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Register a new userA headless tadpole galaxy: the high gas-phase metallicity of the ultra-diffuse galaxy UGC 2162
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The cosmological numerical simulations tell us that accretion of external metal-poor gas drives star-formation (SF) in galaxy disks. One the best pieces of observational evidence supporting this prediction is the existence of low metallicity star-forming regions in relatively high metallicity host galaxies. The SF is thought to be fed by metal-poor gas recently accreted. Since the gas accretion is stochastic, there should be galaxies with all the properties of a host but without the low metallicity starburst. These galaxies have not been identified yet. The exception may be UGC 2162, a nearby ultra-diffuse galaxy (UDG) which combines low surface brightness and relatively high metallicity. We confirm the high metallicity of UGC 2162 (12 + log(O/H) = 8.52+0.27-0.24 ) using spectra taken with the 10-m GTC telescope. GC2162 has the stellar mass, metallicity, and star-formation rate (SFR) surface density expected for a host galaxy in between outbursts. This fact suggests a physical connection between some UDGs and metal-poor galaxies, which may be the same type of object in a different phase of the SF cycle. UGC 2162 is a high-metallicity outlier of the mass-metallicity relation, a property shared by the few UDGs with known gas-phase metallicity.
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Our GMRT HI observations of the ultra diffuse galaxy (UDG) UGC 2162, projected $sim$ 300 kpc from the centre of the M77 group, reveal it to a have an extended HI disk (R$_{HI}$/R$_{25}$ $sim$ 3.3) with a moderate rotational velocity (V$_{rot} sim$ 31 km/s). This V$_{rot}$ is in line with that of dwarf galaxies with similar HI mass. We estimate an M$_{dyn}$ of $sim$ 1.14 $times$ 10$^{9}$ M$_odot$ within the galaxys R$_{HI}$ $sim$ 5.2 kpc. Additionally, our estimates of M$_{200}$ for the galaxy from NFW models are in the range of 5.0 to 8.8 $times$ 10$^{10}$ M$_odot$. Comparing UGC 2162 to samples of UDGs with HI detections show it to have amongst the smallest R$_e$ with its M$_{HI}$/M$_{star}$ being distinctly higher and g -- i colour slightly bluer than typical values in those samples. We also compared HI and dark matter (DM) halo properties of UGC 2162 with dwarf galaxies in the LITTLE THINGS sample and find its DM halo mass and profile are within the range expected for a dwarf galaxy. While we were unable to to determine the origin of the galaxys present day optical form from our study, its normal HI rotation velocity in relation to its HI mass, HI morphology, environment and dwarf mass DM halo ruled out some of the proposed ultra diffuse galaxy formation scenarios for this galaxy.
We describe the structural, stellar population and gas properties of the nearest Ultra Diffuse Galaxy (UDG) discovered so far: UGC2162 (z=0.00392; R$_{e,g}$=1.7$(pm$0.2) kpc; $mu_g(0)$=24.4$pm$0.1 mag/arcsec$^2$; g-i=0.33$pm$0.02). This galaxy, located at a distance of 12.3($pm$1.7) Mpc, is a member of the M77 group. UGC2162 has a stellar mass of $sim$2($^{+2}_{-1}$)$times$10$^7$ M$_odot$ and is embedded within a cloud of HI gas $sim$10 times more massive: $sim$1.9($pm$0.6)$times$10$^8$ M$_odot$. Using the width of its HI line as a dynamical proxy, the enclosed mass within the inner R$sim$5 kpc is $sim$4.6($pm$0.8)$times$10$^9$ M$_odot$ (i.e. M/L$sim$200). The estimated virial mass from the cumulative mass curve is $sim$8($pm$2)$times$10$^{10}$ M$_odot$. Ultra-deep imaging from the IAC Stripe82 Legacy Project show that the galaxy is irregular and has many star forming knots, with a gas-phase metallicity around one-third of the solar value. Its estimated Star Formation Rate (SFR) is $sim$0.01 M$_odot$/yr. This SFR would double the stellar mass of the object in $sim$2 Gyr. If the object were to stop forming stars at this moment, after a passive evolution, its surface brightness would become extremely faint: $mu_g(0)$$sim$27 mag/arcsec$^2$ and its size would remain large R$_{e,g}$$sim$ 1.8 kpc. Such faintness would make it almost undetectable to most present-day surveys. This suggests that there could be an important population of M$_{star}$$sim$10$^7$ M$_odot$ dark galaxies in rich environments (depleted of HI gas) waiting to be discovered by current and future ultra-deep surveys.
