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Register a new userKinematics of dwarf galaxies in gas-rich groups, and the survival and detectability of tidal dwarf galaxies
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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.
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HI line widths are typically interpreted as a measure of ISM turbulence, which is potentially driven by star formation. In an effort to better understand the possible connections between line widths and star formation, we have characterized hi{} kinematics in a sample of nearby dwarf galaxies by co-adding line-of-sight spectra after removing the rotational velocity to produce an average, global hi{} line profile. These superprofiles are composed of a central narrow peak (~6-10 km/s) with higher-velocity wings to either side that contain ~10-15% of the total flux. The superprofiles are all very similar, indicating a universal global HI profile for dwarf galaxies. We compare characteristics of the superprofiles to various galaxy properties, such as mass and measures of star formation (SF), with the assumption that the superprofile represents a turbulent peak with energetic wings to either side. We use these quantities to derive average scale heights for the sample galaxies. When comparing to physical properties, we find that the velocity dispersion of the central peak is correlated with $<Sigma_mathrm{HI}>$. The fraction of mass and characteristic velocity of the high velocity wings are correlated with measures of SF, consistent with the picture that SF drives surrounding HI to higher velocities. While gravitational instabilities provide too little energy, the SF in the sample galaxies does provide enough energy through supernovae, with realistic estimates of the coupling efficiency, to produce the observed superprofiles.
We determined the HI mass function of galaxies in the Ursa Major association of galaxies using a blind VLA-D array survey, consisting of 54 pointings in a cross pattern, covering the centre as well as the outskirts of the Ursa Major volume. The calculated HI mass function has best-fitting Schechter parameters {theta}^* = 0.19+/-0.11 Mpc^{-3}, log(M^*_{HI}/M_{odot}) = 9.8+/-0.8 and {alpha} = -0.92+/-0.16. The high-mass end is determined by a complementary, targeted WSRT survey, the low-mass end is determined by the blind VLA survey. The slope is significantly shallower than the slopes of the HIPASS ({alpha} = -1.37+/-0.03+/-0.05) and ALFALFA ({alpha} = -1.33+/-0.02) HI mass functions, which are measured over much larger volumes and cover a wider range of cosmic environments: There is a relative lack of low HI mass galaxies in the Ursa Major region. This difference in the slope strongly hints at an environmental dependence of the HI mass function slope.
Galaxy evolution by interaction driven transformation is probably highly efficient in groups of galaxies. Dwarf galaxies with their shallow potential are expected to reflect the interaction most prominently in their observable structure. The major aim of this series of papers is to establish a data base which allows to study the impact of group interaction onto the morphology and star-forming properties of dwarf galaxies. Firstly, we present our selection rules for target groups and the morphological selection method of target dwarf member candidates. Secondly, the spectroscopic follow-up observations with the HET are present. Thirdly, we applied own reduction methods based on adaptive filtering to derive surface photometry of the candidates. The spectroscopic follow-up indicate a dwarf identification success rate of roughly 55%, and a group member success rate of about 33%. A total of 17 new low surface brightness members is presented. For all candidates, total magnitudes, colours, and light distribution parameters are derived and discussed in the context of scaling relations. We point out short comings of the SDSS standard pipeline for surface photometry for these dim objects. We conclude that our selection strategy is rather efficient to obtain a sample of dim, low surface brightness members of groups of galaxies within the Virgo super-cluster. The photometric scaling relation in these X-ray dim, rather isolated groups does not significantly differ from those of the galaxies within the Local Volume.
Dwarf galaxies generally follow a mass-metallicity (MZ) relation, where more massive objects retain a larger fraction of heavy elements. Young tidal dwarf galaxies (TDGs), born in the tidal tails produced by interacting gas-rich galaxies, have been thought to not follow the MZ relation, because they inherit the metallicity of the more massive parent galaxies. We present chemical evolution models to investigate if TDGs that formed at very high redshifts, where the metallicity of their parent galaxy was very low, can produce the observed MZ relation. Assuming that galaxy interactions were more frequent in the denser high-redshift universe, TDGs could constitute an important contribution to the dwarf galaxy population. The survey of chemical evolution models of TDGs presented here captures for the first time an initial mass function (IMF) of stars that is dependent on both the star formation rate and the gas metallicity via the integrated galactic IMF (IGIMF) theory. As TDGs form in the tidal debris of interacting galaxies, the pre-enrichment of the gas, an underlying pre-existing stellar population, infall, and mass dependent outflows are considered. The models of young TDGs that are created in strongly pre-enriched tidal arms with a pre-existing stellar population can explain the measured abundance ratios of observed TDGs. The same chemical evolution models for TDGs, that form out of gas with initially very low metallicity, naturally build up the observed MZ relation. The modelled chemical composition of ancient TDGs is therefore consistent with the observed MZ relation of satellite galaxies.
The shallow faint-end slope of the galaxy mass function is usually reproduced in $Lambda$CDM galaxy formation models by assuming that the fraction of baryons that turns into stars drops steeply with decreasing halo mass and essentially vanishes in haloes with maximum circular velocities $V_{rm max}<20$-$30$ km/s. Dark matter-dominated dwarfs should therefore have characteristic velocities of about that value, unless they are small enough to probe only the rising part of the halo circular velocity curve (i.e., half-mass radii, $r_{1/2}ll 1$ kpc). Many dwarfs have properties in disagreement with this prediction: they are large enough to probe their halo $V_{rm max}$ but their characteristic velocities are well below $20$ km/s. These `cold faint giants (an extreme example is the recently discovered Crater 2 Milky Way satellite) can only be reconciled with our $Lambda$CDM models if they are the remnants of once massive objects heavily affected by tidal stripping. We examine this possibility using the APOSTLE cosmological hydrodynamical simulations of the Local Group. Assuming that low velocity dispersion satellites have been affected by stripping, we infer their progenitor masses, radii, and velocity dispersions, and find them in remarkable agreement with those of isolated dwarfs. Tidal stripping also explains the large scatter in the mass discrepancy-acceleration relation in the dwarf galaxy regime: tides remove preferentially dark matter from satellite galaxies, lowering their accelerations below the $a_{rm min}sim 10^{-11} m/s^2$ minimum expected for isolated dwarfs. In many cases, the resulting velocity dispersions are inconsistent with the predictions from Modified Newtonian Dynamics, a result that poses a possibly insurmountable challenge to that scenario.
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