Tidal Dwarf Galaxy Candidates Research Articles

Detection of CO(1−0) Emission at the Tips of the Tidal Tail in the Antennae Galaxies

The tip of the tidal tail, resulting from an encounter between galaxies, features gas concentrations and some star-forming regions, such as tidal dwarf galaxies (TDGs). This region provides a unique laboratory for examining the star formation process in a dynamical environment distinct from that of disk galaxies. Using the Nobeyama 45 m telescope, we conducted 12CO(1−0) position-switching observations at the tips of the southern tidal tail in the Antennae galaxies. We detected CO emission not only from the two star-forming TDG candidates but also in regions with no significant star formation. Adopting a Galactic CO-to-H2 conversion factor without helium correction, the H2 gas surface density is ∼5–12 M ⊙ pc−2. In most regions, the molecular-to-atomic gas ratio is around unity (0.6–1.9), but we find a region with a high ratio with a 3σ lower limit of >7.2. The star formation efficiency (SFE) of molecular gas is notably low (<0.15 Gyr−1), indicating less active star formation than in both nearby disk galaxies (∼0.5–1.0 Gyr−1) and other TDGs previously observed. Including previous observations, the molecular gas SFEs vary widely among TDGs/tidal tails, from 10−2 to 10 Gyr−1, demonstrating significant variations in star formation activity. Potential factors contributing to the low SFE in the Antennae tail tips include extensive tides and/or the young age of the tail.

H i in High Gas-phase Metallicity Dwarf Galaxy WISEA J230615.06+143927.9

We present resolved Giant Metrewave Radio Telescope H i observations of the high gas-phase metallicity dwarf galaxy WISEA J230615.06+143927.9 (z = 0.005) (hereafter J2306) and investigate whether it could be a Tidal Dwarf Galaxy (TDG) candidate. TDGs are observed to have higher metallicities than normal dwarfs. J2306 has an unusual combination of a blue g − r color of 0.23 mag, irregular optical morphology and high-metallicity (12 + log(O/H) = 8.68 ± 0.14), making it an interesting galaxy to study in more detail. We find J2306 to be an H i rich galaxy with a large extended, unperturbed rotating H i disk. Using our H i data we estimated its dynamical mass and found the galaxy to be dark matter (DM) dominated within its H i radius. The quantity of DM, inferred from its dynamical mass, appears to rule out J2306 as an evolved TDG. A wide area environment search reveals J2306 to be isolated from any larger galaxies which could have been the source of its high gas metallicity. Additionally, the H i morphology and kinematics of the galaxy show no indication of a recent merger to explain the high-metallicity. Further detailed optical spectroscopic observations of J2306 might provide an answer to how a seemingly ordinary irregular dwarf galaxy achieved such a high level of metal enrichment.

Catching Tidal Dwarf Galaxies at a Later Evolutionary Stage with ALFALFA

We present deep optical imaging and photometry of four objects classified as “Almost-Dark” galaxies in the Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) survey because of their gas-rich nature and extremely faint or missing optical emission in existing catalogs. They have H i masses of 107–109 M ⊙ and distances of ∼9–100 Mpc. Observations with the WIYN 3.5 m telescope and One Degree Imager reveal faint stellar components with central surface brightnesses of ∼24–25 in the g band. We also present the results of H i synthesis observations with the Westerbork Synthesis Radio Telescope. These Almost-Dark galaxies have been identified as possible tidal dwarf galaxies (TDGs) based on their proximity to one or more massive galaxies. We demonstrate that AGC 229398 and AGC 333576 likely have the low dark matter content and large effective radii representative of TDGs. They are located much farther from their progenitors than previously studied TDGs, suggesting they are older and more evolved. AGC 219369 is likely dark matter dominated, while AGC 123216 has a dark matter content that is unusually high for a TDG, but low for a normal dwarf galaxy. We consider possible mechanisms for the formation of the TDG candidates such as a traditional major merger scenario and gas ejection from a high-velocity flyby. Blind H i surveys like ALFALFA enable the detection of gas-rich, optically faint TDGs that can be overlooked in other surveys, thereby providing a more complete census of the low-mass galaxy population and an opportunity to study TDGs at a more advanced stage of their life cycle.

