required to form molecular hydrogen and stars, spirals become quiescent and passive evolving systems. We speculate that this process would act on the timescale set by the gas depletion rate and might be a first step for the transition between the blue and red sequence observed in the color-magnitude diagram.

[en] We report the discovery of a new dwarf galaxy (NGC 6503-d1) during the Subaru extended ultraviolet disk survey. It is a likely companion of the spiral galaxy NGC 6503. The resolved images, in the B, V, R, i, and Hα bands, show an irregular appearance due to bright stars with underlying, smooth and unresolved stellar emission. It is classified as the transition type (dwarf irregular (dIrr)/dwarf spheroidal (dSph)) between the dIrr and dSph types. Its structural properties are similar to those of the dwarfs in the Local Group, with an absolute magnitude $M_V\sim <!--\; ???\; -->-<!--\; ???\; -->10.5,$ half-light radius $r_e\sim <!--\; ???\; -->400\mathrm{p}\mathrm{c}$, and central surface brightness ${\mathrm{\mu <!--\; ??\; -->}}_{0,V}\sim <!--\; ???\; -->25.2$. Despite the low stellar surface brightness environment, one H ii region was detected, though its Hα luminosity is low, indicating an absence of any appreciable O-stars at the current epoch. The presence of multiple stellar populations is indicated by the color–magnitude diagram of ∼300 bright resolved stars and the total colors of the dwarf, with the majority of its total stellar mass $\sim <!--\; ???\; -->4\times <!--\; ??\; -->{10}^6{M}_{\odot <!--\; ???\; -->}$ in an old stellar population.

[en] We analyze the kinematics of six Virgo cluster dwarf early-type galaxies (dEs) from their globular cluster (GC) systems. We present new Keck/DEIMOS spectroscopy for three of them and re-analyze the data found in the literature for the remaining three. We use two independent methods to estimate the rotation amplitude ( V _rot) and velocity dispersion ( σ _GC) of the GC systems and evaluate their statistical significance by simulating non-rotating GC systems with the same number of GC satellites and velocity uncertainties. Our measured kinematics agree with the published values for the three galaxies from the literature and, in all cases, some rotation is measured. However, our simulations show that the null hypothesis of being non-rotating GC systems cannot be ruled out. In the case of VCC 1861, the measured V _rot and the simulations indicate that it is not rotating. In the case of VCC 1528, the null hypothesis can be marginally ruled out, and thus it might be rotating although further confirmation is needed. In our analysis, we find that, in general, the measured V _rot tends to be overestimated and the measured σ _GC tends to be underestimated by amounts that depend on the intrinsic V _rot/ σ _GC, the number of observed GCs ( N _GC), and the velocity uncertainties. The bias is negligible when N _GC ≳ 20. In those cases where a large N _GC is not available, it is imperative to obtain data with small velocity uncertainties. For instance, errors of ≤2 km s⁻¹ lead to V _rot < 10 km s⁻¹ for a system that is intrinsically not rotating.

[en] It has long been speculated that many starburst or compact dwarf galaxies are resulted from dwarf–dwarf galaxy merging, but unequivocal evidence for this possibility has rarely been reported in the literature. We present the first study of deep optical broadband images of a gas-dominated blue compact dwarf galaxy (BCD) VCC 848 (M _⋆ ≃ 2 × 10⁸ M _⊙) that hosts extended stellar shells and thus is confirmed to be a dwarf–dwarf merger. VCC 848 is located in the outskirts of the Virgo Cluster. By analyzing the stellar light distribution, we found that VCC 848 is the result of a merging between two dwarf galaxies with a primary-to-secondary mass ratio ≲5 for the stellar components and ≲2 for the presumed dark matter halos. The secondary progenitor galaxy has been almost entirely disrupted. The age–mass distribution of photometrically selected star cluster candidates in VCC 848 implies that the cluster formation rate (CFR, ∝ star formation rate) was enhanced by a factor of ∼7–10 during the past ∼1 Gyr. The merging-induced enhancement of CFR peaked near the galactic center a few hundred Myr ago and has started declining in the last few tens of Myr. The current star formation activities, as traced by the youngest clusters, mainly occur at large galactocentric distances (≳1 kpc). The fact that VCC 848 is still (atomic) gas-dominated after the period of the most violent collision suggests that gas-rich dwarf galaxy merging can result in BCD-like remnants with extended atomic gas distribution surrounding a blue compact center, in general agreement with previous numerical simulations.

