arXiv subjects

S. S. McGaugh

Publications and source records attributed to S. S. McGaugh.

24 records · Page 2Linked to original sources

arXiv1998

Testing Modified Newtonian Dynamics with Low Surface Brightness Galaxies --Rotation curve fits-

We present MOND (Modified Newtonian Dynamics) fits to 15 rotation curves of LSB galaxies. Good fits are readily found, although for a few galaxies minor adjustments to the inclination are needed. Reasonable values for the stellar mass-to-light ratios are found, as well as an approximately constant value for the total (gas and stars) mass-to-light ratio. We show that the LSB galaxies investigated here lie on the one, unique Tully-Fisher relation, as predicted by MOND. The scatter on the Tully-Fisher relation can be completely explained by the observed scatter in the total mass-to-light ratio. We address the question of whether MOND can fit any arbitrary rotation curve by constructing a plausible fake model galaxy. While MOND is unable to fit this hypothetical galaxy, a normal dark halo fit is readily found, showing that dark matter fits are much less selective in producing fits. The good fits to rotation curves of LSB galaxies support MOND, especially as these are galaxies with large mass discrepancies deep in the MOND regime.

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arXiv1996

Does low surface brightness mean low surface density?

We compare the dynamical properties of two galaxies at identical positions on the Tully-Fisher relation, but with different surface brightnesses. We find that the low surface brightness galaxy UGC 128 has a higher mass-to-light ratio, and yet has lower mass densities than the high surface brightness galaxy NGC 2403. This is true for the gas surface density, the stellar surface density, and the total mass density.

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arXiv1996

HI observations of low surface brightness galaxies: Probing low-density galaxies

We present Very Large Array ({\sc vla}) and Westerbork Synthesis Radio Telescope ({\sc wsrt}) 21-cm H{\sc i} observations of 19 late-type low surface brightness (LSB) galaxies. Our main findings are that these galaxies, as well as having low surface brightnesses, have low H{\sc i} surface densities, about a factor of $\sim 3$ lower than in normal late-type galaxies. We show that LSB galaxies in some respects resemble the outer parts of late-type normal galaxies, but may be less evolved. LSB galaxies are more gas-rich than their high surface brightness counterparts. The rotation curves of LSB galaxies rise more slowly than those of HSB galaxies of the same luminosity, with amplitudes between 50 and 120~km~s$^{-1}$, and are often still increasing at the outermost measured point. The shape of the rotation curves suggests that LSB galaxies have low matter surface densities. We use the average total mass surface density of a galaxy as a measure for the evolutionary state, and show that LSB galaxies are among the least compact, least evolved galaxies. We show that both $M_{\rm HI}/L_B$ and $M_{\rm dyn}/L_B$ depend strongly on central surface brightness, consistent with the surface brightness--mass-to-light ratio relation required by the Tully-Fisher relation. LSB galaxies are therefore slowly evolving galaxies, and may well be low surface density systems in all respects.

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arXiv1995

Galaxy Selection and the Surface Brightness Distribution

Optical surveys for galaxies are biased against the inclusion of low surface brightness (LSB) galaxies. Disney (1976) suggested that the constancy of disk central surface brightness noticed by Freeman (1970) was not a physical result, but instead was an artifact of sample selection. Since LSB galaxies do exist, the pertinent and still controversial issue is if these newly discovered galaxies constitute a significant percentage of the general galaxy population. In this paper, we address this issue by determining the space density of galaxies as a function of disk central surface brightness. Using the physically reasonable assumption (which is motivated by the data) that central surface brightness is independent of disk scale length, we arrive at a distribution which is roughly flat (i.e. approximately equal numbers of galaxies at each surface brightness) faintwards of the Freeman (1970) value. Brightwards of this, we find a sharp decline in the distribution which is analogous to the turn down in the luminosity function at $L^*$. An intrinsically sharply peaked ``Freeman Law'' distribution can be completely ruled out, and no Gaussian distribution can fit the data. Low surface brightness galaxies (those with central surface brightnesses fainter than $22~B\sb$) comprise $\gtrsim 1/2$ the general galaxy population, so a representative sample of galaxies at $z = 0$ does not really exist at present since past surveys have been insensitive to this component of the general galaxy population.

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arXiv1995

The Tully-Fisher relation for low surface brightness galaxies - implications for galaxy evolution

We present the B band Tully-Fisher relation for Low Surface Brightness (LSB) galaxies. These LSB galaxies follow the same Tully-Fisher relation as normal spiral galaxies. This implies that the mass-to-light ratio (M/L) of LSB galaxies is typically a factor of 2 larger than that of normal galaxies of the same total luminosity and morphological type. Since the dynamical mass of a galaxy is related to the rotation velocity and scale length via M \propto V^2 h, at fixed linewidth LSB galaxies must be twice as large as normal galaxies. This is confirmed by examining the relation between scale length and linewidth for LSB and normal galaxies. The universal nature of the Tully-Fisher relation can be understood if LSB galaxies are galaxies with low mass surface density, σ. The mass surface density apparently controls the luminosity evolution of a galaxy such as to keep the product σM/L constant.

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arXiv1993

Spatial Distribution of Low Surface Brightness Galaxies

Using redshift samples, we calculate the cross-correlation functions of LSB galaxies with normal galaxies in complete samples (i.e. CfA and IRAS), which enables us to compare directly the amplitudes and shapes of the correlation functions. For pair separations $r\gs 2\mpc$, we find that the shape is in agreement with that of the correlation functions for other galaxies. The amplitudes ($A$) of $\xiab (r)$ are lower than those of the autocorrelation functions for the CfA and IRAS samples, with $A_{\rm LSB-CfA}:A_{\rm CfA-CfA}\approx 0.4$ and $A_{\rm LSB-IRAS}:A_{\rm IRAS-IRAS}\approx 0.6$. These results suggest that LSB galaxies are imbedded in the same large scale structure as other galaxies, but are less strongly clustered. This offers the hope that LSB galaxies may be unbiased tracers of the mass density on large scales. For $r\ls 2\mpc$, the cross-correlation functions are significantly lower than that expected from the extrapolation of $\xiab$ on larger scales, showing that the formation and survival of LSB galaxies may be inhibited by interaction with neighboring galaxies. The model which matches these observations suggests that strong luminosity segregation in galaxy clustering is not a necessary consequence of biased galaxy formation, unless the effect of surface brightness is taken in to account. It is also implies a significant mass density in LSB galaxies.

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