Ultra diffuse galaxies in the MATLAS low-to-moderate density fields

Recent advances in deep dedicated imaging surveys over the past decade have uncovered a surprisingly large number of extremely faint low surface brightness galaxies with large physical sizes called ultra diffuse galaxies (UDGs) in clusters and, more recently, in lower density environments. As part of the Mass Assembly of early-Type GaLAxies with their fine Structures (MATLAS) survey, a deep imaging large program at the Canada-France-Hawaii Telescope (CFHT), our team has identified 2210 dwarf galaxies, 59 (∼3%) of which qualify as UDGs. Averaging over the survey area, we find ∼0.4 UDG per square degree. They are found in a range of low to moderate density environments, although 61% of the sample fall within the virial radii of groups. Based on a detailed analysis of their photometric and structural properties, we find that the MATLAS UDGs do not show significant differences from the traditional dwarfs, except from the predefined size and surface brightness cut. Their median color is as red as the one measured in galaxy clusters, albeit with a narrower color range. The majority of the UDGs are visually classified as dwarf ellipticals with log stellar masses of ∼6.5 − 8.7. The fraction of nucleated UDGs (∼34%) is roughly the same as the nucleated fraction of the traditional dwarfs. Only five (∼8%) UDGs show signs of tidal disruption and only two are tidal dwarf galaxy candidates. A study of globular cluster (GC) candidates selected in the CFHT images finds no evidence of a higher GC specific frequencySNfor UDGs than for classical dwarfs, contrary to what is found in most clusters. The UDG halo-to-stellar mass ratio distribution, as estimated from the GC counts, peaks at roughly the same value as for the traditional dwarfs, but spans the smaller range of ∼10 − 2000. We interpret these results to mean that the large majority of the field-to-group UDGs do not have a different formation scenario than traditional dwarfs.

Interacting system NGC 7805/6 (Arp 112) and its tidal dwarf galaxy candidate

We present results from our Giant Metrewave Radio Telescope (GMRT) Hi, Himalayan Chandra Telescope (HCT) Hα, 1 m Sampurnanand Telescope (ST) and 1.3 m Devasthal Fast Optical Telescope (DFOT) deep optical observations of the NGC 7805/6 (Arp 112) system to test KUG 2359+311’s tidal dwarf galaxy (TDG) candidacy and explore the properties of the interacting system. Our GMRT H I map shows no H I detection associated with KUG 2359+311, nor any H I tail or bridge-like structure connecting KUG 2359+311 to the NGC 7805/6 system. Our HCT Hα image, on the other hand, displays strong detections in KUG 2359+311, with net SFR ∼ 0.035 ± 0.009 M ⊙ yr−1. The Hα data constrain the redshift of KUG 2359+311 to 0.00 ≤ z ≤ 0.043, compared to the redshift of NGC 7806 of ∼ 0.015. TDGs detected to date have all been H i rich, and displayed H i, ionised gas and stellar tidal debris trails (bridges or tails) linking them to their parent systems. However, neither our H i data nor our optical imaging, while three magnitudes deeper than SDSS, reveals a tidal trail connecting KUG 2359+311 to NGC 7805/6. Lack of H i, presence of an old stellar population, ongoing star formation and reasonably high SFR compared to normal dwarf galaxies suggest that KUG 2359+311 may not be an Arp 112 TDG. It is most likely a case of a regular gas-rich dwarf galaxy undergoing a morphological transformation after having lost its entire gas content to an interaction with the Arp 112 system. Redshift and metallicity from future spectroscopic observations of KUG 2359+311 would help clarify the nature of this enigmatic structure.