[en] We use deep optical photometry from the Next Generation Virgo Cluster Survey (NGVS) to investigate the color–magnitude diagram for the galaxies inhabiting the core of this cluster. The sensitivity of the NGVS imaging allows us to continuously probe galaxy colors over a factor of ∼2 × 10⁵ in luminosity, from brightest cluster galaxies to scales overlapping classical satellites of the Milky Way ($M_{g^{\mathrm{\prime <!--\; ???\; -->}}}$ ∼ −9; M _* ∼ 10⁶ M _⊙), within a single environment. Remarkably, we find the first evidence that the red sequence (RS) flattens in all colors at the faint-magnitude end (starting between −14 ≤ $M_{g^{\mathrm{\prime <!--\; ???\; -->}}}$ ≤ −13, around M _* ∼ 4 × 10⁷ M _⊙), with the slope decreasing to ∼60% or less of its value at brighter magnitudes. This could indicate that the stellar populations of faint dwarfs in Virgo’s core share similar characteristics (e.g., constant mean age) over ∼3 mag in luminosity, suggesting that these galaxies were quenched coevally, likely via pre-processing in smaller hosts. We also compare our results to galaxy formation models, finding that the RS in model clusters have slopes at intermediate magnitudes that are too shallow, and in the case of semianalytic models, do not reproduce the flattening seen at both extremes (bright/faint) of the Virgo RS. Deficiencies in the chemical evolution of model galaxies likely contribute to the model-data discrepancies at all masses, while overly efficient quenching may also be a factor at dwarf scales. Deep UV and near-IR photometry are required to unambiguously diagnose the cause of the faint-end flattening.

[en] We present the submillimeter spectra from 450 to 1550 GHz of 11 nearby active galaxies observed with the SPIRE Fourier Transform Spectrometer (SPIRE/FTS) on board Herschel. We detect CO transitions from J_up = 4 to 12, as well as the two [C I] fine structure lines at 492 and 809 GHz and the [N II]1461 GHz line. We used radiative transfer models to analyze the observed CO spectral line energy distributions. The FTS CO data were complemented with ground-based observations of the low-J CO lines. We found that the warm molecular gas traced by the mid-J CO transitions has similar physical conditions (n_H2∼ 10^3.2-10^3.9 cm^–3 and T_kin ∼ 300-800 K) in most of our galaxies. Furthermore, we found that this warm gas is likely producing the mid-IR rotational H₂ emission. We could not determine the specific heating mechanism of the warm gas, however, it is possibly related to the star formation activity in these galaxies. Our modeling of the [C I] emission suggests that it is produced in cold (T_kin < 30 K) and dense (n_H2>10³ cm^–3) molecular gas. Transitions of other molecules are often detected in our SPIRE/FTS spectra. The HF J = 1-0 transition at 1232 GHz is detected in absorption in UGC 05101 and in emission in NGC 7130. In the latter, near-infrared pumping, chemical pumping, or collisional excitation with electrons are plausible excitation mechanisms likely related to the active galactic nucleus of this galaxy. In some galaxies, few H₂O emission lines are present. Additionally, three OH⁺ lines at 909, 971, and 1033 GHz are identified in NGC 7130.

[en] We present a detailed study of how the star formation rate (SFR) relates to the interstellar medium (ISM) of M31 at ∼140 pc scales. The SFR is calculated using the far-ultraviolet and 24 μm emission, corrected for the old stellar population in M31. We find a global value for the SFR of 0.25^+0.06_-0.04 M_sun yr^-1 and compare this with the SFR found using the total far-infrared luminosity. There is general agreement in regions where young stars dominate the dust heating. Atomic hydrogen (H I) and molecular gas (traced by carbon monoxide, CO) or the dust mass is used to trace the total gas in the ISM. We show that the global surface densities of SFR and gas mass place M31 among a set of low-SFR galaxies in the plot of Kennicutt. The relationship between SFR and gas surface density is tested in six radial annuli across M31, assuming a power law relationship with index, N. The star formation (SF) law using total gas traced by H I and CO gives a global index of N = 2.03 ± 0.04, with a significant variation with radius; the highest values are observed in the 10 kpc ring. We suggest that this slope is due to H I turning molecular at Σ_Gas ∼ 10 M_☉ pc^–2. When looking at H₂ regions, we measure a higher mean SFR suggesting a better spatial correlation between H₂ and SF. We find N ∼ 0.6 with consistent results throughout the disk—this is at the low end of values found in previous work and argues against a superlinear SF law on small scales.