We present a detailed exploration of the stellar mass vs. gas-phase metallicity relation (MZR) using integral field spectroscopy data obtained from ~1000 galaxies observed by the SAMI Galaxy survey. These spatially resolved spectroscopic data allow us to determine the metallicity within the same physical scale (Reff) for different calibrators. The shape of the MZ relations is very similar between the different calibrators, while there are large offsets in the absolute values of the abundances. We confirm our previous results derived using the spatially resolved data provided by the CALIFA and MaNGA surveys: (1) we do not find any significant secondary relation of the MZR with either the star formation rate (SFR) nor the specific SFR (SFR/Mass) for any of the calibrators used in this study, based on the analysis of the {individual} residuals, (2) if there is a dependence with the SFR, it is weaker than the reported one ($r_csim -$0.3), it is confined to the low mass regime (M*<10$^9$Msun) or high SFR regimes, and it does not produce any significant improvement in the {description of the average population of galaxies. The aparent disagreement with published results based on single fiber spectroscopic data could be due to (i) the interpretation of the secondary relation itself, (ii) the lower number of objects sampled at the low mass regime by the current study, or (iii) the presence of extreme star-forming galaxies that drive the secondary relation in previous results
We report the discovery of two ultra-diffuse galaxies (UDGs) which show clear evidence for association with tidal material and interaction with a larger galaxy halo, found during a search of the Wide portion of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). The two new UDGs, NGC2708-Dw1 and NGC5631-Dw1, are faint ($M_g$=$-$13.7 and $-$11.8 mag), extended ($r_h$=2.60 and 2.15 kpc) and have low central surface brightness ($mu(g,0)$=24.9 and 27.3 mag arcsec$^{-2}$), while the stellar stream associated with each has a surface brightness $mu(g)$$gtrsim$28.2 mag arcsec$^{-2}$. These observations provide evidence that the origin of some UDGs may connect to galaxy interactions, either by transforming normal dwarf galaxies by expanding them, or because UDGs can collapse out of tidal material (i.e. they are tidal dwarf galaxies). Further work is needed to understand the fraction of the UDG population `formed through galaxy interactions, and wide field searches for diffuse dwarf galaxies will provide further clues to the origin of these enigmatic stellar systems.
A small survey of the UV-absorbing gas in 12 low-$z$ galaxy groups has been conducted using the Cosmic Origins Spectrograph (COS) on-board the Hubble Space Telescope (HST). Targets were selected from a large, homogeneously-selected sample of groups found in the Sloan Digital Sky Survey (SDSS). A critical selection criterion excluded sight lines that pass close ($<1.5$ virial radii) to a group galaxy, to ensure absorber association with the group as a whole. Deeper galaxy redshift observations are used both to search for closer galaxies and also to characterize these $10^{13.5}$ to $10^{14.5} M_{odot}$ groups, the most massive of which are highly-virialized with numerous early-type galaxies (ETGs). This sample also includes two spiral-rich groups, not yet fully-virialized. At group-centric impact parameters of 0.3-2 Mpc, these $mathrm{S/N}=15$-30 spectra detected HI absorption in 7 of 12 groups; high (OVI) and low (SiIII) ion metal lines are present in 2/3 of the absorption components. None of the three most highly-virialized, ETG-dominated groups are detected in absorption. Covering fractions $gtrsim50$% are seen at all impact parameters probed, but do not require large filling factors despite an enormous extent. Unlike halo clouds in individual galaxies, group absorbers have radial velocities which are too low to escape the group potential well without doubt. This suggests that these groups are closed boxes for galactic evolution in the current epoch. Evidence is presented that the cool and warm group absorbers are not a pervasive intra-group medium (IGrM), requiring a hotter ($Tsim10^6$ to $10^7$ K) IGrM to be present to close the baryon accounting.
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