Searching for the Magnetized Tidal Dwarf Galaxies in Hickson Compact Groups: HCG 26, 91, and 96

Abstract In this work, archive 1.4 and 4.86 GHz radio continuum data from the Very Large Array were re-reduced and, together with the 1.4 GHz maps from the NRAO VLA Sky Survey, investigated for the presence of detectable, nonthermal continuum radio emission that could be associated with the tidal dwarf galaxy (TDG) candidates in HCG 26, 91, and 96. Radio emission highly coincident with the optical and H α emission maxima of the TDG candidate HCG 91i (estimated physical separation of less than 150 pc) was revealed. Should this emission be intrinsic to this object, it would imply the presence of a magnetic field as strong as 11–16 μG—comparable to that found in the most radio-luminous, star-forming dwarf galaxies of non-tidal origin. However, the star formation rate derived for this object using the radio flux is about two orders of magnitude higher than the one estimated from the H α data. Analysis of the auxiliary radio, ultraviolet, and infrared data suggests that either the radio emission originates in a background object with an aged synchrotron spectrum (possibly a GHz-peaked source), or the estimate is lower due to the fact that it traces the most recent star formation, while most of the detected radio emission originated when what is known as HCG 91i was still a part of its parent galaxy. The latter scenario is supported by a very large stellar mass derived from 3.6 to 4.5 μm data, implying a high star formation rate in the past.

Galaxies lacking dark matter in the Illustris simulation

Context. Any viable cosmological model in which galaxies interact predicts the existence of primordial and tidal dwarf galaxies (TDGs). In particular, in the standard model of cosmology (ΛCDM), according to the dual dwarf galaxy theorem, there must exist both primordial dark matter-dominated and dark matter-free TDGs with different radii. Aims. We study the frequency, evolution, and properties of TDGs in a ΛCDM cosmology. Methods. We use the hydrodynamical cosmological Illustris-1 simulation to identify TDG candidates (TDGCs) and study their present-day physical properties. The positions of galaxies in the radius–mass plane, depending on their nonbaryonic content, are compared with observational data and other simulations. We also present movies on the formation of a few galaxies lacking dark matter, confirming their tidal dwarf nature. Tidal dwarf galaxy candidates can however also be formed via other mechanisms, such as from ram-pressure-stripped material or, speculatively, from cold-accreted gas. Results. We find 97 TDGCs with Mstellar &gt; 5 × 107M⊙ at redshift z = 0, corresponding to a co-moving number density of 2.3 × 10−4 h3 cMpc−3. The most massive TDGC has Mtotal = 3.1 × 109 M⊙, comparable to that of the Large Magellanic Cloud. Tidal dwarf galaxy candidates are phase-space-correlated, reach high metallicities, and are typically younger than dark matter-rich dwarf galaxies. Conclusions. We report for the first time the verification of the dual dwarf theorem in a self-consistent ΛCDM cosmological simulation. Simulated TDGCs and dark matter-dominated galaxies populate different regions in the radius–mass diagram in disagreement with observations of early-type galaxies. The dark matter-poor galaxies formed in Illustris-1 have comparable radii to observed dwarf galaxies and to TDGs formed in other galaxy-encounter simulations. In Illustris-1, only 0.17 percent of all selected galaxies with Mstellar = 5 × 107−109 M⊙ are TDGCs or dark matter-poor dwarf galaxies. The occurrence of NGC 1052-DF2-type objects is discussed.

Refuelled and shielded – the early evolution of tidal dwarf galaxies

We present, for the first time, numerical simulations of young tidal dwarf galaxies (TDGs), including a self-consistent treatment of the tidal arm in which they are embedded. Thereby, we do not rely on idealised initial conditions, as the initial data of the presented simulation emerge from a galaxy interaction simulation. By comparing models which are either embedded in or isolated form the tidal arm, we demonstrate its importance on the evolution of TDGs, as additional source of gas which can be accreted and is available for subsequent conversion into stars. During the initial collapse of the proto-TDG, with a duration of a few 100 Myr, the evolution of the embedded and isolated TDGs are indistinguishable. Significant differences appear after the collapse has halted and the further evolution is dominated by the possible accretion of material form the surroundings of the TDGs. The inclusion of the tidal arm in the simulation of TDGs results in roughly a doubling of the gas mass ($M_\mathrm{gas}$) and gas fraction ($f_\mathrm{gas}$), an increase in stellar mass by a factor of 1.5 and a $\sim3$ times higher star formation rate (SFR) compared to the isolated case. Moreover, we perform a parametric study on the influence of different environmental effects, i.e. the tidal field and ram pressure. Due to the orbit of the chosen initial conditions, no clear impact of the environmental effects on the evolution of TDG candidates can be found.