[en] The first complete submillimeter spectrum (190-670 μm) of the Seyfert 2 galaxy NGC 1068 has been observed with the SPIRE Fourier transform spectrometer on board the Herschel Space Observatory. The sequence of CO lines (J _up = 4-13), lines from H₂O, the fundamental rotational transition of hydrogen fluoride, two o-H₂O⁺ lines, and one line each from CH⁺ and OH⁺ have been detected, together with the two [C I] lines and the [N II] 205 μm line. The observations in both single pointing mode with sparse image sampling and in mapping mode with full image sampling allow us to disentangle two molecular emission components, one due to the compact circumnuclear disk (CND) and one from the extended region encompassing the star-forming ring (SF-ring). Radiative transfer models show that the two CO components are characterized by densities of n(H₂) = 10^4.5 and 10^2.9 cm^–3 and temperatures of T _kin = 100 K and 127 K, respectively. A comparison of the CO line intensities with the photodissociation region (PDR) and X-ray-dominated region (XDR) models, together with the other observational constraints, such as the observed CO surface brightness and the radiation field, indicates that the best explanation for the CO excitation of the CND is an XDR with a density of n(H₂) ∼ 10⁴ cm^–3 and an X-ray flux of 9 erg s^–1 cm^–2, consistent with illumination by the active galactic nucleus, while the CO lines in the SF-ring are better modeled by a PDR. The detected water transitions, together with those observed with the Herschel PACS spectrometer, can be modeled by a large velocity gradient model with low temperature (T _kin ∼ 40 K) and high density (n(H₂) in the range 10^6.7-10^7.9 cm^–3). The emission of H₂O⁺ and OH⁺ are in agreement with PDR models with cosmic-ray ionization. The diffuse ionized atomic component observed through the [N II] 205 μm line is consistent with previous photoionization models of the starburst.

[en] We use imaging from the Next Generation Virgo cluster Survey (NGVS) to present a comparative study of ultra-compact dwarf (UCD) galaxies associated with three prominent Virgo sub-clusters: those centered on the massive red-sequence galaxies M87, M49, and M60. We show how UCDs can be selected with high completeness using a combination of half-light radius and location in color–color diagrams (u*iK_s or u*gz). Although the central galaxies in each of these sub-clusters have nearly identical luminosities and stellar masses, we find large differences in the sizes of their UCD populations, with M87 containing ∼3.5 and 7.8 times more UCDs than M49 and M60, respectively. The relative abundance of UCDs in the three regions scales in proportion to sub-cluster mass, as traced by X-ray gas mass, total gravitating mass, number of globular clusters (GCs), and number of nearby galaxies. We find that the UCDs are predominantly blue in color, with ∼85% of the UCDs having colors similar to blue GCs and stellar nuclei of dwarf galaxies. We present evidence that UCDs surrounding M87 and M49 may follow a morphological sequence ordered by the prominence of their outer, low surface brightness envelope, ultimately merging with the sequence of nucleated low-mass galaxies, and that envelope prominence correlates with distance from either galaxy. Our analysis provides evidence that tidal stripping of nucleated galaxies is an important process in the formation of UCDs

[en] We present measurements of the galaxy luminosity and stellar mass function in a 3.71 deg² (0.3 Mpc²) area in the core of the Virgo Cluster, based on $u^{\ast <!--\; ???\; -->}$ griz data from the Next Generation Virgo Cluster Survey (NGVS). The galaxy sample—which consists of 352 objects brighter than M _g = −9.13 mag, the 50% completeness limit of the survey—reaches 2.2 mag deeper than the widely used Virgo Cluster Catalog and at least 1.2 mag deeper than any sample previously used to measure the luminosity function in Virgo. Using a Bayesian analysis, we find a best-fit faint-end slope of α = −1.33 ± 0.02 for the g-band luminosity function; consistent results are found for the stellar mass function and the luminosity function in the other four NGVS bandpasses. We discuss the implications for the faint-end slope of adding 92 ultracompact dwarfs (UCDs)—previously compiled by the NGVS in this region—to the galaxy sample, assuming that UCDs are the stripped remnants of nucleated dwarf galaxies. Under this assumption, the slope of the luminosity function (down to the UCD faint magnitude limit, M _g = −9.6 mag) increases dramatically, up to α = −1.60 ± 0.06 when correcting for the expected number of disrupted non-nucleated galaxies. We also calculate the total number of UCDs and globular clusters that may have been deposited in the core of Virgo owing to the disruption of satellites, both nucleated and non-nucleated. We estimate that ∼150 objects with M _g ≲ −9.6 mag and that are currently classified as globular clusters might, in fact, be the nuclei of disrupted galaxies. We further estimate that as many as 40% of the (mostly blue) globular clusters in the Virgo core might once have belonged to such satellites; these same disrupted satellites might have contributed ∼40% of the total luminosity in galaxies observed in the core region today. Finally, we use an updated Local Group galaxy catalog to provide a new measurement of the luminosity function of Local Group satellites, α = −1.21 ± 0.05, which is only 1.7σ shallower than measured in the core of the Virgo Cluster.