KUG 0200-096: Dwarf Antennae Hosting a Tidal Dwarf Galaxy

Abstract We study a gas-rich merging dwarf system KUG 0200-096. Deep optical imaging reveals an optically faint tail with a length of 20 kpc, giving a visual impression of tidal antenna similar to NGC 4038/39. The interacting dwarf galaxies have B-band absolute magnitudes of −18.06 and −16.63 mag. We identify a young stellar clump with a stellar mass of 2 × 107 M ⊙ at the tip of the antenna, possibly a tidal dwarf galaxy (TDG). The putative TDG candidate is quite blue with a g − r color index of −0.07 mag, whereas the interacting dwarf galaxies have g − r color indices 0.29 and 0.19 mag. The TDG is currently forming stars at the rate of 0.02 M ⊙ yr−1. We obtained H i 21 cm line data of KUG 0200-096 using the Giant Metrewave Radio Telescope to get a more detailed view of neutral hydrogen (H i) emission in interacting dwarf galaxies and its TDG. Evidence of a merger between the dwarf galaxy pair is also present in H i kinematics and morphology where we find the H i contents of the interacting pair is disturbed, forming an extended tail toward the TDG. The H i velocity field shows a strong gradient along the H i tidal tail extension. We present a comparative study between the Antennae galaxy, NGC 4038/39, and KUG 0200-096 in both optical and H i gas properties and discuss the possible origin of the KUG 0200-096 TDG.

Kinematics and physical properties of the nearby galaxy NGC 4656 and its TDG candidate

Interacting galaxies provide us with an excellent laboratory for studying a number of physical phenomena associated with these processes. In this paper, we present a spectroscopic and kinematic analysis of the interacting galaxy NGC 4656 and its companion tidal dwarf galaxy (TDG) candidate, NGC 4656UV. Using Fabry–Perot and GMOS multislit data, we investigated the possible origin of NGC 4656UV. We found that NGC 4656UV has a low metallicity (12 + log(O/H)∼8.2) and it follows the mass–metallicity relation for normal dwarf galaxies. For NGC 4656, we estimated a flat oxygen abundance gradient of |$\beta = -0.027\, \pm \,$|0.029 dex kpc−1, which suggests the presence of gas flows induced by gravitational interactions. By analysing radial velocity profiles and by fitting a kinematic model of the observed velocity field, we confirm the literature result that NGC 4656 consists of one single body instead of two objects. We estimated a dynamical mass of |$6.8^{1.8}_{-0.6}\times 10^{9}$| M|$\odot$| and R of 12.1 kpc from the kinematic model of NGC 4656. Although the observed velocity field is dominated by rotation at large scales (Vmax/σ ≳2.8), important non-rotational motions are present at small scales. Based on these new results, and on previously published information, we propose that NGC 4656 and 4656UV are a pair of interacting galaxies. NGC 4656UV is a companion of NGC 4656 and it does not have a tidal origin. The interaction between the two could have triggered the star formation in NGC 4656UV and increased the star formation in the north-east side of NGC 4656.

Young stellar clumps in the interacting system Arp 305

We present the results of optical long-slit spectral observations of interacting system Arp305 carried out at the 6m telescope BTA at the SAO RAS: the radial variation of gas kinematics and oxygen abundance. This study continues the series of spectral observations of tidal debris in the interacting galaxies. Here we pay special attention to the star-forming region between the interacting galaxies - a tidal dwarf galaxy (TDG) candidate. This star-forming system appears to be gravitationally bound or close to this condition. We show that TDG is metal poor in comparison to the parental galaxy. It can indicate that either the origin of star-forming gas of TDG was very far from the centre of parent galaxy or its gas has been diluted by the accretion of metal poor intergalactic gas. Nevertheless the region of the brightest emission clump in TDG where a current star formation takes place reveals a noticeable colour excess evidencing a local gas concentration. We pay attention that TDG is situated in the region of intersection of two gas flows and is in a process of formation probably caused by the collision of these flows. Low velocity difference between TDG, NGC 4017 and the gaseous bridge between them evidences that the lifetime of TDG is restricted by the time of its fall back onto the parent galaxy.

Arp 202: a TDG formed in a parent's extended dark matter halo?

We report on H${\alpha}$ + [NII] imaging of the Arp 202 interacting pair and its tidal dwarf galaxy (TDG) candidate as well as a GMOS long slit spectrum from the TDG candidate, observed with the Gemini North telescope. Our H${\alpha}$ + [NII] imaging reveals the TDG to have an elongated structure, $\sim$ 1.9 kpc in length with the two principal star forming knots at either end. Our observations also show the TDG candidate has a recessional V$_{H\alpha}$ $\sim$ 3032 km s$^{-1}$, within 100 km s$^{-1}$ of the parent pair's mean velocity and an oxygen abundance of 12+log(O/H) = 8.10$\pm$0.41. The TDG's oxygen abundance is in good agreement with that of a star forming region in NGC 2719A, one of the parent galaxies, which has an estimated oxygen abundance of 12+log(O/H) = 8.05$\pm$0.41. The TDG's V$_{H\alpha}$ and oxygen abundance confirm previous results validating the candidate as a TDG. The absence of detectable emission from the TDG in $\textit{Spitzer}$ 3.6 $\mu$m and 4.5 $\mu$m images together with the lack of absorption lines and weak continuum in the spectrum is consistent with absence of an old population ($\gtrsim$ 0.5 Gyr). The location of the TDG within the interaction debris and the absence of indicators of an old stellar population in the TDG are consistent with a scenario in which the TDG is formed from HI stripped from the parent galaxies and within the extended dark matter halo of one of the parents as proposed by (Bournaud et al. 2003; Duc et al. 2004).

Tidal dwarf galaxies in cosmological simulations

The formation and evolution of gravitationally bound, star forming substructures in tidal tails of interacting galaxies, called tidal dwarf galaxies (TDG), has been studied, until now, only in idealised simulations of individual pairs of interacting galaxies for pre-determined orbits, mass ratios, and gas fractions. Here, we present the first identification of TDG candidates in fully cosmological simulations, specifically the high-resolution simulations of the EAGLE suite. The finite resolution of the simulation limits their ability to predict the exact formation rate and survival timescale of TDGs, but we show that gravitationally bound baryonic structures in tidal arms already form in current state-of-the-art cosmological simulations. In this case, the orbital parameter, disc orientations as well as stellar and gas masses and the specific angular momentum of the TDG forming galaxies are a direct consequence of cosmic structure formation. We identify TDG candidates in a wide range of environments, such as multiple galaxy mergers, clumpy high-redshift (up to z = 2) galaxies, high-speed encounters, and tidal interactions with gas-poor galaxies. We present selection methods, the properties of the identified TDG candidates and a roadmap for more quantitative analyses using future high-resolution simulations.

H i, star formation and tidal dwarf candidate in the Arp 305 system

We present results from our Giant Metrewave Radio Telescope (GMRT) HI observations of the Arp 305 system. The system consists of two interacting spiral galaxies NGC 4016 and NGC 4017, a large amount of resultant tidal debris and a prominent tidal dwarf galaxy (TDG) candidate projected within the tidal bridge between the two principal galaxies. Our higher resolution GMRT HI mapping, compared to previous observations, allowed detailed study of smaller scale features. Our HI analysis supports the conclusion in Hancock et al. (2009) that the most recent encounter between the pair occurred $\sim$ 4 $\times$ 10$^8$ yrs ago. The GMRT observations also show HI features near NGC 4017 which may be remnants of an earlier encounter between the two galaxies. The HI properties of the Bridge TDG candidate include: M(HI) $\sim$ 6.6 $\times$ 10$^8$ msolar and V(HI) = 3500$\pm$ 7 km/s, which is in good agreement with the velocities of the parent galaxies. Additionally the TDG's HI linewidth of 30 km/s and a modest velocity gradient together with its SFR of 0.2 msolar/yr add to the evidence favouring the bridge candidate being a genuine TDG. The Bridge TDG's \textit{Spitzer} 3.6 $\mu$m and 4.5 $\mu$m counterparts with a [3.6]--[4.5] colour $\sim$ -0.2 mag suggests stellar debris may have seeded its formation. Future spectroscopic observations could confirm this formation scenario and provide the metallicity which is a key criteria for the validation for TDG candidates.

Kinematics of dwarf galaxies in gas-rich groups, and the survival and detectability of tidal dwarf galaxies

We present DEIMOS multi-object spectroscopy (MOS) of 22 star-forming dwarf galaxies located in four gas-rich groups, including six newly-discovered dwarfs. Two of the galaxies are strong tidal dwarf galaxy (TDG) candidates based on our luminosity-metallicity relation definition. We model the rotation curves of these galaxies. Our sample shows low mass-to-light ratios (M/L=0.73$\pm0.39M_\odot/L_\odot$) as expected for young, star-forming dwarfs. One of the galaxies in our sample has an apparently strongly-falling rotation curve, reaching zero rotational velocity outside the turnover radius of $r_{turn}=1.2r_e$. This may be 1) a polar ring galaxy, with a tilted bar within a face-on disk; 2) a kinematic warp. These scenarios are indistinguishable with our current data due to limitations of slit alignment inherent to MOS-mode observations. We consider whether TDGs can be detected based on their tidal radius, beyond which tidal stripping removes kinematic tracers such as H$\alpha$ emission. When the tidal radius is less than about twice the turnover radius, the expected falling rotation curve cannot be reliably measured. This is problematic for as much as half of our sample, and indeed more generally, galaxies in groups like these. Further to this, the H$\alpha$ light that remains must be sufficiently bright to be detected; this is only the case for three (14%) galaxies in our sample. We conclude that the falling rotation curves expected of tidal dwarf galaxies are intrinsically difficult to detect.

Star formation properties of Hickson Compact Groups based on deep Hα imaging

We present deep Hα imaging of seven Hickson Compact Groups (HCGs) using the 4.1-m Southern Astrophysics Research (SOAR) Telescope. The high spatial resolution of the observations allows us to study both the integrated star formation properties of the main galaxies as well as the 2D distribution of star-forming knots in the faint tidal arms that form during interactions between the individual galaxies. We derive star formation rates and stellar masses for group members and discuss their position relative to the main sequence of star-forming galaxies. Despite the existence of tidal features within the galaxy groups, we do not find any indication for enhanced star formation in the selected sample of HCGs. We study azimuthally averaged Hα profiles of the galaxy discs and compare them with the g′ and r′ surface brightness profiles. We do not find any truncated galaxy discs but reveal that more massive galaxies show a higher light concentration in Hα than less massive ones. We also see that galaxies that show a high light concentration in r′, show a systematic higher light concentration in Hα. Tidal dwarf galaxy (TDG) candidates have been previously detected in R-band images for two groups in our sample but we find that most of them are likely background objects as they do not show any emission in Hα. We present a new TDG candidate at the tip of the tidal tail in HCG 91.

H i in the Arp 202 system and its tidal dwarf candidate

We present results from our Giant Metrewave Radio Telescope (GMRT) HI observations of the interacting pair Arp 202 (NGC 2719 and NGC 2719A). Earlier deep UV(GALEX) observations of this system revealed a tidal tail like extension with a diffuse object towards its end, proposed as a tidal dwarf galaxy (TDG) candidate. We detect HI emission from the Arp 202 system, including HI counterparts for the tidal tail and the TDG candidate. Our GMRT HI morphological and kinematic results clearly link the HI tidal tail and the HI TDG counterparts to the interaction between NGC 2719 and NGC 2719A, thus strengthening the case for the TDG. The Arp 202 TDG candidate belongs to a small group of TDG candidates with extremely blue colours. In order to gain a better understanding of this group we carried out a comparative study of their properties from the available data. We find that HI (and probably stellar) masses of this extremely blue group are similar to the lowest HI mass TDGs in the literature. However the number of such blue TDG candidates examined so far is too small to conclude whether or not their properties justify them to be considered as a subgroup of TDGs.

Identification of old tidal dwarfs near early-type galaxies from deep imaging and H i observations

It has recently been proposed that the dwarf spheroidal galaxies located in the Local Group disks of satellites (DoSs) may be tidal dwarf galaxies (TDGs) born in a major merger at least 5 Gyr ago. Whether TDGs can live that long is still poorly constrained by observations. As part of deep optical and HI surveys with the CFHT MegaCam camera and Westerbork Synthesis Radio Telescope made within the ATLAS3D project, and follow-up spectroscopic observations with the Gemini-North telescope, we have discovered old TDG candidates around several early-type galaxies. At least one of them has an oxygen abundance close to solar, as expected for a tidal origin. This confirmed pre-enriched object is located within the gigantic, but very low surface brightness, tidal tail that emanates from the elliptical galaxy, NGC 5557. An age of 4 Gyr estimated from its SED fitting makes it the oldest securely identified TDG ever found so far. We investigated the structural and gaseous properties of the TDG and of a companion located in the same collisional debris, and thus most likely of tidal origin as well. Despite several Gyr of evolution close to their parent galaxies, they kept a large gas reservoir. Their central surface brightness is low and their effective radius much larger than that of typical dwarf galaxies of the same mass. This possibly provides us with criteria to identify tidal objects which can be more easily checked than the traditional ones requiring deep spectroscopic observations. In view of the above, we discuss the survival time of TDGs and question the tidal origin of the DoSs.

CHOIRS H I GALAXY GROUPS: THE METALLICITY OF DWARF GALAXIES

We present a recalibration of the luminosity-metallicity relation for gas-rich, star-forming dwarfs to magnitudes as faint as M$_R$ ~ -13. We use the Dopita et al. (2013) metallicity calibrations to calibrate the relation for all of the data in this analysis. In metallicity-luminosity space we find two sub-populations within a sample of high-confidence SDSS DR8 star-forming galaxies; 52% are metal-rich giants and 48% are metal-medium galaxies. Metal-rich dwarfs classified as tidal dwarf galaxy (TDG) candidates in the literature are typically of metallicity 12 + log(O/H) = 8.70 $\pm$ 0.05, while SDSS dwarfs fainter than M$_R$ = -16 have a mean metallicity of 12 + log(O/H) = 8.28 $\pm$ 0.10, regardless of their luminosity, indicating that there is an approximate floor to the metallicity of low luminosity galaxies. Our hydrodynamical simulations predict that TDGs should have metallicities elevated above the normal luminosity-metallicity relation. Metallicity can therefore be a useful diagnostic for identifying TDG candidate populations in the absence of tidal tails. At magnitudes brighter than M$_R$ ~ -16 our sample of 53 star-forming galaxies in 9 HI gas-rich groups is consistent with the normal relation defined by the SDSS sample. At fainter magnitudes there is an increase in dispersion in metallicity of our sample, suggestive of a wide range of HI content and environment. In our sample we identify three (16% of dwarfs) strong TDG candidates (12 + log(O/H) > 8.6), and four (21%) very metal poor dwarfs (12 + log(O/H) < 8.0), which are likely gas-rich dwarfs with recently ignited star formation.

Intergalactic magnetic fields in Stephan's Quintet

We present results of the VLA radio continuum total power and polarised intensity observations of Stephan's Quintet at 1.43 and 4.86 GHz along with complementary 4.85 and 8.35 GHz Effelsberg observations. Our study shows a large envelope of radio emission encompassing all the member galaxies and hence a large volume of intergalac- tic matter. Infall of the galaxy NGC 7318B produces a ridge of intergalactic, polarised emission, for which the magnetic field strength was estimated as 11.0 \pm 2.2 {\mu}G, with an ordered component of 2.6 \pm 0.8 {\mu}G. The energy density of the field within the ridge area is of the same order as estimates of the thermal component, implying a significant role of the magnetic field in the dynamics of the intergalactic matter. We also report that the tidal dwarf galaxy candidate SQ-B possesses a strong and highly anisotropic magnetic field with the total strength being equal to 6.5 \pm 1.9 {\mu}G and an ordered component reaching 3.5 \pm 1.2 {\mu}G, which is comparable to that found in normal-sized galaxies.